Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Fundamentals of General Embryology
Germ Cells
Female Germ Cells

Female germ Cells — ova, or oocytes (ovum) — are produced in the female Gonads. Over the lifetime of a human and mammals, several hundred of them mature (whereas in amphibians and fish, the number reaches tens of thousands).

Oocytes are spherical in shape, ranging in size from a few micrometers to several centimeters. Unlike spermatozoa, oocytes possess a large volume of Cytoplasm and contain yolk, but lack centrioles, which renders them incapable of independent division (Fig. 21). An oocyte consists of a Nucleus, cytoplasm, and membranes. All oocytes feature a cytolemma (ovolemma), or primary membrane; many are surrounded by a secondary (carbohydrate-protein) membrane, and some by a tertiary membrane (shell and sub-shell layers).

The oocyte cytolemma forms small outgrowths known as microvilli. During its development in the Ovary, the oocyte becomes surrounded by a layer of follicular cells that form the corona radiata around it. These follicular cells participate in nourishing the oocyte. As a result of the METABOLIC ACTIVITY OF both the oocyte and the follicular cells, a glycosaminoglycan-rich layer—the zona pellucida—is formed around the oocyte.

The oocyte nucleus contains a haploid set of Chromosomes. During oocyte growth, intensive synthetic processes occur in The Nucleus, specifically Gene Amplification for RNA Synthesis, resulting in The formation of numerous copies of those chromosomal DNA regions that encode ribosomal RNA. Most of these DNA copies remain blocked by protein molecules until Fertilization takes place.

The cytoplasm (ooplasm) contains a well-developed Endoplasmic reticulum, a moderate number of Cell/35.html">Mitochondria, the Golgi apparatus, and inclusions. Among these inclusions, yolk (lecithos) deserves special attention as a nutrient reserve, The amount of which depends on The Nature of Embryogenesis. The Structure of oocytes is characterized by polarity, which correlates with the amount of yolk in The Cell. The region where yolk accumulates is termed the vegetal pole, whereas the opposite region, toward which the nucleus is displaced, is the animal pole.

Depending on the amount of yolk and its distribution within the cytoplasm, all oocytes are classified into several types (Fig. 22).

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Fig. 21. Structure of a mammalian oocyte (after S. F. Gilbert):

1 — oocyte; 2 — zona pellucida; 3 — corona radiata; 4 — nucleus

Fig. 22. Classification of oocytes