The entire process of spermatogenesis can be broken up into several distinct stages, each corresponding to a particular type of cell in humans. In the following table, ploidy, copy number and chromosome/chromatid counts are for one cell, generally prior to DNA synthesis and division (in G1 if applicable). The primary spermatocyte is arrested after DNA synthesis and prior to division.
Spermatocytogenesis Spermatocytogenesis is the male form of
gametocytogenesis and results in the formation of
spermatocytes possessing half the normal complement of genetic material. In spermatocytogenesis, a diploid
spermatogonium, which resides in the basal compartment of the seminiferous tubules, divides mitotically, producing two diploid intermediate cells called
primary spermatocytes. Each primary spermatocyte then moves into the
adluminal compartment of the seminiferous tubules and duplicates its DNA and subsequently undergoes
meiosis I to produce two haploid
secondary spermatocytes, which will later divide once more into
haploid spermatids. This division implicates sources of genetic variation, such as random inclusion of either parental chromosomes, and
chromosomal crossover that increases the genetic variability of the gamete. The
DNA damage response (DDR) machinery plays an important role in spermatogenesis. The protein
FMRP binds to
meiotic chromosomes and regulates the dynamics of the DDR machinery during spermatogenesis. FMRP appears to be necessary for the
repair of DNA damage. During spermatocytogenesis, meiosis employs special
DNA repair processes that remove DNA damages and help maintain the integrity of the
genome that is passed on to progeny. These DNA repair processes include
homologous recombinational repair and
non-homologous end joining Spermatidogenesis Spermatidogenesis is the creation of
spermatids from secondary spermatocytes. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen in
histological studies.
Spermiogenesis During spermiogenesis, the spermatids begin to form a tail by growing
microtubules on one of the centrioles, which turns into basal body. These microtubules form an
axoneme. Later the centriole is modified in the process of
centrosome reduction. The anterior part of the tail (called midpiece) thickens because mitochondria are arranged around the axoneme to ensure energy supply. Spermatid
DNA also undergoes packaging, becoming highly condensed. The DNA is packaged firstly with specific nuclear basic proteins, which are subsequently replaced with
protamines during spermatid elongation. The resultant tightly packed
chromatin is transcriptionally inactive. The
Golgi apparatus surrounds the now condensed nucleus, becoming the
acrosome. Maturation then takes place under the influence of testosterone, which removes the remaining unnecessary
cytoplasm and
organelles. The excess cytoplasm, known as
residual bodies, is
phagocytosed by surrounding Sertoli cells in the
testes. The resulting spermatozoa are now mature but lack motility. The mature spermatozoa are released from the protective
Sertoli cells into the lumen of the
seminiferous tubule in a process called
spermiation. The non-motile spermatozoa are transported to the
epididymis in
testicular fluid secreted by the Sertoli cells with the aid of
peristaltic contraction. While in the epididymis the spermatozoa gain motility and become capable of fertilization. However, transport of the mature spermatozoa through the remainder of the
male reproductive system is achieved via muscle contraction rather than the spermatozoon's recently acquired motility. == Role of Sertoli cells ==