Animals live by eating other organisms. Thus, despite their remarkable diversity, animals as different as worms, mollusks, insects, and vertebrates share anatomical features that are fundamental to this way of life. Epidermal cells form a protective outer layer; gut cells absorb nutrients from ingested food; muscle cells allow movement; and neurons and sensory cells control behavior. These diverse cell types are organized into tissues and organs, forming a sheet of skin covering the exterior, a mouth for feeding, and an internal gut tube to digest food—with muscles, nerves, and other tissues arranged in the space between the skin and the gut tube. Many animals have clearly defined axes—an anteroposterior axis, with mouth and brain anterior and anus posterior; a dorsoventral axis, with back dorsal and belly ventral; and a left-right axis. In this section, we discuss some fundamental mechanisms underlying animal development, beginning with how the basic animal body plan is established.
conserved Mechanisms establish the Basic animal Body plan
The shared anatomical features of animals develop through conserved mechanisms. After fertilization, the zygote usually divides rapidly, or cleaves, to form many smaller cells; during this cleavage, the embryo, which cannot yet feed, does not grow. This phase of development is initially driven and controlled entirely by the material deposited in the egg by the mother. The embryonic genome remains inactive until a point is reached when maternal mRNAs and proteins rather abruptly begin to be degraded. The embryo’s genome is activated, and the cells cohere to form a blastula—typically a solid or a hollow fluid-filled ball of cells. Complex cell rearrangements called gastrulation (from the Greek “gaster,” meaning “belly”) then transform the blastula into a multilayered structure containing a rudimentary internal gut (Figure 21–3). Some cells of the blastula remain external, constituting the ectoderm, which will give rise to the epidermis and the nervous system; other cells invaginate, forming the endoderm, which will give rise to the gut tube and its appendages, such as lung, pancreas, and liver. Another group of cells moves into the space between ectoderm and endoderm and forms the mesoderm, which will give rise to muscles, connective tissues, blood, kidney, and various other components. Further cell movements and accompanying cell differentiations create and refine the embryo’s architecture.
Figure 21–3 The early stages of development, as exemplified by a frog.
(a) a fertilized egg divides to produce a blastula—a sheet of epithelial cells often surrounding a cavity. During gastrulation, some of the cells tuck into the interior to form the mesoderm (green) and endoderm (yellow). ectodermal cells (blue) remain on the outside. (B) a cross section through the trunk of an amphibian embryo shows the basic animal body plan, with a sheet of ectoderm on the outside, a tube of endoderm on the inside, and mesoderm sandwiched between them. the endoderm forms the epithelial lining of the gut, from the mouth to the anus. It gives rise not only to the pharynx, esophagus, stomach, and intestines, but also to many associated structures. the salivary glands, liver, pancreas, trachea, and lungs, for example, all develop from the wall of the digestive tract and grow to become systems of branching tubes that open into the gut or pharynx. the endoderm forms only the epithelial components of these structures— the lining of the gut and the secretory cells of the pancreas, for example. the supporting muscular and fibrous elements arise from the mesoderm.