Ovum

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 the yolk (protein-lipid inclusions), which provides trophism for the embryo.

According to the amount of yolk, cells are divided into:

• Alecithal (yolkless). Found in some parasitic invertebrates.
• Oligolecithal (low-yolk). Divided into primarily oligolecithal (primitive chordates, e.g., amphioxus) and secondarily oligolecithal. The latter type is characteristic of mammals: during evolution, ancestors accumulated yolk, but with the transition to intrauterine development, the need for it disappeared, and the cell "shrank".
• Polylecithal (high-yolk). Ensure development in the external environment (fish, amphibians, reptiles, birds).

According to the distribution pattern of nutrients, there are:

1. Isolecithal — uniform distribution. Typical of oligolecithal cells (humans, amphioxus).
2. Telolecithal — the yolk is displaced toward the vegetal pole, and the organelles with the nucleus toward the animal pole. In amphibians, they are moderately telolecithal, while in birds and teleost fish, they are strongly telolecithal (cytoplasm with the nucleus compressed into a tiny disc).

Ova and follicular envelopes

The number and structure of envelopes are adapted to the developmental environment. In fish, they are simple (vitelline and mucous), in birds — a complex barrier against desiccation (vitelline, albuminous, shell membranes, and the shell). In mammals, a specific two-tier system has formed, participating in nutrition and implantation.

As the oocyte matures in the ovary, follicles form (from primordial to tertiary). Follicular cells differentiate, forming barriers:

• Zona pellucida (zona pellucida). Consists of glycoproteins and glycosaminoglycans, secreted jointly by the oocyte and its microenvironment.
• Corona radiata (corona radiata, radiating crown). These are follicular cells whose processes penetrate the zona pellucida, entering into direct contact with the oocyte plasmalemma to provide trophism.
• Theca (theca). The outer connective tissue envelope of the follicle, located beyond the basement membrane.

Biological status and meiosis asymmetry

Technically, females do not have an independent stage of a mature haploid ovum. The ovary contains oocytes whose nuclei are tetraploid in DNA content.

At the moment of ovulation, the follicle ruptures, and a secondary oocyte, arrested in prophase of the second meiotic division, leaves the ovary wall. Along with it, the zona pellucida and cells of the corona radiata emerge. 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 inside, the oocyte dies. Meanwhile, meiosis itself is highly asymmetrical: to preserve the entire supply of yolk in a single zygote, the cytoplasm does not divide equally. One giant cell is formed along with small polar bodies that receive chromosomes but are devoid of cytoplasm. They remain lying beneath the egg envelopes.

Cytoplasmic ultrastructure

Specific structures are visualized inside the cytoplasm on electron micrographs:

• Yolk granules. Bounded by a membrane, they have a dense crystalline center made of phosphoproteins and lipoproteins (which are supplied, among other ways, exogenously from the liver).
• Multivesicular bodies. Large membrane-bound sacs with small vesicles inside (lysosome derivatives).
• Cortical granules. Localized directly beneath the plasmalemma. They contain hydrolytic enzymes that are released outward upon fertilization (cortical reaction) and block the penetration of other spermatozoa.

Immediately after fertilization, a powerful protein-synthesizing apparatus (ribosomes, various RNAs) is sharply activated in the cell to meet the needs of the embryo at early stages of embryogenesis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Histophysiology and cell differentiation in the developing follicle
• Mechanisms of formation and composition of the Zona pellucida
• Molecular bases of the cortical reaction
• Light microscopy of polar bodies (using Ascaris as an example)
• Chemical composition of yolk granules and pathways of their synthesis

Other topics in the section "Germ cells, fertilization, cleavage, blastula"

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