Comparison of gastrulation in vertebrates

Embryonic development is a continuum – every stage is essential, and dependent on preceding ones – but if one stage were to be singled out as of central importance, probably most would say it is gastrulation. This is because gastrulation is the process leading to the establishment of the germ layers – ectoderm, mesoderm and endoderm – from which all of the body’s tissues are derived.

It is during gastrulation that the fundamental plan of the vertebrate body is brought into existence. [2]

So from an evolutionary perspective we would surely expect gastrulation to be ‘conserved’ – substantially the same throughout the vertebrates which have these three germ layers. But in fact we find that even for this key stage of embryonic development, for almost all of the major classes of vertebrates, the mechanism of gastrulation is substantially different from any of the others.

Archetypal gastrulation

The early embryonic development of many invertebrate phyla results in a blastula which comprises a hollow ball of cells, and gastrulation involves an invagination of this ball. Cells from the outer layer then move over the lip of the invagination by a process called involution, which internalises cells that are to form internal tissues. Amphioxus, an invertebrate chordate which is generally considered to resemble vertebrate ancestors, resembles this, although recent work has shown that invagination predominates, and involution is 'negligible'.[3]

The edge or rim of the invagination then constricts to form a pouch which is called an archenteron. This develops into the animal’s gut – the mouth at one end and anus at the other – which is why this stage is called gastrulation: formation of the stomach.

Gastrulation by invagination

Amphibians

No vertebrates gastrulate in this archetypal way. Perhaps the closest to it are the amphibians. However, a major difference is that, although the blastopore starts as an invagination, this is a slit on one side of the blastula which progressively extends around the blastula, at the same time encircling the presumptive endoderm, i.e. the invagination is an anular slit rather than a pouch.

Reptiles

Gastrulation in reptiles is via a blastopore which arises by invagination. However, although it develops from the epiblast, it extends right through the embryonic disc, including the hypoblast, to the subgerminal cavity; and it does not develop into the animal’s gut.

Unfortunately, it is still common to find gastrulation in reptiles described as being via a primitive streak (see below), but this is incorrect. There is no case known of gastrulation in a reptile being via a primitive streak: all are via a blastopore (for a recent review see Bertochinni et al. [4]).

Gastrulation by involution, but not invagination

Gastrulation via a blastopore involves involution i.e. a layer or sheet of surface or epithelial cells moves over the lip and into the blastopore. In some other classes, gastrulation is primarily via involution, but not into a blastopore.

Teleosts

In teleosts (most bony fish) gastrulation proceeds by cells of the enveloping layer (EVL) and epiblast spreading (epiboly) over the yolk and, as they do so, cells at the margin of the epiblast fold underneath (involute) to form an underlying layer of cells. (This lower layer of cells which develops into mesoderm and endoderm is generally called hypoblast; but it is different from the hypoblast of amniotes which forms before gastrulation and is substantially displaced by cells of the epiblast which become mesoderm and endoderm.)

It should be noted that, although the substance of gastrulation is the spread and involution of epiblast, the driving force for epiboly and the vegetal-ward advance of the margin of the epiblast is the overlying EVL (even though subsequently it is shed, i.e. does not become part of the embryo).

Gastrulation by cells rolling over an extension of the epiblast

Chondrichthyans

Gastrulation in chondrichthyans begins by the formation of an extension and overhang of the posterior part of the epithelial surface blastomeres; and then proceeds by cells rolling over the lip of the extension.

Although this rolling over of a sheet of cells resembles involution, a significant difference is that the cells that roll over are not internalised: the epithelial cells that were on the upper surface form the ventral epidermis. Similarly, although some authors refer to the space below the overhang as an archenteron (the name given to the amphibian invagination) it is quite different: it is not an invagination into the embryo, but merely a space between the overhanging embryo and the underlying yolk.

Gastrulation via a primitive streak

Birds and mammals

A key feature of gastrulation via a primitive streak is that cells move from the epiblast into and through underlying layers by ingression rather than involution. In involution, cells retain their epithelial character, and move as a layer or sheet of cells. In contrast, because ingression entails moving through embryonic tissue, it is necessary for the epiblast cells to lose their epithelial character (and associated basement membrane), and become mesenchymal (epithelial to mesenchymal transition, EMT), and they move individually rather than as a sheet of cells. Cells entering the endoblast (but not those becoming mesoderm) must reverse this, and become epithelial again i.e. undergo mesenchymal to epithelial transition (MET).

Go to Gastrulation in the Overview of diverse early embryonic development of vertebrates.

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Page created October 2020, revised June 2022.