Microscopic Anatomy Of The Male Reproductive
Tract
The testes have two distinct
functions: spermatogenesis and androgen production. Spermatogenesis occurs
within distinct structures called seminiferous tubules (Fig. 8.1). These
tubules lie coiled within lobules whose ducts all exit the testis into the epididymis.
Androgen production occurs within pockets of specialized cells that lie in the
interstitium between the tubules.
The seminiferous tubules are
surrounded by a basement membrane. Juxtaposed to the medial side of this
basement membrane are the progenitor cells for sperm production. The epithelium
containing the developing spermatozoa that line the tubules is known as the seminif-
erous epithelium or germinal epithelium. In a cross-section of the
testis, spermatocytes within a given tubule are in varying stages of
maturation. Mixed among the spermatocytes are Sertoli cells. These are
the only nongerminal cells in the seminiferous epithelium. Sertoli cells were
aptly called “nurse cells” when first described by Sertoli in 1865. They are
responsible for the metabolic and structural support of the developing
spermatozoa. All Sertoli cells make contact with the basement membrane at one
pole and surround the developing sperma- tozoa at the other. Sertoli cells have
large, complex cytoplasmic “fingers” that extend around many spermatozoa at one
time.
A wide variety of substances that
are normally present in the circula- tion are excluded from the fluid within
the seminiferous tubule. This phenomenon is similar to that seen in the brain
as the result of the blood–brain barrier. The
male reproductive system displays its own blood–testis barrier.
This barrier allows the testis to be one of very few immune-privileged sites in
the human body. While the function of this barrier is incompletely described,
its ultrastructural basis is known to be the tight junctions that form between
adjacent Sertoli cells. The barriers created by these tight junctions divide
the germinal epithelium into basal and luminal compartments. The basement compartment
contains the spermatogonia and the adluminal compartment, the maturing germinal
cells.
Spermatogenesis can be divided
into three phases: (i) mitotic proliferation to produce large numbers of cells;
(ii) meiotic division to produce genetic diversity; and (iii) maturation. The
latter involves extensive cellular morphologic remodeling aimed at facilitating
sperm transit to, and penetration of, the oocyte in the female tract. Primitive
spermatogonial stem cells remain dormant in the testis until puberty. At
puberty, they are activated and maintained in rounds of mitoses at the basement
membrane of the seminiferous tubule. From this reservoir of self-regenerating
stem cells emerges several subtypes of spermatogonial clones until, after the
final division, they exit mitosis as primary spermatocytes. Primary
spermatocytes then undergo two meiotic cell divisions. These important
divisions halve the number of chromosomes in the daughter cells. Cells
undergoing the first of these meiotic divisions have very characteristic
differences in their nuclear morphology that has led to a specific nomenclature
(resting, leptotene, zygotene,
pachytene and diplotene; Chapter 4). The first meiotic division produces
secondary spermatocytes (II) and the second, early haploid spermatids. The
spermatids then undergo remarkable cytoplasmic remodeling, during which a tail,
mitochondrial midpiece and acrosome all develop. Almost all of the spermatid
cytoplasm is expelled as residual bodies during this remodeling; only a small
droplet of cytoplasm remains within the head of the mature spermatozoon. The
surrounding Sertoli cells phagocytose the residual bodies, a process that may
transmit information about the developing sperm cell to the Sertoli cell.
Development of the spermatozoa within
the seminiferous epithelium is a complex and highly ordered sequence of events
in most mammalian species. In humans, the process appears somewhat less
orderly, but still follows the general principles found in other species. In
each, the number of mitotic divisions the spermatogonia undergo is fixed. In
humans, four mitotic divisions occur. The length of time for an early
spermatogonium to develop into a spermatozoon ready to enter the epididymis is
also fixed and species-specific. In humans, it takes 64 ± 4 days for this
process. As the spermatocytes move through the maturation process, they also
move in waves toward the lumen of the seminiferous tubule.
The Sertoli cells enveloping the
developing spermatozoa are homologs of the granulosa cells in the ovary.
Sertoli cells phagocytose the extruded spermatid cytoplasm. They also function
in aromatization of androgen precursors to estrogen, a product that exerts
local feed-back regulation on the androgen-producing (Leydig) cells. Sertoli
cells also produce androgen-binding proteins.
Leydig cells perform the
other major function of the testes – androgen production. The Leydig cells are
homologous with the theca cells of the ovary. They produce large amounts of androgen
from either circulating cholesterol or cholesterol made internally within their
own smooth endoplasmic reticulum. Leydig cells are very large and, consistent
with their intracellular activities, appear foamy by standard histologic
assessment.
The most easily damaged cells in
the testis are the spermatogonia. Irradiation, excessive alcohol intake,
dietary deficiencies and local inflammation can rapidly induce degenerative
changes in these cells. Excess heat also induces extensive spermatogonial cell degeneration
but does not affect the length of the spermatogenic cycle.
Epididymis and vas (ductus) deferens
The ducts forming the epididymis
and vas deferens have muscular coats composed of an inner layer of circularly
directed fibers and an outer layer of longitudinally directed fibers. The
muscle component of these structures is responsible for peristalsis that moves
the spermatozoa along the ducts. The ducts are lined with a mixture of
secretory and ciliated cells. The former aid in the generation of intratubal
fluids; the latter assist in directed transit of intratubal fluids and cellular
components.
Seminal vesicles
The alveoli of the seminal
vesicles are lined with a pseudostratified epithelium whose cells contain
numerous granules and clumps of yellow pigment. Some of the epithelial cells
have flagella. The secretion of the seminal vesicles is a yellowish, viscous
liquid containing globulin and fructose. This secretion provides the majority
of the ejaculate volume.
Prostate gland
The tubuloalveolar glands of the
prostate are lined with an epithelium that is highly responsive to androgens.
The acini of the central glandular zone that surrounds the ejaculatory ducts
are large and irregular. By contrast, the acini of the peripheral glandular
zone are small and regular. These striking differences in glandular
architecture, along with the observation that several unique enzymes present in
the seminal vesicles are present in the central but not the peripheral glandular
zone, suggests different embryologic tissue origins for these two parts of the
prostate (Chapter 6). The epithelium of the prostatic tubuloalveolar glands
produces the acid phosphatase and citric acid normally found in semen.
Penis
The erectile tissue of the penis
is a vast, sponge-like system of irregular vascular spaces fed by the afferent
arterioles and drained by the efferent venules. A pair of cylindrical bodies,
the corpora cavernosa, is surrounded by a thick fibrous membrane called the
tunica albuginea and separated by an incomplete fibrous septum. The veins
draining the cavernous bodies lie just beneath the tunica. The interior of the
cavernous bodies contains many partitions called trabeculae. Trabeculae are
comprised of elastic fibers and smooth muscle embedded within thick bundles of collagen and covered by
endothelial cells.
