. 1. Primary spermatocyte 2. Secondary spermatocytes
3. Spermatids 4. Sperm ]]
Spermiogenesis is the final stage of spermatogenesis, during which the develop into mature spermatozoon. At the beginning of the stage, the spermatid is a more or less circular cell containing a cell nucleus, Golgi apparatus, centriole and mitochondria; by the end of the process, it has radically transformed into an elongated spermatozoon, complete with a head, midpiece, and tail.
Phases
The process of spermiogenesis is traditionally divided into four stages: the Golgi phase, the cap phase, formation of the tail, and the maturation stage.
[ ANAT D502 – Basic Histology]
Golgi phase
The
, which up until now have been mostly radially symmetrical, begin to develop polarity. The head forms at one end, where the
Golgi apparatus creates enzymes that will become the
acrosome. At the other end, it develops a thickened midpiece, where the
mitochondria gather and the distal centriole begins to form an
axoneme.
Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is first packaged with specific nuclear basic proteins, which are subsequently replaced with during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive.
Cap/acrosome phase
The Golgi apparatus surrounds the condensed nucleus, becoming the
acrosome.
Formation of tail
One of the
of the cell elongates to become the tail of the sperm. A temporary structure called the "manchette" assists in this elongation.
During this phase, the developing spermatozoa orient themselves so that their tails point towards the center of the lumen, away from the epithelium.
Maturation phase
The excess
cytoplasm, known as
residual body of Regaud,
is
phagocytosed by surrounding
Sertoli cells in the
testes.
Spermiation
The mature spermatozoa are released from the protective
into the lumen of the seminiferous tubule and a process called
spermiation then takes place, which removes the remaining unnecessary
cytoplasm and
organelles.
The resulting spermatozoa are now mature but lack motility, rendering them sterile. The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the Sertoli cells, with the aid of peristalsis.
Whilst in the epididymis, they acquire motility. However, transport of the mature spermatozoa through the remainder of the male reproductive system is achieved via muscle contraction rather than the spermatozoon's motility. A glycoprotein coat over the acrosome prevents the sperm from fertilizing the egg prior to traveling through the male and female reproductive tracts. Capacitation of the sperm by the enzymes FPP (fertilization promoting peptide, produced in the prostate gland) and heparin (in the female reproductive tract) removes this coat and allows sperm to bind to the egg.
Genome integrity
During spermiogenesis, the
ploidy post-meiotic stages of gametogenesis in males, several fundamental challenges are encountered.
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After completion of the two meiosis divisions, chromatids become vulnerable to DNA double-strand damages, since accurate DNA repair ordinarily requires availability of a sister chromatid or homologous chromosome, but these are now unavailable for long periods, i.e. days or weeks.
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The sperm genome is unable to undergo transcription during spermiogenesis, impeding its ability to respond to new challenges, such as DNA damage.
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Associated with proper genome packaging to create mature germ cells there is a transition from histone protein binding to protamine protein binding and this transition is associated with production of DNA double-strand breaks.
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How these challenges are overcome is still not well understood.
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