In-Depth Notes on Oogenesis and Fertilization Process

Cellular Process Leading to Oogenesis

• Spongal Chainesis: Cellular process regulating the production of sponges, controlled by the granulomatous HGG axis.

Overview of Oogenesis

• Oogenesis: The process leading to the production of ova.
• Primordial Germ Cells: These cells differentiate into eugonia in females during fetal development.
• Meiosis in Females:
  • Primordial germ cells develop into oocytes (eugonia).
  • Meiosis I occurs, resulting in a primary oozyte that remains in a dormant state until puberty.
  • Meiosis II occurs after fertilization, only if the ovum comes into contact with sperm.

Follicle Development and Ovulation

• Follicular Cells: These cells release estrogens and provide support for the developing ovum.
• Meiosis Division:
  • Primary oocyte undergoes meiosis to form a secondary oocyte and a polar body.
  • Only one large secondary oozyte matures; polar bodies degenerate.
• Ovulation Process:
  • The rupturing of the follicle releases the secondary oocyte and the polar body.
  • Completion of Meiosis II occurs after fertilization.

Hormonal Regulation during Oogenesis

• Follicular Phase:
  • Follicle grows and produces estradiol, which provides negative feedback to the hypothalamus.
  • Anterior pituitary releases FSH and LH, stimulating follicle growth.
  • Increased estradiol has positive feedback just before ovulation, leading to a surge in LH and FSH.
• Postovulation (Luteal Phase):
  • The ruptured follicle forms the corpus luteum, producing progesterone and estradiol, which prepare the uterus for potential pregnancy.
  • Progesterone supports uterine lining, while inhibin prevents further egg development if fertilization does not occur.

Fertilization Process

• Fertilization Mechanism:
  • External vs. Internal Fertilization: Terrestrial animals generally use internal fertilization for protection from desiccation.
  • Spawning: Coordinated release of gametes in aquatic animals for external fertilization.
  • Sperm's acrosome releases enzymes to penetrate the ovum's jelly coat (zona pellucida) to enable fertilization.
  • Sperm and Egg Fusion: The sperm nucleus enters the egg, fusing to form a diploid zygote (one-celled embryo).

Block to Polyspermy

• Fast Block: Occurs immediately after sperm binding, leading to a depolarization of the egg membrane to prevent additional sperm entry.
• Slow Block: Involves cortical granules that harden the egg’s outer layer, creating a fertilization membrane.

Developmental Pathways Post-Fertilization

• Different Types of Development:
  • Oviparous: Embryos develop outside the mother's body, e.g., birds, reptiles, most fish.
  • Viviparous: Animals like mammals that develop embryos inside the mother, connected via umbilical cord to receive nutrients.
  • Ovoviviparous: Eggs remain inside the mother until hatching, seen in some reptiles and sharks.

Amniotic Egg Structure

• Amniotic Structure: The significance of the amniotic egg in providing a protective environment for the embryo, important for terrestrial vertebrates.
  • Amnion: Fluid-filled membrane surrounding the embryo.
  • Yolk Sac: Supplies nutrients.
  • Allantois: Collects waste products.

Importance of Egg Structure

• Egg structure critical for understanding fertilization dynamics; layers like the zona pellucida and vitelline require proper interaction with sperm for successful fertilization.