What is gametogenesis?

What is gametogenesis?

Every living being is unique in its form with the basic difference determined by its genetic information (DNA). In plants and animals, DNA is tightly packed into thread-liked structures called chromosomes. The term “ploidy” tells about the number of chromosome sets found in a nucleus. Depending on the number of chromosomes, cells are identified into a haploid and a diploid type. Diploid describes a cell containing two sets of each chromosome (2n). In a human body, nearly all cells are diploid excepts cells of the germline, which develop into gametes, i.e., an ovum or a sperm cell. The human diploid cell has 46 chromosomes (2n), with one set inherited from each parent. Among these, two pairs are sex-determining chromosomes and the other twenty-two pairs are non-sex determining chromosomes. Sex determining chromosomes for a female are two copies of X-chromosomes while a male has an X and a Y- chromosome. Diploid cells are formed through mitosis.

A haploid cell is a cell that contains only a single set of chromosomes (n). Haploid cells are formed through meiosis. The haploid cell is mostly referred to as the gametes chromosomes of an ovum or sperm cell with each containing 23 chromosomes, each of which is derived from the pair of diploid cells. Through the stages of gametes formation, cells undergo mitosis and meiosis.

Gametogenesis is a biological process by which diploid and haploid parent cells undergo differentiation and cell division to form gametes. Depending upon the biological life cycle of the organism, types of cells, gametogenesis occur by meiotic division of the diploid gametocyte into the gametes or by mitosis. Gametogenesis of diploid cells in animals and other higher plants occurs through the germline tissue in which the formation of gametes from germ cells occurs through the gonads.

A broad explanation of the stages involved in gametogenesis can be considered regarding the animal gametes’ formation from primordial germ cells. The complete formation of the male and female gametes from the germ cells is generally termed as Gametogenesis, specifically Spermatogenesis for males and Oogenesis for females.  The stage of gametogenesis is initiated commonly in both types of embryos. The cells multiply by mitosis and towards the late embryonic stage when they reach the urogenital ridge, they are referred to as gametogonia. Once they are the gametogonia, they are classified into male and female gametogonia. Male gametogonia called spermatogonia (singular spermatogonium) locate and store themselves in the testes, throughout the embryonic development stage till adulthood. Female gametogonia also called oogonia (singular oogonium) is located in the ovaries. Gametogonia (singular gametogonium) starts shortly after fertilization where the stem cells that formed from the primordial germ cells at the dorsal endoderm of the yolk sac migrate along the epigastric towards the urogenital ridge.

In males, gametogonia multiply mitotically to form primary gametocytes and further undergo meiosis to create secondary gametocytes. The complete stage of gametocyte creation involving mitosis and meiosis is called male Gametocytogenesis or spermatocytogenesis. Primary spermatocytes are diploid cells which after Meiosis-I, two secondary spermatocytes are formed. Each secondary spermatocyte that is a haploid cell will undergo Meiosis-II to form two spermatids. Thereafter, spermatids undergo morphologic modification to become mature male gametes or spermatozoa. Maturation of spermatids or Spermiogenesis takes place in the seminiferous tubules within the testes for a period of two and half months.  Spermiogenesis occurs in four phases:

  1. The Golgi phase: The spermatid acquires its head and tail and DNA is highly condensed but inactive.
  2. Cap phase: An acrosome, a membrane-bound compartment capping the tip of the spermatid is formed.
  3. Formation of the tail: The tail is fully elongating from the microtubules on one of the centrioles
  4. Maturation stage: The testis- determining factor, testosterone is formed and packed into each spermatozoon.

Through the process of oogenesis, an immature ovum exists as an oogonium, an oocyte, or an ootid. Oogonia that are created during the embryonic stage will all turn to primary oocytes at the late fetal period. The primary oocyte undergoes Meiosis-I during the process of ootid genesis, duplicating its DNA to form a secondary oocyte and a polar body. Most of the primary oocytes spend most of their lifetime in the late fetal age, which eventually degenerate with only a few going through the ovulation age. Therefore, a few immature ova can stay as primary oocytes till before menopause. The secondary oocyte, through ovulation, continues meiosis-II to form daughter cells, one ootid, and one polar body. While polar bodies of both meiosis stages disintegrate, the ootid undergoes maturation into an ovum. The ovum will wait in the ovary to either be fertilized or at limited intervals will disintegrate and, in few animals, the ovum is released through menstruation. During this stage, the next secondary oocyte has not completed Meiosis-II. The time as a secondary oocyte is counted as days.

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