Fusion and formation of a zygote

Both the oocyte and the sperm go through transformations, as a reaction to the fusion of cell membranes, preparing for the fusion of their genetic material.

Entry of the sperm triggers completion of the oocyte second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called a female pronucleus in this process, to distinguish it from the nuclei that are the result of fertilization.

The head of engulfed sperm swells, forming a male pronucleus. The sperm’s tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.

This process ultimately leads to the formation of a diploid cell called a zygote. The zygote is the fertilized ovum, and is the first form of a new human being.

The male pronucleus, its centrioles and female pronucleus migrate towards the center of the oocyte, rapidly replicating their DNA as they do so to prepare the new human for its first mitotic (cleavage) division.

The male and female pronuclei don’t fuse, although their genetic material do so. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms around them to catch the chromosomes before they get lost in the egg cytoplasm. The first cleavage division follows, producing two identical daughter cells.

BIOLOGICAL SIGNIFICANCE of FERTILIZATION

• the penetration by the sperm cell reconstitutes the diploid number of chromosomes;

• the zygote contains a new combination of chromosomes and genetic material that is different from those of the parents;

• the embryo sex is determined at fertilization by kind of the sperm that fertilizes the ovum (X-bearing sperm or Y-bearing sperm);

• the penetration by the sperm cell serves as a stimulus for the ovum to complete the second meiotic division and cast off the second polar body;

• fertilization initiates a series of rapid mitotic divisions called the zygote cleavage

Comparative Vertebrate Embryology

CLASSIFICATION of EGG CELLS

Morphological characteristics of egg cells of different representatives of the phylum Chordata vary. The principal modifications are associated with varying amounts of yolk within ovum. Yolk decelerates or even prevents division of the ovum cytoplasm. Thus, amounts of yolk as well as regional differences in yolk allocation are important for ways of subsequent development. So, classification of ova is based on amounts of yolk and its allocation within ovum (Tabl.2; Fig.6).

Tabl.2 TYPES of OOCYTES

A) primary isolecithal egg cell; B) mesolecithal egg cell; D) pronounced telolecithal egg cell; E) secondary isolecithal egg cell: