Male Anatomy
Meiosis takes place in the testes, which
are located in the scrotum. Sperm move
from the testis to the epididymis, where
they mature. Upon ejaculation, the sperm
move through the ductus deferens and
then to the urethra. The seminal vesicles,
prostate gland, and bulbourethral gland
add fluids to the sperm to form semen.
The testes also produce testosterone,
the male sex hormone. The testes, found
in the scrotum, are located outside the
body cavity. If the testes are retained
within the body cavity (a condition known
as cryptorchidism), sperm production, but
not testosterone production, will either
cease or be greatly decreased, it is too
warm in the pelvic cavity for sperm to be produced.
Histology of the Testis
(a)Gross anatomy of the testis with a
section cut away to reveal internal
structures. (b) Cross section of a
seminiferous tubule. Spermatogonia are
near the periphery, and mature sperm
cells are near the lumen of the
seminiferous tubule. (c) Mature sperm cell.
(d) Head of a mature sperm cell. Note:
Ductus Deferens is sometimes called vas
deferens.
Histology of the testis, showing the seminiferous tubules and
interstitial cells.
Meiosis
Sertoli cells secrete inhibin which selectively inhibits FSH without reducing testosterone secretion.
Sexually reproducing organisms are diploid; they have two of every type
of chromosome, (ie. homologus pairs). Prior to sexual reproduction meiosis
reduces the number of chromosomes by half, (one of each pair) and
fertilization restores the original number.
Chromosomes come in homologous pairs in diploid organisms. Unless the cell
is dividing they are found in an unduplicated state. As illustrated by the left
pair of chromosomes, homologues carry the same genes, but not necessarily
the same forms or alleles of those genes. One homologue is inherited from
each parent. The one from the father is termed the paternal chromosome.
The one from the mother is the maternal chromosome. At the beginning of
cell division the DNA replicates and each chromosome is now a pair of
identical sister chromatids. The ultimate goal of the process of meiosis is
to reduce the number of chromosomes by half. This must occur prior to
sexual reproduction. The cell at the top contains two homologous pairs of
chromosomes, for a total of four chromosomes. The final products of
meiosis are four daughter cells. Each cell contains one chromatid from each
original homologous pair, for a total of two chromosomes..
Meiosis I reduces the number of chromosomes by half, but each
chromosome still contains two sister chromatids.
Meiosis is the process by which a diploid nucleus divides twice to produce four haploid nuclei. The divisions are called meiosis I
and meiosis II.
In the life cycles of diploid organisms meiosis precedes sexual reproduction. Among animals, the products of meiosis are
gametes—eggs or sperm. DNA is replicated prior to the start of meiosis. The identical sister chromatids are joined at the
centromere as in mitosis. Unlike in mitosis, homologous chromosomes pair with one another. These pairs intertwine during early
prophase of the first meiotic division and may exchange segments. This exchange is called crossing over. During prophase I, the
nuclear envelope disappears and the spindle forms. The homologous pairs lie side by side as they reach the midplane of the
spindle and attach to spindle fibers in metaphase I. Metaphase ends and anaphase I begins as the partners in each pair of
homologous chromosomes separate as they are pulled toward opposite poles of the spindle. These chromosomes still consist of
sister chromatids joined at their centromeres. During telophase I the spindle disappears, nuclear membranes may re-form and
the two nuclei, each containing a haploid set of chromosomes, are separated as cytokinesis divides the cytoplasm. Prophase II
begins with the formation of a spindle and the still duplicated chromosomes move toward its midplane. At metaphase II they
are lined up and attached to spindle fibers. Anaphase II begins when centromeres separate and sister chromatids, now
considered chromosomes, begin moving in opposite directions. During telophase II the nuclear membrane re-forms, the spindle
disappears, and cytokinesis divides the cytoplasm. The result is four haploid cells.
Crossing-over may occur during prophase I of meiosis. (a) A
pair of replicated homologous chromosomes. (b)
Chromatids of the homologous chromosomes form a tetrad.
The chromatids are crossed in two places. The chromatids
may break at the points of crossing and become fused to
the opposite chromosome, resulting in crossing-over. (c)
Genetic material has been exchanged following crossingover of the chromatids. Crossing-over contributes greatly
to genetic variability in the offspring.
Male Physiology
Meiosis occurs within the seminiferous tubules. As meiosis
proceeds, the haploid cells move toward the lumen of the
tube. A mature sperm cell has a head, with a cap called the
acrosome, a tail, and very little cytoplasm. The acrosome
contains enzymes that will be used to penetrate the egg.
Spermatogenesis involves two successive meiotic divisions.
The testis is organized into a network of seminiferous
tubules. Within these tubules spermatogenesis occurs.
Spermatogenesis is the production of mature haploid
sperm cells through meiotic cell division.
A spermatogonium deep in the epithelium of the
seminiferous tubules gives rise to a primary
spermatocyte. This cell contains 46 chromosomes. A
primary spermatocyte undergoes the first meiotic
division, producing two identical secondary
spermatocytes. Each secondary spermatocyte contains 23
chromosomes. The secondary spermatocytes complete
meiosis, giving rise to haploid spermatids. Each spermatid
contains 23 chromosomes. The spermatids then develop
into mature sperm.
There are three columns of erectile tissue in the penis.
Along the ventral side of the penis is a single erectile
body, the corpus spongiosum. On the dorsal side of the
penis there are a pair of corpora cavernosa (singular
corpus cavernosum). Increased blood flow to this
tissue causes erection. Ejaculation is a reflex that
causes contractions that expel the seminal fluid
through the urethra
The penis is filled with vascular erectile tissues that allow it to exist in a flaccid or erect state. An erection is achieved through
regulation of blood pressure within the penis. Upon psychic or tactile sexual stimulation, venous blood flow out of the penis
decreases while arterial flow into the erectile tissue increases. The size of the erectile tissues is limited by a dense convering
called the tunica albuginea. Engorgement of these tissues with blood results
in a stiffening of the penis.
Circumcision is the removal of the foreskin or prepuce of the penis. A
circumcision is generally performed for hygienic purposes because the glans
penis is easier to clean if it is exposed. The first of two incisions is made
vertically across the prepuce. The second incision is made with the help of
special surgical tools. This incision is located around the base of the
prepuce. Now the prepuce is removed, exposing the glans penis.