The formation of sex cells begins before birth; spermatozoa form in males and oocytes in females. Spermatogenesis (sperm cell production) occurs in the seminiferous tubules. Spermatogonia (stem cells) line these tubules at birth and contain 46 chromosomes (genetic material). After birth, spermatogonia continue to divide during mitosis. This cell division process produces two daughter cells with the same chromosome number (46) as the parent.
At puberty onset, some spermatozoa grow to become primary spermatocytes. These cells undergo meiosis, the cell division process that cuts back the number of chromosomes from 46 to 23. Each primary spermatocyte undergoes the first meiotic division to produce two secondary spermatocytes. Each secondary spermatocyte undergoes the second meiotic division to produce two spermatids. Each spermatid develops into a mature spermatozoon (sperm cell). In this way, meiosis produces millions of sperm every day.
Oogenesis is the formation of the ovum (female sex cells), which begin as hundreds of thousands of oogonia (stem cells) in the fetal ovaries. During prenatal development, the oogonia grow to become primary oocytes that contain 46 chromosomes. Each oocyte undergoes meiosis; at birth, oocytes are in prophase. During this first meiotic division, oocytes enter a resting phase that lasts until the oocyte resumes development during the ovarian cycle (puberty). The female is born with all the oocytes she will ever have.
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