Reproduction 705important function of masculinizing the embryonic structures.
Testosterone levels rise again in newborn boys until the age of
3 months and then fall to almost undetectable levels by ages
7–12 months. From the age of 12 months until adolescence, the
sex hormone levels are the same in both sexes.
As the testes develop, they move within the abdominal cavity
and gradually descend into the scrotum. Descent of the testes is
sometimes not complete until shortly after birth. The temperature
of the scrotum is maintained at about 35 8 C—about 3 8 C below
normal body temperature. This cooler temperature is needed for
spermatogenesis. The fact that spermatogenesis does not occur
in males with undescended testes—a condition called cryptor-
chidism ( crypt 5 hidden; orchid 5 testes)—demonstrates this
requirement.during the meiotic cell division that formed the sperm cell.
Similarly, rare female babies with XY genotypes were found to
be missing the same portion of the Y chromosome erroneously
inserted into the X chromosome of XX males.
Through these and other observations, it has been shown
that the gene for the testis-determining factor is located on the
short arm of the Y chromosome. This gene was named SRY (sex-
determining region Y), and was found to code for a 204-amino-
acid protein that binds to DNA. However, other Y-specific genes
may also be needed for the proper development of the testes and
spermatogenesis.
The structures that will eventually produce sperm within the
testes, the seminiferous tubules, appear very early in embryonic
development—between 43 and 50 days following conception.
The tubules contain two major cell types: germinal and nonger-
minal. The germinal cells are those that will eventually become
sperm through meiosis and subsequent specialization. The non-
germinal cells are called Sertoli (or sustentacular ) cells. The
Sertoli cells appear at about day 42. At about day 65, the Leydig
(or interstitial ) cells appear in the embryonic testes. The Ley-
dig cells are clustered in the interstitial tissue that surrounds the
seminiferous tubules. The interstitial Leydig cells constitute the
endocrine tissue of the testes. In contrast to the rapid develop-
ment of the testes, the functional units of the ovaries—called the
ovarian follicles —do not appear until the second trimester of
pregnancy (at about day 105).
CLINICAL APPLICATION
In Kleinfelter’s syndrome, the affected person has 47
instead of 46 chromosomes because nondisjunction (failure
to separate during meiosis) of the X and Y chromosomes in
the father caused the zygote to be formed with the geno-
type XXY. Usually also because of a similar nondisjunction,
a person with Turner’s syndrome gets only one sex chro-
mosome, the X from the mother, and thus has a total of 45
chromosomes. These conditions illustrate that the Y chro-
mosome determines the sex, because a person with Klein-
felter’s syndrome (XXY) is male and a person with Turner’s
syndrome (X) is female. However, people with Kleinfelter’s
and Turner’s syndromes are generally infertile, and have
lower than normal amounts of testosterone and estrogen,
respectively, as well as other consequences of these chro-
mosomal abnormalities.FITNESS APPLICATION
Cremaster muscles travel with the spermatic cord to insert
on each teste. Their primary function is to maintain the cor-
rect temperature of the testes for optimal spermatogenesis.
They contract and elevate the testes closer to the body in
cold weather, and relax to lower the testes in warm weather.
They also contract when a male is frightened and during
sexual intercourse. The cremasteric reflex, involving con-
traction of the cremaster and elevation of its attached teste,
can be elicited by lightly stroking the upper inside portion of
a male’s thigh. This can be a medically useful test, because
the cremasteric reflex is absent if testicular torsion (the
twisting of the spermatic cord, perhaps as a result of exer-
cise) has occurred.The early-appearing Leydig cells in the embryonic tes-
tes secrete large amounts of male sex hormones, or androgens
( andro 5 man; gen 5 forming). The major androgen secreted
by these cells is testosterone. Testosterone secretion begins
as early as 8 weeks after conception, reaches a peak at 12 to
14 weeks, and then declines to very low levels by the end of
the second trimester (at about 21 weeks). Testosterone secre-
tion during embryonic development in the male serves the very
Development of Accessory Sex
Organs and External Genitalia
In addition to testes and ovaries, various internal accessory sex
organs are needed for reproductive function. Most of these are
derived from two systems of embryonic ducts. Male accessory
organs are derived from the wolffian (mesonephric) ducts,
and female accessory organs are derived from the müllerian
(paramesonephric) ducts ( fig. 20.5 ). Interestingly, the two duct
systems are present in both male and female embryos between
day 25 and day 50, and so embryos of both sexes have the poten-
tial to form the accessory organs characteristic of either sex.
Experimental removal of the testes (castration) from male
embryonic animals results in regression of the wolffian ducts
and development of the müllerian ducts into female accessory
organs: the uterus and uterine (fallopian) tubes. Female acces-
sory sex organs, therefore, develop as a result of the absence of
testes rather than as a result of the presence of ovaries.
In a male, the Sertoli cells of the seminiferous tubules secrete
müllerian inhibition factor (MIF), a polypeptide that causes
regression of the müllerian ducts beginning at about day 60. The
secretion of testosterone by the Leydig cells of the testes sub-
sequently causes growth and development of the wolffian ducts