UROGENITAL
SYSTEM
REFERENCES: Balinsky, An
introduction to embryology. Also any dissection manual from freshman
biology lab for fetal pigs.
Urogenital system stands for urinary
plus genital systems. They are interrelated because the male genital ducts
arise from the mesonephric kidney and ducts and the exit of both kinds of ducts
occurs at the UG-sinus. The kidneys and ducts arise from the intermediate
mesoderm. The nephrotome is separated into three areas which give rise to
different structures: anterior part giving rise to the pronephric regions which
in mammals are very minimal, but which give rise to the pronephric duct. It is
induced by the mesenchyme of the nephrotome to form branches as it grows back
along the rest of the mesenchyme of the nephrotome and these branches induce
the mesonephric kidney nephrons, forming a separate organ. This functions in
fetal pigs because they have very non-intimate placentas so the fetal pig makes
its own urinary products to concentrate and store in the allantois. When the
pronephric duct reaches the cloaca, it opens up in the UG sinus region. The
pronephric kidney deteriorates rapidly early and the duct becomes the
mesonephric duct as the induced mesonephric nephron tubules fuse with it. Look
at 10-20 mm pig cross sections, see the diagrams which follow, and label the
numbered structures of UG system and skeleton. You will also be able to see the
mesonephros organ in the littlest whole pigs which you make a sagital section
of and dissect.
The metanephric kidney will be induced as a separate organ which will
become the adult kidney by the ureteric buds which grow out of the base of the
mesonephric duct in response to the metanephric mesenchyme induction at the
posterior end of the nephrotome. Nephrons will develop at the cortex of the
metanephros and will fuse with collecting ducts induced as branches from the
ureteric bud at the pelvis of the kidney. The branching and formations of
neprhons may result from NGF stimulus on receptors of kidney cells. Look at the
metanephros and mesonephros in 10-20 mm pigs and their sections, look for
gonads in all the little pigs.
The ureter will move from opening into the UG sinus to opening into the
base of the allantois which becomes the bladder. That allows the mesonephric
duct to open esparately into the UG sinus for males where sprem will not be
forced into the bladder, but will go directly to the urethra.
The UG system is also related to the gut because the allantois base has
grown out from the hindgut. The prostate and bulbourethral glands will develop
at the juncture by outgrowth of branches from the endodermally lined urethra
into the splanchnic mesoderm, to secrete lubricating seminal fluids and to close
off the urethra from the bladder during erection. Find where the hindgut and UG
ducts come together and where the allantois is given off. The base of the
allantois will become the bladder.
The testes will descend from their position in the dorsal body wall into
the scrotum via a shortening of the gubernaculum and descend through the
inguinal canal.
In the male, the same duct (the urethra will release urinary and genital
products. The old mesonephric duct becomes the Wolffian duct or vas deferens
and where the connection is made to the old kidney nephrons, it becomes
elongated and coiled to become the epididymus. The converted mesonephric
tubules become the vasa efferentia connecting the testis seminiferous tubules
and epididymus. Male sex hormones are necessary as well as hormone receptors on
the somatic cells to make the male structures (mesonephric ducts and tubules)
remain. They die in female embryos. Male hormones in males also kill off the
female reproductive ducts which developed earlier, even in males (Mullerian or
oviducts.)
In the female: genital products (eggs and embryos) are delivered out
through the oviducts, uterus, vagina whereas urinary products go out the
urethra. There is a complex development of cloaca and then separation into two
parts- hindgut and UG sinus, then urethra (the connection between the allantois
and the UG sinus) and vagina form separately from an opening-up of the base of
the UG sinus so that now both open independently to the outside. The oviduct
bases fuse with the uterus and cervex and that fuses with the vagina which
forms partly by an evagination from the UG sinus and has a dual endodermal and
mesodermal origin.
The external genitalia are in an indifferent stage at first with genital
tubercle and UG sinus; then if there is no male hormone the default condition
is the female pattern of development.The cloacal folds around the cloaca help
separate it into anus and UG sinus with the genital tubercle at the front of
it. The urethral folds separate the UG sinus into vagina and urethra behind the
clittoris which develops from the tubercle. In the male, the urethral folds
extend along the entire length of the growing penis and fuse along it, carrying
the opening of the urethra to the tip of the penis. The UG folds also fuse, and
instead of forming labia as in the female, they form scrotal sacs which close
over the rest of the UG sinus opening.
Questions to answer:
1. At what size pig has the
mesonephros disappeared or fused with the gonad?
2. At which size pig does the UG
sinus separate from the anus?
3. How does the bladder arise?
4. What is the difference between
ureter and urethra?
5. How can you tell male from female,
in the pigs?
PLACENTA
Examine the preparation of the PIG fetus with fetal membranes: measure
the length of the whole placenta. Can you see the folds on its surface which
interdigitate with folds of the uterus)? There are no villi on pig chorion
which correspond to those on the disc of the human placenta. The connection
with the mother is much less intimate, so there is little bleeding at birth of
pigs. In order to see these folds, use the dissection microscope. Examine the
fetal pig in the jar which has an injected placenta, with arteries in red and
veins in blue. Compare it to placentas of sheep, cat, other mammals shown
mounted in jars and to the human placentas we have seen. Next, cut open the fetal membranes (the
chorion vesicle is outermost) carefully so you can keep them, BEING CAREFUL NOT
TO CUT A SECOND MEMBRANE INSIDE, then examine the connection of the umbilical
cord with the membranes and see if you can tell the difference between the
allantois, (which is fused with the chorion and is used to store urine in
pig development, and to bring the circulation to the uterine lining to get
oxygen and release wastes) and the amnion, which contains the fluid
which protects the embryo from drying and mechanical injury, and the outer chorion, and yolk sac.
Look at the slide of human placenta villi. The round or
irregularly shaped structures are sections of the villi. Each villus
after the fourth month has:
1. a blood vessel in it which will probably contain blood
cells.
2. Around the blood vessel is connective tissue which contains a
few cells and a lot of extracellular material which is fibers and matrix
material.
3. Around the connective tissue is the syncytiotrophoblast
(syntrophoblast) which separates the mother's blood from the rest of the
villus, since the mother's blood is in the
4. intervillous space or lacuna between the villi.