Classification of Ova: Yolk as a Criterion
The morphological diversity of female gametes directly depends on the conditions of upcoming embryogenesis. The key factor is yolk (protein-lipid inclusions), which provides nutrition for the embryo.
Based on the amount of yolk, cells are divided into:
- Alecithal (yolk-less). Found in some parasitic invertebrates.
- Oligolecithal (low yolk). Divided into primary oligolecithal (primitive chordates, e.g., amphioxus) and secondary oligolecithal. The latter type is characteristic of mammals: ancestors accumulated yolk during evolution, but with the transition to intrauterine development, the need for it disappeared, and the cell "shrank."
- Polylecithal (high yolk). Ensures development in the external environment (fish, amphibians, reptiles, birds).
Based on the distribution of nutrients, they are classified into:
- Isolecithal — uniform distribution. Typical of oligolecithal cells (humans, amphioxus).
- Telolecithal — yolk is displaced toward the vegetal pole, while organelles and the nucleus are displaced toward the animal pole. In amphibians, they are moderately telolecithal, whereas in birds and teleost fish, they are extremely telolecithal (cytoplasm and nucleus are compressed into a tiny disc).
Coverings of the Ovum and Follicle
The number and structure of envelopes are adapted to the environment of development. In fish, they are simple (vitelline and mucous membranes); in birds, they form a complex barrier against desiccation (vitelline membrane, albumen, shell membranes, and the shell). In mammals, a specialized two-tier system has evolved to support nutrition and implantation.
As the oocyte matures in the ovary, follicles form (ranging from primordial to tertiary). Follicle cells differentiate to form barriers:
- Zona pellucida. Consists of glycoproteins and glycosaminoglycans, secreted jointly by the oocyte and its surrounding cells.
- Corona radiata (radiant crown). Follicular cells whose cytoplasmic processes penetrate the zona pellucida, making direct contact with the oocyte plasmalemma to ensure nutrition.
- Theca (theca folliculi). The outer connective tissue layer of the follicle located outside the basement membrane.
Biological Status and Meiotic Asymmetry
Technically, humans do not have an independent stage of a mature haploid ovum. The ovary contains oocytes, whose nuclei are tetraploid in DNA content.
At ovulation, the follicle ruptures, and a secondary oocyte, arrested in prophase of the second meiotic division, leaves the ovary. It is released along with the zona pellucida and corona radiata cells. The basement membrane and mural follicular cells remain in the ovary.
The cell can complete meiosis only in the event of fertilization. If a spermatozoon does not penetrate, the oocyte degenerates. Furthermore, meiosis itself is highly asymmetrical: to preserve the entire yolk supply within a single zygote, the cytoplasm does not divide equally. One giant cell is formed along with small polar bodies, which receive chromosomes but lack cytoplasm. They remain positioned beneath the ovum envelopes.
Ultrastructure of the Cytoplasm
Electron micrographs reveal specific structures within the cytoplasm:
- Yolk granules. Membrane-bound structures with a dense crystalline core composed of phosphoproteins and lipoproteins (derived in part exogenously from the liver).
- Multivesicular bodies. Large membrane-bound sacs containing small vesicles (lysosomal derivatives).
- Cortical granules. Localized directly beneath the plasmalemma. They contain hydrolytic enzymes that are released extracellularly upon fertilization (cortical reaction) to block polyspermy.
Immediately after fertilization, a powerful protein-synthesizing apparatus (ribosomes, various RNAs) is rapidly activated within the cell to meet the demands of early embryonic development.