Biology · MU Pleven

30.Ovogenesis. Characteristics and types of the mature eggs.

Explains oogenesis, the process by which a diploid primary oocyte gives rise to a single haploid ovum through meiosis, contrasting it with spermatogenesis. It takes the stages in order: multiplication of oogonia, the long growth phase with species-specific growth figures for mammals, amphibians and birds, and the two meiotic maturation divisions that produce polar bodies. It closes with notes on where the second division happens and a final outcome summary.

Inside: 3 sections · 2 labelled pictures · 10 flashcards · 3 min

Topic 30 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 433 words (3 min of reading), with 10 flashcards.

In this note

  1. Ovogenesis. Characteristics and types of the mature eggs.
  2. Oogenesis (formation of ova)
  3. Stages of oogenesis

Oogenesis (formation of ova)

Oogenesis is the process by which a diploid primary oocyte gives rise to a single functional haploid ovum (egg) by meiosis. Primary oocytes are stored in the female ovaries and, unlike spermatogenesis, oogenesis produces one large ovum plus smaller polar bodies rather than four equivalent gametes. The three smaller products (polar bodies) typically degenerate, leaving the single mature egg. In humans this process normally produces one mature egg about once a month from puberty until menopause.

Key differences from spermatogenesis

  • One functional ovum per primary oocyte (vs four sperm per spermatogonium).
  • Three small polar bodies are produced and normally degenerate; only the large oogonium-derived cell becomes the ovum.

Stages of oogenesis

Multiplication phase

  • Primordial germ cells divide repeatedly to form oogonia.
  • Oogonia are diploid (2n) and called egg-mother cells.
  • Only a subset of oogonia enters oogenesis; others become follicle (nurse or nutritive) cells that support the developing oocyte.

Growth phase

  • This is the longest stage and involves cell growth, not division.
  • Oogonia enlarge into primary oocytes (diploid) and accumulate yolk and cytoplasmic components.
  • The extent and duration of growth varies by species (examples from your notes):
  • Mammals: ~20× increase; growth period ~14 days in the example given.
  • Amphibians: ~200× increase; growth period ~3 days (example).
  • Birds: ~4000× increase; hens ~6–14 days (example).
  • The primary oocyte then arrests (usually in prophase I) until maturation resumes.

Maturation phase (meiotic divisions)

  • Maturation involves meiosis I and II and produces one large ootid/ootocyte and polar bodies.
  • Final output: one functional ootid (which matures into the ovum) and up to three polar bodies (these usually degenerate).

Maturation I (Meiosis I)

  • The diploid primary oocyte completes meiosis I to produce:
  • One large haploid secondary oocyte (retains most cytoplasm)
  • One small haploid first polar body (polocyte I)

Maturation II (Meiosis II)

  • The secondary oocyte undergoes meiosis II to yield:
  • One functional ootid (which becomes the ovum)
  • One small second polar body (polocyte II)
  • The first polar body may also divide during meiosis II to produce additional tiny polar bodies (sometimes labelled polocyte Ia & Ib).

Timing / location notes (species variation)

  • The second meiotic division often completes outside the ovary: in many animals it finishes after ovulation and may be completed only after fertilization (this is true for humans).
  • In some non-chordates both meiotic divisions occur inside the ovary.
  • Thus, whether Meiosis II happens before or after fertilization depends on the species.

Final outcome:

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