Chapter 3. Gamete (germ Cell) Formation, or Gametogenesis: Spermatogenesis - Review of Medical Embryology Book

The gametes are formed in the gonads, which also have hormonal functions. The sperm and oocyte (the male and female germ cells or gametes) are specialized sex cells containing one-half the regular number of chromosomes
Gametogenesis is a delicate, specialized maturation process called spermatogenesis in males and oogenesis in females and has 2 major functions

REDUCTION TO HALF THE NUMBER OF CHROMOSOMES AS WELL AS REDISTRIBUTION OF THE HEREDITARY MATERIAL: accomplished by meiosis (the combination of 2 divisions involving a single synthesis of DNA and an exchange of chromosome segments)

Each daughter cell formed by meiosis (, secondary spermatocyte) has only half the number of chromosomes of the parent cell
There are 2 successive meiotic divisions

In the first meiotic division, homologous chromosomes pair during prophase and segregate at anaphase
At the second meiotic division, the centromere of each chromosome divides, and chromatids are drawn to opposite poles

Constancy of chromosome number from generation to generation by producing haploid sex cells
Allows independent assortment of maternal and paternal chromosomes

Crossing over (relocating maternal and paternal chromosomes) allows genetic mixing and recombination of genetic material

ACQUISITION OF SPECIAL FORM AND FUNCTION BY THE REPRODUCTIVE CELLS prepares them and makes them suitable for fertilization

SPERMATOZOA are formed in the seminiferous tubules of the testis, from basic cells called spermatogonia which have been dormant in the tubules since the fetal period. Production of spermatozoa is continuous from puberty until death. Transformation of an immature germ cell or spermatogonium into a mature sperm takes about 64 days

Spermatogonia begin to increase in number at puberty (13-16 years of age in male)
After several mitotic divisions, spermatogonia grow and undergo changes to form primary spermatocytes (largest germ cells in the tubules)
Each primary spermatocyte undergoes a reduction division called the first meiotic division to form 2 haploid secondary spermatocytes (half the size of primary)
Secondary spermatocytes undergo a second meiotic division to form 4 haploid spermatids (half the size of secondary spermatocytes)
The spermatids gradually form 4 mature sperm or spermatozoa via spermiogenesis