In this article we will discuss about the characteristics of metabolism during gastrulation period in animals.
Throughout gastrulation, the volume of the embryo does not change appreciably. Every expansion in one direction occurs at the expense of a contraction in another direction or directions. What has been said about the absence of growth during cleavage applies in the same way to the period of gastrulation.
Division of cells by mitosis continues, however, throughout gastrulation, and thus there is an increase of nuclear material at the expense of the cytoplasmic substances. Breakdown and assimilation of reserve materials are also proceeding, but here a new feature is observed that makes the metabolism of the gastrula different from the metabolism of a blastula.
The morphogenetic movements during gastrulation could be expected to cause an increased expenditure of energy and consequently increased oxidation. This is what is actually found; the oxygen consumption during gastrulation shows a further increase as compared with the cleavage stages and with the blastula. A similar sharp increase in total oxygen consumption is also observed in sea urchin eggs.
One of the substances particularly involved in the supply of energy during gastrulation in amphibians is glycogen. It has been discovered that the amount of glycogen becomes considerably diminished in the invaginating cells of the dorsal lip of the blastopore. This was first discovered by histochemical methods, by using a specific stain for glycogen on sections of gastrulating embryos.
Later, by methods of chemical analysis, the exact amounts of glycogen consumed were determined; it was found that in the dorsal lip of the blastopore 31 per cent of the glycogen is lost during gastrulation, whereas in other parts of the embryo only from 1 per cent to 9 per cent is lost during the same time.
Rapid breakdown of glycogen in the dorsal lip suggests particularly active respiration in this area. Direct measurements of respiration, however, showed that the dorsal lip region is by no means the part of the gastrula which respires at the highest rate. In order to compare respiration in different parts of the embryo, frog gastrulae were cut into several regions.
Two big pieces were made of the vegetal hemisphere (a dorsal and a ventral one) and four pieces of the marginal zone and the animal hemisphere (pieces 1 to 4), starting with the dorsal lip of the blastopore. The oxygen consumption was determined for each piece as well as the dry weight, total nitrogen, and extractable nitrogen—the latter, is equivalent to the nitrogen of the active cytoplasm (total nitrogen less the nitrogen contained in the yolk).
When the oxygen consumption is related to dry weight of the fragments, it is seen that there is a great difference in the oxygen consumption of various parts of the embryo.
The highest oxygen consumption, as related to dry weight or to total nitrogen—that is, to the whole mass of the embryonic tissue—is found at the animal pole of the gastrula, and the lowest oxygen consumption is at the vegetal pole. The dorsal side, including the dorsal lip, has distinctly higher oxygen consumption than the ventral side.
This would account for the difference in the breakdown of glycogen between the dorsal and the ventral lips. We must realize, however, that not all parts of the cells of a frog gastrula respire; the yolk presumably does not respire at all, whereas it contributes to the dry weight and to the total nitrogen of parts of the embryo.
It would be desirable to eliminate the yolk from calculations of embryonic respiration, and this was done by calculating the oxygen consumption per unit of extractable nitrogen (= nitrogen of active cytoplasm). In other words, the observed differences in oxygen consumption are due to different amounts of active cytoplasm in relation to yolk, that is, to the gradient of yolk distribution, and not to a local specifically higher rate.
The second peculiarity of the metabolism during the gastrulation period is a sharp increase in protein turnover, and particularly in protein synthesis. In a newt embryo (Triturus), the rate of synthesis of protein, as measured by the uptake of radioactive precursors, increases fivefold between the beginning of gastrulation and the late tail-bud stage when gastrulation is completed and most of the organ rudiments are formed.
In the same way, the rate of radioactive precursor intake into proteins in the sea urchin embryo increases roughly threefold between the earliest gastrula, with primary mesenchyme migrating into the blastocoele, and the middle gastrula.
The source of materials for the protein synthesis is mainly the protein yolk, contained in the eggs of most animals. The breakdown of yolk granules has been investigated in amphibian embryos both electrons microscopically and chemically. With the electron microscope, it can be seen that in amphibian embryos the first change in the yolk platelets consists in the disappearance of the amorphous or granular peripheral layer which contains, besides protein, considerable quantities of ribonucleic acid.
The disappearing material goes into solution in the cytoplasm and becomes available for synthetic processes. The solubilization of the peripheral layer occurs in the invaginating chordomesoderm during gastrulation, in the neural plate during late gastrulation and early neurulation, and still later in the epidermis. The solubilization of the crystalline core (“main body”) of the yolk platelets occurs considerably later, and in endoderm, it is delayed till just preceding the stage when the larvae start feeding.
Chemically, the solubilization of the yolk platelets can be recorded either spectrophotometrically by the decrease of light absorption of the yolk platelets in a microscopic preparation or by separating the yolk platelets from homogenates of embryos and measuring their protein content. By both methods it was shown that there is a rapid decrease of yolk platelet protein in the invaginating chordomesoderm starting from the beginning of gastrulation and a slower decrease in the ectoderm in neurulation stages.