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In this article we will discuss about the gastrulation and the formation of primary organ rudiments in fishes.
In fishes the yolk formed in the oocytes during oogenesis initially does not differ substantially from the yolk in amphibian oocytes and eggs. It consists of yolk platelets lying in the cytoplasm and intermingled with other cytoplasmic components (mitochondria and the like).
The cores of the yolk platelets, similar to those of amphibians, may show a crystalline structure, although this structure is lost by the time the eggs are ripe. In the immature egg the surface is formed by yolk-free cytoplasm containing numerous cortical granules.
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After fertilization, however, the cytoplasm moves toward the animal pole of the egg, and there it forms a cytoplasmic “cap.” This is particularly the case in bony fishes. Active cytoplasm is withdrawn both from the surface and from the interior of the egg.
As a result, the yolk platelets are pressed closer together. The process is a gradual one and occurs earlier or later in different fishes. In the more primitive fishes the condensation of the yolk does not go too far; cleavage remains complete, and development follows essentially the pattern found in amphibians.
In the Teleostei, however, the yolk platelets are pressed together in the central parts of the egg to such an extent that the cytoplasm is completely squeezed out from between them. The fertilized egg then consists of the cytoplasmic cap on the animal pole containing the nucleus or nuclei, the central mass of yolk, and a thin layer of cytoplasm surrounding the yolk.
Cleavage occurs only in the cytoplasmic cap on the animal pole; the yolk and the layer of cytoplasm surrounding it remain un-cleaved. Even the cytoplasmic cap of the animal pole is not completely subdivided into cells; the deeper parts of the cytoplasm are provided with nuclei in the process of cleavage, but the mass of cytoplasm fails to divide and remains in the form of a syncytial layer adjoining the un-cleaved yolk.
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This layer is called the periblast. The cellular mass on top of the periblast represents the blastoderm. The superficially lying cells of the blastoderm adhere more firmly to one another than the cells lying in the interior, thus forming a “covering layer”.
Only the blastoderm is responsible for the formation of the body of the embryo. The other parts—the periblast, the yolk, and the surface layer of cytoplasm surrounding the yolk (where it is not covered by the periblast and the blastoderm)—do not contribute directly to the construction of the body of the embryo and, for this reason, are designated as extra-embryonic.
The three germinal layers are formed within the blastoderm. How this occurs is a subject of long-standing controversy. It has been claimed that the endomesoderm arises from the edge of the blastodisc, and that therefore the edges of the blastodisc correspond to the lips of the blastopore, as found in amphibians, cyclostomes, and the more primitive fishes.
Experiments have furnished evidence against this view by showing that there is no significant sinking in of the superficial material into the interior and that the endomesodermal layer is formed by a rearrangement of the deep-lying cells of the blastodisc. Whatever the mechanism of its formation, the endomesodermal layer is found along the edges of the blastodisc, where it is continuous with the superficial (ectodermal) layer.
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Furthermore, the endomesodermal layer is best developed at what will be the posterior edge of the blastodisc. Here the blastodisc is thickened, forming an embryonic shield, and it is here that the primary organ rudiments are formed – the neural plate and tube, the notochord, and laterally, the somites.
The endoderm is separated from the mesoderm rather late and is then represented as an elongated plate of cells stretched longitudinally underneath the notochord. It does not contain a cavity until a much later stage. After the separation of the endoderm, the remaining layer becomes equivalent to the chordomesodermal mantle.
In fishes the formation of the primary organ rudiments starts with the most anterior parts and progresses backward. The lateral edges of the blastodisc are gradually involved in the body formation; they are drawn toward the midline and contribute to the formation of the more posterior parts of the body and the tail.
Even before the primary organs of the embryo are laid down, the extra-embryonic yolk is brought into the orbit of development of the new organism by the blastodisc’s spreading over the surface of the yolk and eventually surrounding it completely.
Immediately after fertilization the cytoplasmic cap rises quite steeply over the surface of the yolk, acquiring a very nearly hemispherical shape. During cleavage the mass of blastomeres is still markedly elevated, but at the end of cleavage it spreads out over the yolk and becomes flatter; it can then be likened to an inverted saucer and is very well characterized by the term “embryonic disc.”
During the formation of the germinal layers the disc spreads over a wider area covering a greater part of the surface; this continues during organ formation. As a result, an ever increasing part of the yolk is covered by the blastodisc. It has been shown that the spreading of the embryonic disc occurs at the expense of a thinning of the cellular layer, without an appreciable increase in its mass. It is therefore a process of epiboly, similar to the spreading out of the animal hemisphere of the amphibian embryo during gastrulation.
The advancing edge of the disc consists of three layers – the periblast, the mesoderm, and the ectoderm. No endoderm is present at the edge of the blastodisc, except in a small mid-posterior portion; thus, endoderm does not participate in the overgrowth of the yolk.
If the cellular part of a fish blastula is removed, the remaining periblast nevertheless spreads out over the yolk. It may therefore be concluded that this syncytial layer is essential for the overgrowth of the yolk and that it provides a suitable substrate over which the germinal layers (mesoderm and ectoderm) may also expand and shift over the surface of the yolk.
Eventually the edges of the blastodisc, having surrounded the entire yolk, converge and close at the posterior end of the embryo, similar to the closure of the blastopore in an amphibian or Amphioxus. Just before the yolk is completely closed in, a yolk plug may be seen projecting between the constricting edges of the blastodisc.
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This yolk plug looks very much like that of the amphibians but differs from it insofar as it consists only of non-segmented yolk, whereas the yolk plug in amphibians consists of cells of the vegetal region. The yolk is thus enclosed in an envelope consisting of periblast, mesoderm, and ectoderm.
The process of neurulation in the bony fishes differs considerably from that in amphibians. Although a neural plate is found, the neural folds are only slightly indicated. The neural plate does not roll into a tube but narrows gradually, at the same time sinking deeper and deeper into the underlying tissues.
Eventually it separates from the epidermis, which becomes continuous over the dorsal surface of the embryo. No cavity appears in the nervous system rudiment while it is being separated from the remainder of the ectoderm. The ventricles of the brain and the central canal of the spinal cord are formed later by the separation of cells in the middle of these organs.
Gastrulation and the formation of the primary organ rudiments in elasmobranchs follow the same pattern as in the bony fishes—with the difference, however, that an archenteron is formed at the posterior edge of the blastodisc. Likewise, the rudiment of the nervous system is not solid, as in bony fishes, but the neural plate is rolled into a tube having a distinct cavity right from the start.