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1. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Chapter 21:
Male Reproductive
System
Textbook of Histology, 4th ed.
Gartner
Copyright © 2017 by Elsevier, Inc. All rights reserved.

2. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Male Reproductive System
The male reproductive system consists of the two testes
suspended in the scrotum, a system of intratesticular and
extratesticular genital ducts, associated glands, and the male
copulatory organ, the penis. The testes are responsible for the
formation of the male gametes, known as spermatozoa, as
well as for the synthesis, storage, and release of the male sex
hormone, testosterone.
The glands associated with the male reproductive tract are the
paired seminal vesicles, the single prostate gland, and the
two bulbourethral glands (of Cowper). These glands form
the non-cellular portion of semen (spermatozoa suspended in
the secretions of the accessory glands), which not only
nourishes the spermatozoa but also provides a fluid vehicle
for their delivery into the female reproductive tract. The penis
has a dual function: It delivers semen to the female
reproductive tract during copulation and serves as the conduit
of urine from the urinary bladder to outside the body.
The male reproductive system.
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3. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Testis
Each testis is surrounded by a capsule known as the tunica
albuginea. Immediately deep to this layer is a highly vascularized
loose connective tissue, the tunica vasculosa. The posterior aspect
of the tunica albuginea is somewhat thickened, forming the
mediastinum testis, from which connective tissue septa radiate to
subdivide each testis into approximately 250 compartments known
as the lobuli testis.
Each lobule has one to four blindly ending seminiferous tubules
and small conglomerations of endocrine cells, the interstitial cells
(of Leydig) which synthesize testosterone.
Spermatozoa, produced by the seminiferous epithelium of the
seminiferous tubules, enter short straight ducts, tubuli recti, that
connect the open end of each seminiferous tubule to the rete testis,
a system of labyrinthine spaces housed within the mediastinum
testis. The spermatozoa leave the rete testis through 10 to 20 short
tubules, the ductuli efferentes, which eventually fuse with the
epididymis.
The vascular supply of each testis is derived from the testicular
artery, which descends with the testis into the scrotum
accompanying the ductus deferens (vas deferens).
The capillary beds of the testes are collected into several veins, the
pampiniform plexus of veins, which are wrapped around the
testicular artery.
Blood in the pampiniform plexus of veins is cooler than that in the
testicular artery, reduces the temperature of the arterial blood, thus
forming a countercurrent heat exchange system, keeping the
temperature of the testes a few degrees lower (35˚ C), than that of
the remainder of the body. At this cooler temperature spermatozoa
develop normally; at body temperature, spermatozoa that develop
are sterile.
The testis and epididymis. Lobules and their contents are not drawn to scale.
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4. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Seminiferous Tubule
The wall of the seminiferous tubule is composed of a
slender connective tissue layer, the tunica propria, and
a thick seminiferous epithelium, separated from each
other by a well-developed basal lamina.
The seminiferous epithelium is several cell layers thick
and is composed of two types of cells: Sertoli cells and
spermatogenic cells. The latter cells are in various
stages of maturation.
The lateral cell membranes of adjacent Sertoli cells
form occluding junctions with each other, thus
subdividing the lumen of the seminiferous tubule into
basal and adluminal compartments. Thus, the
zonulae occludentes of these cells establish a blood-
testis barrier that isolates the adluminal compartment
from connective tissue influences, thereby protecting the
developing gametes from the immune system.
Sertoli cells function in: supporting the developing
spermatogenic cells; establishing the blood-testis barrier;
phagocytosis of cytoplas shed by developing
spermatogenic cells; manufacturing the following
substances: androgen binding protein, antimullerian
hormone, inhibin, testicular transferrin, and a fructose-
rich medium.
Seminiferous epithelium.
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5. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Spermatogenesis
Most of the cells composing the thick seminiferous epithelium
are spermatogenic cells in various stages of maturation. Some
of these cells, spermatogonia, are located in the basal
compartment, whereas most of the developing cells—primary
spermatocytes, secondary spermatocytes, spermatids, and
spermatozoa—occupy the adluminal compartment.
Spermatogonia are diploid cells that undergo mitotic division
to form more spermatogonia as well as primary spermatocytes,
which migrate from the basal into the adluminal compartment.
Primary spermatocytes enter the first meiotic division to
form secondary spermatocytes, which undergo the second
meiotic division to form haploid cells known as spermatids.
Spermatids are transformed into spermatozoa by shedding of
much of their cytoplasm, rearrangement of their organelles, and
formation of flagella.
The maturation process is divided into three phases:
– Spermatocytogenesis: spermatogonia
differentiate into primary spermatocytes
– Meiosis: reduction division whereby diploid
primary spermatocytes reduce their chromosome
complement, forming haploid spermatids
– Spermiogenesis: transformation of spermatids
into spermatozoa (sperm)
Seminiferous epithelium.
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6. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Spermatozoon
Spermatids discard much of their cytoplasm and form a flagellum to
become transformed into spermatozoa, a process known as
spermiogenesis.
The spermatozoa (sperm) are long cells (~65 μm), composed of a head,
housing the nucleus, and a tail, which accounts for most of its length
The tail of the spermatozoon is subdivided into four regions: neck, middle
piece, principal piece, and end piece. The plasmalemma of the head is
continuous with the tail’s plasma membrane.
The neck (~5 μm long) connects the head to the remainder of the tail. It is
composed of the cylindrical arrangement of the nine columns of the
connecting piece that encircles the two centrioles, one of which is usually
fragmented. The posterior aspects of the columnar densities are
continuous with the nine outer dense fibers.
The middle piece (~5 μm long) is located between the neck and the
principal piece. It is characterized by the presence of the mitochondrial
sheath, which encircles the outer dense fibers and the centralmost
axoneme. The middle piece stops at the annulus. Two of the nine outer
dense fibers terminate at the annulus; the remaining seven continue into
the principal piece.
The principal piece (~45 μm long) is the longest segment of the tail and
extends from the annulus to the end piece. The axoneme of the principal
piece is continuous with that of the middle piece. Surrounding the
axoneme are the seven outer dense fibers that are continuous with those of
the middle piece and are surrounded, in turn, by the fibrous sheath.
The end piece (~5 μm long) is composed of the central axoneme
surrounded by plasmalemma. The axoneme is disorganized in the last 0.5
to 1.0 μm.
Spermiogenesis and a mature spermatozoon.
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7. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Leydig Cells
Luteinizing hormone (LH), a gonadotropin released from
the anterior pituitary gland, binds to LH receptors on the
Leydig cells, activating adenylate cyclase to form cyclic
adenosine monophosphate (cAMP). Activation of protein
kinases of the Leydig cells by cAMP induces inactive
cholesterol esterases to become active and cleave free
cholesterol from intracellular lipid droplets. The first step
in the pathway of testosterone synthesis is also LH-
sensitive because LH activates cholesterol desmolase, the
enzyme that converts free cholesterol into pregnenolone.
The various products of the synthetic pathway are shuttled
between the smooth endoplasmic reticulum and
mitochondria until testosterone, the male hormone, is
formed and is ultimately released by these cells.
Testosterone synthesis by the interstitial cells of Leydig. ATP, adenosine triphosphate; cAMP,
cyclic adenosine monophosphate; CoA, coenzyme A; LH, luteinizing hormone; SER, smooth
endoplasmic reticulum.
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8. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Hormonal Relationships
Because blood testosterone levels are not sufficient to
initiate and maintain spermatogenesis, FSH induces Sertoli
cells to synthesize and release androgen-binding protein
(ABP). ABP binds testosterone, thereby preventing the
hormone from leaving the region of the seminiferous
tubule and elevating the testosterone levels in the local
environment sufficiently to sustain spermatogenesis.
Release of LH is inhibited by increased levels of
testosterone and dihydrotestosterone, whereas release of
FSH is inhibited by the hormone inhibin, which is
produced by Sertoli cells.
Testosterone is also required for the normal functioning of
the seminal vesicles, prostate, and bulbourethral glands as
well as for the appearance and maintenance of the male
secondary sexual characteristics. The cells that require
testosterone possess 5α-reductase, the enzyme that
converts testosterone to its more active form,
dihydrotestosterone.
Hormonal control of spermatogenesis. FSH, follicle-stimulating hormone; LH, luteinizing hormone;
LHRH, luteinizing hormone–releasing hormone. (Adapted from Fawcett, DW: Bloom and Fawcett’s A
Textbook of Histology, 10th ed. Philadelphia, WB Saunders, 1975.)
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9. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Prostate
The prostate gland, the largest of the accessory glands, is
pierced by the urethra and the ejaculatory ducts. The slender
capsule of the gland is composed of a richly vascularized,
dense irregular collagenous connective tissue interspersed with
smooth muscle cells. The connective tissue stroma of the
gland is derived from the capsule and is, therefore, also
enriched by smooth muscle fibers in addition to their normal
connective tissue cells.
The prostate gland, a conglomeration of 30 to 50 individual
compound tubuloalveolar glands, is arranged in three
discrete, concentric layers mucosal, submucosal, and main.
The mucosal glands are closest to the urethra and thus are the
shortest of the glands. The submucosal glands are peripheral
to the mucosal glands and are consequently larger than the
mucosal glands. The largest and most numerous of the glands
are the peripheralmost main glands, which compose the bulk
of the prostate.
The lumina of the tubuloalveolar glands frequently house
round to oval prostatic concretions (corpora amylacea),
composed of calcified glycoproteins, whose numbers increase
with a person’s age.
The prostatic secretion constitutes a part of semen. It is a
serous, white fluid rich in lipids, proteolytic enzymes, acid
phosphatase, fibrinolysin, and citric acid. The formation,
synthesis, and release of the prostatic secretions are regulated
by dihydrotestosterone, the active form of testosterone.
Human prostate gland.
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10. Copyright © 2017 by Elsevier, Inc. All rights reserved.
Penis
The penis is composed of three columns of erectile tissue, each
enclosed by its own dense, fibrous connective tissue capsule, the
tunica albuginea.
Two of the columns of erectile tissue, the corpora cavernosa, are
positioned dorsally; their tunicae albugineae are discontinuous in
places, permitting communication between their erectile tissues. The
third column of erectile tissue, the corpus spongiosum, is positioned
ventrally. Because the corpus spongiosum houses the penile portion
of the urethra, it is also called the corpus cavernosum urethrae.
The corpus spongiosum ends distally in an enlarged, bulbous
portion, the glans penis (head of the penis). The tip of the glans
penis is pierced by the end of the urethra as a vertical slit.
The three corpora are surrounded by a common loose connective
tissue sheath, but no hypodermis, and are covered by thin skin. Skin
continues distal to the glans penis to form a retractable sheath, the
prepuce. When an individual is circumcised, it is the prepuce that is
removed.
Erectile tissue of the penis contains numerous variably shaped,
endothelially lined spaces. The vascular spaces of the corpora
cavernosa are larger centrally and smaller peripherally, near the
tunica albuginea. However, the vascular spaces of the corpus
spongiosum are similar in size throughout its extent.
The penis in cross section.
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