Gametogenesis in Humans

Development in humans: 

Gametogenesis in humans: 

Gametogenesis in humans refers to the process of the formation of mature gametes, which are the reproductive cells involved in sexual reproduction. In males, this process is known as spermatogenesis, and in females, it is called oogenesis. 

Spermatogenesis:

  • Spermatogenesis is the process by which sperms are produced in the testis.
  • Immature male germ cells called spermatogonia undergo spermatogenesis in the seminiferous tubules of the testis.
  • Spermatogenesis begins at puberty.
  • Spermatogonia undergo mitotic divisions to increase in number.
  • Primary spermatocytes, derived from spermatogonia, undergo meiosis.

Meiosis I: Primary spermatocytes complete the first meiotic division, resulting in the formation of two equal, haploid cells called secondary spermatocytes. Each secondary spermatocyte has 23 chromosomes.

Meiosis II: Secondary spermatocytes undergo the second meiotic division, producing four equal, haploid cells called spermatids. Each spermatid also has 23 chromosomes.

  • Spermiogenesis: Spermatids undergo spermatogenesis, a process of transformation into mature spermatozoa (sperms). During this process, the spermatids develop a head, midpiece, and tail.


  • Chromosome Number in Spermatids: Like spermatocytes, spermatids also have 23 chromosomes. They are haploid cells.

 


Schematic representation of spermatogenesis

 

  • Spermiation: After spermatogenesis, sperm heads become embedded in Sertoli cells and are released from the seminiferous tubules in a process called spermiation.
  • Sertoli cells provide essential nutrients and metabolic support to developing sperm cells.

 


 

Seminiferous tubule - enlarged

(Sectional view)

 

  • Hormonal Regulation: Spermatogenesis is regulated by hormones. Increased secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus at puberty stimulates the anterior pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
  • LH acts on Leydig cells, stimulating the synthesis and secretion of androgens, such as testosterone. Androgens play a role in spermatogenesis.
  • FSH acts on Sertoli cells, stimulating the secretion of factors that aid in the process of spermiogenesis.

  • Structure of Sperm:

Sperm Structure: Sperm is a microscopic structure consisting of a head, neck, middle piece, and tail.

 

-Head: The head contains an elongated haploid nucleus. The anterior portion of the head is covered by a cap-like structure called the acrosome, which contains enzymes for fertilisation of the ovum.

 

- Neck: The neck region connects the head to the middle piece.

 

- Middle Piece: The middle piece of the sperm contains numerous mitochondria. These mitochondria produce energy required for the movement of the tail and sperm motility, which is essential for fertilisation.

 

- Tail: The tail, also known as the flagellum, is a long, whip-like structure that enables the sperm to swim and move towards the ovum.

 

- Plasma Membrane: The entire sperm is enveloped by a plasma membrane.

 


Structure of a sperm 

 

  • Sperm Count and Quality: During ejaculation, the human male releases approximately 200 to 300 million sperms. For normal fertility, at least 60% of the sperms should have a normal shape and size, and at least 40% of them should exhibit vigorous motility.
  • Role of Accessory Ducts and Glands: Sperms released from the seminiferous tubules are transported by the male sex accessory ducts. The secretions from the epididymis, vas deferens, seminal vesicle, and prostate are essential for the maturation and motility of the sperms.
  • Semen: The seminal plasma, along with the sperms, constitutes semen, the fluid ejaculated during orgasm.
  • Hormonal Regulation: The functions of the male sex accessory ducts and glands are maintained by testicular hormones, specifically androgens.

Oogenesis:

Oogenesis: The process of formation of a mature female gamete (ovum) is called oogenesis.

Initiation of Oogenesis: Oogenesis begins during embryonic development when a couple of million gamete mother cells called oogonia are formed within each foetal ovary.

Division and Arrest: The oogonia start division and enter prophase-I of meiotic division, getting temporarily arrested at this stage as primary oocytes.

Primary Follicle: Each primary oocyte is surrounded by a layer of granulosa cells and is called a primary follicle.

Secondary Follicle: Primary follicles develop more layers of granulosa cells and a new theca, becoming secondary follicles.

Tertiary Follicle: The secondary follicle transforms into a tertiary follicle characterised by the presence of a fluid-filled cavity called antrum. The theca layer is organised into the inner theca interna and the outer theca externa.

Meiotic Division and Formation of Secondary Oocytes: Within the tertiary follicle, the primary oocyte grows in size and completes its first meiotic division, resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.

Retention of Cytoplasm: The secondary oocyte retains the bulk of the nutrient-rich cytoplasm from the primary oocyte.

Advantage of Retained Cytoplasm: The retained cytoplasm provides nourishment and resources for the developing embryo in case fertilisation occurs.

First Polar Body: The fate of the first polar body, whether it further divides or degenerates, is currently uncertain.

 

Schematic representation of oogenesis

 

 

  • Mature Follicle or Graafian Follicle: The tertiary follicle further transforms into the mature follicle or Graafian follicle.

 


 

Schematic representation of ovarian follicle development and ovulation.

 

  • Zona Pellucida: The secondary oocyte forms a new membrane called the zona pellucida surrounding it.

 


Zona pellucida

 

  • Ovulation: The Graafian follicle ruptures, releasing the secondary oocyte (ovum) from the ovary in a process called ovulation.