Role of Uterus in Reproduction
The uterus plays a crucial role in reproduction in animals by serving as the site for implantation, fetal development, and nourishment of the embryo.
The uterus consists of the two uterine horns (cornua), a body and a cervix (neck). The degree to which the uterine horns are developed constitutes the basis of classification of the mammalian uterus.
Type of Uterus
There are three distinct anatomical types of uteri namely:
- Duplex uterus
- Bicornuate uterus
- Simplex uterus
(1) Duplex Uterus
Duplex uterus is characterized by two cervical canals which separate each uterine horn into distinct compartments. In this there are two types.
The first is characterized by having a single vaginal canal opening to the exterior. On the interior it bifurcates (splits) into two vagina and two cervices. Eg. Marsupials-Opossum in this female will be inseminated by the male with a forked penis and deposits semen in each of the two sides of the reproductive tract simultaneously.
In the second, less complex type of duplex uterus, there are two uterine horns with two distinct cervical canals connecting with a single vaginal canal. Eg. Rabbit. So it is possible to inseminate rabbit with two different male semen and obtain offsprings that will represent two different genetic types.
(2) Bicornuate Uterus
Bicornuate uterus is characterized as having two uterine horns and a small uterine body. The length of the uterine horns is dependant on the degree of fusion between the paramesonephric ducts in the developing female foetus. In mare, where there is a high degree of fusion, there are short uterine horns and a relatively large uterine body. In cow, ewe and goat, there is moderate degree of fusion resulting in uterine horns of intermediate length. In sow, very little fusion takes place between adjacent paramesonephric ducts resulting in long uterine horns. In all the types the uterus opens into the vagina through a single cervical canal.
(3) Simplex Uterus
Fusion of the paramesonephric duct is almost complete resulting in a single chambered uterus with out horns. Eg. Primates including humans.
Histology of Uterus
The uterus consists of a thin outer layer, the perimetrium, a thick myometrium composed of inner circular and outer longitudinal muscle layers and an inner layer, the endometrium.
The outer perimetrium is the serous layer which is part of the peritoneum.
In the myometrium, in between the inner circular and outer longitudinal layer of muscle fibres lies a vascular layer made of blood and lymph vessels, nerves and connective tissue. During pregnancy the amount of muscle tissue in the uterine wall increases, both cell enlargement and cell number.
Uterine glands develop from the endometrial mucosa, penetrate into the submucosa and become coiled. They secrete material into the lumen of the uterus (arrows).
The endometrium is a highly glandular structure consisting of an epithelial lining of the lumen, a glandular layer and connective tissue.
The epithelial lining of the lumen is of columnar cell type that undergo structural change during pregnancy to facilitate the attachment of the blastocyst. The endometrium is characterized by numerous openings of the endometrial glands.
These are simple branched tubular glands that are more or less coiled, especially towards their ends. The glands possess a lamellar connective tissue and are lined by simple or ciliated columnar epithelium.
These glands are scattered throughout the endometrium with the exception of the caruncles. These uterine glands reach full secretary capacity under the influence of progesterone.
These secretions called uterine milk or historoph are important to the survival and function of the preimplantation embryo.
The function, thickness and the vascularity of the structures of the endometrium are regulated by the changes in ovarian hormones throughout the estrous cycle and pregnancy.
Uterus of Various Animals
Uterus of Cows
The uterus is of bipartite bicornuate type. The horns are joined by the dorsal and ventral intercornual ligaments.
The uterus is located either on the floor of the pelvis, on the pelvic brim or in parous cows mostly on the caudal floor of the abdominal cavity. The uterus is attached dorso-laterally by the broad ligament and the mesometrium.
During pregnancy the uterus enlarges greatly and becomes located further cranially and ventrally on the abdominal cavity.
Internally the inner surface of the uterus in ruminants contains non-glandular convex shaped projections called the caruncles which are arranged in four rows. They are 20 to 120 in number and during pregnancy they increase in size from 15mm to 10cms and appear spongy due to the numerous crypts that receive the placental chorionic villi. These villi of the cotyledons of the placental side invade the caruncles and together they are called as placentomes.
Uterus of Mare
The nonpregnant uterus is cruciform or ‘T’ shaped and the body is about equal in size to each horn.
The uterus is suspended in the pelvic and abdominal cavity dorsal to the bladder by the broad ligaments.
These ligaments are attached dorsally to the sublumbar region. Internally the endometrium of mare have no caruncles but characterized by many endometrial folds.
Uterus of Ewe
The uterus is similar to cow each horn measuring 10 to 12 cms long. Internally there are 88 to 96 caruncles which are concave shaped and the placental villi dip into the caruncles.
Uterus of Sow
The uterine horns of sows are long and tortuous that are freely movable because of the long broad ligaments.
Uterus of Bitch and queen
The uterus is of bicornuate type with the horns that diverge at an acute angle towards the pole of each kidney. Suspended from the sublumbar region by the broad ligaments, the uterus lies entirely in the abdominal cavity.
Functions of the Uterus
- Sperm transport: The contraction of the myometrium is essential for the transport of spermatozoa from the site of ejaculation to the site of fertilization.
- Luteolysis and control of cyclicity: the uterus produces the PGF2α that lysis the corpus luteum and thereby controls the oestrus cyclicity.
- Environment and pre-attachment embryo: uterus (endometrial glands) secrets the uterine milk which nourishes the free living zygote proceding implantation.
- Maternal contribution to the placenta gives attachment to the placenta and plays a role in the nutrition, excretion and exchange of carbondioxide and oxygen.
- Expulsion of the foetus and foetal placenta: at the time of parturition the strong muscular contractions of the uterus helps to expel the foetus and placenta.
