454 Chapter 19
Key Terms (continued)
Spermatogenesis........... 454. Testosterone........... 457 .... Uterus.............. 466......
Spermatogonia............ 454. Tunica albuginea........... 454. Vagina............... 468 .....
Spermatozoa.............. 457. Umbilical cord............ 471 .. Vaginal orifice............. 469.
Spongy/cavernous urethra.... 460 Urethra.................. 460 .. Vasectomy............... 459 ..
Straight tubules........ 458 .... Urethral orifice.......... 469 ... Vestibule.............. 469 ....
Testes.................. 454 ... Uterine cavity........... 466 ... Vulva/pudendum........ 469 ...
Testis.............. 454 .... Uterine/fallopian... tubes..... 465. Zygote................ 471 ....
Introduction
At its most basic level, reproduction is the process by
which a single cell duplicates its genetic material. This is
the process of mitosis, discussed in Chapter 4. Mitosis
allows us to grow and repair damaged or old tissues. In this
sense, cellular reproduction enables us to maintain
ourselves. However, reproduction is also the process by
which our genetic material is passed on from one gen-
eration to the next. This process requires a special kind of
cellular reproduction that produces special cells, the sperm
from the male and the egg from the female. These join in
the process of fertilization to produce a fertilized egg, or
zygote. The special type of cellular division that produces
the sex cells is called meiosis. Meiosis, you will recall from
Chapter 4, is a reduction division of the genetic material.
This results in an egg carrying 23 chro-mosomes and the
sperm carrying 23. When fertilization occurs, the resulting
zygote will possess the full comple-ment of 46
chromosomes.
This chapter discusses the organs of the reproductive
system that produce the sex cells, transport and nurture
their development. Once an egg is fertilized by a sperm, the
resulting zygote will develop into an embryo in the uterus
of the female and grow by the process of mitosis into a
fetus. The fetus will continue to develop until birth. The
purpose of the reproductive system is to produce off-spring
and ensure the perpetuation of the human species.
The Female Reproductive System.
The primary sex organs of the male reproductive system
are the testes (TES-teez) or male gonads. These organs
produce sperm and the male sex hormones. There are also
accessory organs, like the scrotum, that support the testes.
Other accessory structures nurture the develop-ing sperm
cells and various ducts store or transport the sperm to the
exterior or into the female reproductive tract. Accessory
glands add secretions that make up the semen. A
transporting and supporting structure is the penis. See
Concept Map 19-1: The Male Reproductive System.
The Scrotum
The scrotum (SKROH-tum) is an outpouching of the
abdominal wall. It consists of loose skin and superficial
fascia. It is the supporting structure of the testes.
Externally, it appears as a single pouch of skin separated
into lateral portions by a median ridge known as the raphe
(RAY-fee) (Figure 19-1). Internally, it is separated into two
sacs by a septum. Each sac contains a single testis (TES-
tis). The testes (plural) produce the sperm and the male sex
hormones. Because sperm and hormone production and
survival require a temperature lower than normal body
temperature, the scrotum lies outside the body cavity. Its
environment is about 3°F below body temperature.Exposure to cold, as in winter, causes contraction of
the smooth muscle fibers, moving the testes closer to the
pelvic cavity so that they can absorb more body heat. The
whole scrotal sac contracts and a muscle, the cremaster
(kree-MASS-ter) muscle, located in the spermatic cord,
elevates the testes. Exposure to heat, as on a hot summer
day, reverses the process, and the scrotal sac hangs well
below the pelvic cavity to avoid body warmth.The Testes
The testes are paired oval glands measuring about 5 cm (2 inches)
in length and 2.5 cm (1 inch) in diameter (Figure 19-2). They are
covered by a dense layer of white fibrous connective tissue called
the tunica (TYOO-nih-kah) albuginea (al-byoo-JEN-ee-ah)
that extends inward and divides each testis into a number of
smaller, internal compartments known as lobules. Each lobule
contains one to three tightly coiled tubules called the convoluted
seminiferous (sem-in-IF-er-us) tubules. These seminiferous
tubules actually produce the sperm by a process called
spermatogenesis (spur-mat-oh-JEN-eh-sis) -(Figure 19-3).Spermatogenesis begins in the seminiferous tubules, as
the most immature sperm cells called spermatogonia
(spur-mat-oh-GO-nee-ah) begin to divide by mitosis to
produce daughter cells called