The cloaca is a structure in the development of the urinary and reproductive organs. The

hind-gut The hindgut (or epigaster) is the Posterior (anatomy), posterior (Caudal (anatomical term), caudal) part of the alimentary canal. In mammals, it includes the Anatomical terms of location#Proximal and distal, distal one third of the transverse colo ...

is at first prolonged backward into the

body-stalk The connecting stalk, or body stalk is an embryonic structure that is formed by the third week of development and connects the embryo to its shell of trophoblasts. The connecting stalk is derived from the extraembryonic mesoderm. Initially it lies ...

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

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. Stru ...

and Müllerian duct open into its ventral portion. The cloaca is, for a time, shut off from the anterior by the

cloacal membrane The cloacal membrane is the membrane that covers the embryonic cloaca during the development of the urinary and reproductive organs. It is formed by ectoderm and endoderm coming into contact with each other. As the human embryo grows and caudal ...

, 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

birth defect A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

can arise known as a

persistent cloaca A persistent cloaca is a symptom of a complex anorectal congenital disorder, in which the rectum, vagina, and urinary tract meet and fuse, creating a cloaca, a single common channel. Signs and symptoms Cloacas appear in a wide spectrum of variati ...

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 In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...

, 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 Cloacal exstrophy (EC) is a severe birth defect wherein much of the abdominal organs (the bladder and intestines) are exposed. It often causes the splitting of the bladder, genitalia, and the anus. It is sometimes called OEIS complex. Diagnosti ...

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 {{developmental-biology-stub