Download Accessory Sex Organs - Academic Script

[Academic Script]
Accessory Sex Organs, Sex determination
& Sex differentiation
Subject:
Zoology
Course:
B.Sc. 2nd Year
Paper No. & Title:
Z-203B
Vertebrate
Endocrinology and
Reproductive Biology
Topic No. & Title:
Topic – 10
Accessory Sex Organs
and their dependence
as steroid hormones.
Sex determination &
Sex differentiation
Lecture Title:
Accessory Sex Organs,
Sex determination &
Sex differentiation
Academic Script
1. Introduction
The male reproductive tract can be divided into a sequence of
ducts and three major secretory glands. Moving from testes to
penis, sperm pass through the epididymis, ductus deferens,
ejaculatory duct, and urethra. The major accessory sex glands,
which provide most of the semen, are the seminal vesicles,
prostate, and bulbourethral gland. The female reproductive tract
consists of two oviducts, the uterus and the vagina. The
oviducts (or fallopian tubes), which connect the ovaries to the
uterus, can be divided into three segments. The most lateral
part is the fimbriated infundibulum, which picks up the mature
ovum when it leaves the ovary. Next is the ampulla, which is the
site of fertilization; it leads into the isthmus, a relatively short
straight tube running into the uterus. The major accessory sex
glands are bartholin’s glands and mammary glands.
ACCESSORY
ORGANS
OF
THE
MALE
REPRODUCTIVE
SYSTEM
EPIDIDYMIS
It is the start of the male reproductive tract, is a coiled tube
bound to the posterior border of the testis. It is almost 7 meters
long, the epididymis is divided into caput, corpus and cauda
segments.
It stores and protects Sperm and facilitates their Functional
Maturation. A spermatozoon passes through the epididymis in
about two weeks and completes its functional maturation at that
time under the strict control of hormones.
THE DUCTUS DEFERENS OR VAS DEFERENS
It is 40–45 cm long. It begins at the tail (cauda) of the
epididymis. Just before the ductus deferens reaches the prostate
gland and seminal vesicles, its lumen enlarges. This expanded
portion is known as the ampulla (distae part). The junction of
the ampulla with the duct of the seminal vesicle marks the start
of the ejaculatory duct. This short passageway penetrates the
muscular wall of the prostate gland and empties into the urethra
near the opening of the ejaculatory duct from the opposite side.
In addition to transporting spermatozoa, the ductus deferens
can store spermatozoa for several months. During this time, the
spermatozoa remain in a state of suspended animation and have
low metabolic rates.
URETHRA
Urethra shares by urinary and reproductive systems. It has 3
regions: 1. prostatic urethra: connects to urinary bladder and
ejaculatory duct, passes through prostate 2. membranous
urethra: passes through body wall (urogenital diaphragm) 3.
spongy/penile urethra: length of penis, opens at external
urethral orifice.
SEMINAL VESICLES
The Seminal Vesicles are glands embedded in connective tissue
on either side of the midline, sandwiched between the posterior
wall of the urinary bladder and the rectum. It contributes about
60 percent of the volume of semen. In particular, the secretion
of the seminal vesicles contains (1) higher concentrations of
fructose, which is easily metabolized by spermatozoa; (2)
prostaglandins, which can stimulate smooth muscle contractions
along the male and female reproductive tracts; and (3)
fibrinogen, which after ejaculation forms a temporary clot within
the vagina.
PROSTATE GLAND
The Prostate Gland is a small, muscular, rounded organ about 4
cm in diameter and encircles the proximal portion of the urethra
as it leaves the urinary bladder. It produces prostatic fluid, a
slightly acidic solution that contributes 20–30 percent of the
volume of semen. These secretions are ejected into the prostatic
urethra by peristaltic contractions of the muscular wall.
THE BULBOURETHRAL GLANDS, OR COWPER'S GLANDS
They are situated at the base of the penis, covered by the fascia
of the urogenital diaphragm and are compound, tubuloalveolar
mucous glands that secrete thick, alkaline mucus. The secretion
helps neutralize any urinary acids that may remain in the
urethra and lubricates the glans.
ACCESSORY ORGANS OF THE FEMALE REPRODUCTIVE
SYSTEM
THE UTERINE (FALLOPIAN) TUBES, OR OVIDUCTS
These are bilateral muscular ducts. They are about 4 in long,
and the margin of the dilated end, or ampulla, is surrounded by
a number of fringelike processes called fimbriae.
The functions of the uterine tubes are to convey the ova from
the ovaries to the uterus, to aid in the upward passage of the
spermatozoa, and to provide circular folds within which the
ovum
is
nourished
and
delayed
of
the
elapse
between
fertilization and implantation so that the uterine wall will be
properly prepared for growth and development of the embryo.
THE UTERUS OR WOMB
The uterus is a hollow, thick-walled, pear-shaped muscular
organ about 3in long, situated in the pelvis cavity between the
rectum and the bladder. Three parts of the uterus can be
distinguished: (1) the body with its superior expanded portion
called the fundus extending above the entrance of the uterine
tubes; (2) the isthmus, or middle, slightly constricted portion;
and (3) the cervix, or cylindrical lower part, that surround the
cervical canal and projects into the vagina.
The cavity of the uterus is small because of the thickness of its
walls. The part of the cavity within the body is triangular and
has three openings. The uterus is the organ of the reproductive
tract in which the embryo grows and develops until the time of
delivery.
THE VAGINA
The vagina is a fibro muscular tube, 7.5 to 10cm in length,
situated anterior to the rectum and anal canal and posterior to
the bladder and urethra. It is the organ of copulation, for the
deposition of semen in the female, and during parturition it
serves as the exit from the uterus.
BARTHOLIN’S GLANDS
It secrete mucus-like lubricating fluid.
DEPENDENCE OF SEX ORGANS ON STEROID HORMONES
Androgens and estrogens play a major role in the development
of
both
sex
secondary
characteristics.
Androgens,
or
testosterone give the male its sex characteristics during puberty
and for promoting tissue and muscle growth. Androgens play a
key role in maintenance of accessory sex organs, and their
functions. Similarly, estrogens are synthesized in the ovaries,
which control female secondary characteristics and regulation of
the menstrual cycle. Another steroid hormone is needed for
preparing the uterus for implantation of the ovum, this hormone
is progesterone.
SEX DETERMINATION
Surprisingly, it is only in the last 50 years that we have begun
to understand the nature of the biological events which
determine our sex. It is not so long ago that women were
blamed if they failed to produce a son for their husband and
clearly it was thought that the power of sex determination lay
within the body of the woman. Sex determination (male/female)
at the biological level is determined by the presence or absence
of the Y chromosome. "Female" is the default sex; due to the
absence of the Y.
There are many possible sex determination mechanisms in the
animal kingdom. Here we consider only chromosomal sex
determination mechanism in human.
Chromosomal Sex Determination
The basic mammalian sex determination mechanism arises from
the possession of a small but important gene located on the Ychromosome. The Y-chromosome has very few functional genes
and nearly all-important genes for development of early
embryonic form are spread among the remaining somatic
chromosome or on the X chromosome. There are indeed
hundreds of genes that are necessary to construct the basic
reproductive system of vertebrates during early development,
but the trigger to take on a female or a male form arises
from testes determining factor, the TDF genes. For years,
biologists speculated that such a factor must reside on the Y
chromosome and it has now been isolated in at least a few
vertebrates more specifically present in p arm of Y chromosome
called as sex determining region (SRY) on Y chromosome.
The presence (XY) or absence (XX) of this gene takes the
embryo down two alternative pathways male versus female. If
the gene is absent, then the organism develops into a female
with ovaries and a system of endocrine glands that regulate
female reproduction and behaviors. If the gene is present, the
organism develops testes rather than ovaries, and the testes in
turn produce testosterone, which further organizes the neuroendocrine system.
Human Sex Determination
In humans, sex is determined by a specific set of chromosomes.
Females have two X chromosomes (XX), whereas males have an
X and a Y chromosome (XY). A mature female will produce
eggs, each with one X chromosome; a mature male will produce
sperm with either an X chromosome or a Y chromosome. When
an egg and a sperm fuse during reproduction, the chromosome
that the sperm carries determines the sex of the child.
SEX DIFFERENTIATION
After the determination of sex, a series of events occur to
promote the appearance of three things: 1. Right type of gonadThe appearance of ovaries or testis occurs early in development.
This establishes the "gonadal sex". 2. Right set of duct systemAdult sexual phenotype requires, besides the gonad, the
presence of proper system capable of transporting and storing
gametes, and adaptations that ensure gametes are brought
together.3.The
right
genitals-proper
external
genitalia
for
copulation.
Sex differentiation in humans
1. Differentiation of gonad
At 5th week of gestation, gonads develop as paired bulges of
tissue near the midline at the back of the abdominal cavity
forms genital ridges. The gonad at this stage is "indifferent" or
"bipotential",
and
has
no
germ
cells
(oogonia
spermatogonia). Both cortex and medulla are present.
or
If Y chromosome is not present, the cortex grows into "cortical
cords". Germ cells migrate from elsewhere and reside in cortical
cords
(future
follicles),
and
medulla
largely
degenerates.
Enlarging cortex becomes the ovary, and primordial germ cells
become oogonia. Mitosis occurs in oogonia to give rise to
primary oocytes. Human female infants are born with about
4,00,000-5,00,000 primary oocytes in primordial follicles at
birth. From 7th-9th months, the ovaries descended from
abdomen to pelvis.
If Y chromosome (SRY) is PRESENT, at 43-50 days, the medulla
grows
into
"medullary
cords".
Germ
cells
migrate
from
elsewhere and reside in medullary cords (future seminiferous
tubules), and the cortex degenerates largely. Leydig cells
appear at about 60 days and can be stimulated to secreted
testosterone. From 3rd-9th months, the testes descended into
scrotum
(stimulated
by
testosterone).Thus,
SRY
of
Y
chromosome plays a pirotal role in testis formation, whole
absence leads to ovary.
2. Differentiation of ducts
At 6th week of gestation, the embryo also has indifferent sex
ducts, consisting of two pairs. One pair, the Wolffian duct
(mesonephric duct, or archinephric duct as a more general
term), is derived from ducts of the primitive embryonic kidney
(the mesonephros) which later form the male duct system.
Second pair, the Mullerian duct (paramesonephric duct), is
derived from the dorsal lateral lining of body cavity which gives
rise to the female duct system. If testes develop-testosterone
secreted by testes will cause the development of Wolffian duct
into: epididymis, vas deferens, seminal vesicle and ejaculatory
duct. Prostate and bulbourethral glands develop from another
duct (urogenital sinus, which gives rise to urethra) and not
Wolffian ducts. Embryonic testes also secrete Mullerian inhibiting
substance (MIS). MIS causes the regression of the Mullerian
ducts.
If ovaries develop no testosterone is secreted, and Wolffian
ducts regress. No MIS is secreted, so Mullerian ducts are
retained and develop into: oviduct, uterus cervix, upper vagina.
Lower vagina is developed from another duct (Urogenital sinus,
which also gives rise to urinary bladder and urethra).
3. Differentiation of external genitalia
If testes are present in embryos. Testosterone supports wolffian
duct development, and is converted to DHT i.e 5-alpha
dihydrotestosterone by 5-α-reductase to support development
of external genitalia and prostate. DHT stimulates the male
external
genitalia
to
development. Genital
develop
tubercle
during
form
the
most
8th
of
week
the
of
penis,
urogenital folds forms ventral aspect of penis shaft, labioscrotal
swelling
form
scrotum.
Both
bulbourethral
and prostate
glands enlarge. If no testes are present (e.g., ovaries present),
no testosteron. Genital tubercle form clitoris, urogenital folds
forms labia minora, labioscrotal swelling form labia majora ,this
is a process that is estrogen-independent.
Summary
The male accessory sex organs can be divided into a sequence
of ducts and three major secretory glands. Moving from testes
to penis, sperm pass through the epididymis, ductus deferens,
ejaculatory duct, and urethra. The major accessory sex glands,
which provide most of the semen, are the seminal vesicles,
prostate, and bulbourethral gland. The female accessory sex
organs consist of two oviducts, the uterus and the vagina. The
major accessory sex gland is bartholin’s glands.
Chromosomal sex determination in humans (male/female) at
the biological level is determined by the presence or absence of
the Y chromosome (SRF). "Female" is the default sex; due to
the absence of the Y. After the determination of sex, in sex
differentiation
a
series
of
events
occur
to
promote
the
appearance of three things: Right type of gonad, Right set of
ductwork, the right genitals. Thus in this manner complete
accessory
sex
organs
and
their
dependence
on
steroid
hormones. Sex determination and sex differentiation has been
studied.