VAN 211: Veterinary Histology and Embryology (2+2)

OOGENESIS

• Oogenesis is the process by which the female gamete or ova is produced. It takes place in the ovary.
• The ovary is a compact mass of connective tissue, smooth muscle fibres blood vessels, nerves and ovarian follicles at different stages of development. The entire ovary is covered by a thin germinal epithelium. From this, the germinal cells dip into the substance of the ovary and these are the oogonia. Like spermatogenesis, the oogenesis takes place in three phases namely,
  • Multiplication phase
  • Growth phase
  • Maturation phase
• The process of oogenesis is more complicated. It involves not only the production of the ova but also the acquisition of food reserves for the developing embryo.

Multiplication phase

• The ova are derived from the primordial germ cells, which undergo proliferation by mitotic division and the resulting cells are called Eugenia or egg mother cells.
• When the division stops, the cells are termed primary oocyte which enter a period of growth.
• The nucleus of primary oocyte is in diploid state (2n).

Growth phase

• Growth plays a much greater role in oogenesis than in spermatogenesis.
• During this period, nutritive substances and other materials necessary for the development of embryo are synthesized.

Maturation phase

• The primary oocyte under goes the I meiotic division to form two haploid unequal cells- a large secondary oocyte with more cytoplasm and a small polar body with little cytoplasm.
• Then the secondary occtye undergoes II meiotic division resulting in a large cell ootid and a small second polar body.
• Sometimes the I polar body divides into two polar bodies. Thus in oogenesis, one ootid and three polar bodies are formed from a single primary oocyte.
• The ootid then becomes the mature ovum. The three polar bodies are soon degenerates.