Page 75 - 86 human physiology part-2

Chapter 15

Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg
from fertilization. The first barrier to sperm is made up of so-called cumulus cells embedded in a gel-
like substance made primarily of hyaluronic acid. The cumulus cells develop in the ovary with the egg
and support it as it grows.

The second barrier coating the oocyte is a thick shell formed by glycoproteins called the zona
pellucida. One of the proteins that make up the zona pellucida binds to a partner molecule on the
sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of
different species from fusing. There is some evidence that this binding is what triggers the acrosome to
release the enzymes that allow the sperm to fuse with the egg.

When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes weaken the
shell, allowing the sperm cell to penetrate it and reach the plasma membrane of the egg. Part of the
sperm's cell membrane then fuses with the egg cell's membrane, and the sperm cell sinks into the egg
(at which point the sperm tail falls off).

Upon penetration, the egg cell membrane undergoes a change and becomes impenetrable,
preventing further fertilization.

The binding of the sperm to an ovum is called a zygote. A zygote is a single cell, with a complete
set of chromosomes, that normally develops into an embryo.

Puberty

In addition to producing sperm, the male reproductive system also produces sex hormones, which
help a boy develop into a sexually mature man during puberty. When a baby boy is born, he has all the
parts of his reproductive system in place, but it isn't until puberty that his reproductive organs mature
and become fully functional. As an newborn FSH and LH levels are high and afrter a few weeks levels
drop to extremly low. When puberty begins, usually between the ages of 10 and 14, the pituitary gland
- which is located in the brain - secretes hormones that stimulate the testicles to produce testosterone.
The production of testosterone brings about many physical changes. Although the timing of these
changes is different for each individual male, the stages of puberty generally follow a set sequence.

• First stage: the scrotum and testes grow larger, the apocrine glands develop (see
explanation of apocrine glands in glossary).

• Second stage: the penis becomes longer, and the seminal vesicles and prostate gland grow.
Hair begins to grow in the pubic region. Reproductive capacity has usually developed by this
stage.

• Third stage: hair begins to appear on the face and underarms. During this time, a male's
voice also deepens. Fertility continues to increase.

Testicular size, function, and fertility

In boys, testicular enlargement is the first physical manifestation of puberty (and is termed
gonadarche). Testes in prepubertal boys change little in size from about 1 year of age to the onset of

290 | Human Physiology