In males, spermatogenesis, or the production of sperm, occurs. In females, oogenesis, or the production of ova, occurs. These germ cells provide the continuity of life between generations.

The precursors of the germ cells are the primordial germ cells. They arise from cells initially set aside in the cytoplasm of the fertilized egg which is referred to as the germ plasm. The primordial germ cells migrate to the developing gonads during embryogenesis. Once they arrive at the gonads, they can undergo differentiation to form the mature sperm (spermatogenesis) or mature egg (oogenesis).

For gametogenesis to occur, the processes of both mitosis and meiosis must take place.

Mitosis = cell division in which the chromosomes replicate before nuclear division such that each daughter cell receives a full set of chromosomes.

Meiosis = cell division in which daughter cells receive only one half of chromosomes present in the original cells.

Mitosis results in identical daughter cells and is the process that facilitates the equal partitioning of replicated chromosomes into two identical groups. Before partitioning can occur, the chromosomes must become aligned so that the separation process can occur in an orderly fashion. This alignment of replicated chromosomes and their separation into two groups can be observed in virtually all eukaryotic cells.

There are several different stages of mitosis:

The first stage is the interphase during which DNA is doubled in preparation for cell division.
During prophase, the second phase, the nuclear envelope breaks down, the chromosomes undergo extensive condensation becoming thicker and shorter, and spindles form between two centrioles.
In prometaphase, the chromosomes attach to the spindle fibers.
In metaphase the chromosomes align at the equator of the cell.
The next phase is anaphase where the duplicated chromosomes (chromatids) are separated.
In the last stage (telophase), the chromosomes reach the mitotic poles and the cell begins to pinch in.

• At each pole are the same number and type of chromosomes as were present in the cell before it divided.

Meiosis, in contrast to mitosis, is the process that produces haploid gametes having only half the chromosomes present in the original cells. The way that this happens is that two cell divisions occur referred to as meiosis I and meiosis II.

The first division produces two daughter cells as occurs in mitosis, but each of the two daughter cells then go through meiosis II which consists of a metaphase 2, anaphase 2, and telophase 2.

• The result is that four daughter cells are produced from one original cell and each of the daughter cells has only one chromosome (haploid) instead of two (diploid).

In comparing mitosis and meiosis:

• Meiosis undergoes two cell divisions whereas mitosis only undergoes one cell division.
• Mitosis produces two daughter cells, each with two sets of chromosomes, and meiosis produces four daughter cells each with only one chromosome.
• In mitosis the two daughter cells which are produced are genetically identical to each other and the parent cell.
• By contrast, meiosis produces four haploid daughter cells which differ genetically not only from the parent, but from each other.
• Meiosis II is similar to mitosis but no DNA replication occurs.
• Thus, in meiosis, DNA replicates once but the cells divide twice.
• Another important difference between meiosis and mitosis is that meiosis occurs in the gametes and mitosis occurs in somatic cells.

In returning to gametogenesis, the gametes are derived from the primordial germ cells which enter the gonads during development.

When the primordial germ cells become established in the gonad, they become stem cells that initially divide by mitosis to produce the gametes that are required for reproduction.
The germ cells, once in the gonad, may associate with specific somatic cells that support, nuture, and protect them.
In the male, these cells are called Sertoli cells.
During the proliferative phase, the germ cells are called gonia (spermatogonia) and act as a stem cell population that divides by mitosis.

• Spermatogenesis is the developmental pathway from spermatogonial stem cell to mature sperm and this process occurs in the recesses of the Sertoli cells in the seminiferous tubules.

When the organism reaches maturity, the germ cells acquire the ability to differentiate into functional gametes and undergo meiosis to produce haploid cells for reproduction. Gametes are produced through this process of gametogenesis.

The various stages of spermatogenesis consists of replication by mitosis followed by meiosis.

• After reaching the gonads, the primordial germ cells divide to produce the type A1 spermatogonia.
  
• The cells then undergo changes from A1 to A2 to A3 to A4 spermatogonia having subtle differences in cell morphology.
  
• The type A4 spermatogonia can then differentiate into the intermediate spermatogonia.
  
• To this point the cells are capable of self-renewal but after intermediate spermatogonia are formed, the cells are committed to form spermatozoa.
  
• They divide mitotically again to produce type B spermatogonia which are the last cells to undergo mitosis.
  
• The cells divide once to produce primary spermatocytes that enter meiosis.
  
• Each primary spermatocyte (with two chromosomes) produces a secondary spermatocyte which complete the second division of meiosis.
  
• The haploid cells that are formed are referred to as spermatids.

During the divisions from Type A1 spermatogonia to spermatid, the cells move farther away from the basement membrane of the seminiferous tubule and closer to its lumen.

In humans, the progression from spermatogonial stem cell to mature sperm takes 65 days.

Spermiogenesis:

• During spermiogenesis, the round spermatid undergoes maturation through differentiation. Spermiogenesis differentiates the sperm for motility and interaction with the egg.
  
• The first step is the formation of the acrosome which forms a cap that covers the sperm nucleus.
• The nucleus flattens and condenses and the sperm enter the lumen of the seminiferous tubule.

As a result of spermatogenesis [mitosis, meiosis, and spermiogenesis (differentiation)], four haplid sperm result from each diploid spermatogonium.

• Each day about 100 million sperm are produced and each ejaculation releases about 200 million sperm.
• Unused sperm are resorbed or passed out of the body in urine.