Gametogenesis

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Gametogenesis is the process by which gametes form.  The process is controlled by hormones and results in haploid cells – either egg or sperm.

Spermatogenesis

Sperm are the male gametes.  Spermatogenesis is the process that results in the production of haploid sperm cells.  Beginning at puberty, human males start producing millions of sperm every day.  The sperm survive inside the male for only a short time, which is why they need to continually be produced.  The process by which this happens is outlined here:

Spermatogonia are cells that contain 46 chromosomes.  They are the starting point for spermatogenesis, which occurs in the seminiferous tubules.  The spermatogonia line the walls of the seminiferous tubules and go through mitosis.  So, each spermatogonium produces a cell called a primary spermatocyte, which also contains 46 chromosomes.

The primary spermatocyte goes through meiosis I to produce two secondary spermatocytes, which contain 23 chromosomes.

Secondary spermatocytes go through meiosis II and produce four spermatids, each of which also contain 23 chromosomes.

Each spermatid develops into a spermatozoa, which is just the technical name for what you know as a sperm. To become a full fledged sperm, the spermatid must mature to the point where it has a tail, middle piece, and a head.  The tail is the flagellum that moves the sperm through the body fluids.  The middle piece contains many mitochondria, which supply the energy for the sperm’s tail to move, and the head contains the 23 chromosomes inside the nucleus.

 

Oogenesis 

Eggs are the female gamete.  Oogenesis is the process that results in haploid eggs inside the female.  A human female is born with all the eggs she will ever have.  From birth until puberty, the eggs lie dormant in the ovaries.  The hormones generated during puberty kick-start the eggs into the menstrual cycle, which continues monthly from puberty until the woman begins menopause. 

When a female is a developing fetus, oogenesis begins in the ovaries.  An oogonium is the initial cell, and it contains 46 chromosomes.  It grows in size, eventually maturing into a primary oocyte.  The primary oocyte goes through meiosis, but the entire process is paused until the female enters puberty.  Once the hormones of sexual development start flooding the ovaries, the menstrual cycle begins. 

At the midpoint of the menstrual cycle, ovulation – the release of an egg from an ovary – takes place.  To prepare for release, and possible fertilization, the primary oocyte continues through meiosis.  The first meiotic division produces:

• a daughter cell known as a secondary oocyte, which receive most of the cytoplasm, and
• a polar body, which has minimal cytoplasm.

Both the daughter cell and the polar body have 23 chromosomes.  The second meiotic division results in the daughter cell producing the egg and a second polar body while the first polar body produces two more polar bodies.  

So, out of one original oogonium, only one functional egg is produced; the three polar bodies that are also produced just wither away.