Spermiogenesis (spermatohistogenesis) and structure of the sperm cell
The differentiation of the spermatids into sperm cells is called spermiogenesis. It corresponds to the final part of spermatogenesis and comprises the following individual processes that partially proceed at the same time:
- Nuclear condensation: thickening and reduction of the nuclear size, condensation of the nuclear contents into the smallest space.
- Acrosome formation: Forming a cap (acrosome) containing enzymes that play an important role in the penetration through the pellucid zone of the oocyte.
- Flagellum formation: generation of the sperm cell tail.
- Cytoplasma reduction: elimination of all unnecessary cytoplasm
- Axonemal structure, first flagellar primordium
- Golgi complex
- Acrosomal vesicle
- Pair of centrioles (distal and proximal)
- Mitochondrion
- Nucleus
- Flagellar primordium
- Microtubules
- Sperm cells tail
- Acrosomal cap
Three differing stages of spermiogenesis:
on the left: a fresh spermatid
on the right: an immature sperm cell
in the middle: an in-between stage.
A rotation of the nucleus causes a repositioning of the acrosomal vesicle to occur. This inverts itself like a cap over the nucleus that continues to be condensed (dotted line). The cytoplasm cell components that are no longer needed are discarded and phagocytized by Sertoli's cells. The mitochondria are packed thickly (tightly) together around the beginning part of the flagellum (mid-piece). As a sign of its immaturity, the sperm cell (on the right) that has issued into the lumen still has a bit of cytoplasm around its neck (compare with fig. 16 below).
Nuclear condensation
The nucleus becomes smaller, denser and takes on a characteristic, flattened form. Seen from above, the nucleus is oval and, from the narrow side, is pear-shaped. The acrosome lies over the tip. Nucleus and acrosome form the sperm cell's head that is bound to the mid-piece by a short neck.
Acrosome formation
The Golgi complex engender the vesicles, which then merge into a larger formation that settles close to the cell nucleus and finally inverts itself like a cap over the largest part of the nucleus. The acrosome corresponds functionally to a lysosome and thus contains lysosomal enzymes (hyaluronidase among others).
Development of the flagellum
The future axonemal structure grows out of one centriole (distal). This consists of a bundle of nine peripheral double microtubules and two single ones in the center. During its development, through the rotation of the nucleus and acrosomal vesicle, the flagellum primordium comes to lie on the opposite side of the acrosome.
Four parts of the finished flagellum can be distinguished:
- The neck contains the two centrioles (proximal and distal) among other things.
- The mid piece consists of a sheath of ring-shaped mitochondria grouped around the axoneme to provide the energy for the flagellar movement.
- The principle piece has a sheath of ring fibers around the axoneme.
- The tail consists of only the 9+2 structure of the axoneme.
The mature sperm cell is approximately 60 µm long and completely enveloped by the plasma membrane.
- Plasma membrane
- Outer acrosomal membrane
- Acrosome
- Inner acrosomal membrane
- Nucleus
- Proximal centriole
- Rest of the distal centriole
- Thick outer longitudinal fibers
- Mitochondrion
- Axoneme
- Anulus
- Ring fibers
- A
- Head
- B
- Neck
- C
- Mid piece
- D
- Principal piece
- E
- Endpiece
The mature sperm cell is slender; in the middle part, the mitochondria are thick and ring-shaped. The DNA in the nucleus is maximally condensed.
Cytoplasmic reduction
The cytoplasm of the spermatids that is no longer needed is phagocytized by Sertoli's cells or is disposed of in the lumen of the tubules. A clump of cytoplasm, though, can remain hanging on the neck and mid piece of the sperm cell for a little while.
During sperm cell production considerable individual variations exist that are also partially influenced by psychological factors. Per day roughly 100 million sperm cells are produced. It is said that in each ejaculate an average number of 50-200 million sperm cells are present (WHO standard value: over 40 million)