Reproduction
605
II. When functioning male gonads are present, they secrete
testosterone and MIS, so a male reproductive tract and
external genitalia develop. In the absence of testes, the female
system develops.
SECTION A KEY TERMS
SECTION A CLINICAL TERMS
accessory reproductive
organ
600
androgen
600
aromatase
600
Barr body
603
bivalent
601
crossing-over
602
dihydrotestosterone
(DHT)
603
estradiol
600
estriol
600
estrogen
600
estrone
600
follicle-stimulating hormone
(FSH)
600
gamete
600
gametogenesis
600
genotype
602
germ cell
601
gonad
600
gonadal steroid
600
gonadotropin
600
gonadotropin-releasing
hormone (GnRH)
600
inhibin
600
karyotype
603
luteinizing hormone (LH)
600
meiosis
601
mitosis
601
Müllerian duct
603
Müllerian-inhibiting substance
(MIS)
603
ovary
600
ovum
600
phenotype
603
progesterone
600
secondary sexual
characteristic
600
sex chromatin
603
sex chromosome
602
sex determination
602
sex differentiation
603
sex hormone
600
sperm
600
spermatozoan
600
SRY
gene
603
testis
600
testosterone
600
Wolffi
an duct
603
X chromosome
602
Y chromosome
602
androgen insensitivity
syndrome
603
cryptorchidism
603
testicular feminization
603
SECTION A REVIEW QUESTIONS
1. Describe the stages of gametogenesis and how meiosis results
in genetic variability.
2. State the genetic difference between males and females and a
method for identifying genetic sex.
3. Describe the sequence of events, the timing, and the control
of the development of the gonads and the internal and external
genitalia.
SECTION B
Male Reproductive Physiology
Anatomy
The male reproductive system includes the two testes, the sys-
tem of ducts that store and transport sperm to the exterior,
the glands that empty into these ducts, and the penis. The
duct system, glands, and penis constitute the male accessory
reproductive organs.
The testes are suspended outside the abdomen in the
scrotum,
which is an outpouching of the abdominal wall and
is divided internally into two sacs, one for each testis. During
fetal development, the testes are located in the abdomen, but
during the seventh month of
gestation,
they usually descend
into the scrotum. This descent is essential for normal sperm
production during adulthood, since sperm formation requires
a temperature ~2°C lower than normal internal body tempera-
ture. Cooling is achieved by air circulating around the scro-
tum and by a heat-exchange mechanism in the blood vessels
supplying the testes. In contrast to spermatogenesis, testos-
terone secretion can usually occur normally at internal body
temperature, and so failure of testes descent usually does not
impair testosterone secretion.
The sites of
spermatogenesis
(sperm formation) in the
testes are the many tiny, convoluted
seminiferous tubules
(
Figure 17–4
). The combined length of these tubes is 250 m
(the length of over 2.5 football fi elds). Each seminiferous
tubule is bounded by a basement membrane. In the center
of each tubule is a fl
uid-fi
lled lumen containing spermato-
zoa. The tubular wall is composed of developing germ cells
and another cell type, to be described later, called Sertoli
cells.
The
Leydig cells,
or interstitial cells, which lie in small
connective tissue spaces between the tubules, are the cells that
synthesize and release testosterone. Thus, the sperm-producing
and testosterone-producing functions of the testes are car-
ried out by different structures—the seminiferous tubules and
Leydig cells, respectively.
The seminiferous tubules from different areas of a tes-
tis converge to form a network of interconnected tubes, the
rete testis
(
Figure 17–5
). Small ducts called efferent duct-
ules leave the rete testis, pierce the fi brous covering of the tes-
tis, and empty into a single duct within a structure called the
epididymis
(plural,
epididymides
). The epididymis is loosely
attached to the outside of the testis. The duct of the epididy-
mis is so convoluted that, when straightened out at dissection,
it measures 6 m. The epididymis draining each testis leads to
a
vas deferens,
a large, thick-walled tube lined with smooth
muscle. The vas deferens and the blood vessels and nerves sup-
plying the testis are bound together in the
spermatic cord,
which passes to the testis through a slitlike passage, the ingui-
nal canal, in the abdominal wall.
After entering the abdomen, the two vas deferens—one
from each testis—continue to the back of the urinary blad-
der base (
Figure 17–6
). The ducts from two large glands,
the
seminal vesicles,
which lie behind the bladder, join the
two vas deferens to form the two
ejaculatory ducts.
The
ejaculatory ducts then enter the
prostate gland
and join
the urethra, coming from the bladder. The prostate gland
is a single donut-shaped gland below the bladder and sur-
rounding the upper part of the urethra, into which it secretes
uid through hundreds of tiny openings in the side of the
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