BIOL 1051H Chapter Notes - Chapter 27-28: Sex Steroid, Anterior Pituitary, Underarm Hair
The Reproductive System
Human beings undergo sexual reproduction which means that we form new individuals
through a process of combining two cells with a half set of chromosomes together to form a new
cell with a full set of chromosomes. The cells with a half set of chromosomes are haploid. In
humans a haploid cell has 23 chromosomes. After two haploid cells join (fertilization) the cell
now has a full set of chromosomes or is diploid. In humans this is 46 chromosomes. The haploid
cells are called gametes. These are either sperm (from the male) or eggs (from the female). The
fertilized diploid cell is called a zygote. The process of haploid cell production is called meiosis.
This is different from cells replicating which is called mitosis.
The control of reproductive events is largely performed by the endocrine system. The
hypothalamus controls the release of gonadotropin releasing hormone (GnRH) which in turn
promotes release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) f from the
anterior pituitary. LH and FSH stimulate production of sperm and eggs, and gonadal sex
steroids. Sex steroids provide negative feedback on the hypothalamus and anterior pituitary.
The gonads also secrete a hormone called inhibin which negatively feeds back specifically on
FSH secretion.
The initial development of gonads takes place early in gestation but then stops until
puberty. Development of the gonads is promoted through the action of FSH and LH.
Consequently, these hormones are at high levels in the fetus between 1 and 6 months gestation
and then drop to very low levels until puberty.
At puberty the hypothalamus increases GnRH secretion which stimulates the secretion
of LH and FSH. Higher levels of LH and FSH then stimulate sex steroid secretion. The sex
steroids (estrogen and progesterone in the female and testosterone in the male) drive changes
in secondary sex characteristics and menarche (first menstrual flow).
Although estrogen and progesterone are the most prominent sex steroids in females,
and testosterone is the most important in males, both males and females have all these
hormones in their bodies, just in different amounts. For instance, the growth of pubic and axillary
hair is stimulated by androgens (testosterone) that is actually secreted by the adrenal gland.
The extra testosterone in males makes this change more dramatic in males than females.
Growth is also a function of both sex and age. At the beginning of puberty the increased
secretion of estrogen stimulates growth. Males experience a surge in testosterone at puberty
which also causes a growth spurt, but this occurs a little later than the growth spurt experienced
by females on average. The age of puberty in girls depends on the percent of body fat and
physical activity of the girl. Girls with low body fat and high activity enter puberty at an older age
than those with high body fat and low activity.
The testes contain the seminiferous tubules (STs) where spermatogenesis occurs.
Within the STs are the developing sperm as well as the Sertoli cells. Sertoli cells contain
receptors for FSH, the binding of which stimulates spermatogenesis. The interstitial tissue of the
STs contains the Leydig cells. The leydig cells contain LH receptors, the binding of which
causes the secretion of testosterone.
The control of LH and FSH secretion is by negative feedback. Testosterone inhibits LH
and GnRH secretion, but not FSH. Inhibin produced by Sertoli cells provides negative feedback
on FSH. Spermatogenesis takes place in the ST. Germ cells (cells that produce gametes)
migrate from the yolk sac during development and become spermatogonia (stem cells that
produce sperm). The spermatogonia replicate themselves throughout the adult life of the man
by mitosis. These give rise to haploid sperm by meiosis.
The spermatogonia and primary spermatocytes are located in the outer part of the ST.
Spermatids and mature spermatozoa are located toward the lumen. The tails of the
spermatozoa are in the lumen. Spermiogenesis is the maturation of spermatids into
spermatozoa. This step requires the participation of Sertoli cells (sustentacular cell). Most of the
cytoplasm of the spermatids is pinched off and ingested by the surrounding Sertoli cells.
Sertoli cells not only house the developing sperm, and produce hormones; they also
form the blood-testes barrier. This barrier protects the developing sperm from the immune
system. Remember that the cells produced here are not the same as those in the rest of the
body. They have half the chromosomes so they can produce proteins that are not normally seen
in the rest of the body. If recognized by the immune system they would be destroyed.
The spermatogonia and developing spermatozoa are embedded in and nurtured by the
adjacent Sertoli cells. Sertoli cells also secrete a FAS ligand that triggers apoptosis (controlled
cell death) of T lymphocytes.
Spermatozoa have an oval-shaped head that contains DNA and the acrosome (a cap of
digestive enzymes). They also have a midpiece and a flagella tail. The tail becomes motile in
the epididymis. The formation of primary spermatocytes and entry into early prophase I, begins
during embryonic development. Spermatogenesis is then arrested until puberty. Testosterone
and its metabolites are required for completion of meiosis and spermatid maturation. FSH itself
is not essential for spermatogenesis. It does enhance testosterone-stimulated spermatogenesis
and at puberty, FSH acts with Testosterone to stimulate the proliferation of sertoli cells.
The male has several accessory sex organs which function primarily in the production of
fluids that are added to the sperm and produce semen. Spermatozoa and fluids exit the
seminiferous tubules via the rete testis. The fluid then passes through the efferent ductules to
the epididymis and leaves through the vas deferens.
Document Summary
Human beings undergo sexual reproduction which means that we form new individuals through a process of combining two cells with a half set of chromosomes together to form a new cell with a full set of chromosomes. The cells with a half set of chromosomes are haploid. In humans a haploid cell has 23 chromosomes. After two haploid cells join (fertilization) the cell now has a full set of chromosomes or is diploid. These are either sperm (from the male) or eggs (from the female). The fertilized diploid cell is called a zygote. The process of haploid cell production is called meiosis. This is different from cells replicating which is called mitosis. The control of reproductive events is largely performed by the endocrine system. The hypothalamus controls the release of gonadotropin releasing hormone (gnrh) which in turn promotes release of luteinizing hormone (lh) and follicle stimulating hormone (fsh) f from the anterior pituitary.
