31.9 : Sperm Structure and Semen Composition

During ejaculation, males release around 2-5 milliliters of semen, which is a complex mixture of mature sperm and various fluids produced by accessory glands. The mature sperm cells measure approximately 60 micrometers in length and consist of a head, neck, midpiece, and tail. The head is flattened and tapered, measuring about 4 to 5 micrometers in length. It contains a nucleus with condensed chromosomes and an acrosome, a cap-like structure filled with enzymes essential for penetrating the egg's protective layers during fertilization.

The neck of the sperm is short and located between the head and middle piece, containing centrioles that form microtubules, which are continuous with the middle piece and tail. The midpiece of the sperm contains spirally arranged mitochondria that provide the energy required for the sperm's movement and metabolism. Finally, the tail, or the flagellum, propels the sperm through a whiplash movement that enables it to swim towards the egg.

Semen's volume primarily comprises secretions from the seminal vesicles, prostate gland, and bulbourethral glands, with sperm making up a small fraction of the total volume. This slightly alkaline, whitish fluid protects and nourishes the sperm and facilitates their transport. Seminal fluid neutralizes the acidity of the vagina, creating a more favorable environment for sperm survival. Additionally, enzymes in the semen, such as fibrinolysin, liquefy coagulated semen after ejaculation to aid sperm mobility.