15.2 - Gametogenesis in Humans

The Necessity of Gametogenesis

Gametogenesis is the process of producing gametes/reproductive cells/sex cells. The process occurs in the gonads/reproductive organs in both male and female. Testes (singular, testis) produce the male gametes/sperms, while ovaries produce the female gametes/ova. The formation of gametes ensures that the diploid chromosomal number is maintained from one generation to the next.

Spermatogenesis

Spermatogenesis is the process of sperm formation. It begins when a male reaches his puberty stage. It takes place in the seminiferous tubules. The process is stimulated by follicle-stimulating hormone (FSH) and testosterone. Luteinizing hormone (LH) secretion helps to promote testosterone secretion in the testes.

Figure 3 - The primordial germ cells of the seminiferous tubules will undergo cell division to produce sperms. Sertoli cells that line the tubules provide nutrients for the growth of sperms. The sperms pass from the seminiferous tubules through the epididymis and vas deferens to be stored.

Oogenesis

Oogenesis is the formation of secondary oocyte which occurs in the ovaries. It begins in the developing ovaries of a female fetus before birth.

1. Primordial germ cells undergo multiple mitotic division to form oogonium (diploid).

2. Oogonium grows to form primary oocytes. Each primary oocyte is surrounded by one or more layers of follicular cells, forming primary follicle. The development of the follicle is stimulated by FSH. As fetal development continues, all primary oocytes undergo meiosis but stop at prophase I. At birth, a human female body has millions of primary oocytes that remain dormant in prophase I of meiosis I. However, this number is reduced at puberty.

3. Upon puberty, the primary oocytes become active and meiosis resumes. The diploid primary oocyte completes meiosis I to form haploid secondary oocyte and a first polar body. The secondary oocyte starts meiosis II but stops at metaphase II. The first polar body goes on to complete meiosis II to form two second polar bodies.

4. At this stage, the secondary oocyte surrounded by layers of follicular cells is called secondary follicle. The secondary follicle increases in size and mature to form Graafian follicle , which releases estrogen. The Graafian follicle merges with the wall of the ovary to release the secondary oocyte into the Fallopian tube. This process is called ovulation.

5. If a sperm penetrates the secondary oocyte during fertilization, the secondary oocyte will complete meiosis II to produce ovum (n) and first polar body (n). The nuclei of the sperm cell and ovum fuse to form a diploid zygote (2n). The remaining polar bodies eventually degenerate. Upon fertilization, the Graafian follicle develops into corpus luteum, which continues to grow and release estrogen and progesterone. If fertilization does not occur, the corpus luteum and the secondary oocyte will degenerate and die. They are removed through menstruation.

Figure 4 - Oogenesis in the ovaries

Structure of Sperm and Graafian Follicle

A mature sperm consists of 3 main parts.

• Head , which contains a large nucleus
• Midpiece , which contains large number of mitochondria
• Tail , that propels the sperm towards the Fallopian tube for fertilization.

Figure 5 - Sperm

A secondary oocyte is a large cell that contains gel-like substance and follicular cells around it. The secondary oocyte together with the follicular cells form the Graafian follicle.

Figure 6 - Structure of a Graafian follicle

Spermatogenesis and Oogenesis

• Similarities

  • Both processes occur in the reproductive organs
  • Both produce haploid gametes that are involved in fertilization

• Differences

  • Spermatogenesis

    • Place of formation: Testes
    • Gametes produced: Millions of sperms produced everyday after puberty
    • Size of gametes: Small
    • Number of gametes produced during meiosis: Four sperms
    • Differentiation: Spermatids differentiate to become sperms
    • Germinal epithelial cells: Production of sperm is continuous throughout lifetime of a male.
    • Meiotic division:
      • 2 secondary spermatocytes are produced after meiosis I.
      • Meiosis is completed.

  • Oogenesis

    • Place of formation: Ovaries
    • Gametes produced: Only one secondary oocyte is formed from the ovaries in every menstrual cycle
    • Size of gametes: Large
    • Number of gametes produced during meiosis: One secondary oocyte (three non-functioning polar bodies)
    • Differentiation: No differentiation
    • Germinal epithelial cells: Production of oocytes is not continuous. It begins when the female fetus is in her mother's womb and becomes dormant when the baby is born. Oogenesis continues when the female is sexually matured and stops after menopause.
    • Meiotic division:
      • 1 secondary oocyte and 1 polar body are produced after meiosis I.
      • Meiosis II completes only if a sperm fertilizes the secondary oocyte.