Oogenesis and Spermatogenesis Overview

  • Oogenesis vs. Spermatogenesis

    • Definition:

    • Oogenesis is the production of eggs (ova) occurring in the ovary whereas spermatogenesis is the production of sperm occurring in the testes.

  • Starting Point:

    • Oogenesis: Begins prior to birth with a diploid cell called oogonium.

    • Spermatogenesis: Starts with diploid cells called spermatogonia.

  • Meiosis in Oogenesis:

    • Oogonium undergoes meiosis I to produce two haploid primary oocytes.

    • Due to unequal cytoplasmic division, most cytoplasm goes to primary oocyte, and one polar body is formed (a non-functional cell).

    • Primary oocyte undergoes meiosis II to form a secondary oocyte.

    • This secondary oocyte is ovulated.

    • Another polar body (secondary polar body) may be formed.

      • Both polar bodies generally disintegrate and do not participate in fertilization.

  • Spermatogenesis Process:

    • Mitosis: Diploid spermatogonia undergo mitosis.

    • Produces two types of cells: one will replace itself (stem cell), and the other will undergo meiosis.

    • Meiosis I: Forms secondary spermatocyte.

    • Meiosis II: Forms four haploid spermatids.

    • Happens in the testes and involves further maturation to produce functional sperm (spermiogenesis).

  • Timeline of Oogenesis Events:

    • Before Birth:

    • Oogonia undergo meiosis and stop at prophase I.

    • Childhood to Puberty:

    • Ova remain inactive.

    • Puberty:

    • Oocytes are recruited; meiosis I completes producing secondary oocytes which then arrest in metaphase II.

    • The secondary oocyte is ovulated in this arrested state and completes meiosis II upon fertilization.

  • Follicle Development:

    • Females are born with primordial follicles (primary oocytes+supportive follicular cells).

    • Follicle Stimulating Hormone (FSH):

    • Stimulates growth of the follicle, transitioning from primordial to primary to secondary follicle.

    • Mature Follicle (Graafian Follicle):

    • Develops monthly from puberty to menopause.

    • The ovulated secondary oocyte is surrounded by follicular cells known as corona radiata.

  • Corpus Luteum:

    • After ovulation, remaining follicular cells form the corpus luteum, an endocrine structure that supports early pregnancy if fertilization occurs.

  • Key Stages of Meiosis:

    • Meiosis consists of two main divisions: Meiosis I and Meiosis II, each having four stages: Prophase, Metaphase, Anaphase, Telophase.

  • Arresting Stages:

    • Oocyte arrests at prophase I before birth and metaphase II before ovulation.