Meiosis and Oogenesis

Introduction

• Primordial germ cells: Migrate from the endoderm of the yolk sac to the gonadal ridge (developing ovary).
• Gametogenesis: Process by which germ cells form gametes via meiosis.
• Oogenesis: Process of forming a mature oocyte (ovum) from an oogonium via meiosis.

Cell Division Recap

• Mitosis: Somatic cell division producing 2 diploid cells (2n).
  • Diploid cells (2n): 46 chromosomes, 23 pairs.
  • Process: Prophase, Prometaphase, Metaphase, Anaphase, Telophase, Cytokinesis.
  • Interphase: G1, S (DNA replication), G2 phases.
• Meiosis: Division of germ cells producing 4 haploid cells (n) after two divisions.
  • First meiotic division: Reduction division.
  • Second meiotic division: Similar to mitosis, no DNA replication before.

Stages of Mitosis

1. Prophase: Chromatids condense.
2. Prometaphase: Spindles destroy nuclear membrane and attach to chromatids.
3. Metaphase: Chromatids line up at equatorial plate.
4. Anaphase: Chromatids split at centromere and move to opposite poles.
5. Telophase: Nuclear membrane reforms, cytokinesis completes cell division.

Meiosis Details

• Meiosis I (First Division)
  1. Prophase I: Homologous chromosomes pair (synapsis), crossing over occurs (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis stages).
  2. Metaphase I: Chromatids line up along the equatorial plate.
  3. Anaphase I: Homologous chromosomes move to opposite poles without splitting at centromere.
  4. Telophase I: Spindles disappear, nuclear membrane reforms, cytokinesis completes division.
  • Result: Two cells, each with half the chromosome number (haploid).
• Meiosis II (Second Division)
  • Similar steps as mitosis without DNA replication prior.
  1. Prophase II: Chromatids condense, spindles form.
  2. Metaphase II: Chromatids line up again.
  3. Anaphase II: Chromatids split at centromere and move to opposite poles.
  4. Telophase II: Nuclear membrane reforms, cytokinesis completes division.
  • Result: Four haploid gametes.

Oogenesis Process

• Primordial germ cells: Divide via mitosis to form more germ cells during migration.
• Oogonia: Differentiate and divide by mitosis until about 5 months of gestation.
• Primary oocytes: Enter first meiotic division, arrested in prophase I (diplotene stage) until puberty.
• At puberty: Surge of luteinizing hormone causes primary oocyte to complete meiosis I, forming a secondary oocyte and the first polar body.
• Secondary oocyte: Enters meiosis II and is arrested at metaphase II until fertilization.
• Fertilization: Meiosis II completes, forming a mature oocyte (ovum) and second polar body.
• Result: One functional mature oocyte and degenerated polar bodies.

Folliculogenesis and Ovarian Cycle

• Follicles surround and protect the developing oocytes.
• Developmental stages of follicles contribute to the ovarian cycle under hormonal influence.