Terminology and Classification
Strictly speaking, using the term "egg cell" (ovum) for 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 refers to one of two developmental stages:
- Primary oocyte. Located within a late secondary or tertiary ovarian follicle during the major growth phase.
- Secondary oocyte. The ovulated cell released into the lumen of the uterine (fallopian) tube. This is the stage most commonly depicted in anatomical diagrams.
From a classification standpoint, the human female gamete is secondary oligolecithal (containing a small amount of yolk) and isolecithal (yolk is evenly distributed throughout the cytoplasm). The "secondary" designation has evolutionary significance: during phylogeny, a return to low yolk content occurred, first observed in amphioxus. Despite the sparse yolk, the total volume of the female gamete exceeds that of a spermatozoon by thousands of times due to a massive volume of cytoplasm and organelles.
Internal Cytoplasmic Structure
The internal content of the oocyte is highly specialized. Yolk granules are evenly dispersed throughout the cytoplasm, each surrounded by its own membrane. Chemically, they are lipoprotein and phosphoprotein complexes (the main components are phosvitin and lipovitellin). The substances for these granules are synthesized not only by the cell itself but also supplied via the bloodstream from the maternal liver.
Directly beneath the plasmalemma, within the cortical layer, cortical granules are localized. These are specialized membrane-bound vesicles filled with enzymes. Their primary task is participating in the cortical reaction upon fertilization, a crucial defense mechanism against polyspermy.
The cytoplasm also contains multivesicular bodies—membrane structures formed by the digestion of phagocytized particles. The robust protein-synthesizing machinery warrants special attention. The oocyte pre-accumulates a large reserve of ribosomes, transfer RNAs, and messenger RNAs. This reserve is vital for ensuring rapid protein synthesis during the first hours after fertilization, before the zygote's own genome begins functioning.
Cell Center and Surface Features
A critical feature of the human oocyte (secondary oocyte) is the complete absence of a cell center (centrosome). Upon completion of the first meiotic division, the female gamete loses its centrioles. This has significant biological and clinical implications: the oocyte cannot undergo division independently. The capacity for mitosis is restored exclusively after a spermatozoon contributes its centrioles during fertilization.
The cell surface is formed by a plasmalemma bearing numerous microvilli. Their presence significantly increases the total surface area of the cell, ensuring intensive metabolism.
Oocyte Coverings
Externally, the female gamete is surrounded by two protective layers: the zona pellucida and the corona radiata.
- Zona pellucida (Zp). Directly surrounds the oocyte plasmalemma. It is secreted by both the oocyte itself and the surrounding follicular cells. Chemically, it consists of glycosaminoglycans and specific glycoproteins (fractions Zp1, Zp2, Zp3). Fraction Zp3 acts as a species-specific receptor for spermatozoa, ensuring gamete binding. Glycoproteins Zp2 undergo structural changes following the cortical reaction, blocking the penetration of extra sperm (the mechanism preventing polyspermy).
- Corona radiata. Formed by follicular cells lacking a basement membrane. These cells extend long processes that penetrate the zona pellucida, forming the characteristic "radiated crown".
Located beneath or within these layers are the polar bodies (reductional bodies). These are the products of the first (occurring prior to fertilization) and second (following fertilization) meiotic divisions.