Conception and Embryology

Question 1

Describe the maturation process of the ovum.

Answer 1

PRENATAL OOGENESIS

1. Primordial germ cells differentiate to oogonia in the foetus, and oogonia then proliferate by mitotic division in the foetal ovary until 20/40.
2. Oogonia differentiate to primary oocytes. Primary oocyte begins the first meiotic division at birth and is then arrested in prophase of the first meiotic division.
   
   • After each meiotic division a polar body is extruded and degenerates, so that one primary oocyte results in one mature ovum at the completion of the process.

POSTNATAL OOGENESIS

1. At the onset of puberty, a few follicles at a time recommence growth and one of these is selected for ovulation each cycle.
2. The primary oocyte resumes meiosis I and differentiates into a secondary oocyte, which begins meiosis II and arrests in metaphase II.
3. Meiosis is resumed at ovulation in response to the luteinising hormone surge, which stimulates follicle release and ovulation. The secondary oocyte has now become a mature ovum.

Question 2

Describe the follicle and folliculogenesis.

Answer 2

• The follice is a structure of mesenchymal cells which surround the female germ cell.
• The follicle develops from a primordial follicle, which contains the primordial germ cell, to the larger and more established pre-antral, antral and pre-ovulatory follicles sequentially.
• Each cycle a few follices recommence growth though only one is selected to become a pre-ovulatory follicle and ovulate.
• The antral follice is composed of inner granulosa cells, which secrete oestrogen, and outer thecal cells, which secrete androgens.
• The high oestrogen concentration produced by the inner granulosa cells is what triggers the luteinising hormone surge leading to ovulation.

Question 3

What is the main difference is spermatocyte and oocyte division?

Answer 3

Both occur via meiosis. Spermatocytes yield four haploid mature spermatozoa, whereas oocytes result in just one mature haploid ovum.

This is because the oocyte extrudes a polar body with each meiotic cycle which then degenerates.