Gametogenesis is the process by which diploid precursor cells undergo division and differentiation to become haploid gametes (sex cells)

• In males, this process is called spermatogenesis and produce spermatozoa (sperm)

• In females, this process is called oogenesis and produce ova (eggs)

The process of gametogenesis occurs in the gonads and involves the following steps:

• Multiple mitotic divisions and cell growth of precursor germ cells

• Two meiotic divisions (meiosis I and II) to produce haploid daughter cells
  

• The haploid daughter cells then undergo differentiation to produce functional gametes

Spermatogenesis:

• Sperm is produced within the seminiferous tubules of the testes 

• The tubules are lined by germline cells (spermatogonia) which move towards the lumen as they divide and differentiate

• The spermatogonia grow to form spermatocytes, before undergoing meiotic division to form spermatids

• The spermatids differentiate into functional spermatozoa, which are transported via the tubules to the epididymis

Oogenesis

• Egg cells (ova) are produced within follicles stored inside the ovaries

• These follicles develop over the course of a menstrual cycle and so may exist in various stages of development

• Primordial follicles contain primary oocytes that have been arrested during meiosis from birth

• Each month, some of these follicles continue to develop and produce secondary oocytes

• During ovulation, a mature follicle will rupture and release its oocyte – this will develop into an ovum if it is fertilised by a sperm

Gametogenesis Comparisons

While the processes of spermatogenesis and oogenesis follow the same basic pathway, several key differences exist between them

• The two processes differ in the number of gametes produced, the size of the cells and the timing of the processes

1. Number of cells produced:

• In spermatogenesis, the cells divide equally during meiosis to produce four functional gametes

• In oogenesis, the cells do not divide equally and as a result only one functional gamete is formed (plus 2 – 3 polar bodies)

2. Size of cells:

• In spermatogenesis, the cells that are formed following differentiation are all of equal size with equal amounts of cytoplasm

• In oogenesis, one daughter cell (the ovum) retains all of the cytoplasm, while the other daughter cells form polar bodies
  

  • The polar bodies remain trapped within the surrounding layer of follicle cells until they eventually degenerate

3. Timing of the process:

• In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death

• In oogenesis, the production of gametes is a staggered and finite process:
  

  • It begins before birth (prenatally) with the formation of a fixed number of primary oocytes (~40,000)

  • It continues with the onset of puberty according to a monthly menstrual cycle

  • It ends when hormonal changes prevent the further continuance of the menstrual cycle (menopause)

Spermatogenesis vs Oogenesis