spermiogenesis

Complete diagram of a human spermatozoon

Spermiogenesis is the final stage of spermatogenesis, which sees the maturation of spermatids into mature, motile spermatozoa.

Phases

The process of spermiogenesis is traditionally divided into four stages: the Golgi phase, the cap phase, the acrosomal phase, and the maturation stage.^[1]

Golgi phase

The spermatids, which up until now have been mostly radially symmetrical, begin to develop polarity.

• The head forms at one end, and the Golgi apparatus creates enzymes that will become the acrosome.
• At the other end, it develops a thickened mid-piece, where the mitochondria gather and form an axoneme.

Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is packaged first, with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive.

Cap phase

The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosomal cap.

Note how the tails of the sperm point inward. This orientation occurs during the acrosomal phase.

Acrosomal phase

One of the centrioles 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 bodies, is phagocytosed by surrounding Sertoli cells in the testes.

Spermiation

The mature spermatozoa are released from the protective Sertoli cells 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 peristaltic contraction.

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 recently acquired 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 by the male) and heparin (in the female reproductive tract) remove this coat and allow sperm to bind to the egg.

References

1. ^ ANAT D502 – Basic Histology

External links

• Swiss embryology (from UL, UB, and UF) cgametogen/spermato05
• Images and video of spermiogenesis - University of Arizona
• Overview at yale.edu