The cloaca 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
The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...
; but, with the growth and flexure of the tail-end of the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, 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
The mesonephric duct (also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct) is a paired organ that forms during the embryonic development of humans and other mammals and gives rise to male reproductive organs.
Struc ...
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
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
and
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
, 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.
Clinical significance
A
birth defect can arise known as a
persistent cloaca where the
rectum
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
,
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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