The cloaca (: cloacae) is a structure in the
development of the urinary and reproductive organs.
The
hind-gut is at first prolonged backward into the
body-stalk as the tube of the
allantois; but, with the growth and flexure of the tail-end of the
embryo, the body-stalk, with its contained allantoic tube, is carried forward to the ventral aspect of the body, and consequently a bend is formed at the junction of the
hind-gut and allantois.
This bend becomes dilated into a pouch, which constitutes the endodermal cloaca; into its dorsal part the hind-gut opens, and from its ventral part the allantois passes forward.
At a later stage, the
Wolffian duct and
Müllerian duct open into its ventral portion.
The cloaca is, for a time, shut off from the anterior by the
cloacal membrane, formed by the apposition of the
ectoderm and
endoderm, and reaching, at first, as far forward as the future
umbilicus.
Behind the umbilicus, however, the mesoderm subsequently extends to form the lower part of the
abdominal wall and
pubic symphysis.
By the growth of the surrounding tissues, the cloacal membrane comes to lie at the bottom of a depression, which is lined by ectoderm and named the ectodermal cloaca.
Outcome
While most
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s retain a
cloaca, an opening for the
reproductive,
urinary and
digestive systems, the embryonic cloaca splits into the following in
placental mammals:
Males: the
urethra opening into the
penis
A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate.
The term ''pen ...
for both urination and reproduction
Females: the urethra and
vagina
In mammals and other animals, the vagina (: vaginas or vaginae) is the elastic, muscular sex organ, reproductive organ of the female genital tract. In humans, it extends from the vulval vestibule to the cervix (neck of the uterus). The #Vag ...
opening into the
vestibule/
vulva
In mammals, the vulva (: vulvas or vulvae) comprises mostly external, visible structures of the female sex organ, genitalia leading into the interior of the female reproductive tract. For humans, it includes the mons pubis, labia majora, lab ...
for urination and reproduction respectively
Both sexes: the
rectum opening as the
anus for
defecation
Clinical significance
A
birth defect can arise known as a
persistent cloaca where the rectum, vagina, and
urinary tract fuse to create a common channel or cloaca.
A rare birth defect which leaves much of the abdominal organs exposed is known as
cloacal exstrophy.
Additional images
File:Gray977.png, Human embryo about fifteen days old.
File:Gray983.png, Front view of two successive stages in the development of the digestive tube.
File:Gray991.png, Tail end of human embryo from fifteen to eighteen days old.
File:Gray992.png, Cloaca of human embryo from twenty-five to twenty-seven days old.
File:Gray1115.png, Tail end of human embryo twenty-five to twenty-nine days old.
File:Gray1118.png, Primitive kidney and bladder, from a reconstruction.
File:Development of external sexual organs in the male and female Gray1119.png, Stages in the development of the external sexual organs in the male and female.
References
External links
*
Embryology of digestive system
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