Human ovum

Terminology and classification

Strictly speaking, using the term "ovum" in relation to mammals and humans is somewhat conventional. During the reproductive cycle, a truly mature female gamete in the classical biological sense is not formed. In medicine and embryology, this term is used to denote one of two developmental stages:

1. Primary oocyte. Located inside a late secondary or tertiary ovarian follicle during the period of intensive growth.
2. Secondary oocyte. This is already an ovulated cell that has entered the lumen of the uterine tube. This exact stage is most commonly depicted in anatomical diagrams.

From a classification standpoint, the human female gamete is secondary oligolecithal (contains a small amount of yolk) and isolecithal (yolk is distributed evenly throughout the cytoplasm). The concept of "secondary" has evolutionary significance: in the course of phylogenesis, there was a reversion to a small amount of yolk inclusions, which was first observed in amphioxus in the history of living organisms. Despite the scanty amount of yolk, the total volume of the female gamete exceeds the size of a spermatozoon by thousands of times due to a huge amount of cytoplasm and organelles.

Internal cytoplasmic structure

The internal contents of the oocyte are characterized by a high degree of specialization. Yolk granules are evenly dispersed in the cytoplasm, each surrounded by its own membrane. Chemically, they are complexes of lipoproteins and phosphoproteins (the main components are phosvitin and lipovitellin). The substances for these granules are not only synthesized by the cell itself but also supplied from the bloodstream by the mother's liver.

Directly beneath the plasmalemma, in the cortical layer, cortical granules are localized. These are specific membrane vesicles filled with enzymes. Their main task is participation in the cortical reaction at the moment of fertilization, which is a crucial defense mechanism against polyspermy.

Also present in the cytoplasm are multivesicular bodies—membrane structures formed as a result of the digestion of phagocytosed particles. Of particular note is the robust protein-synthesizing apparatus. The oocyte pre-accumulates a high reserve of ribosomes, transfer RNAs, and messenger RNAs. This reserve is vital for ensuring rapid protein synthesis in the first hours after fertilization, before the zygote's own genome begins to function.

Features of the cell center and surface

A critical feature of the ovum (secondary oocyte) is the complete absence of the cell center. Upon completion of the first meiotic division, the female gamete loses its centrioles. This has important biological and clinical significance: the ovum is incapable of independent division. The capacity for mitosis is restored exclusively after the spermatozoon introduces its centrioles during the fertilization process.

The cell surface is formed by the plasmalemma, which bears numerous microvilli. Their presence significantly increases the total surface area of the cell, ensuring intensive metabolism.

Ovum envelopes

Externally, the female gamete is surrounded by two protective envelopes: the zona pellucida and the corona radiata.

• Zona pellucida (zona pellucida, Zp). Directly surrounds the oocyte plasmalemma. It is a product of secretion by both the ovum itself and follicular cells. Chemically, it consists of glycosaminoglycans and specific glycoproteins (Zp1, Zp2, Zp3 fractions). The Zp3 fraction acts as species-specific receptors for spermatozoa, ensuring gamete binding. After the cortical reaction, Zp2 glycoproteins change their structure and block the penetration of extra spermatozoa (a defense mechanism against polyspermy).
• Corona radiata (radiate crown, corona radiata). Formed by follicular cells that lack a basement membrane. These cells extend long processes penetrating the zona pellucida, thereby forming the characteristic "corona radiata".

Underneath the envelopes or within their composition lie polar bodies. These are the products of the first (occurring before fertilization) and second (after fertilization) meiotic divisions.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of the cortical reaction and biochemistry of polyspermy prevention
• The process of yolk granule synthesis and the role of the maternal liver
• Histological structure of the corona radiata and functions of its processes
• Dynamics of meiosis in oogenesis and formation of polar bodies
• Functions of the oocyte protein-synthesizing apparatus prior to zygote genome activation

Other topics in the section "Early stages of human intrauterine development"

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