The Reproductive System
ĕ-sis; “sperm formation”)
is the sequence of events in the seminiferous tubules of the tes-
tes that produces male gametes—
process begins around the age of 14 years (and oFen earlier) in
males, and continues throughout life. Every day, a healthy adult
male makes about 400 million sperm. It seems that nature has
made sure that the human species will not be endangered for
lack of sperm!
Before we describe spermatogenesis, let’s deﬁne some terms.
±irst, having two sets of chromosomes, one from each parent,
is a key factor in the human life cycle. Te normal chromo-
some number in most body cells is the
loid) of the organism, symbolized as
. In hu-
mans, the diploid chromosomal number is 46, and diploid cells
contain 23 pairs of similar chromosomes called
ŏ-gus), or homologues. One mem-
ber of each pair is from the male parent (the
), and the other is from the female parent (the
Generally speaking, the two homologues of each chromo-
some pair look alike and carry genes that code for the same
traits, though not necessarily for identical expression of those
traits. (Consider, for example, the homologous genes control-
ling the expression of freckles. Te paternal gene might code
for an ample sprinkling of freckles and the maternal gene for no
freckles.) In Chapter 29, we will consider how our mom’s and
dad’s genes interact to produce our visible traits.
Te number of chromosomes in human gametes is 23, re-
ferred to as the
haploid chromosomal number
. Gametes contain only one member of each homologous pair.
When sperm and egg fuse, they form a fertilized egg that rees-
tablishes the typical diploid chromosomal number of human
Gamete formation in both sexes involves
“a lessening”), a unique kind of nuclear division that, for the
most part, occurs only in the gonads. Recall that
process by which most body cells divide) distributes replicated
chromosomes equally between the two daughter cells. Conse-
quently, each daughter cell receives a full set of chromosomes
identical to that of the mother cell. Meiosis, on the other hand,
consists of two consecutive nuclear divisions that follow one
round of DNA replication. Its product is four daughter cells in-
stead of two, each with
as many chromosomes as typical
(diploid) body cells.
In other words, meiosis reduces the diploid chromosomal
number by half (from
) in gametes. Additionally, meiosis
introduces genetic variation because each of the haploid daugh-
ter cells has only some of the genes of each parent, as explained
Meiosis Compared to Mitosis
Te two nuclear divisions of meiosis, called
, are divided into phases for convenience. Tese phases
have the same names as those of mitosis (prophase, metaphase,
anaphase, and telophase), but some events of meiosis I are quite
diﬀerent from those of mitosis.
Te reproductive ducts and accessory glands contract,
emptying their contents into the urethra.
Semen in the urethra triggers a spinal reﬂex through so-
matic motor neurons. Te bulbospongiosus muscles of the
penis undergo a rapid series of contractions, propelling
semen from the urethra at a speed of up to 500 cm/s (200
inches/s, close to 11 mi/h). Tese rhythmic contractions
are accompanied by intense pleasure and many systemic
changes, such as generalized muscle contraction, rapid
heartbeat, and elevated blood pressure.
Te entire ejaculatory event is called
gasm is quickly followed by
, a period of muscular
and psychological relaxation. Activity of sympathetic nerve ﬁ-
bers constricts the
internal pudendal arteries
(and penile arte-
rioles), reducing blood ﬂow into the penis, and activates small
muscles that squeeze the cavernous bodies, forcing blood from
the penis into the general circulation. Although the erection
may persist a little longer, the penis soon becomes ﬂaccid again.
AFer ejaculation, there is a
from minutes to hours, during which a man is unable to achieve
another orgasm. Te latent period lengthens with age.
Check Your Understanding
What is erection and which division of the ANS regulates it?
What occurs during resolution and what brings it about?
For answers, see Appendix H.
Erectile dysfunction (ED)
, the inability to attain an erection
when desired, usually occurs when the parasympathetic nerves
serving the penis do not release enough NO. Approximately
50% of American men over age 40 (about 32 million) have some
degree of erectile dysfunction. Psychological factors, alcohol, or
certain drugs (antihypertensives, antidepressants, and others)
can cause temporary ED.
When chronic, ED is largely the result of problems with hor-
mones (diabetes mellitus), blood vessels (arteriosclerosis, vari-
cose veins), or the nervous system (stroke, penile nerve damage,
multiple sclerosis). In men with varicose veins, incompetent
valves that fail to retain blood in the penis during the erection
phase may be the major cause.
Until recently, most ED remedies entailed using a vacuum
pump to suck blood into the penis, or implanting a device into
the penis to make it rigid. Viagra and similar drugs (e.g., Cialis
and Levitra) potentiate the eﬀect of the existing NO, a remedy
that most men can live with. Tese drugs are taken orally, work
within minutes to an hour to produce a sustained blood ﬂow
to the penis, and have essentially no signiﬁcant side eﬀects in
healthy males. However, to avoid a fatal result, those with preex-
isting heart disease or diabetes mellitus must heed the warning
that these drugs reduce systemic blood pressure.
Deﬁne meiosis. Compare and contrast it to mitosis.
Outline the events of spermatogenesis.