The gastrulation is the most active phase of embryonic development during which extensive or profound morphogenetic activities of different types of blastomeres occur and rate of metabolism of gastrula as a whole becomes rapidly increased. Its metabolism can be studied under following headings:
(a) Catabolism- The morphogenetic movements during gastrulation cause an increased expenditure of energy rich ATP molecules and consequently, an increase oxidation, an increase oxidation. It has been found that the oxygen consumption during the gastrulation of frog and sea urchin shows a further increase as compared with the blastula stage. During gastrulation the food reserves (e.g., glycogen and yolk) are oxidized for the manufacturing of ATP molecules.
(ii) Oxidation of glycogen- One of the substances particularly involved in the supply of energy during gastrulation in amphibians and other animals, is glycogen. It has been shown in frogs that amount of glycogen becomes considerably diminished in the invaginating cells of the dorsal lip of the blastopore. Rapid breakdown of the glycogen in the dorsal lip suggests a particularly active respiration in this area.
(ii) Breakdown of yolk- Besides glycogen, the yolk of the blastomeres is broken down for energy metabolism or assimilation. The breakdown of yolk granules has been investigated in amphibian embryos both electron microscopically and biochemically. With the electron microscopically, it can be seen that in amphibian embryos, the first change in the yolk platelets consists in the disappearance of the amorphous of granular peripheral layer which contains, besides proteins, considerable quantities of ribonucleic acids. 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 chorda-mesoderm 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.
(b) Anabolism- Throughout gastrulation, the volume of the embryo does not change appreciably. Every expansion in one direction occurs at the expense of contraction in another direction. Division of cells by mitosis continues throughout gastrulation and this mean, that there is an increase of nuclear material at the expense of the cytoplasmic substances. The anabolic chemical activities of gastrulation includes following important functions:
(i) Nucleic acid synthesis- During gastrulation, synthesis of different kinds of nucleic acid molecules occurs. The replication of DNA is needed for the duplication of chromosomes during each mitosis of gastrulation.
The onset of gastrulation actually recognized at the molecular level by the start of transcription of DNA dependent rRNA molecules. During gastrulation, the rate of transcription of new mRNA molecules. During gastrulation, the rate of transcription of new mRNA and tRNA molecules become greatly increased which indicates towards the possibility of increased protein synthesis in the cytoplasm.
(ii) Protein synthesis- During the gastrulation, a sharp increase in protein turn over and in particular protein synthesis takes place. The source of materials for the protein synthesis is mainly the protein yolk, contained in the eggs of most animals. The protein turn over after the beginning of gastrulation seems to be essentially different from what had been going on during cleavage. The new proteins are qualitatively different from those present in the egg. These different kinds of proteins are manufactured in gastrula, only due to active participation of paternal genes in the production of new mRNA molecules during this phase.
In sea urchins and amphibians, it has been found that their gastrulae contain antigen proteins, capable of causing the formation of antibodies, which were not present before, (Clayton, 1951)