In this article we will discuss about the process of maturation of eggs in the female gamete.
After the oocyte completes its growth, it is ready for the reduction divisions. By this time, the chromosomes of the oocyte have reached the stage of diakinesis. The meiotic division in the female gametes cannot, however, proceed in the same way as in the male gametes, that is, by division of the original cell into four cells of equal size.
It is found that the ability of the egg cytoplasm to respond to the spermatozoon (to become activated) is in some animals dependent on the intermingling of the nuclear sap with cytoplasm after the breakdown of the nuclear membrane.
Such a division would largely defeat the purpose of the accumulation of food reserves during the period of growth, for each of the four cells would receive only one quarter of the stockpile of food supplies. What actually occurs is that during the meiotic divisions the cytoplasm is partitioned very unequally, the bulk of the cytoplasm with the yolk remaining intact at each division.
When the oocyte is ready to undergo reduction divisions, the nuclear membrane breaks up, and the contents of the nucleus become intermingled with the surrounding cytoplasm. The rupture of the nuclear membrane, apart from liberating the chromosomes for the subsequent division, is of great importance because substances accumulated in the nucleoplasm are released and become mixed with the rest of the cytoplasm.
Frog oocytes can be enucleated prior to germinal vesicle breakdown and then maturation effected by hormone treatment. If a nucleus is supplied to such eggs by injecting into them nuclei from blastula cells the egg starts developing but stops after a few abortive cleavages. On the other hand, if some material from a germinal vesicle is injected into the egg prior to the injection of a blastula nucleus, normal development takes place.
After the breakdown of the nuclear membrane, the chromosomes, which have become greatly contracted and concentrated toward the center of the germinal vesicle, are carried to the periphery of the oocyte. Here an achromatic figure is formed, which takes up a position perpendicular to the surface of the oocyte.
The bivalents are placed on the equatorial plate and in due course separate into the two component chromosomes. A bulge now appears at the surface of the oocyte, and the outer pole of the spindle with half of the chromosomes during anaphase enters into this cytoplasmic bulge. The part of the cytoplasm with the chromosomes, which is bulging out, is then pinched off from the rest of the oocyte. The resulting small cell is called the first polar body.
The set of chromosomes contained in the first polar body is equivalent to the set of chromosomes remaining in the oocyte (which can now be distinguished as the secondary oocyte), just as the two sets of chromosomes in secondary spermatocytes are equivalent to each other. The first polar body, however, receives only a very small quantity of cytoplasm, while all the rest goes to the secondary oocyte.
The second meiotic division is carried out in the same way; that is, an achromatic figure is again formed at the surface, and when division takes place, half of the chromatids are given off, together with a small quantity of cytoplasm, as the second polar body. The large cell, which inherits the bulk of the cytoplasm together with one half of the chromatids, is now the fully mature egg. The chromosome set of the egg, as the result of two meiotic divisions, is a haploid one.
In the meantime, the first polar body may sometimes divide, so that eventually four cells are produced from one oocyte- one cell is the mature egg, the second and third cells are produced by the division of the first polar body, and the fourth cell is the second polar body.
These four cells can be compared to the four spermatids which are produced from a primary spermatocyte after two meiotic divisions, the difference being that each of the four spermatids later becomes a functional spermatozoon, while of the four cells produced by meiotic division in the female only one cell, the egg, is a functional gamete, and the other three cells (or two cells, if the first polar body does not divide further) later die off. Having so little cytoplasm, with no food reserves, they are not able to start developing. The similarities (and differences) between oogenesis and spermatogenesis.
At this stage we should again point out that, after maturation (= reduction divisions), the spermatids must still go through the process of differentiation before they become spermatozoa, that is, functional male gametes. On the other hand, the egg, after reduction divisions, does not undergo any further changes; it is quite ready to start developing.
A further distinction between the male and the female gametes is that the male gametes, the spermatozoa, become capable of fertilizing the egg sometime after they have completed the reduction divisions and subsequently have become differentiated. The eggs become capable of being fertilized sometimes before they have completed reduction divisions and before they have given off the polar bodies.