Overview

Total length≈55–60 µm

Head length≈4–5 µm

Survival in female tractUp to ≈5 days

The head (≈4–5 µm long, flattened and oval) contains the tightly condensed haploid nucleus (protamine-packed chromatin) and, over its anterior half to two-thirds, the acrosome — a cap-like lysosome-derived vesicle containing hydrolytic enzymes (hyaluronidase, acrosin) used to penetrate the egg coats. The neck contains the proximal centriole, which in humans becomes the centrosome of the zygote. The midpiece has ≈50–75 mitochondria wound helically around the axoneme, supplying ATP; paternal mitochondria are destroyed after fertilisation, so mitochondrial DNA is inherited maternally. The tail (principal and end pieces) contains a 9 + 2 axoneme of microtubules powered by dynein ATPase, surrounded by outer dense fibres and a fibrous sheath.

Sperm leaving the testis cannot swim; they gain progressive motility and the capacity to bind the zona pellucida during epididymal maturation. In the female tract they undergo capacitation (hours), which enables hyperactivated motility and the acrosome reaction at the zona pellucida. Sperm can survive in cervical mucus and the uterine tubes for up to ≈5 days.

Location: Formed in the seminiferous tubules; matured and stored in the epididymis (especially the tail) and ampulla of the ductus deferens; deposited in the vagina at ejaculation.

Parts & subdivisions

Head
Haploid nucleus with condensed chromatin; carries X or Y chromosome, determining the sex of the embryo.
Acrosome
Enzyme-filled cap (hyaluronidase, acrosin); released in the acrosome reaction.
Neck
Proximal centriole — contributes the zygote's centrosome.
Midpiece
Helical mitochondrial sheath producing ATP for movement.
Principal and end pieces (tail)
9 + 2 axoneme with dynein arms, outer dense fibres and fibrous sheath; flagellar beating propels the cell.

Function

Deliver the paternal haploid genome and centrosome to the oocyte.
Determine genetic sex.
Activate the oocyte (via phospholipase C zeta).

Clinical significance

Semen analysis terms

Oligozoospermia (low count), asthenozoospermia (poor motility), teratozoospermia (abnormal forms), azoospermia (no sperm — obstructive or non-obstructive). Sperm DNA fragmentation and antisperm antibodies may also impair fertility.

Primary ciliary dyskinesia

Defects of axonemal dynein arms cause immotile sperm and male infertility, together with chronic sinusitis, bronchiectasis and, in Kartagener syndrome, situs inversus.

Test yourself

Approximately how long does human spermatogenesis take?

≈7 days
≈2–3 weeks
≈64–74 days
≈6 months

Answer: ≈64–74 days

From spermatogonium to released spermatozoon takes ≈64–74 days, plus ≈1–2 weeks in the epididymis. Effects of illness or drugs on semen therefore appear about 3 months later, and repeat semen analysis is spaced accordingly.