Urogenital
system
The
Urogenital system is a combination of the two closely related systems - the
excretory and the reproductive. The primitive kidney is built for
osmoregulation and functions in marine, freshwater and land environments.
Nitrogenous waste can be removed by diffusion through the skin in aquatic
environments, but on land, the removal of nitrogenous waste is a primary
function of kidneys. Kidneys consist of several renal corpuscles (glomerulus in
a Bowman’s capsule) each connected to a tubule leading to a common tubule which
is segmentaly attached to a longitudinal collecting duct.
Gonads
produce gametes and maintain secondary sex organs (reproductive ducts and
glands) and secondary sex characteristics. They arise from genital ridges
medial to the nephric tissue. Germ cells migrate into the genital ridges. Genes
and hormones determine whether the germ cells will form ovaries or testes. Eggs
are released into the coelom before entering the oviduct. Sperm is always in a
closed system of vessels.
Once
you have studied the reproductive structures of your animal, you must trade
animals with a group that has an animal of the opposite sex. It is therefore
essential that you do not destroy any of the reproductive features
Dogfish
Urogenital System
Excretory
System
Locate
the opisthonephric kidneys on the dorsal body wall. They are flat grey bands running
the full length of the body cavity behind all of the other organs and the
peritoneum. They are therefore retroperitoneal. The kidneys developed from a
strip of nephrogenic tissue derived from mesomere mesoderm. Sharks retain urea, which
makes them hyperosmotic. Water is drunk or flows in from the environment. Rectal
(salt) glands secrete excessive salts to maintain osmotic balance. The kidneys
and gonad ducts enter a common urogenital chamber, which is partly separated by
the urogenital papillae from the rectum in the cloaca.
Genital
System
In
the male, locate the two large testes, which look like small
lobes of the liver. They lead by a series of small ducts, vasa efferentia, which are
undeveloped kidney tubules. They lead into the opisthonephric (mesonephric) or Wolffian
duct, which is convoluted and on top of the kidney. The urine also
goes through this duct. When the duct widens and straightens into a seminal
vesicle, the ‘ureter’ forms a bypass for the urine. The sperm sac, which may
be full or empty depending on the season, stores mature sperm. These sacs are
remnants of muellerian ducts. The male uses the claspers for
internal fertilization. Inside the claspers is a siphon sac that adds sugars to
the sperm.
In
the female, locate the two large ovaries, which look like small lobes
of the liver. They may be swollen with eggs covered in yolk. The eggs are shed
into the body cavity and then into the opening (ostia) of the two
oviducts, called Muellerian ducts. They are silvery tubes on top of
the kidneys. The oviductual gland provides albumen and makes
egg cases in some species. It may be large or small depending on the season.
The 4-5 embryos grow in the uterus for 20-22 months being nourished by
the yolk, some of which is still present in a sac when they are born.
Necturus Urogenital System
Excretory
System
The
kidneys are long flat strips, narrower anteriorly, which lie
dorsally close to the midline, in the posterior of the pleuroperitoneal cavity.
Unlike other vertebrate kidneys, they are not retroperitoneal, but are
supported by a peritoneal fold. Urine is produced and collected by tubules, which
enter the mesonephric duct (ureters). In males this duct is large and coiled in
the anterior half. In females, it is small, on the outside edge of the kidney.
The
bladder is first found in amphibians. It forms as a pocket in the floor
of the cloaca. The ureters (mesonephric ducts) open into the
cloaca and urine backs up into the bladder when the vent is closed.
Reproductive
System
In
males, the muellerian ducts do not mature, but
persist as a rudiment. It can be seen anterior to the kidney as a thin,
pigmented duct close to the midline. The testis is
attached to the dorsal wall by a peritoneal sheet, the mesorchium. Sperm
passes through the upper (genital) portion of the kidney by efferent ducts, which
can only be seen when filled with white sperm. The tubules are collected in a
longitudinal duct and by more tubules to the mesonephric duct which
joins the cloaca. The papillate cloacal gland
secretes gelatinous envelopes, which form around clumps of sperm and make spermatophores for
sperm transfer in water. The male deposits these spermatophores in front of a
female, who picks them up with her cloacal lips.
In
females, the ovaries are folded, thin walled sacs,
containing many large eggs. Eggs rupture into the coelom and are wafted by
cilia to the large ostia (openings) of the oviduct. The
linings of the oviducts have glands, which produce jelly to coat the eggs. The
convoluted oviducts are attached to the kidney and body wall by a peritoneal
fold, the mesovarium. Eggs are accumulated in the terminal end,
called ovosacs until they are laid. The oviducts enter the cloaca.
Spermatophores from the male are collected in the fall and stored in the cloaca
until spring when the eggs are laid. Tadpole larva hatch out. Mudpuppies are neotenic and
retain gills and some other larval characteristics as adults.
Mammal
Urogenital System
Excretory
System
The
structures of the excretory system are paired kidneys and ureters from mesomere
mesoderm, and an unpaired urinary bladder and urethra. Locate the kidneys on the
dorsal part of the abdomen on either side of the vertebral column, embedded in
fat. Note a small adrenal gland anterior to each kidney. This is an endocrine
gland and is not part of the excretory system. The kidneys have a duct or tube,
the ureters leading from them. The kidneys are bean shaped and are metanephric kidneys,
which are a new formation. They are retroperitoneal (outside the coelomic
space) and came from a strip of mesomere, which formed the caudal end of
nephrogenic mesoderm. This piece becomes displaced and moves laterally and
anteriorly. The notch or hilus of the kidney is the site of
entrance and exit of the blood vessels and the ureters. Section a kidney and
identify the cortex (outer layer), medulla (inner
layer), the pelvis (a cavity) and the papilla of the calyxes, which
project into the pelvis. There are also some models to show these parts.
Excretory products in the urine pass into the pelvis, thence to the
ureter. Urine may either be hyperosmotic (higher solute, excess
salts) to the blood or hypoosmotic (higher water). The ureters pass
posteriorly and enter the neck of the urinary bladder. The
urinary bladder is a pear-shaped sac supported by ligaments (remnants of the
ventral mesentery), with a narrowing neck, which terminates in a tube, the urethra that opens
to the outside. In mammals, the bladder comes from the stalk of the allantois
(evagination of the embryonic cloaca) and is endodermal in origin.
Hormones may allow water reabsorption from the bladder.
Reproductive
System
The
male reproductive system consists of testes, ducts, glands, urethra
and penis. The testes (from splanchnic hypomere mesoderm) are located
in an external pouch, the scrotum, which is a projection from the
peritoneal cavity. Scrotal sacs have descended outside the abdominal cavity to
keep the sperm cool. An epididymis lies on the dorsal surface of each
testis, consisting of a head or caput epididymis (which receives sperm from the
seminiferous tubules of the testis proper via the vasa efferentia or efferent
ductules), and a body of the epididymis (which matures sperm) and a caudal
portion (which holds sperm like the seminal vesicles of dogfish). The
epididymis joins a convoluted tube, the vas deferens. The vas
deferens from each testis carries sperm to the urethra. It is looped over the
ureters in the region of the neck of the urinary bladder, then turns
posteriorly, and enters the urethra a short distance posterior to the
bladder.
Identify
the prostate glands, at the point of entrance of the vas deferens, and Cowper’s
glands (bulbo-urethral glands) at the base of the penis. Cowper's glands
are located between the posterior and lateral muscles at the base of the penis.
The prostate and Cowper’s glands both contribute seminal fluid to the urethra.
In
the male rat, there are a number of additional prostatic glands. Use the
diagram to identify the vesicular, ampullary and coagulating glands.
Trace
the urethra posteriorly, and identify the penis, glans
penis (acorn shaped tip) and the prepuce (foreskin).
Note that the neck of the urinary bladder, in males, continues as the urethra,
through the penis to the exterior. The sperm uses the same passageway as the
urine in the urethra, but not at the same time.
The
female reproductive system consists of a pair of ovaries, a pair of
fallopian tubes, a pair of uteri and a vagina. The ovaries, from
splanchnic hypomere mesoderm, lay posteriorly to the kidneys, held in position
by a mesentery, the mesovarium. From each ovary eggs are shed into
the coelom and a fallopian tube with a wide mouth, or ostium receives
ova. The tubes coil, and then enter the uterus. The uteri
from both sides join in the midline to form a "V’ shaped structure. Each
arm of the Y is formed by the cornua, or horns of the uterus, and the base is
the body of the uterus. Such a type of uterus is termed a bicornuate uterus.
The uterus joins the vagina at the posterior end of the short
body of the uterus. The projection of the uterus into the vagina is known as
the cervix. The urethra is joined ventrally onto the vagina. The common
passageway housing the separate ducts, urethra and uterus is the vestibule. Cut into
it and trace the ducts. The external portion of the vagina is the vulva. At the
ventral side of the vestibule near its orifice is a small structure, the clitoris, which is
the homologue of the penis of the male. In rats, it encloses the end of the
urethra and conducts urine to the outside. At the edges of the vestibule are
the labia minora derived from genital folds and the labia majora derived
from genital swellings, which are homologues to the scrotal sac of males.
Examine
the enlarged uterine horns of a pregnant female. Cut them open to see the fetus
in the amniotic sac. The fetus is attached to the uterine wall by the umbilical
cord, which contains two arteries and a vein. The umbilical cord ends
in a zonary placenta for both cats and rats.
Table of Glands in the
Reproductive System
|
Cat |
Rat |
Function |
|
mammary |
mammary |
produce milk |
|
prostate(2) |
prostate(2) |
seminal fluid
(neutralizes ph in female) |
|
Cowpers(2) |
Cowpers (2) vesicular
(2) |
seminal fluid |
|
preputal |
preputal |
penis lubrication |
|
Assignment: Fill in
the table comparing the Urogenital Systems of a mammal with a Dogfish and
Mudpuppy. Comparison of the Urogenital Tract
|