- A spermatozoon is a highly specialized, free swimming, actively motile cell which measures about 60 µm in length.
- The spermatozoon is made up of three parts. They are: a head, a neck and a tail or principal piece.
- Head is piriform in shape and measures 4 µm in length.
- It is derived from the nucleus, consisiting 23 highly condensed chromosomes.
- A cap covers the head of the spermatozoon known as acrosomic cap, anterior nuclear cap or galea capitis.
- Acrosome is a glycoprotein also regarded as a large lysosome containing numerous enzymes like proteases acid phosphatase, neuraminidase, hyaluronidase.
- These enzymes help in penetration of the spermatozoon into the ovum during fertilization.
- The neck of spermatozoon is narrow containing a funnel shaped basal body and a spherical centriole.
- An axial filament or axoneme starts just behind the centriole which passes through the middle piece and extends into the tail.
- This axial filament passes through a ring-like annulus at the point where the middle piece joins the tail.
- A spiral sheath made up of mitochondria surrounds the part of the axial filament that lies in the middle piece.
- When seen from the front the spermatozoon appears oval as the head of the human spermatozoon is flattened from before backwards, but it appears to be pointed when seen from one side or in section.
- It consists of chromatin (mostly DNA) that is extremely condensed and therefore appears to have a homogeneous structure even when examined by EM due to which it is highly resistant to various physical stresses.
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- Basal body is the main structure to be seen in the neck which is also called connecting piece as it helps to establish an intimate union between the head and the remainder of the spermatozoon.
- There are nine segmented rod like structures each of which is continuous distally with one coarse fibre of the axial filament that makes the basal body.
- The basal body has a convex articular surface on its proximal side (towards the head of spermatozoon) that fits into a depression called the implantation fossa present in the head.
- The axial filament passing through the middle piece and most of the tail is really composed of several fibrils.
- There is a pair of central fibrils surrounded by nine pairs (doublets) arranged in a circle around the central pair.
- This type of arrangement of fibrils is found also in cilia.
- Nine coarser petal shaped fibrils of unequal size in addition to these doublets are also present in which one such fibril lies just outside each doublet.
- These coarse fibrils are present in the middle piece and most of the tail without extending into the terminal part of the tail.
- The whole system of fibrils is kept in position by series of coverings.
- There is a fibrous sheath immediately outside the fibrils.
- In the region of the middle piece the fibrous sheath is surrounded by spirally arranged mitochondria.
- Finally the entire sperm is enclosed in a plasma membrane.
- It is seen that one of the coarse fibril is larger than the other. This is known as fibril 1 whereas, the others being numbered in a clockwise direction from it.
- The fibrous sheath is adherent to fibrils 3and 8.
- The tail is divided into a major compartment containing 4 fibrils and a minor compartment containing 3 fibrils by the line joining fibril 3 and 8.
- This is the line which also passes through both the central fibrils and provides an axis in reference to which sperm movements can be analyzed.
Spermatozoon and Its Structure