Question

Describe the structure of human sperm.

Answer 1

Structure of Human Sperm:

• The male gamete is called as spermatozoon or sperm. It is a single cell structure and it bears a haploid set of chromosomes in the nucleus.
• Human sperm has a head, neck, midpiece and tail.

Head:

• It is the anterior part of the sperm. Its shape varies from species to species.
• The anterior most part of the head is occupied by the acrosome.
• The acrosome is formed by the Golgi-body and it is bounded by a unit membrane.
• It is full of sperm lysins and it helps in fertilization.
• There is a nucleus just behind the acrosome which is slightly elongated and its posterior end has a notch.
• The nucleus has a haploid set of chromosomes which have protamines in addition to DNA.
• The sperms of some species have a vacuole-like structure between the acrosome and nucleus which is called as a perforator.
• The posterior notch of the nucleus harbours a proximal centriole which is to be contributed to the egg at the time of fertilization.
• It helps at the beginning of the first cleavage.

Neck:

• It is a small part which connects the head to the mid-piece.
• It bears a distal centriole which remains at 90° to the proximal centriole.
• The distal centriole gives rise to an axial filament which extends throughout the length of the mid-piece & tail.
• It exhibits a fibrilar arrangement of 9 (doublet) + 2 (singlet).

Middle piece:

• In the middle piece, the axial filament is surrounded by a sheath called manchette.
• The manchette is made up of nebenkern (fused mitochondria) and little condensed cytoplasm.
• In the sperms of some species, the posterior end of the middle piece may have a ring centriole. Its role is not clear.

Tail:

• In the tail, the axial filament is covered by a sheath which is made up of 9 (singlet) fibres.
• This the sheath is absent around the terminal end of the tail.
• The whole sperm is bounded by a plasmalemma.
• Most of the sperms are mono-flagellated.