• It is the process of formation of respective gametes (sperm and ova) in respective gonads. It involves Spermatogenesis and Oogenesis.

Spermatogenesis

• It is the process of formation of sperm in testis.
• Testis: it is the primary male reproductive organ.
• Shape and size: pinkish oval bodies occurring in pair, size is about 4.5cm long, 2.5 cm wide and 3 cm thick.
• Location: situated outside of abdominal cavity in scrotal sac. (Temperature of 2-3°C below body temperature is required for spermatogenesis)
• Each testis is surrounded by three layers.

1. Tunical vaginalis: double membrane outer covering, made up of fibrous connective tissue
2. Tunica albuginea: it is the middle layer below the tunica vaginalis.
3. Tunica vasculosa: it is the innermost highly vascular layer with network of blood capillaries.

Histology of testis:

• Each testis consists of 200-300 lobules, and each lobule contains 1-4 convulated loops called Seminiferious tubules. In between the seminiferous tubules, there is a group of interstitial cell called Leydig cell, which secrete testosterone, a male sex hormone. Each seminiferous tubules lined with germinal epithelium produce sperm by the process Spermatogenesis

Spermatogenesis occur in three phase

Figure: stages of spermatogenesis

i) Multiplication phase: the germinal epithelium of seminiferous tubules produce primodial germ cell. These cell multiplies repeatedly by mitosis to produce large number of spermatogonia.

ii) Growth or Maturation phase: The spermatogonia undergoes maturation. It is a diploid cell. After maturation spermatogonia is known as Sperm mother cell because it will eventually develop into the mature sperm.

iii) Meiotic phase: Duplication of homologous chromosome in sperm mother cell occur and become ready for meiosis. First meiotic division produce two Primary spermatocyte with haploid number of chromosome. The first meiotic division separates the homologous chromosomes from each parent. The second meiotic division of each primary spermatocytes occur resulting altogether of 4 haploid secondary spermatocytes. The secondary spermatocytes after maturation is known as spermatids. Each Spermatids goes on metamorphosis into sperm by the process of Spermiogenesis.

Spermiogenesis:  Sperm is a motile male gamete with head, neck and tail. During Metamorphosis of spermatids into sperm, following changes occurs

• Spermatids elongates and its Nucleus
• Cytoplasm extended to develop Flagella
• Golgi body produces Acrosome
• Mitochondria aggregate to form super mitochondria around base of flagella, providing energy for sperm motility
• By tubulobular process, sertoli cell phagocytose the sheded cytoplasm

Oogenesis-

• It is the process of formation of ova or egg in ovary.
• Ovary: it is the primary female reproductive organ.
• Shape and size: grayish pink almond shaped structure, size is 2.5-3.5 cm long, 2 cm wide and 1 cm thick
• Location: in the abdominal cavity, one on either side of vertebral column behind kidney.
• Each ovary can be differentiated into 3 parts

1. Outer germinal epithelium
2. Tunica albuginea: it is middlelayer of delicate connective tissue
3. Stroma: it is the inner mass of connective tissue. It is further differentiated into 2 layer-outer cortex and inner medulla. It is lined with germinal epithelium which form ovarian follicle. Each ovary is composed of about 400000 ovarian follicle.

• Ovaries are inactive before puberty, but stroma already contain immature follicle; Primordial follicle. Priomrdial follicle mature in about 28 days, rapture and release ova; process known as ovulation. 
• The germinal epithelium of ovarian follicle give ova germ cell called Oogonia.
• Oogenesis occur in 3 phages

  

                           Figure: stages of Oogenesis

i) Multiplication: The initial phase of Oogenesis starts during fetal stage. The primary germ cell, Oogonia develop from stem cell by mitosis cell division. In adult ovaries, primordial follicle contains a primary oocyte. Primary oocyte is also known as ova mother cell, which eventually produce ova.

ii) Growth or maturation phase: the Oogonia undergoes maturation. It is a diploid cell. Mature Oogonia is knownas primary oocytes, which undergoes meiosis, howerer, meiosis stopped at Prophase-I.

iii) Meiotic phase: Completion of meiosis-I produces a secondary oocyte and a polar body. The second meiosis division os Secondary Oocyte occur with unequal distribution of cytoplaswm producing large egg and a small second polar body. Eventually 1 egg and 3 polar bodies are produced.

Gametogenesis in Human-Spermatogenesis and Oogenesis

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