Semen and Sperm
In the male, semen is the fluid expelled during ejaculation. In addition to plasma, the semen ejaculate contains secretions from the seminal vesicles and other glands to support and nourish the living sperm cells (spermatozoa) contained within the semen. Sperm cells are haploid sex cells of the male. Unlike eggs (oocytes and the mature ovum) that are large, non-motile, and generally ovulated one at a time, sperm are tiny, motile, and produced in the millions. While the human sperm contain a relatively long tail (flagella), the volume of an entire sperm, tail and all, is only 1/85,000 of the mature ovum.
Reproduction in humans may occur when semen—containing sufficient numbers of living and healthy (viable) sperm cells--is deposited in the vagina of a female near the cervical opening of the uterus. The haploid sperm move through the cervix, into the uterus and then migrate into the Fallopian (uterine) tube where, if a mature ovum is encountered, fertilization may occur. With fertilization, the egg finishes its second meiotic division to become haploid. The haploid sperm and mature egg together form a diploid zygote (single-celled embryo).
The male gonad (testis singular, testes plural; testicle is derived from the diminutive of testis and perhaps is best used to describe the gonads of a sexually immature boy) produces the hormone testosterone and sex cells. Early in embryonic development, primordial germ cells, which are diploid, migrate to the embryonic gonad. The primordial germ cells give rise to the diploid stem cells of the testis, known as spermatogonia. Each of the many spermatogonia, after the first meiotic division, form two primary spermatocytes that in turn, after the second meiotic division, form four haploid spermatids. In the process of forming mature sperm the spermatids lose much of their cytoplasm and develop a long, propulsive tail.
Motility of the sperm is due to the long tail which is a modified flagellum. Cilia and flagella, from protozoa through humans, all have a similar structure that has been intensively investigated since first described in early electron microscope studies. Microtubules that run the length of the sperm tail are arranged in a ring of nine pairs surrounding a pair in the center. Ciliary dynein is associated with each of the nine microtubule pairs. It is the interaction of the dynein with the microtubules which causes flagellar bending and thus propulsion.
It is estimated that a quarter of a billion sperm are released in a single ejaculate of semen in a healthy male human. Tin addition to a nutrient function the semen plays an important role in thermal and hydration regulation that promotes viable sperm cells. The semen also provides initial protection against the acidic gradient of the vagina and cervical region.
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