Chapter 6 Gastrulation, or germ layer formation, is a stage of embryological development during which the single‐layered blastula is converted into a trilaminar structure consisting of an outer ectodermal, a middle mesodermal and an inner endodermal layer. These changes occur through a series of orderly cell migrations from the surface of the blastula into its interior. Cells arising from each germ layer ultimately give rise to specific tissues and organs. Ectoderm differentiates into the epidermis of the skin and into neural tissue, endoderm forms the lining of the gastrointestinal and respiratory tracts, and from the middle mesodermal layer the urogenital, circulatory and supportive muscular and skeletal systems are formed. Using labelling techniques, it is possible to identify the cells in the blastula from which the germ layers arise and from which specific organ primordia develop. Data compiled in this manner can be used to construct a diagrammatic illustration of the migration of cells from their origin within the blastula to specific tissues or organs in later stages of development. Such diagrammatic illustrations are termed fate maps. Despite marked differences in the manner in which gastrulation proceeds in diverse animal species, the arrangement of germ layers at the end of gastrulation is comparable in all vertebrates. The process of gastrulation in higher mammals can be more readily appreciated by comparing the process of gastrulation in primitive chordates, amphibians and avian species. The pattern of gastrulation in Amphioxus represents a comparatively simple model for illustrating the major cellular events in germ layer formation observed in more evolutionarily advanced species. Gastrulation in Amphioxus begins when the blastoderm at the vegetal pole flattens and invaginates (Fig 6.1). The embryo then undergoes a series of morphological changes. As cells at the vegetal pole invaginate, the spherical shape of the embryo changes with the sequential formation of a cavity referred to as the archenteron or primitive gut. The opening of the archenteron to the exterior is known as the blastopore. The outer layer of cells form the ectoderm, and the inner layer the endoderm. Cells responsible for the formation of the notochord and other mesodermal structures originally occupy a position at the edge of the blastopore. Later, these cells migrate to a position between the ectoderm and endoderm. Thus, the endodermal and mesodermal structures relocate from the surface of the embryo to its interior, forming a trilaminar embryo referred to as a gastrula. Figure 6.1 Sections showing sequential stages of gastrulation in Amphioxus from the blastula stage A to the gastrula stage E. The section shown in E is at the level indicated in the embryo at the gastrula stage in F. Because of the presence of yolk‐filled cells in the vegetal hemisphere of the amphibian blastula, invagination, as observed in Amphioxus, cannot occur. At the junction of the animal and vegetal hemispheres, cells from the surface move to the interior forming a cleft, the forerunner of the primitive gut. Following an influx of endodermal cells from below the cleft and mesodermal cells from above, the cleft deepens. With the constant movement of cells from the surface to the interior, a circular blastopore is formed (Fig 6.2). The blastocoele becomes obliterated and the yolk‐laden cells at the vegetal pole move to the interior. Finally, a trilaminar embryo, similar to that observed in Amphioxus, is formed. Figure 6.2 Sequential stages of gastrulation in amphibians from the blastula stage to the gastrula stage. The avian blastoderm consists of two parts, the area pellucida and the area opaca (Fig 6.3). The cells of the area pellucida give rise to two layers, an upper epiblast, which comprises prospective ectoderm, endoderm and mesoderm, and a lower hypoblast destined to become the extra‐embryonic endoderm. The bilaminar region of the blastoderm of the chick embryo is a flat structure which corresponds to the spherical blastula observed in Amphioxus and in amphibians. The avian epiblast corresponds to the cells of the animal pole of the blastula; the hypoblast corresponds to the cells of the vegetal pole and the intervening space corresponds to the blastocoele. The organ‐forming part of the blastoderm in birds is confined to a region of the area pellucida extending cranially from its caudal edge for about three‐fifths of its length. Thickening of the blastoderm, which results from the convergence of cells in the surface layer of the blastoderm towards the midline, forms the primitive streak. At the cranial end of the primitive streak, an increased concentration of cells forms a structure referred to as the primitive node or Hensen’s node (Fig 6.3
Gastrulation
Primitive chordates
Amphibians
Avian species
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