Dimensión Cambio :

The fi nal deuterostome feature concerns the origin of the coe-lom. In enterocoely (Gr. enteron, gut, ⫹ koilos, cavity), both mesoderm and coelom are made at the same time. In entero-coely, gastrulation begins with one side of the blastula bending inward to form the archenteron or gut cavity. As the archen-teron continues to elongate inward, the sides of the archenarchen-teron push outward, expanding into a pouchlike coelomic compart-ment (see Figure 8.10 ). The coelomic compartcompart-ment pinches off to form a mesodermally bound space surrounding the gut (see Figure 8.10 ). Fluid collects in this space. Notice that the cells

Figure 8.11

Regulative and mosaic cleavage. A, Regulative cleavage. Each of the early blastomeres (such as that of a sea urchin) when separated from the others develops into a small pluteus larva. B, Mosaic cleavage. In a mollusc, when blastomeres are separated, each gives rise to only a part of an embryo. The larger size of one defective larva is the result of the formation of a polar lobe (P) composed of clear cytoplasm of the vegetal pole, which this blastomere alone receives.

Regulative (Sea urchin) Mosaic (Mollusc)

Separate blastomeres Separate blastomeres

Normal larvae (plutei)

Normal

larva Defective larvae P P

A B

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w w w . m h h e . c o m / h i c k m a n i p z 1 4 e CHAPTER 8 Principles of Development 167

that form the coelom during enterocoely come from a differ-ent region of the endoderm than do those making the coelom during schizocoely (see Figure 8.10 ).

Examples of Deuterostome Development

The general outline of deuterostome development just given var-ies in some of its details depending upon the animal being stud-ied. The presence of large amounts of yolk in some embryos further complicates the developmental sequence. A few examples of specifi c developmental sequences illustrate this variation.

Variations in Deuterostome Cleavage The typical deu-terostome pattern is radial cleavage, but ascidian chordates (also called tunicates) exhibit bilateral cleavage. In ascidian eggs, the anteroposterior axis is established prior to fertiliza-tion by asymmetrical distribufertiliza-tion of several cytoplasmic com-ponents ( Figure 8.12 ). The fi rst cleavage furrow passes through the animal-vegetal axis, dividing the asymmetrically distributed cytoplasm equally between the fi rst two blastomeres. Thus, this fi rst cleavage division separates the embryo into its future right and left sides, establishing its bilateral symmetry (hence the name bilateral holoblastic cleavage). Each successive division orients itself to this plane of symmetry, and the half-embryo formed on one side of the fi rst cleavage is the mirror image of the half embryo on the other side.

Most mammals possess isolecithal eggs and a unique cleav-age pattern called rotational cleavage, so called because of the orientation of blastomeres with respect to each other dur-ing the second cleavage division (see mouse development in Figure 8.7E ). Cleavage in mammals is slower than in any other animal group. In humans, the fi rst division is completed about 36 hours after fertilization (compared with about an hour and a half in sea urchins), and the next divisions follow at 12- to 24-hour intervals. As in most other animals, the fi rst cleavage plane runs through the animal-vegetal axis to yield a two-cell embryo. However, during the second cleavage one of these blastomeres divides meridionally (through the animal-vegetal axis) while the other divides equatorially (perpendicular to the animal-vegetal axis). Thus, the cleavage plane in one blasto-mere is rotated 90 degrees with respect to the cleavage plane of the other blastomere (hence the name rotational cleavage).

Furthermore, early divisions are asynchronous; not all blasto-meres divide at the same time. Thus, mammalian embryos may

not increase regularly from two to four to eight blastomeres, but often contain odd numbers of cells. After the third division, the cells suddenly close into a tightly packed confi guration, which is stabilized by tight junctions that form between outer-most cells of the embryo. These outer cells form the tropho-blast. The trophoblast is not part of the embryo proper but will form the embryonic portion of the placenta when the embryo implants in the uterine wall. Cells that actually give rise to the embryo proper form from the inner cells, called the inner cell mass (see blastula stage in Figure 8.13E ).

Telolecithal eggs of reptiles, birds, and most fi sh divide by discoidal cleavage. Because of the great mass of yolk in these eggs, cleavage is confi ned to a small disc of cytoplasm lying atop a mound of yolk (see chick development in Figure 8.7D ). Early cleavage furrows carve this cytoplasmic disc to yield a single layer of cells called the blastoderm. Further cleavages divide the blastoderm into fi ve to six layers of cells ( Figure 8.13D ).

Variations in Deuterostome Gastrulation In sea stars, gastrulation begins when the entire vegetal area of the blastula fl attens to form a vegetal plate (a sheet of epithelial tissue).

This event is followed by a process called invagination, in which the vegetal plate bends inward and extends about one-third of the way into the blastocoel, forming the archenteron ( Figure 8.13A ). Coelomic formation is typical of enterocoely.

As the archenteron continues to elongate toward the animal pole, and its anterior end expands into two pouchlike coelo-mic vesicles, which pinch off to form left and right coelocoelo-mic compartments ( Figure 8.13A ).

The ectoderm gives rise to the epithelium of the body sur-face and to the nervous system. The endoderm gives rise to the epithelial lining of the digestive tube. The outpocketing of the archenteron is the origin of mesoderm. This third germ layer will form the muscular system, reproductive system, peritoneum (lining of the coelomic compartments), and the calcareous plates of the sea star’s endoskeleton.

Frogs are deuterostomes with radial cleavage but morpho-genetic movements of gastrulation are greatly infl uenced by the mass of inert yolk in the vegetal half of the embryo. Cleavage divisions are slowed in this half so that the resulting blastula consists of many small cells in the animal half and a few large cells in the vegetal half (see Figures 8.7B and 8.13B ). Gastrula-tion in amphibians begins when cells located at the future dor-sal side of the embryo invaginate to form a slitlike blastopore.

Thus, as in sea stars, invagination initiates archen-teron formation, but amphibian gastrulation begins in the marginal zone of the blastula, where ani-mal and vegetal hemispheres come together, and where there is less yolk than in the vegetal region.

Gastrulation progresses as sheets of cells in the marginal zone turn inward over the blastopore lip and move inside the gastrula to form mesoderm and endoderm (see opening fi gure of this chapter, p. 158). The three germ layers now formed are the primary structural layers that play crucial roles in further differentiation of the embryo.

Figure 8.12

Bilateral cleavage in tunicate embryos. The fi rst cleavage division divides the asymmetrically distributed cytoplasm evenly between the fi rst two blastomeres, establishing the future right and left sides of the adult animal. Bilateral symmetry of the embryo is maintained through subsequent cleavage divisions.

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In bird and reptile embryos (see Figure 8.13D ), gastrulation begins with a thickening of the blastoderm at the caudal end of the embryo, which migrates forward to form a primitive streak ( Figure 8.14 ). The primitive streak becomes the anteroposterior axis of the embryo and the center of early growth. The primi-tive streak is homologous to the blastopore of frog embryos, but in chicks it does not open into the gut cavity because of the obstructing mass of yolk. The blastoderm consists of two layers (epiblast and hypoblast) with a blastocoel between them.

Cells of the epiblast move as a sheet toward the primitive streak, then roll over the edge and migrate as individual cells into the blastocoel. These migrating cells separate into two streams. One stream of cells moves deeper (displacing the hypoblast along the midline) and forms endoderm. The other stream moves between the epiblast and hypoblast to form mesoderm. Cells on the sur-face of the embryo compose the ectoderm. The embryo now has three germ layers, at this point arranged as sheetlike layers with ectoderm on top and endoderm at the bottom. This arrange-ment changes, however, when all three germ layers lift from the underlying yolk ( Figure 8.14 ), then fold under to form a three-layered embryo that is pinched off from the yolk except for a stalk attachment to the yolk at midbody (see Figure 8.22 ).

Gastrulation in mammals is remarkably similar to gastrula-tion in reptiles and birds (see Figure 8.13E ). Gastrulagastrula-tion move-ments in the inner cell mass produce a primitive streak. Epiblast cells move medially through the primitive streak into the blas-tocoel, and individual cells then migrate laterally through the blastocoel to form mesoderm and endoderm. Endoderm cells

Figure 8.13

Blastula and gastrula stages in embryos of sea star, frog, nemertean worm, chick, and mouse.

Blastula Gastrula

Blastula

Blastocoel Blastocoel

Gastrula

Blastula Gastrula

Blastopore (becomes mouth) Blastopore (becomes anus)

Yolk plug (blastopore) Coelomic vesicles

Archenteron Archenteron Blastocoel

Prospective mesoderm cells Blastocoel

Blastocoel

Inner cell mass Trophoblast Blastula Yolk

Blastula (blastocyst)

Gastrula

Gastrula

Primitive streak

Amniotic cavity Amnion Ectoderm

Endoderm Migrating cells Yolk sac Migrating cells A Sea star

B Frog

C Nemertean worm

D Chick

E Mouse

Figure 8.14

Gastrulation in a chick. Transverse sections through the heart-forming region of the chick show development at 18, 25, and 28 hours of incubation.

18 hours

25 hours

28 hours

Yolk Yolk

Yolk

These layers form extra-embryonic membranes

Forming heart tube

Coelomic (pericardial space) Heart rudiments Superficial ectoderm Blastocoel

Hypoblast

Migrating cells

Foregut Neural tube

Notochord Neural groove

Primitive streak

Subgerminal space Epiblast

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w w w . m h h e . c o m / h i c k m a n i p z 1 4 e CHAPTER 8 Principles of Development 169

(derived from the hypoblast) form a yolk sac devoid of yolk (since mammalian embryos derive nutrients directly from the mother via the placenta).

Amphibians, reptiles, and birds, which have moderate to large amounts of yolk concentrated in the vegetal region of the egg, have evolved derived gastrulation patterns in which the yolk does not participate in gastrulation. Yolk is an impediment to gastrulation and consequently the gastrulation process occurs around (amphibians) or on top (reptiles and birds) of the vegetal yolk. Mammalian eggs are isolecithal, and thus one might expect them to have a gastrulation pattern similar to that of sea stars.

Instead they have a pattern more suited to telolecithal eggs. The best explanation for this feature of mammalian egg development is common ancestry with birds and reptiles. Reptiles, birds, and mammals share a common ancestor whose eggs were teloleci-thal. Thus, all three groups inherited their gastrulation patterns from this common ancestor, and mammals subsequently evolved isolecithal eggs but retained the telolecithal gastrulation pattern.

A further developmental complication in vertebrates is that coelom formation occurs by a modifi ed form of schizocoely (see Figure 8.10 ), not enterocoely. The nonvertebrate chordates form the coelom by enterocoely, as is typical of deuterostomes.