Oogenesis and Follicle Development Overview

Oogenesis Overview

• Oogenesis is the formation of ova from female germ cells called oogonia.
• Oogonia are the stem cells of the female reproductive system, which differentiate into primary oocytes.
• Primary oocytes are the only form of oogonia present in the ovary; they are not visible under a microscope.
• Oogenesis starts during embryonic development and completes when the primary oocyte travels to the fallopian tube for fertilization.

Follicle Development

• An oocyte and its surrounding follicle cells are collectively termed a follicle.
• Follicles undergo maturation, resulting in the release of mature oocytes during ovulation.
• Follicle cells proliferate to form multiple layers around the oocyte, supporting its growth.

Structure Surrounding Oocyte

• Vitelline membrane: A thin membrane enclosing the primary oocyte.
• Zona Pellucida: A glycoprotein layer secreted by the primary oocyte and radiata, present uniquely in mammals.
• Corona Radiata: Follicular cell layer surrounding the zona pellucida.
• Cumulus Oophorus: Several layers of follicular cells anchoring the primary oocyte to the stratum granulosum.
• Antrum: Cavity containing fluid that separates the oocyte from the stratum granulosum.
• Stratum Granulosum: Layer of follicular cells that secrete estrogen and form the follicle’s wall.
• Theca Folliculi: Capsule surrounding the follicle, supported by inner (theca interna) and outer (theca externa) layers.

Types of Ovum in Chordates

• Eggs classified by yolk amount and distribution, correlated with offspring quantity.
• Microlecithal: Minimal yolk, evenly distributed; characteristic of placental mammals.
• Mesolecithal: Moderate yolk, with gradient distribution; found in amphibians.
• Macrolecithal: Abundant yolk at the vegetal pole; observed in birds.

Oogonia and Primary Oocytes

• Oogonia are diploid cells that multiply through mitosis and develop into primary oocytes.
• Oocytes are large cells with a small centrally-placed nucleus, protected by follicular cells.
• Primary oocytes are arrested in prophase I until puberty, eventually evolving into secondary oocytes during the first meiotic division.

Secondary Oocytes and Ovum

• Secondary oocytes are formed alongside the first polar body, which receives the extra set of chromosomes.
• After fertilization, the secondary oocyte completes meiosis II, forming the ovum and second polar body.
• The ovum is the mature female reproductive cell containing yolk and genetic material, ready for fertilization.

Follicular Development Stages

• Primordial Follicle: Located at the ovary's periphery, consists of a primary oocyte surrounded by flat follicular cells.
• Primary Follicle: Larger than primordial, with cuboidal or columnar follicular cells around the primary oocyte.
• Follicular development progresses alongside ovarian maturation, with follicular cells becoming stratified and secreting components like zona pellucida.

Corpus Luteum and Remnants

• Following ovulation, remaining follicular cells form the corpus luteum, an endocrine organ secreting progesterone.
• If fertilization does not occur, the corpus luteum regresses to become the corpus albicans.

Fertilization Process

• Fertilization is the union of a haploid secondary oocyte and a sperm cell, forming a diploid zygote.
• The process initiates with sperm approaching the egg and culminates in the zygote beginning mitosis.
• Fertilization occurs in the fallopian tube, crucial for the development of a new individual.

Summary of Key Terms

• Oogonia: Diploid precursor germ cells
• Primary Oocyte: Egg cell halted in prophase I
• Secondary Oocyte: Egg cell formed after meiosis I
• Corpus Luteum: Temporary endocrine structure after ovulation
• Zygote: Fertilized egg capable of developing into a new organism