Preview Extract
This document contains Chapters 8 to 9 CHAPTER 8 PRINCIPLES OF DEVELOPMENT CHAPTER OUTLINE 8.1. History A. Primary Organizer 1. Spemann’s and Mangold’s Research a. Research in 1920–1930s centered on discovering how one tissue influenced the fate of another: induction. b. Hans Spemann and Hilde Mangold noted the capacity of tissue from the dorsal lip of the gastrula could induce development of an entirely new salamander joined to the host salamander at the site of the transplant. c. Dorsal lip tissue was considered the primary organizer that set the axis of the secondary embryo. d. Recent advances in molecular biology have inaugurated the second golden age of embryology. Secretion of certain molecules triggers or represses the activity of a combination of genes in nearby cells. Cells of the Spemann organizer secrete proteins such as noggin, chordin, and follistatin. Similar proteins occur in both invertebrates and vertebrates. These scientific breakthroughs ushered in the new science of evolutionary developmental biology. 2. All of the knowledge for constructing a complex organism must be in the nucleus and cytoplasm. 3. Genetics and molecular biology techniques have made a breakthrough in embryology in the last two decades. 8.2 Early Concepts: Preformation Versus Epigenesis 1.. Preformation is the concept of a miniature adult being present in the sperm or egg, waiting to unfold. a. Some claimed they could see a miniature adult in the egg or sperm. (Figure 8.1) b. A young animal is merely unfolding the structures that are already there. 2. In 1759, Kaspar Friederich Wolff showed there was no preformed chick in the early egg. a. Undifferentiated granular material became arranged into layers. b. The layers thickened, thinned, and folded to produce the embryo. c. Epigenesis is the concept that the embryo contains building materials that are assembled. 3. Development is a series of progressive changes. (Figure 8.2) a. This begins when a fertilized egg divides mitotically. b. Specialization occurs as a hierarchy of developmental “decisions.” c. Cell types do not unfold but arise from conditions created in preceding stages. d. Interactions become increasingly restrictive; each stage limits developmental fate. e. With each new stage, cells lose the option to become something different—it becomes determined. f. Both cytoplasmic localization and induction cause this feature. 8.3 Fertilization A. Initial Event 1. Fertilization is the union of male and female gametes. a. Fertilization provides for recombination of paternal and maternal genes, restoring the diploid number. b. Fertilization activates the egg to begin development. B. Oocyte Maturation (Figure 8.3) 1. Contrast with sperm. a. The sperm eliminates nearly all cytoplasm and condenses its nucleus. b. The egg grows in size by accumulating yolk; it also contains much mRNA, ribosomes, tRNA and elements for protein synthesis. 2. Morphogenetic determinants direct the activation and repression of specific genes in the egg. 3. The egg nucleus grows in size, bloated with RNA and is called the germinal vesicle. 4. Preparations in the egg occur during the prolonged prophase. 5. After meiosis resumes, the egg is ready to fuse its nucleus with the sperm nucleus. C. Fertilization and Activation 1. A century of research has been conducted on marine invertebrates, especially sea urchins. 2. Contact and Recognition Between Egg and Sperm (Figures 8.4, 8.5) a. Marine organisms release enormous numbers of sperm in the ocean to fertilize eggs. b. Many marine eggs release a chemotactic molecule to attract sperm of the same species. c. Sea urchin sperm first penetrate the jelly layer before contacting the vitelline envelope. d. Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope. e. In the marine environment, many species may be spawning at the same time. f. Similar recognition proteins are found on sperm of vertebrate species. 3. Prevention of Polyspermy (Figures 8.5, 8.6, 8.7) a. A fertilization cone forms where the sperm contacts the vitelline membrane. b. Important changes in the egg surface block entrance to any additional sperm. c. Polyspermy, the entry of more than one sperm, would cause a triploid nucleus. d. In the sea urchin, an electrical potential rapidly spreads across the membrane; this is the “fast block.” e. This is followed by the cortical reaction. 1) Thousands of enzyme-rich cortical granules below the egg membrane fuse with the membrane. 2) The cortical granules release contents between the membrane and vitelline envelope. 3) This lifts the envelope and forms a moat. 4) One cortical granule enzyme causes the vitelline envelope to harden, becoming the fertilization membrane. 4. Fusion of Pronuclei and Egg Activation (Figure 8.7) a. After sperm and egg membranes have fused, the sperm disconnects from its flagellum. b. The enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus. c. Fusion forms a diploid nucleus. 1) Nuclear fission takes 12 minutes in sea urchins; about 12 hours in mammals. 2) The fertilized egg is now properly called a zygote. d. Fission removes one or more inhibitors that blocked metabolism and kept the egg quiescent. e. DNA and protein synthesis undergoes a burst of activity, using a supply of mRNA in the egg cytoplasm. f. Fertilization initiates reorganization of cytoplasm and repositions determinants that begin development and cleavage. D. What Can We Learn from Development? 1. Biologists study development for different reasons. a. One reason is to understand how a single cell (the zygote) can produce the variety of body parts in an organism. 2. Another reason is the search for commonalities among organisms. 8.4. Cleavage and Early Development A. Blastomeres 1. The embryo undergoes cleavage to convert the large cytoplasmic mass into small maneuverable cells. 2. No cell growth occurs, only subdivision until cells reach regular somatic cell size. 3. At the end of cleavage, polychaete worms have 1000 cells, amphioxus has 9000, and frogs have 700,000. 4. Polarity—a polar axis—establishes the direction of cleavage and differentiation. B. Patterns of Cleavage (Figure 8.8) 1. The pattern of cleavage is affected by: a. quantity and distribution of yolk present, and b. genes controlling the symmetry of cleavage. 2. The yolk-rich end of the embryo is the vegetal pole while the other end is the animal pole. 3. Cleavage is an orderly sequence of cell divisions. 4. During each division, a cleavage furrow is visible in the cell. C. How Amount and Distribution of Yolk Affects Cleavage 1. Isolecithal yolk describes eggs with very little yolk and the yolk is distributed evenly. (Figure 8.8) a. In such eggs, cleavage is holoblastic. b. The cleavage furrow extends completely through the egg. Isolecithal eggs are widespread and present in: echinoderms, tunicates, cephalochordates, molluscs and mammals. Cleavage is slowed in the yolk-rich vegetal pole. 2. Mesolecithal eggs have a moderate amount of yolk concentrated in the vegetal pole. a. The animal pole is opposite the vegetal pole and contains cytoplasm and very little yolk. b. These eggs cleave holoblastically, but cleavage is retarded in yolk-rich vegetal pole. c. The cleavage furrow progresses much more slowly through the vegetal pole; thus cleavage is faster in the animal region. d. Amphibians have mesolecithal eggs. 3. Telolecithal eggs have much yolk concentrated at the vegetal pole. a. Actively dividing cytoplasm is confined to a narrow disc-shaped mass on the yolk. b. Cleavage is partial or meroblastic; the furrow does not cut through the heavy yolk. c. Birds, reptiles, most fishes and a few amphibians have telolecithal eggs. 4. Centrolecithal eggs have a large mass of centrally located yolk. (Figure 8.9) a. Cytoplasmic cleavage is limited to a surface layer of yolk-free cytoplasm; yolk-rich inner cytoplasm is uncleaved. b. They have meroblastic cleavage. c. Insects and many other arthropods have centrolecithal eggs. d. Yolk is therefore an impediment to cleavage. 5. Amount of yolk affects developmental mode. a. In most animals, a mother does not directly nourish embryonic development but has provisioned the egg with yolk. b. The amount of yolk is related not only to cleavage pattern, but also to whether a larval stage occurs during development. c. The cleavage furrows cut into the cytoplasm in either radial, spiral, discoidal, or rotational ways. d. Animals in which the zygote is telolecithal generally have direct development. e. Species with isolecithal or mesolecithal zygotes generally have indirect development. (Figure 8.10) f. Indirect development is characteristic of animals where the larval stage is between embryo and adult. g. Direct development can occur when there is enough yolk to support growth as juveniles; this occurs in reptiles and birds. 8.5 An Overview of Development Following Cleavage A. Blastulation 1. Cleavage creates a cluster of cells called the blastula (Figure 8.11).; in mammals the cluster of cells is called a blastocyst (Figure 8.20E). 2. In most animals, these cells are arranged around a fluid-filled cavity called the blastocoel. 3. The blastula stage typically consists of a few hundred to several thousand cells. 4. Blastula formation, with a single germ layer, occurs in all multicellular animals. 5. The germ layers formed during development ultimately produce all structures of the adult body. B. Gastrulation and Formation of Two Germ Layers (Figure 8.11) 1. Gastrulation results in the formation of a second germ layer. 2. Typically, gastrulation involves an invagination of one side of the blastula. 3. This invagination forms a new internal cavity, the archenteron or gastrocoel. 4. The opening into the cavity is the blastopore. 5. The gastrula has an outer layer of ectoderm and an inner layer of endoderm. The only opening into this embryonic gut is at the blastopore. 6. Some animals retain a blind, or incomplete gut (gastrovascular cavity), but most have a complete gut with a second opening, the anus. C. Formation of a Complete Gut (Figure 8.11) 1. In the formation of a complete gut, the inward movement of the archenteron continues until the end of the archenteron reaches the ectodermal wall of the gastrula. 2. An endodermal tube, the gut, is surrounded by the blastocoel. 3. The endodermal tube has two openings, the blastopore and a second opening formed by the merging of the archenteron tube with the ectoderm. D. Formation of Mesoderm, a Third Germ Layer (Figures 8.11, 8.20A, 8.20C) 1. Animals with two germ layers are diploblastic. 2. Most animals add a third germ layer and are triploblastic. 3. The third layer, mesoderm, lies between the endoderm and the ectoderm. 4. Mesoderm can form in two different ways: the proliferation of cells from near the lip of the blastopore into the space between the archenteron and the outer body wall, or by the pushing of the central region of the archenteron wall into the space between the archenteron and the outer body wall. 5. Regardless of method of mesoderm formation, initial mesoderm cells arise from endoderm. E. Formation of the Coelom (Figure 8.11) 1. A coelom is a body cavity surrounded by mesoderm. 2. The coelomic cavity appears within the mesoderm by either schizocoely or enterocoely. 3. The method by which the coelom forms is an inherited character and is important in grouping organisms based on developmental characters. 4. Upon completion of coelom formation, the body has three tissue layers and two cavities. 8.6. Mechanisms of Development A. Nuclear Equivalence 1. Roux-Weismann Hypothesis a. An early—but wrong—explanation of differentiation was based on division of nuclear material along the cell lineage. b. Early embryologists saw this as an explanation of differentiation; hereditary material was parceled out to cells. c. Hans Driesch separated the two-celled sea urchin stage; both developed into normal larvae but this did not disprove eventual progressive modification. 2. Hans Spemann Disproves Roux-Weismann Hypothesis, Discovers Gray Crescent a. Spemann used human hair to almost tie-off a salamander zygote. b. The nucleus was on one side; only cytoplasm on the other. c. The nucleated side divided many times: only when one nucleus wandered across did the cytoplasmic side divide. d. Its normal development showed no nuclear material had been lost after many nuclear divisions. e. Occasionally the nucleated side only developed into a ball of “belly” tissue; it was missing the pigment-free gray crescent area that appears at fertilization. f. This disproved the Roux-Weismann Hypothesis and showed that cytoplasm in the gray crescent contained essential information. Spemann’s experiment demonstrated that every blastomere contains sufficient genetic information for the development of a complete animal. Cloning is still technically difficult. 3. Methods of Differential Cell Differentiation a. Except for insects, cells become committed to particular fates from cytoplasmic segregation of determinative molecules during cleavage, and from interaction among neighboring cells (induction). B. Cytoplasmic Specification (Figure 8.12) 1. Cytoplasmic components are unevenly distributed in a zygote. 2. Different components contain morphogenetic determinants that control commitment to cell type. 3. Different determinants are partitioned among different blastomeres by cleavage and determine cell fate. 4. Some tunicate species have different colored types of cytoplasm. a. Yellow cytoplasm gives rise to muscle cells. b. Gray equatorial cytoplasm produces the notochord and neural tube. c. Clear cytoplasm produces larval epidermis. d. Gray vegetal cytoplasm gives rise to the gut. C. Conditional Specification (Figure 8.13) 1. Induction is the capacity of some cells to evoke a specific developmental response in other cells. a. Following Hans Spemann’s work described before, a graft of the dorsal blastopore lip could induce host ectoderm to form a neural tube. b. Such a graft is formed of partly grafted tissue and partly induced host tissue. c. But only grafts of dorsal lip blastopore tissue could cause induction. d. Only ectoderm of the host would respond by developing nervous tissue. e. Dorsal lip was the primary organizer because it was the only tissue to induce growth. f. Spemann called this primary induction the first inductive event; there are other cell types that originate from secondary induction. 2. Cells that have differentiated act as inductors for adjacent undifferentiated cells. a. Timing is critical; primary induction sets in motion secondary induction. b. The sequence includes cell movement, changes in adhesion, and cell proliferation. c. There is no “hard-wired” master control panel directing development. D. Syncytial Specification 1. A syncytium is a single cell membrane surrounding multiple nuclei, which occurs in the centrolecithal egg of Drosophila. 2. Some development occurs before the syncytium is cellularized. 3. Syncytial specification is similar to conditional specification except that the molecules the influence cell fate diffuse within the cytoplasm, not among cells. 8.7 Gene Expression During Development A. Pattern Formation 1. After fertilization, proteins are translated from stored mRNA transcribed from maternal genome. 2. In many animals, maternal mRNA directs protein synthesis through cleavage and to mid-blastula stage. 3. After this, protein synthesis switches from maternal to zygotic control as the nucleus transcribes its own mRNA. B. Homeotic Genes and Hox Genes (Figures 8.14, 8.15) 1. Gene expression is regulated to ensure orderly development. 2. Mutations of homeotic genes in fruit flies revealed they controlled overall body plan of legs, wings, etc. 3. The homeobox is a 180-nucleotide DNA sequence that occurs in most animals. 4. These are master genes that control expression of subordinate genes. 5. Homeotic genes are remarkably similar across diverse species; evolution “solved” the development problem only once. 6. Proteins coded by homeobox genes contain a highly conserved 60-amino acid sequence: the homeodomain. 7. The homeodomain proteins all bind to specific promoter sequences of DNA; they switch subordinate genes on or off. 8. Mice and humans have four clusters of homeobox-containing genes on separate chromosomes; all are homologues of the fruit fly’s homeotic genes. 9. Hox and homeobox genes play a role in shaping individual organs and limbs. (Figures 8.15, 8.16) C. Morphogenesis of Limbs and Organs 1. New limb buds can be induced to grow from the side of a chick by implanting a bead soaked in fibroblast growth factor (FGF). 2. FGF forms a gradient from the apical ectodermal ridge to the base of the limb bud. 3. This gradient helps establish a proximodistal axis — one of the three axes that guide limb development. (Figure 8.17). 8.8 Developmental patterns in animals A. There are 34 phyla of naimls in the clodogram (see inside front cover). 1. Triphlobastic bilaterally symmetrical animals are divided into two major clades: Protostomia and Deutrostomia. B. Protostome Development 1. Cleavage Patterns a. Spiral cleavage occurs in most protostomes. (Figure 8.18) b. Blastomeres cleave obliquely to the animal-vegetal axis. c. Upper layer of cells appear offset relative to lower layer. d. Mosaic development is characteristic of most protostomes. (Figure 8.19) e. In mosaic development, cell fate is determined by the distribution of morphogenetic determinants, in the egg cytoplasm. f. In superficial cleavage, the centrally located mass of yolk restricts cleavage to the cytoplasmic rim of the egg. 2. Fate of Blastopore a. Protostomes are so named because the blastopore becomes the mouth. 3. Coelom Formation (Figure 8.18) a. In protostomes, a band of mesoderm forms around the gut before a coelom forms. b. If a coelom exists, it is formed by schizocoely. c. Endodermal cells move by ingression into the space between the archenteron walls and the ectoderm. d. These cells divide and produce mesodermal precursors; the proliferating cells become the mesoderm. (Figure 8.20) e. Studies indicate that these mesodermal precursors originate from a single large blastomere, the 4d cell, present in a 29- to 64- cell embryo. f. Some protostomes, like flatworms, are acoelomate; g. In others, mesoderm lines only one side of the blastocoel, leaving a fluid-filled cavity around the gut called a pseudocoelom. h. The pseudocoelom is lined on the outer edge by mesoderm and on the inner edge by the endodermal gut lining. i. In most protostomes, mesodermal cells all derive from the 4d cell. j. In some nemertean worms, mesoderm derives from an earlier blastomere. (Figure 8.13) C. Deuterostome Development 1. Cleavage Patterns (Figures 8.18, 8.19) a. Radial cleavage is named because the embryonic cells are arranged in radial symmetry around the animal-vegetal axis. b. After the third cleavage, an upper tier of cells sits directly on top of the tier of cells below it. c. Most deuterostomes have regulative development in which the fate of a cell depends on interactions with neighboring cells. d. Early in the development of these embryos, each cell is capable of producing an entire embryo if separated from the other cells. 2. Fate of Blastopore a. Deuterostome embryos develop a complete gut. b. The blastopore becomes the anus and second opening becomes the mouth. 3. Coelom Formation (Figure 8.18) a. In enterocoely, the mesoderm and the coelom form at the same time. b. In enterocoely, gastrulation begins with one side of the blastula bending inward to form the archenteron. c. As the archenteron elongates, its sides push outward and expand into a pouch-like coelomic compartment. d. This pouch-like compartment pinches off to form a mesodermally bound space surrounding the gut. 4. Examples of Deuterostome Development — Variations in Deuterostome Cleavage a. Radial cleavage is characteristic of the Deuterostomia, including echinoderms, hemichordates and chordates. b. In bilateral cleavage, the egg is defined by unequal cytoplasmic components. (Figure 8.12) c. The first cleavage furrow of bilateral cleavage passes through the animal vegetal axis dividing the asymmetrically divided cytoplasm between the two blastomeres. d. This first cleavage pattern determines the future right and left side. e. The half-embryo formed on one side is the mirror image of the half embryo on the other side. f. Ascidians (tunicates) demonstrate this cleavage pattern. g. In rotational cleavage, the first cleavage plane is aligned with the animal vegetal axis. h. However, in the second cleavage, one blastomere divides meridionally while the other divides equatorially, rotated 90 degrees to the first. i. Early divisions may be asynchronous and possess odd numbers of cells below the 2-4-8-16 series that would occur with synchronous division. j. After the third division, cells form a tightly packed cluster stabilized by outer cells with tight junctions, the trophoblast. k. The trophoblast will form the embryonic portion of the placenta. l. Cells that give rise to the embryo are the inner cell mass. m. This type of cleavage is present in mammals and is slower than in other animal groups. n. Telolecithal eggs may divide by discoidal cleavage. o. In telolecithal eggs, there is a large mass of yolk in each egg; cleavage is confined to a small disc of cytoplasm. p. Early cleavage furrows carve the disc into a single layer of cells called the blastoderm. 5. Examples of Deuterostome Development — Variations in Deuterostome Gastrulation a. In sea stars, gastrulation begins with the flattening of the vegetal area of the blastula to form the vegetal plate. b. The archenterons elongates toward the animal pole and its anterior end expands to form two pouchlike coelomic vesicles. (Figure 8.20) c. The ectoderm will give rise to the epithelium of the body surface and to the nervous system. d. Endoderm will give rise to the epithelial lining of the digestive tube. e. The mesoderm, developed from the outpocketing of the archenterons, will form the muscular system, the reproductive system, the peritoneum, and the calcareous plates of the sea star’s endoskeleton. f. Frogs are deuterostomes with radial cleavage in which the movements during gastrulation are influenced by the large mass of yolk. (Figures 8.8B, 8.20B) g. Slower cleavage in the vegetal half of the blastula results in an asymmetry, with many small cells in the animal half and fewer, larger ones in the vegetal half. h. Gastrulation in amphibians begins with invagination of cells located on the future dorsal side of the embryo with the formation of a slitlike blastopore. i. Gastrulation progresses as sheets of cells turn inward and move inside the gastrula to form mesoderm and ectoderm. j. The primitive streak forms the anterioposterior axis and center of early growth in bird and reptile embryos. (Figure 8.21) k. The blastoderm consists of two layers: epiblast and hypoblast with a blastocoel between them. l. The epiblast sheet moves toward the primitive streak and over the edge, migrating as cells into the blastocoel. m. One stream of cells moves deeper, displacing midline hypoblast, and forms endoderm. n. Surface cells form ectoderm. o. All three layers lift from the yolk and pinch off, leaving a stalk attachment to the yolk at midbody. (Figures 8.21, 8.23) p. In mammalian gastrulation (Figure 8.20E), epiblast cells move medially through the primitive streak into the blastocoel; cells migrate laterally through the blastocoel to form mesoderm and endoderm. q. Endoderm derived from the hypoblast forms a yolk sac without yolk; mammals utilize nutrients from a placenta. r. Reptiles, birds and mammals share a common ancestor whose eggs were telolecithal; all inherited this gastrulation pattern and mammals then evolved isolecithal eggs with a telolecithal pattern. 8.9 Evolutionary Developmental Biology Zoologists have long looked to embryology for clues to evolutionary history. Evolutionary developmental biology has contributed several concepts to our understanding, but many questions remain. Are the body plans of all bilaterally symmetric animals fundamentally similar? Insects and amphibians share a similar control of dorsoventral patterning, except that one is upside down relative to the other. The French naturalist St. Hilaire suggested in 1822 that vertebrates were essentiallyinverted invertebrates. His idea was rejected, but zoologists are reconsidering whether protostomes and deuterostomes are simply inverted with respect to each other. D. Can the anatomy of extinct ancestral species be inferred from the developmental genes shared by their descendants? 1. Similar dorsoventral patterning in protostomes and deuterostomes suggests a common ancestor with similar patterning. 2. The same can be said for the similarity of HOM/Hox clusters in insects and chordates. E. Instead of evolution proceeding by the gradual accumulation of numerous small mutations, could it proceed by relatively few mutations in a few developmental genes? 1. The induction of legs or eyes by a mutation in one gene suggests that organs can develop as modules. 2. This raises the question of whether entire limbs or organs could be lost or gained during evolution as the result of one or a few mutations. 3. The existence of developmental genes that could bring about major changes would challenge Darwin’s theory of gradualism. 8.10 Vertebrate Development A. Common Vertebrate Heritage (Figure 8.22) 1. One outcome of shared ancestry in vertebrates is the similarity of postgastrula embryos. a. For a short time, all vertebrate embryos share: dorsal neural tube, notochord, pharyngeal gill pouches with aortic arches, ventral heart and postanal tail. b. This similarity is extraordinary considering the variety of eggs and developmental patterns. B. Amniotes and the Amniotic Egg (Figures 8.23, 8.24) 1. Amniotes are a monophyletic grouping of vertebrates. a. Their embryos develop within the amnion, a membranous sac that provides an aquatic environment and protects it from shock. b. The amniotic egg contains four extraembryonic membranes including the amnion. c. The amniotic egg allowed development away from water. d. The yolk sac is the first membrane and pre-dates the amniotes by millions of years; it is extraembryonic and is discarded. e. The yolk sac in animals that give live birth becomes vascular, however, and functions to transfer nutrients and gases from the mother. f. The allantois is a sac that grows out of the hindgut and holds metabolic wastes during development. g. The chorion lies beneath the eggshell and encloses the rest of the embryo. h. With growth, the allantois and chorion fuse to form the chorioallantoic membrane, a provisional “lung.” i. Evolution of the shelled amniotic egg made fertilization a requirement before a shell was developed. C. Mammalian Placenta and Early Mammalian Development (Figures 8.23, 8.25, 8.26) 1. Mammals inherited the amniotic egg but retained it in the mother’s body. a. Monotremes are examples of primitive mammals that lay eggs. b. In marsupials, embryos develop but do not “take root” in the uterus; they climb out and enter an external abdominal pouch. 2. Placental mammals represent 94% of the Class Mammalia. a. Evolution of a placenta required considerable restructuring. b. Extraembryonic membranes had to restructure to form the placenta. c. The maternal oviduct had to evolve to develop a uterus to house the embryo. 3. Early Stages of Mammalian Development a. The blastocyst travels down the oviduct toward the uterus, propelled by ciliary action and peristalsis. b. At about the sixth day, the human blastocyst composed of about 100 cells contacts the endometrium. c. On contact, the trophoblast cells proliferate rapidly and produce enzymes to break down epithelium of the endometrium. d. By the twelfth day, the blastocyst is totally buried, surrounded by maternal blood. e. The trophoblast thickens, sending out tiny fingers of chorionic villi. f. The surface area of chorionic villi eventually expands to 13 square meters in the human placenta. g. The amnion remains unchanged; the embryo floats in this “pond.” h. The yolk sac contains no yolk but has stem cells that give rise to blood and lymphoid cells; they later migrate into the developing embryo. i. The allantois is not needed to store wastes; it contributes to the umbilical cord. j. The chorion forms the embryo’s side of the placenta. k. The germinal period is the first two weeks. l. The embryonic period includes the next eight weeks when all major organs and body shape form; this is a period that is sensitive to drugs that may cause malformation in the embryo. m. The fetal period begins when embryo becomes a fetus at about two months; mainly a growth phase, but the endocrine and nervous systems continue to differentiate. 8.11 Development of Systems and Organs (Figure 8.27) A. Germ Layers 1. Germ layers should not be confused with germ cells (eggs and sperm). 2. Germ layers do not alone determine differentiation but rather the position of embryonic cells. B. Derivatives of Ectoderm: Nervous System and Nerve Growth (Figure 8.28) 1. Just above the notochord, the ectoderm thickens to form a neural plate. 2. Edges of the neural plate fold up to create an elongated, hollow neural tube. a. The anterior end of neural tube enlarges and forms the brain and cranial nerves. b. The posterior end forms the spinal cord and spinal motor nerves. c. Neural crest cells pinch off from the neural tube. 3. Neural crest cells form many structures. a. They become portions of cranial nerves, pigment cells, cartilage, bone, ganglia of the autonomic system, medulla of the adrenal gland, and parts of other endocrine glands. b. It is unique to vertebrates and was important in evolution of the vertebrate head and jaws. 4. In 1907, Ross Harrison cultured nerve cells; each axon grows from one cell. 5. Additional research revealed that a nerve axon grows in response to guidance molecules secreted into its path. (Figure 8.29) C. Derivatives of Endoderm: Digestive Tube and Survival of Gill Arches (Figures 8.22, 8.27, 8.30) 1. During gastrulation, the archenteron forms as the primitive gut. 2. This endodermal cavity eventually produces the digestive tract, lining of pharynx and lungs, most of the liver and pancreas, thyroid and parathyroid glands and thymus. 3. The alimentary canal develops from primitive gut; ends are lined with ectoderm. 4. Lungs, liver and pancreas arise from the foregut. 5. During development, endodermally-lined pharyngeal pouches interact with overlying ectoderm to form gill arches. 6. In fish, gill arches become gills. 7. Gill arches remain as necessary primordia for a variety of other structures in terrestrial vertebrates. a. The first arch and its endoderm-lined pouch form the upper and lower jaws. b. Second, third and fourth gill pouches become the tonsils, parathyroids and thymus. c. The original function has been abandoned but the structure is retained for new purposes; this provides a view of evolutionary history. D. Derivatives of Mesoderm: Support, Movement and the Beating Heart (Figure 8.31) 1. With an increase in size and complexity, mesodermally derived structures take up a greater proportion. 2. Muscles arise from mesoderm along each side of the neural tube. a. The mesoderm divides into a linear series of somites (38 in humans). b. The splitting, fusion and migration of somites produce the: 1) axial skeleton, 2) dermis of dorsal skin, and 3) muscles of the back, body wall and limbs. c. Limbs begin as buds from the side of the body; projections become fingers and toes. 3. Mesoderm gives rise to the embryonic heart. a. Guided by underlying endoderm, two clusters of precardiac mesodermal cells move to either side of the gut. b. These clusters differentiate into a pair of double-walled tubes that fuse into a single thin tube. c. The primitive heart begins beating on the second day of the 21-day incubation period; there is no blood or vessels at this time. d. Twitching becomes rhythmical as the ventricle and atrium develops; each chamber has a faster intrinsic beat. e. A specialized sinoatrial node in the sinus venosus eventually takes charge as pacemaker. f. The circulatory system becomes critical to deliver food from the yolk and return wastes to extraembryonic tissues. Lecture Enrichment 1. Describe how cellular differentiation occurs as different genes are turned on and off, and how different proteins and enzymatic products are produced in the different types of cells. Most of biology can be reduced to protein chemistry, and development is change in that chemistry. 2. In explaining various kinds of differentiation and how dissimilar cells could come from a single zygote, a visual image of animal phylogeny is useful to show the evolution of cleavage patterns, etc. 3. A balloon can be pinched many times to illustrate how the cleavage process can go through many rounds of cell division with no cell growth, starting with the egg as the largest single cell in a species. Emphasize that differentiation may begin with the first cell division. 4. Keep students aware that a chick embryo has requirements other than the nutrients in the yolk. Ask what they are and how they are dealt with—such as oxygen requirements (enters the shell) and removal of wastes (accumulate as uric acid in an extraembryonic membrane and are discarded with the eggshell). 5. Amniotic fluid is used to examine a fetus for genetic defects, since cells of the fetus are shed from the skin and from the respiratory and urinary tracts into the fluid. Chorionic villi are one of the first fetal tissues that can be sampled in prenatal diagnosis. Commentary/Lesson Plan Background: Most students will have virtually no direct experiences with embryology beyond general biology abstractions. A few students may have seen fertilized eggs and recognized that development occurs on the surface of the yolk. Developmental biology is rapidly expanding and modifying its knowledge base, perhaps second only to our rapidly expanding understanding about the brain and nervous system. Some of this chapter is critical to citizenry understanding future controversies concerning use of fetal tissues, etc. The complexities of developmental biology research likewise will require using visuals. The history story line (Spemann et al.) may be an excellent way to carry the otherwise abstract and complex topic. Misconceptions: Some students may have been led to believe that there is some point in development that is the “beginning of life”; since the germ line is continuous and living from sperm/egg through zygote, this is a non-biological question and relates to when society assigns personhood. Since it is not usually stated otherwise, there is the assumption that all cells in the zygote become the embryo and then the born organism. As the text makes clear, many of the cells at the early 64-cell stage become placenta which has to proliferate rapidly to become part of the support system for the developing embryo. Schedule: If extensive history illustrations are used, add an additional class period. HOUR 1 8.1. History A. Primary Organizer B. Early Concepts: Preformation Versus Epigenesis 8.2. Fertilization A. Initial Event B. Oocyte Maturation C. Fertilization and Activation 8.3. What Can We Learn From Developmnet? 8.4 Cleavage and Early Development A. Blastomeres B. Patterns of Cleavage C. Amount and Distribution of Yolk Affects Cleavage D. Amount of Yolk Affects Developmental Mode E. Cleavage Affected by Different Inherited Patterns F. Blastulation G. Gastrulation and the Formation of Germ Layers H. Formation of the Coelom HOUR 2 8.5 Overview of Development Following Cleavage 8.6 Mechanisms of Development A. Nuclear Equivalence B. Cytoplasmic Specification C. Embryonic Induction D. Gene Expression During Development HOUR 3 8.7 Development patterns in animals 8.8 Evolutionary Developmental Biology 8.9. Vertebrate Development A. Common Vertebrate Heritage B. Amniotes and the Amniotic Egg C. Mammalian Placenta and Early Mammalian Development 8.10. Development of Systems and Organs A. Germ Layers B. Derivatives of Ectoderm: Nervous System and Nerve Growth C. Derivatives of Endoderm: Digestive Tube and Survival of Gill Arches D. Derivatives of Mesoderm: Support, Movement and Beating Heart ADVANCED CLASS QUESTIONS: 1. In a classic experiment in the 1980s, the cells of three mouse embryos were intermingled at the 8-64 cell stage; they clustered and became one mouse called a chimera with different parts showing the features of its six different parents! What does this indicate relative to the independence of one individual cell as the smallest unit of life? When researchers intermingled the cells of four or five embryos, only traits from three parental lineages were expressed in the chimera. Why? Why would the distribution of mouse embryo-versus-placenta cells not be 50-50 or even less for the placenta? Answer: 1. Independence of Individual Cells: • The chimera experiment demonstrates that individual cells are not necessarily autonomous units of life. Rather, they are part of a larger, coordinated system. • The ability of cells from different embryos to combine and form a single individual with features of multiple parents indicates that individual cells can work together and contribute to the development of a multicellular organism. 2. Expression of Traits in Chimeras: • When cells from multiple embryos are intermingled, the resulting chimera may express traits from all parental lineages. • In the case of intermingling cells from four or five embryos, traits from only three parental lineages were expressed in the chimera. This suggests that certain cell populations may have a competitive advantage during development. 3. Distribution of Embryo-versus-Placenta Cells: • The distribution of mouse embryo-versus-placenta cells in a chimera is not necessarily 50-50 because some cells may have a competitive advantage in contributing to specific tissues. • Additionally, the placenta may require a higher proportion of cells from certain parental lineages to ensure proper development and function. 2. In a rural hospital in Eastern Europe, a mother with type O blood gave birth to her biological child with type AB blood. Based on genetics, this is impossible. A check of her parents revealed she was a chimera, a fusion of two different sibling embryos that developed as one individual but with two tissue types. Her husband had type B blood. What are her two tissue types? How does embryology place a caveat on the mathematics of genetics in paternity cases? Answer: 1. Two Tissue Types: • The mother is a chimera, meaning she has two different tissue types derived from two different sibling embryos that fused during early development. • One tissue type has type O blood, while the other tissue type has type AB blood. 2. Caveat in Paternity Cases: • In paternity cases, the presence of chimerism can complicate genetic analysis and interpretation. • If the mother is a chimera, the child may inherit genetic material from either tissue type, potentially leading to discrepancies between the child's genetic profile and the genetic profiles of the mother's known relatives. 3. When an insect egg reproduces many nuclei without cytoplasmic cleavage, does this violate the cell theory? Answer: No, this process does not violate the cell theory. The cell theory states that all living organisms are composed of one or more cells, and that cells arise from pre-existing cells through cell division. In insects, the process of nuclei division without cytoplasmic cleavage, known as syncytial blastoderm, is a specialized form of cell division where nuclei divide rapidly without forming individual cells. However, each nucleus still contains genetic material and functions as a separate unit, maintaining the integrity of the cell theory. 4. Why do you inherit all of your mitochondria from your mother? [Careful: The sperm tail does enter the egg, contrary to some textbooks, but carries far fewer mitochondria than egg mitochondria; they are “worn out” and are marked for destruction.] Answer: You inherit all of your mitochondria from your mother because: 1. During fertilization, the egg contributes the majority of cellular components to the zygote, including mitochondria. 2. Sperm mitochondria are typically marked for destruction upon entry into the egg, as they are considered "worn out" and may carry mutations that could be harmful to the developing embryo. 3. Therefore, the mitochondria inherited by an individual come exclusively from the mother. 5. Combining information from both Chapters 8 and 9, why are “Siamese twins” joined? [The answer should combine information about twinning, implantation, and developmental signals.] Answer: Siamese twins, or conjoined twins, are joined because: 1. They result from incomplete separation of identical twins during early embryonic development. 2. Identical twins arise from the splitting of a single fertilized egg (zygote) during early embryonic development. 3. If the separation of the developing embryos is incomplete, the twins remain physically connected. 4. The specific point of connection depends on when and how the embryos divide, as well as the location of the blastocyst during implantation. 6. Chemical signals are involved in development. Should these chemical signals be considered hormones? Why or why not? Answer: Chemical signals involved in development can be considered hormones if they meet the criteria for hormone classification: 1. Long-distance Signaling: Hormones are typically produced by specialized cells or glands and are transported through the bloodstream to target tissues. 2. Regulation of Physiological Processes: Hormones regulate various physiological processes, including growth, development, metabolism, and reproduction. 3. Specific Binding: Hormones bind to specific receptors on target cells, initiating a cellular response. 4. Feedback Regulation: Hormone secretion is often regulated by feedback mechanisms to maintain homeostasis. Chemical signals involved in development often meet these criteria, as they are produced by specialized cells, regulate specific developmental processes, and act on target cells to elicit a response. Therefore, they can be considered hormones. 7. Discuss the genetics of pattern formation in embryogenesis. Be sure to include a discussion of morphogens (both specific and general). Answer: Pattern formation in embryogenesis is governed by complex genetic and molecular mechanisms, including the action of morphogens: 1. Morphogens: • Morphogens are signaling molecules that regulate the pattern and organization of cells and tissues during embryonic development. • Specific morphogens, such as bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and Wnt proteins, play key roles in establishing positional information along the embryonic axes. • General morphogens, such as retinoic acid and Sonic hedgehog (Shh), regulate broader patterning events and cell differentiation. 2. Genetic Control: • The expression and activity of morphogens are tightly regulated by genetic pathways, including transcription factors and signaling cascades. • Gradients of morphogen concentration provide positional information to developing cells, guiding their differentiation and patterning. • Homeobox (Hox) genes are a class of transcription factors that play a critical role in specifying regional identity along the body axes. 3. Cellular Response: • Cells within the developing embryo respond to morphogen gradients by activating specific genetic programs that determine their fate and function. • Cell-to-cell communication, mediated by morphogens and their receptors, ensures coordinated patterning and tissue differentiation. In summary, pattern formation in embryogenesis is a highly coordinated process regulated by genetic pathways and morphogen gradients. These mechanisms ensure the precise organization and differentiation of cells and tissues during development, ultimately giving rise to the complex body plan of the organism. PART III DIVERSITY OF ANIMAL LIFE 9 Architectural Pattern of an Animal 10 Taxonomy and Phylogeny of Animals 11 Unicellular Eukaroyates 12 Sponges and Placozoans 13 Radiate Animals 14 Acoelomorpha, Platyzoa, and Mesozoa 15 Polyzoa and Kryptrochozoa 16 Molluscs 17 Annelids and Allied Taxa 18 Smaller Ecdysozoans 19 Trilobites, Chelicerates, and Myriapods 20 Crustaceans 21 Hexapods 22 Chaetognaths, Echinoderms, and Hemichordates 23 Chordates 24 Fishes 25 Early Tetrapods and Modern Amphibians 26 Amniote Origins and Nonavian Groups 27 Birds 28 Mammals CHAPTER 9 ARCHITECTURAL PATTERN OF AN ANIMAL CHAPTER OUTLINE 9.1. New Designs for Living A. Levels of Organization in Organismal Complexity 1. Zoologists recognize 34 major phyla of living multicellular animals. 2. 600 million years ago in the Cambrian, nearly 100 phyla had evolved representing nearly all major modern body plans. 3. Major body plans are the result of extensive selection and are a limiting determinant of future adaptational variants. 4. Animals share structural complexities that reflect common ancestry. 9.2. Hierarchical Organization of Animal Complexity (Table 9.1) A. Grades of Organization 1. Unicellular groups are the simplest animal-like organisms. a. Within the cell, they perform all basic functions. b. Diversity is achieved by varying architectural patterns of subcellular structures, organelles and the whole cell. 2. Metazoa are multicellular animals. a. Cells become specialized parts of a whole organism; these cells cannot live alone as do unicellular organisms. b. Simplest metazoans show a cellular grade of organization and are not strongly associated to perform a collective function. c. More complex metazoans have an organization with cells working closely together as a unit, called tissues. d. Organisms that are at or beyond the cell-tissue grade of organization are called eumetazoans. e. Chief functional cells of an organ are called parenchyma; supportive tissues are its stroma. f. Many tissues work together in an organ; most metazoans operate at the organ system level. 9.3. Animal Body Plans A. Animal Symmetry (Figure 9.1) 1. Spherical symmetry occurs when any plane divides the body into mirrored halves, as in cutting a globe in half. 2. Radial symmetry occurs when any plane passing through the longitudinal axis divides the body into mirrored halves, as in cutting a pie; the Cnidaria and Ctenophora are the Radiata. 3. Biradial symmetry occurs in an animal that is radial, except for some paired feature that allows only two mirrored halves. 4. In bilateral symmetry, an organism can be cut in a sagittal plane into two mirror halves; this usually provides for a head (cephalization) in bilateral animals classified in the Bilateria. 5. Cephalization (Figure 9.2) 6. Differentiation of the head, or cephalization, is mainly found in bilaterally symmetrical animals. B. Body Cavities and Germ Layers (Figures 9.3, 9.4) 1. A body cavity is an internal space. a. Most obvious is a gut cavity. b. Most animals have a second cavity outside the gut. c. When fluid-filled this cavity may cushion and protect the gut. d. Animals differ in the presence and number of body cavities. e. A coelom provides more space for organs and surface area for exchange. f. Worms rely on the coelom for a hydrostatic skeleton to aid in burrowing. 2. Sponges have no body cavity. a. Like all metazoans, sponges develop from a zygote to a blastula stage. b. In sponges, after the formation of a blastula, the cells reorganize to form the adult animal. 3. In animals other than sponges, development proceeds from a blastula to a gastrula stage. a. The depression becomes a gut cavity, also called a gastrocoel or archenteron. The external opening is the blastopore; it typically becomes the adult mouth. The gut lining is endoderm, while the outer layer of cells surrounding the blastocoel is ectoderm. While the fluid-filled blastocoel persists in diploblasts, in most it is filled with mesoderm. Triploblasts possess an ectoderm, mesoderm, and endoderm. In protostomes, mesoderm forms as endodermal cells migrate into the blastocoel. Following this migration, three body plans are possible. In the acoelomate plan, mesodermal cells completely fill the blastocoel. In the pseudocoelomate plan, mesodermal cells line the outer edge of the blastocoel, leaving a persistent blastocoel and a gut cavity. (Figure 9.4) In the eucoelomate animals, the blastocoel is completely lined with mesoderm forming a true coelom. There are two different eucoelomate plans, schizocoelous and enterocoelous. In the schizocoelous plan, mesodermal cells fill the blastocoel; then programmed cell death leads to the opening of a space inside the mesodermal band forming a coelom. In the enterocoelous plan, cells from the central portion of the gut lining begin to grow outward as pouches. The expanding pouch walls form a mesodermal ring and enclose a space which becomes a coelomic cavity. Enterocoelous and schizocoelous coeloms are functional equivalent. C. Developmental Origins of Body Plans in Triploblasts (Figure 9.5) 1. Diploblastic organisms typically have radial symmetry as adults and develop from two germ layers. 2. Tripoblastic organisms follow one of several major developmental pathways. 3. Radial cleavage pathway results in the blastopore becoming the anus, a new opening forms a mouth, the coelom forms by enterocoely, and cleavage is regulative. a. Animals with these features are deuterostomes; examples are sea urchins and chordates. 4. The spiral cleavage pathway results in the blastopore becoming the mouth, cleavage is mosaic, and mesoderm forms from a particular cell in the embryo. a. The body may become acoelomate, pseudocoelomate, or coelomate. b. Animals with these features are called lophotrochozoan protostomes; examples are mollusks and segmented worms. D. Complete Gut Design and Segmentation (Figures 9.5, 9.6) 1. Eumetazoans show great variety in symmetry, number of body layers, and gut structure. 2. Metamerism (Segmentation) (Figure 9.7) a. Metamerism is serial repetition of similar body segments. b. Each segment is a metamere or somite. C True metamerism is found in Annelida, Arthropoda and Chordata; other groups show a superficial segmentation. 9.4 Components of Animal Bodies A. Extracellular Components 1. Body fluids and extracellular structural elements are noncellular components of metazoan animals. 2. In contrast to intracellular fluids, extracellular fluids are outside the cells; extracellular fluids are divided into blood plasma and interstitial fluid. 3. Extracellular structural elements are the supportive material of the organism, including connective tissue, cartilage, bone, and cuticle. B. Cellular Components: Tissues (Figure 9.7) 1. Histology is the study of types of tissues. 2. Epithelial Tissue (Figures 9.8-9.10) a. Epithelium is a sheet of cells that covers an internal or external surface. b. It provides outside protection and internal linings, often modified to produce lubricants, hormones or enzymes. c. Simple epithelia are found in all metazoa. d. Stratified epithelia are restricted to vertebrates. e. All epithelia have an underlying basement membrane. f. Blood vessels never penetrate epithelial tissues. 3. Connective Tissue (Figure 9.11) a. Connective tissues are nearly everywhere in the body. b. It is made up of few cells, many extracellular fibers and a ground substance or matrix. c. In vertebrates, there are two types of connective tissue proper: loose and dense d. Much fibrous tissue is made of protein collagen, the most abundant protein in the animal kingdom. e. Connective tissue also includes blood, lymph and adipose. f. Cartilage is semirigid connective tissue composed of a firm matrix containing chondrocytes, collagen and/or elastic fibers. g. Bone is calcified matrix containing salts organized around collagen fibers. 4. Muscular Tissue (Figure 9.12) a. Muscle is the most abundant tissue in most animals. b. Muscle originates from mesoderm. c. The cell is the muscle fiber, specialized for contraction. d. Striated muscles include skeletal and cardiac muscles. e. Invertebrates contain a type of striated muscle called obliquely striated muscle. f. Smooth muscles lack the alternating bands seen in striated muscle. g. Myofibrils are contractile elements and the unspecialized cytoplasm is sarcoplasm. 5. Nervous Tissue (Figure 9.13) a. Nervous tissue receives and conducts impulses. b. Nervous tissue cell types are neurons and neuroglia that support the neurons. 9.5 Complexity and Body Size (Figures 9.14, 9.15) A. More complex grades of metazoan organization permit and promote evolution of large body size. B. Surface-area-to-volume ratios have important consequences for animal respiration, heat, etc. 1. Surface area increases are the square of body length; volume is the cube of body length. 2. A large animal has less surface area compared to its volume than does a smaller animal. 3. Flattening or infolding the body increases surface area, as in flatworms. 4. Most animals had to develop internal transport systems to shuttle nutrients, gases and waste products, as they became larger. C. Benefits of Being Large 1. Larger size buffers against environmental fluctuations in temperature, etc. 2. Size provides protection against predators and promotes offensive tactics. 3. Cost of maintaining body temperature is less per gram of weight in large than in small animals. 4. Energy costs of moving a gram of body over a given distance are less for larger animals. Lecture Enrichment 1. Why are insects so successful on land, and why there are so many different species of insects? Why are there so many more entomologists than there are malacologists? 2. We do not have huge spiders or giant ants that carry away people. Query why? The essay by J.B.S. Haldane on “Being the Right Size” can be used to provide background examples for an instructor to use. 3. Compare the body structure of the flatworm, the roundworm and a vertebrate with emphasis on body layers, symmetry and coelomic development; this provides some concrete examples of the concepts. 4. Assigning students to construct a geometrical “humbug” based on instructions using the terms “basal” and “distal” and “lateral” etc. can reinforce the descriptive terminology. 5. Emphasize that all kinds of tissues are present in nearly every organ. For example the stomach is composed of multiple tissues: the epithelium lining the stomach’s interior and exterior surfaces, muscle that allows stomach contractions to occur, nerves that supply the impulses for those contractions and connective tissues such as blood and connections binding the various tissues together. 6. Contrast the presence of sensory receptors in the head of some annelids such as sandworms with their absence in the earthworm; ask what this suggests about the environments in which these worms live. 7. Veteran faculty vary in the extent they wish to cover the generalizations in this chapter. Some may choose to integrate these concepts with the in-depth coverage of organisms provided by the later chapters. Commentary/Lesson Plan Background: Because many of the organisms mentioned (e.g., flatworms, cnidaria, etc.) are mostly microscopic or remote from inland temperate populations, few students will be directly familiar with any of these aside from previous specific targeted biology labwork. Coastal students may have some experience with marine cnidaria and flatworms. International students from tropical countries may be willing to relate the features of some of the forms familiar to them. Due to the small size of arthropods, most students have some experience with them, particularly insects and spiders. Nevertheless, the astounding diversity of animal groups will require substantial visuals. Misconceptions: Some students may believe that the basic patterns of symmetry are a major phylogenetic feature and early zoology charts did place echinoderms directly after Cnidaria (then known as coelenterates); however, the symmetry provides a descriptive discussion but little phylogenetic information of detailed nature. Often discussions of the value of various evolutionary inventions leads students to believe that they are always better (“adaptationism”) but some primitive organisms dominate their niche because they are better fitted than any subsequent animals. Many students are surprised to find that primitive insects, for example, had more segments and more wing veins that modern species, and that replication of metameres and veins occurred early and subsequent evolution involved a reduction in segments and veins. Schedule: HOUR 1 9.1. New Designs for Living A. Levels of Organization in Organismal Complexity 9.2. Hierarchical Organization of Animal Complexity A. Grades of Organization 9.3. Animal Body Plans A. Animal Symmetry B. Body Regions C. Body Cavities D. Metamerism E. Cephalization HOUR 2 9.4 Developmental Origins of Body Plans in Triploblasts A. Diploblastic Organisms B. Triploblastic Organisms 9.5 A Complete Gut Design and Segmentation A. Eumetazoans B. Metamerism Hour 3 9.6 Components of Metazoan Bodies A. Extracellular Components B. Cellular Components 9.7 Complexity and Body Size ADVANCED CLASS QUESTIONS: 1. An animal’s body has levels of organization. How is each level of organization more than the sum of its parts? Answer: Each level of organization in an animal's body is more than the sum of its parts because: 1. Cellular Level: • Cells are the basic structural and functional units of an organism. However, individual cells cannot perform all necessary functions for survival. • Cells specialize and organize into tissues, each with specific functions and properties. 2. Tissue Level: • Tissues are groups of similar cells working together to perform a specific function. • Tissues have properties and capabilities that individual cells do not possess, such as contractility in muscle tissue or conductivity in nerve tissue. 3. Organ Level: • Organs are composed of different types of tissues working together to perform a particular function. • Organs have complex structures and functions that cannot be accomplished by any single tissue type alone. 4. Organ System Level: • Organ systems are groups of organs working together to perform coordinated functions essential for the survival of the organism. • Organ systems integrate the functions of multiple organs to carry out complex physiological processes necessary for life, such as digestion, respiration, and circulation. In summary, each level of organization in an animal's body represents an increase in complexity and specialization, with emergent properties and capabilities that are not present at lower levels of organization. 2. All organisms carry out certain life processes. How do the various organ systems of animals in general contribute features that are similar or different from plants? Answer: The various organ systems of animals contribute features that are similar or different from plants in the following ways: Similar Features: 1. Nutrition: • Both animals and plants require nutrients for energy and growth. • Animals have specialized digestive systems to ingest, digest, and absorb nutrients from food, while plants use specialized structures like roots and leaves to absorb nutrients from the soil and air. 2. Transport: • Animals have circulatory systems composed of a heart and blood vessels to transport nutrients, oxygen, and waste products throughout the body. • Plants have vascular tissues (xylem and phloem) to transport water, nutrients, and sugars throughout the plant. 3. Respiration: • Animals have respiratory systems, such as lungs or gills, to exchange gases (oxygen and carbon dioxide) with the environment. • Plants have stomata and specialized cells for gas exchange during photosynthesis and respiration. Different Features: 1. Reproduction: • Animals typically have complex reproductive systems involving internal fertilization and specialized reproductive organs. • Plants have reproductive structures such as flowers, cones, and seeds for reproduction, often involving pollination and fertilization. 2. Support and Movement: • Animals have skeletal and muscular systems for support, movement, and protection. • Plants have structural support provided by cell walls and specialized tissues like wood, but they lack muscles and a skeletal system. 3. Animals are a major group of organisms alive today. What features might you expect animals to have in common because they are heterotrophic by ingesting food? How might this common trait cause them to be differently adapted? Answer: Common Features of Animals due to Heterotrophy: 1. Digestive Systems: • Animals have specialized digestive systems to ingest, digest, and absorb nutrients from food. • They possess organs such as mouth, esophagus, stomach, and intestines for the breakdown and absorption of nutrients. 2. Mobility: • Many animals have evolved mobility to search for, capture, and consume food. • Mobility can vary from simple movements in sessile animals to complex locomotion in vertebrates. 3. Sensory Organs: • Animals have evolved specialized sensory organs to detect and locate food sources. • These may include organs such as eyes, antennae, noses, and taste buds. Differences in Adaptations: 1. Feeding Strategies: • Animals have evolved diverse feeding strategies, including herbivory, carnivory, omnivory, filter-feeding, and scavenging. • Different adaptations have evolved based on the type of food available and the animal's ecological niche. 2. Structural Adaptations: • Animals have diverse structural adaptations related to feeding, including beaks, teeth, claws, proboscises, and specialized digestive structures. • Adaptations are often specialized based on the type of food consumed and the animal's feeding behavior. 3. Behavioral Adaptations: • Animals exhibit various behavioral adaptations related to feeding, such as hunting, foraging, grazing, and food storage. • Behavioral adaptations are shaped by factors such as food availability, competition, and predation pressure. 4. Why are giant ants and spiders the size of humans or elephants not possible? Answer: Giant ants and spiders the size of humans or elephants are not possible due to physiological constraints related to their exoskeleton, respiratory system, and metabolic rate: 1. Exoskeleton: • Insects and arachnids have exoskeletons that provide structural support and protection. • However, as they grow larger, the exoskeleton becomes heavier and more cumbersome, limiting mobility and increasing the risk of injury. 2. Respiratory System: • Insects and arachnids have a simple respiratory system that relies on diffusion of gases through their exoskeleton. • As their size increases, the surface area-to-volume ratio decreases, making efficient gas exchange more difficult. 3. Metabolic Rate: • Insects and arachnids have high metabolic rates relative to their size, allowing them to quickly generate energy for movement and other activities. • However, maintaining a high metabolic rate becomes increasingly challenging as size increases, limiting the ability to support the energy requirements of a larger body. Overall, the structural, respiratory, and metabolic constraints of insects and arachnids make it physiologically impossible for them to reach the size of humans or elephants. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214
