Oogenesis in Female: Process, Phases, Yolk Formation and Maturation
What is Gametogenesis?
Gametogenesis is the process of formation of male and female gametes. In humans it occurs as spermatogenesis and oogenesis.
What is Oogenesis?
Oogenesis is the process of formation, growth and maturation of female gametes (ova) in the ovary.
It involves development of a haploid egg along with accumulation of food reserves required for embryonic development.
Structure of Ovary
The ovary is attached to the broad ligament and consists of cortex and medulla.
- Cortex contains developing ovarian follicles.
- Medulla contains blood vessels and connective tissue.
- Corpus luteum develops after ovulation.
- Corpus albicans forms after degeneration of corpus luteum.
Events in Oogenesis
Oogenesis occurs in three major phases:
- Proliferation Phase
- Growth Phase
- Maturation Phase
Proliferation Phase
Primordial germ cells divide mitotically to form oogonia.
Oogonia later become primary oocytes which contain diploid chromosomes.
Growth Phase
This is the longest phase of oogenesis where the oocyte increases greatly in size and accumulates nutrients.
Pre-Vitellogenesis
Growth of Nuclear Substances
- Formation of germinal vesicle.
- Development of lampbrush chromosomes.
- Increased RNA synthesis.
- Growth of nucleolus.
Growth of Cytoplasmic Substances
- Mitochondrial multiplication.
- Golgi body development.
- Endoplasmic reticulum expansion.
- Cortical granule formation.
Vitellogenesis
Vitellogenesis is the phase during which yolk is synthesized and deposited in the growing oocyte.
In vertebrates, yolk precursors are often synthesized in the liver and transported to the ovary.
Physico-Chemical Nature of Yolk
Yolk contains proteins, phospholipids and fats that provide nutrition for embryo development.
Types of Yolk
- Granular Yolk – Found in many invertebrates.
- Yolk Platelets – Common in amphibians.
- Yolk Spheres – Present in birds and reptiles.
Functions of Yolk
- Stores nutrients.
- Influences cleavage pattern.
- Determines egg size.
- Supports gastrulation.
- Controls embryonic development.
Role of Follicle Cells in Oogenesis
Follicle cells surround the developing oocyte and provide nutrition, protection and metabolic support.
- Form Graafian follicles.
- Secrete follicular fluid.
- Facilitate nutrient transfer.
- Support oocyte maturation.
Role of Nurse Cells
Nurse cells occur mainly in invertebrates and provide nutrients directly to the developing oocyte.
They are especially prominent in insects such as Drosophila.
Maturation Phase
The primary oocyte undergoes meiosis to produce a mature haploid ovum.
First Meiotic Division
- Produces secondary oocyte.
- Forms first polar body.
Second Meiotic Division
- Produces mature ovum.
- Forms second polar body.
Significance of Polar Bodies
Polar bodies allow the ovum to retain maximum cytoplasm and nutrient reserves required for embryo development.
Frequently Asked Questions
What is oogenesis?
Formation and maturation of female gametes in the ovary.
How many phases are present in oogenesis?
Three phases: proliferation, growth and maturation.
What is vitellogenesis?
The process of yolk formation and deposition in the developing oocyte.
Why are polar bodies formed?
To retain most cytoplasm within the mature ovum.