8 CHAPTER 1 FIGURE 1.2 Early development of the frog Xenopus laevis. (A) As the egg matures, it accumulates yolk (here stained yellow and green) in the vegetal cytoplasm.(B)Frogs mate by mplexus, the male grasping the female around the belly and fertilizing the eggs as they are released. (C) A newly laid clutch of eggs. The brown area of each egg where the eggs nucleus resides. (D) Cytoplasm rearrangement seen during first cleav age. Compare with the initial stage seen in(A). (E)A 2-cell embryo near the end of its first cleavage. (F)An 8-cell embryo. (G)Early blastula. Note that the cells get smaller but the volume of the egg remains the same. (H)Late blastula. (1) Cross section.o late blastula, showing the blastocoel (cavity), (A-H courtesy of Michael Danilchik ar Kimberly Ray; I courtesy of ). Heasman) (G) (I) Blastocoel
CHAPTER 1 FIGURE 1.2 Early development of the frog Xenopus laevis. (A) As the egg matures, il accumulates yolk (here stained yellow and green) in the vegetal cytoplasm. (B) Frogs mate by amplexus, the male grasping the female around the belly and fertilizing the eggs as they are released. (C) A newly laid clutch of eggs. The brown area of each egg is the pigmented animal hemisphere. The while spot in the middle of the pigment is where the egg's nucleus resides. (D) Cytoplasm rearrangement seen during first cleavage. Compare with the initial stage seen in (A). (E) A 2-cell embryo near the end of its first cleavage. (F) An 8-cell embryo. (G) Early blastula. Note that the cells get smaller, but the volume of the egg remains the same. (H) Late blastula. (I) Cross section of a late blastula, showing the blastococl (cavity). (A-H courtesy of Michael Danilchik and Kimberly Ray; I courtesy of J. Heasman.) 1 1
DEVELOPMENTAL ANATOMY 9 FIGURE 13 Continued development of Xenopus laevis. (A) Gastrulation begins with (A) an invagination, or slit, in the future dorsal (top) side of the embryo. (B)This slit, the dor- sal blastopore lip, as seen from the ventral surface (bottom) of the embryo (C) The slit becomes a circle, the blastopore. Future mesoderm cells migrate into the interior of the embryo along the blastopore edges, and the ectoderm (future epidermis and nerves) is eventually encircled. (D) Neural folds begin to form on the dorsal surface. (E), migrates down the outside of the embryo. The remaining part, the yolk-filled endoder groove can be seen where the bottom of the neural tube will be. (F)The neural folds come together at the dorsal midline, creating a neural tube. (G)Cross section of the s em bryo at the neurula stage. (H)A pre- hatching tadpole, as the protrusions of the forebrain begin to induce eyes to form. (D)A mature tadpole, having swum awa from the egg mass and feeding independently. Courtesy of Michael Danilchik an Kimberly Ra Dorsal blast Dorsal blastopore lip (C) Yolk plug Blastopore (G) Dorsa Notochord omite (future gut Mesoderm ectoderm) Ventral (1) Somites Gill area Expansion of forebrain to touch urface ectoderm (induces eyes to form) Stomodeum(mouth) Tailbud
DEVELOPMENTAL ANATOMY FIGURE 1.3 Continued development of Xcnopus laevis. (A) Gastrulation begins with an invagination, or slit, in the future dorsal (top) side of the embryo. (B)This slit, the dorsal blastopore lip, as seen from the ventral surface (bottom) of the embryo. (C)The slit oecomes a circle, the blastopore. Future mesoderm cells migrate into the interior of the embryo along the blastopore edges, and the ectoderm (future epidermis and nerves) "ligrates down the outside of the embryo. The remaining part, the yolk-filled endoderm, is eventually encircled. (D) Neural folds begin to form on the dorsal surface. (E) A groove can be seen where the bottom of the neural tube will be. (F) The neural folds come together at the dorsal midline, creating a neural tube. (G) Cross section of the \enopus embryo at the neurula stage. (H) A pre-hatching tadpole, as the protrusions of the forebrain begin to induce eyes to form. (I) A mature tadpole, having swum away Tom the egg mass and feeding independently. (Courtesy of Michael Danilchik and Kimberly Ray.) Dorsal blastopore lip (C) Yolk plug Blastopore H I 1 UufiH (G) Notochord Dorsal (back) Neural tube Somite Neural groove Brain Gill area Expansion of forebrain to touch surface ectoderm (induces eyes to form) Stomodeum (mouth) Open neural tube Epidermis Mesoderm (ectoderm) Ventral (belly) Somites Tailbud
10 CHAPTER 1 FIGURE 1.4 Metamorphosis of the frog (A) Huge changes are obvious when one con- trasts the tadpole and the adult bullfrog. Note especially the differences in jaw structure growth.(D)Onset of metamorphic climax as forelimbs emerge. (E, F) Climax stager mb and limbs. (B) Premetamorphic tadpole. (C) Prometamorphic tadpole, showing hindi (A O Patrice Ceisel/Visuals Unlimited. derm, and this outer layer expands to enclose the entire embryo. The large, yolky cells that remain in the vegetal Organogenesis hemisphere(until they are encircled by the expanding ecto- Organogenesis begins when the notochord-a rod of derm)become the endoderm. Thus, at the end of gastrula- mesodermal cells in the most dorsal portion of the tion, the ectoderm(precursor of the epidermis, brain, and embryo*signals the ectodermal cells above it that they nerves)is on the outside of the embryo, the endoderm (pre- cursor of the gut and respiratory systems)is on the inside" The notochord consists of cells such as those mentioned on p 2of of the embryo, and the mesoderm(precursor of the connec- tive tissue, blood, heart, skeleton, gonads, and kidneys)is embryo but which, having performed their tasks, die.Although between them adult vertebrates do not have notochords, this embryonic organ ical for establishing Meiosis 1: Separation of homologous chromosomes Nuclear Homologous Homologou chromatids Interphase Early prophase I Mid prophase I Late prophase I Metaphase l DNA replicates The nuclear envelope breaks down and homologous chromosomes (each chromosome being double, with the chromatids joined at the kinetochore) align in pairs. Chromosomal rearrangements can occur between the four homologous chromatids at this time
10 CHAPTER 1 /'-•-^ ^ -**»• FIGURE 1.4 Metamorphosis of the frog. (A) Huge changes are obvious when one contrasts the tadpole and the adult bullfrog. Note especially the differences in jaw structure and limbs. (B) Premetamorphic tadpole. (C) Prometamorphic tadpole, showing hindlimb growth. (D) Onset of metamorphic climax as forelimbs emerge. (E,F) Climax stages. (A © Patrice CeiselA/isuals Unlimited.) ',- ' i^>> derm, and this outer layer expands to enclose the entire embryo. The large, yolky cells that remain in the vegetal hemisphere (until they are encircled by the expanding ectoderm) become the endoderm. Thus, at the end of gastrulation, the ectoderm (precursor of the epidermis, brain, and nerves) is on the outside of the embryo, the endoderm (precursor of the gut and respiratory systems) is on the inside of the embryo, and the mesoderm (precursor of the connective tissue, blood, heart, skeleton, gonads, and kidneys) is between them. Organogenesis Organogenesis begins when the notochord—a rod of mesodermal cells in the most dorsal portion of the embryo'"—signals the ectodermal cells above it that they The notochord consists of cells such as those mentioned on p. 2 of the Introduction—i.e., cells that are important for constructing the embryo but which, having performed their tasks, die. Although adult vertebrates do not have notochords, this embryonic organ is critical for establishing the fates of the ectodermal cells above it, as we shall sec in Chapters 7-9. Meiosis I: Separation of homologous chromosomes Nuclear envelope Nucleus Chromatin Homologous chromosomes Homologous chromatids Interphase DNA replicates Early prophase I Mid prophase I Late prophase I Metaphase I The nuclear envelope breaks down and homologous chromosomes (each chromosome being double, with the chromatids joined at the kinetochore) align in pairs. Chromosomal rearrangements can occur between the four homologous chromatids at this time
EVELOPMENTAL ANATOMY 11 are not going to become epidermis. Instead, these dorsal tongue muscle of the frog develops. Meanwhile, the tad ectoderm cells form a tube and become the nervous sys pole's lengthy intestine-a characteristic of herbivores- tem. At this stage, the embryo is called a neurula. The neu- shortens to suit the more carnivorous diet of the adult frog ral precursor cells elongate, stretch, and fold into the The gills regress and the lungs enlarge. The speed of meta embryo, forming the neural tube(Figure 1.3D-F); the morphosis is carefully keyed to environmental pressures future epidermal cells of the back cover the neural tube In temperate regions, for instance, Rana metamorphosis Once the neural tube has formed, it and the notochord must occur before ponds freeze in winter. An adult leop induce changes in their neighbors, and organogenesis con- ard frog can burrow into the mud and survive the winter tinues. The mesodermal tissue adjacent to the neural tube its tadpole cannot and notochord becomes segmented into somites( Figure As metamorphosis ends, the development of the germ 1.3G, H), the precursors of the frog's back muscles, spinal cells begins. Gametogenesis can take a long time In Rana vertebrae, and dermis( the inner portion of the skin). The pipiens, it takes 3 years for the eggs to mature in the embryo develops a mouth and an anus, and it elongates females ovaries ( Sperm take less time; Rana males are ito the familiar tadpole structure(Figure 1.31). The neu- often fertile soon after metamorphosis )To become mature, rons make their connections to the muscles and to other the germ cells must be competent to complete meiosis neurons, the gills form, and the larva is ready to hatch from. Meiosis(Figure 1.5)is one of the most important evolu- its egg jelly. The hatched tadpole will feed for itself as soon tionary processes characteristic of eukaryotic organisms as the yolk supplied by its mother is exhausted It makes fertilization possible and is critical in recombin- See VADE MECUM ing genes from the two parents. Genetics, development, The amphibian life cycle and evolution throughout the animal kingdom are predi cated on meiosis. We will discuss meiosis more thorough ly in Chapter 16, but the most important things to remem- ber about meiosis are: Metamorphosis and gametogenesis Metamorphosis of the fully aquatic tadpole larva into an 1. The chromosomes replicate prior to cell division, so that dult frog that can live on land is one of the most striking each gene is represented four times transformations in all of biology. In amphibians, metamor 2. The replicated chromosomes(each called a chromatid phasis is initiated by hormones from the tadpole's thyroid are held together by their kinetochores(centromeres) land.(The mechanisms by which thyroid hormones and the four homologous chromatids pair together es compe lish these changes will be discussed in Chapter 15.) In frogs, almost every organ is subject to modification, and FIGURE 1.5 Summary of meiosis. The DNA replicates during the resulting changes in form are striking and very obvi- interphase. During first meiotic prophase, the nuclear envelope ous(Figure 1.4). The hindlimbs and forelimbs the adult will breaks down and the homologous chromosomes (each chromo- recedes. The cartilaginous tadpole skull is replaced by the align together. Chromosome rearrangements p" crossing over") can predominantly bony skull of the young frog. The horny teeth the tadpole uses to tear up pond plants disappear as sorted into different cells. During the second meiotic division, the he mouth and jaw take a new shape, and the fly-catching kintochore splits and the sister chromatids are moved into sepa rate cells, each with a haploid set of chromosomes Meiosis II: Separation of the chromatids 当→( Anaphase I Telophase I Metaphase II Anaphase Il Telophase lI The two original homo The kinetochore splits Each new cell has logous chromosomes copyof each
DEVELOPMENTAL ANATOMY 11 are not going to become epidermis. Instead, these dorsal ectoderm cells form a tube and become the nervous system. At this stage, the embryo is called a neurula. The neural precursor cells elongate, stretch, and fold into the embryo, forming the neural tube (Figure 1.3D-F); the future epidermal cells of the back cover the neural tube. Once the neural tube has formed, it and the notochord induce changes in their neighbors, and organogenesis continues. The mesodermal tissue adjacent to the neural tube and notochord becomes segmented into somites (Figure 1.3G,H), the precursors of the frog's back muscles, spinal vertebrae, and dermis (the inner portion of the skin). The embryo develops a mouth and an anus, and it elongates into the familiar tadpole structure (Figure 1.31). The neurons make their connections to the muscles and to other neurons, the gills form, and the larva is ready to hatch from its egg jelly. The hatched tadpole will feed for itself as soon as the yolk supplied by its mother is exhausted. See VADE MECUM The amphibian life cycle Metamorphosis and gametogenesis Metamorphosis of the fully aquatic tadpole larva into an adult frog that can live on land is one of the most striking transformations in all of biology. In amphibians, metamorphosis is initiated by hormones from the tadpole's thyroid gland. (The mechanisms by which thyroid hormones accomplish these changes will be discussed in Chapter 15.) In frogs, almost every organ is subject to modification, and the resulting changes in form are striking and very obvious (Figure 1.4). The hmdlimbs and forelimbs the adult will use for locomotion differentiate as the tadpole's paddle tail recedes. The cartilaginous tadpole skull is replaced by the predominantly bony skull of the young frog. The horny teeth the tadpole uses to tear up pond plants disappear as the mouth and jaw take a new shape, and the fly-catching tongue muscle of the frog develops. Meanwhile, the tadpole's lengthy intestine—a characteristic of herbivores— shortens to suit the more carnivorous diet of the adult frog. The gills regress and the lungs enlarge. The speed of metamorphosis is carefully keyed to environmental pressures. In temperate regions, for instance, Rana metamorphosis must occur before ponds freeze in winter. An adult leopard frog can burrow into the mud and survive the winter; its tadpole cannot. As metamorphosis ends, the development of the germ cells begins. Gametogenesis can take a long time. In Rana pipiens, it takes 3 years for the eggs to mature in the female's ovaries. (Sperm take less time; Rana males are often fertile soon after metamorphosis.) To become mature, the germ cells must be competent to complete meiosis. Meiosis (Figure 1.5) is one of the most important evolutionary processes characteristic of eukaryotic organisms. It makes fertilization possible and is critical in recombining genes from the two parents. Genetics, development, and evolution throughout the animal kingdom are predicated on meiosis. We will discuss meiosis more thoroughly in Chapter 16, but the most important things to remember about meiosis are: 1. The chromosomes replicate prior to cell division, so that each gene is represented four times. 2. The replicated chromosomes (each called a chromatid) are held together by their kinetochores (centromeres), and the four homologous chromatids pair together. FIGURE 1.5 Summary of meiosis. The DNA replicates during interphase. During first meiotic prophase, the nuclear envelope breaks down and the homologous chromosomes (each chromosome is double, with its two chromatids joined at the kinetochore) align together. Chromosome rearrangements ("crossing over") can occur at this stage. After the first metaphase, the kinetochore remains unsplit and the pairs of homologous chromosomes are sorted into different cells. During the second meiotic division, the kinlochore splits and the sister chromatids are moved into separate cells, each with a haploid set of chromosomes. Meiosis II: Separation of the chromatids Telophase I The two original homologous chromosomes are segregated into different cells Anaphase II The kinetochore splits Telophase II Each new cell has one copy of each chromosome
12 CHAPTER 1 3. The first meiotic division separates the chromatid pairs mammals--originate from eggs. Ex ovo omnia("All from from one another the egg)was the motto on the frontispiece of Harveys On 4. The second meiotic division splits the kinetochore such the Generation of Living Creatures, and this precluded the that each chromatid becomes a chromosome spontaneous generation of animals from mud or excre- 5. The result is four germ cells, each with a haploid ment. This statement was not made lightly, for Harvey nucleus knew that it went against the views of Aristotle, whom Having undergone meiosis, the mature sperm and egg Harvey still venerated. (Aristotle had thought that men- nuclei can unite in fertilization, restoring the diploid chro- strual fluid formed the material of the embryo, while the mosome number and initiating the events that lead to semen gave it form and animation. Harvey also was the development and the continuation of the circle of life first to see the blastoderm of the chick embryo(the small region of the egg containing the yolk-free cytoplasm that " How Are You? slands" of blood tissue form before the heart does. Har The fertilized egg has no heart. It has no eye. No limb is vey also suggested that the amniotic fluid might function found in the zygote. So how did we become what we are? as a"shock absorber"for the embryo What part of the embryo forms the heart? How do the cells As might be expected, embryology remained little but that form the eye s retina migrate the proper distance from speculation until the invention of the microscope allowed the cells that form the lens? How do the tissues that form a detailed observations. In 1672, Marcello Malpighi pub- birds wing relate to the tissues that form fish fins or the lished the first microscopic account of chick development hand? Wha Here, for the first time, the neural groove(precursor of the particular genes? These are the types of questions asked neural tube), the muscle-forming somites, and the first cir- by developmental anatomists culation of the arteries and veins--to and from the yolk Several strands weave together to form the anatomical were identified(Figure 1.6) approaches to development. The first strand is compara tive embryology, the study of how anatomy changes dur ing the development of different organisms. The second Epigenesis and preformation strand, based on the first, is evolutionary embryology, the With Malpighi begins one of the great debates in embry- study of how changes in development may cause evolu- ology: the controversy over whether the organs of the tionary change and of how an organisms ancestry may embryo are formed de novo("from scratch")at each gen- constrain the types of changes that are possible. The third eration, or whether the organs are already present, in strand of the anatomical approach to developmental biol- miniature form, within the egg(or sperm). The first view, ogy is teratology, the study of birth defects called epigenesis, was supported by Aristotle and Harvey The second view, called preformation, was reinvigorated Comparative embryology with Malpighi's support Malpighi showed that the unin- cubated chick egg already had a great deal of structure, The first known study of comparative developmental and this observation provided him with reasons to ques- ry BCE. In The Generation of Animals(ca 350 BCE), he noted the organs of the adult were prefigured in miniature with- ome of the variations on the life cycle themes: some ani- in the sperm or(more usually)the egg. Organisms were mals are born from eggs(oviparity, as in birds, frogs, and not seen to be "constructed"but rather"unrolled most invertebrates); some by live birth(viviparity, as in The preformationist hypothesis had the backing of eigh placental mammals); and some by producing an egg that teenth-century science, religion, and philosophy( Gould hatches inside the body(ovoviviparity, as in certain rep- 1977; Roe 1981 Pinto-Correia 1997). First, if all organs were tiles and sharks). Aristotle also identified the two major prefigured, embryonic development merely required the cell division patterns by which embryos are formed: the growth of existing structures, not the formation of new holoblastic pattern of cleavage(in which the entire egg is ones. No extra mysterious force was needed for embryon divided into smaller cells, as it is in frogs and mammals) ic development. Second, just as the adult organism was and the meroblastic pattern of cleavage(as in chicks, prefigured in the germ cells, another generation already wherein only part of the egg is destined to become the existed in a prefigured state within the germ cells of the embryo, while the other portion--the yolk-serves as first prefigured generation. This corollary, called emboit nutrition for the embryo). And should anyone want to ment (encapsulation), ensured that the species would know who first figured out the functions of the placenta and the umbilical cord, it was aristotle There was remarkably little progress in embryology for As pointed out by Maitre- Jan in 1722, the eggs Malpighi examined may technically be called"unincubated, but as they were left sit the two thousand years following Aristotle. It was only in ting in the Bolognese sun in August, they were not unheated. Such 1651 that William Harvey concluded that all animals-even eggs would be expected to have developed into chicks
12 CHAPTER 1 3. The first meiotic division separates the chromatid pairs from one another. 4. The second meiotic division splits the kinetochore such that each chromatid becomes a chromosome. 5. The result is four germ cells, each with a haploid nucleus. Having undergone meiosis, the mature sperm and egg nuclei can unite in fertilization, restoring the diploid chromosome number and initiating the events that lead to development and the continuation of the circle of life. "How Are You?" The fertilized egg has no heart. It has no eye. No limb is found in the zygote. So how did we become what we are? What part of the embryo forms the heart? How do the cells that form the eye's retina migrate the proper distance from the cells that form the lens? How do the tissues that form a bird's wing relate to the tissues that form fish fins or the human hand? What organs are affected by mutations in particular genes? These are the types of questions asked by developmental anatomists. Several strands weave together to form the anatomical approaches to development. The first strand is comparative embryology, the study of how anatomy changes during the development of different organisms. The second strand, based on the first, is evolutionary embryology, the study of how changes in development may cause evolutionary change and of how an organism's ancestry may constrain the types of changes that are possible. The third strand of the anatomical approach to developmental biology is teratology, the study of birth defects. Comparative embryology The first known study of comparative developmental anatomy was undertaken by Aristotle in the fourth century BCE. In The Generation of Animals (ca. 350 BCE), he noted some of the variations on the life cycle themes: some animals are bom from eggs (oviparity, as in birds, frogs, and most invertebrates); some by live birth (viviparity, as in placental mammals); and some by producing an egg that hatches inside the body (ovoviviparity, as in certain reptiles and sharks). Aristotle also identified the two major cell division patterns by which embryos are formed: the holoblastic pattern of cleavage (in which the entire egg is divided into smaller cells, as it is in frogs and mammals) and the meroblastic pattern of cleavage (as in chicks, wherein only part of the egg is destined to become the embryo, while the other portion—the yolk—serves as nutrition for the embryo). And should anyone want to know who first figured out the functions of the placenta and the umbilical cord, it was Aristotle. There was remarkably little progress in embryology for the two thousand years following Aristotle. It was only in 1651 that William Harvey concluded that all animals—even mammals—originate from eggs. Ex ovo omnia ("All from the egg") was the motto on the frontispiece of Harvey's On the Generation of Living Creatures, and this precluded the spontaneous generation of animals from mud or excrement. This statement was not made lightly, for Harvey knew that it went against the views of Aristotle, whom Harvey still venerated. (Aristotle had thought that menstrual fluid formed the material of the embryo, while the semen gave it form and animation.) Harvey also was the first to see the blastoderm of the chick embryo (the small region of the egg containing the yolk-free cytoplasm that gives rise to the embryo), and he was the first to notice that "islands" of blood tissue form before the heart does. Harvey also suggested that the amniotic fluid might function as a "shock absorber" for the embryo. As might be expected, embryology remained little but speculation until the invention of the microscope allowed detailed observations. In 1672, Marcello Malpighi published the first microscopic account of chick development. Here, for the first time, the neural groove (precursor of the neural tube), the muscle-forming somites, and the first circulation of the arteries and veins—to and from the yolk— were identified (Figure 1.6). Epigenesis and preformation With Malpighi begins one of the great debates in embryology: the controversy over whether the organs of the embryo are formed dc novo ("from scratch") at each generation, or whether the organs are already present, in miniature form, within the egg (or sperm). The first view, called epigenesis, was supported by Aristotle and Harvey. The second view, called preformation, was reinvigorated with Malpighi's support. Malpighi showed that the unincubated* chick egg already had a great deal of structure, and this observation provided him with reasons to question epigenesis. According to the preformationist view, all the organs of the adult were prefigured in miniature within the sperm or (more usually) the egg. Organisms were not seen to be "constructed" but rather "unrolled." The preformationist hypothesis had the backing of eighteenth-century science, religion, and philosophy (Gould 1977; Roe 1981; Pinto-Correia 1997). First, if all organs were prefigured, embryonic development merely required the growth of existing structures, not the formation of new ones. No extra mysterious force was needed for embryonic development. Second, just as the adult organism was prefigured in the germ cells, another generation already existed in a prefigured state within the germ cells of the first prefigured generation. This corollary, called embditment (encapsulation), ensured that the species would "As pointed out by Maitre-Jan in 1722, the eggs Malpighi examined may technically be called "unincubated," but as they were left sit- ting in the Bolognese sun in August, they were not unheated. Such eggs would be expected to have developed into chicks