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

OLIGOLECITHAL		POLYLECITHAL
ISOLECITHAL		TELOLECITHAL
PRIMARY (amphioxus)	SECONDARY (mammals)	MESOLECITHAL (Pisces, amphibious)	PRONOUNCED-TELOLECITHAL (reptiles, birds)

A) primary isolecithal egg cell; b) mesolecithal egg cell; d) pronounced telolecithal egg cell; e) secondary isolecithal egg cell:

1) Nucleus; 2) yolk (vitelline) granules; 3) follicular cells

Oligolecithal ovum is characterized by small amounts of yolk. In any oligolecithal ovum yolk is equally distributed within cytoplasm. Thus, all oligolecithal ova are isolecithal ova. A primitive vertebrate (such as Amphioxus, lancelet) leading an aquatic existence produces many small, yolk-poor eggs, which form free swimming larvae. Their ova are known as primary isolecithal ova. Through phylogenesis in eutherian mammals yolk has been secondarily reduced to the oligolecithal form, because their development occurs in a uterus. Thus, the human egg cell can be referred to oligolecithal, secondary isolecithal oocyte.

Polylecithal ovum is characterized by large amounts of yolk. Telolecithal ovum is characterized by the polarity of yolk allocation. One of the cell poles - the vegetative pole - is laden heavily with yolk, while another pole – the animal pole – is not so heavily yolked. The nucleus is placed at this animal pole. This polarity in yolk allocation may be pronounced more moderately – in mesolecithal ova, or more sharply – in pronounced-telolecithal ova. Most amphibians have large, moderately heavily yolked (mesolecithal) eggs, which form free swimming larvae (tadpoils). In adaptation to a terrestrial existence, reptiles and birds produce few, large, yolk-rich (pronounced-telolecithal) eggs, with protective membranes and a shell, and have no larval stage.