Uterus
The uterus is the tubular hollow structure that connects the oviduct to the cervix. It is called the organ of pregnancy as the implantation and development of the foetus occur in the uterus.
The uterus consists of a corpus (body), cervix (neck) and two horns (cornua). The mammalian uteri can be classified into four types based on the development of uterine horns, viz. bicornuate, bipartite, simplex and duplex (Fig. 21.3). The bicornuate type uterus is characterised by two well-developed uterine horns and a small uterine body connected with a single cervix. The small uterine body results from the poor fusion of paramesonephric ducts during embryonic development. Bicornuate type uterus is found in pigs, dogs, cats, elephants, whales and dolphins. The uterus of ruminants and mare are characterised by moderate to poorly developed horns with a relatively large uterine body. This type of uterus is called the bipartite uterus. In the uteri of the mare, the uterine horn is shorter with a large uterine body than the ruminants due to a high degree of paramesonephric duct fusion. The simplex type of uterus is seen in primates and humans. A single uterus and cervix characterise it without a uterine horn. Two uterine horns form two different compartments in the duplex uterus that open into two cervical canals. The duplex uterus is of two types. In the first category, two cervical canals terminate into two vaginas. This type of uterus is seen in marsupials (opossum). The males of this species have characteristics forked penis that simultaneously deposits semen in each of the cervical canals during mating.The second category of the duplex uterus is characterised by two cervical canals with a single vaginal canal, as seen in the rabbits, rats, mice and guinea pigs. Generally, the length of the uterine horns is related to litter size. The large uterine horns are characteristic of polytocous species like pigs and bitch to accommodate large litters.
The uterus of sow, mare, bitch and queen has longitudinal folds. The morphometric features of the uterus of different animals have beenFig. 21.3 Types of the uterus in mammals. Figures show four types of the uterus, viz. (a) Bicornuate uterus having two well-developed horns of the uterus, a single body of uterus and cervix; (b) Bipartite uterus comprising of two moderate size uterine horns and single well- developed uterus and cervix; (c) Simplex without horns, a single body of uterus and cervix and (d) Duplex having two horns of the uterus, body of uterus and cervixes
presented in Table 21.1. In artificial insemination, semen is deposited in the body of the uterus. In natural mating, the semen is generally deposited either in the body of the uterus or the cervical canal in dogs, pigs and horses.
21.4.1 Histological Features
The histological features of the uterus reveal a thin outer layer, perimetrium, thick myometrium and inner endometrium. The perimetrium is continued with the suspensory ligaments and supports the uterus to remain in position. The myometrium is made up of a thinner outer longitudinal and thick inner circular layer of smooth muscles. The muscles exhibit hypertrophy and hyperplasia during pregnancy to support the foetus. The endometrium is composed of an epithelial lining of the lumen followed by a glandular layer and connective tissue layer. The epithelium lining of the lumen is columnar and exhibits structural modification under the influence of hormones during different phases of the reproductive cycle. The endometrial glands are simple, branched and tubular, which coiled progressively towards their ends. These glands are lined by ciliated columnar epithelium. In ruminants, some mushroom-shaped fleshy vascular growths are present over the endometrium, called caruncles, where the lobules of the foetal membrane, the cotyledons, are attached.
The mother’s nutrients and the foetus’s metabolites are transported through these caruncles. The secretion of the uterine glands is called uterine milk or histotrophs, which supports the survival of the periimplantation embryo. The secretion of uterine milk is under the influence of progesterone.21.4.2 Functions
The uterus has the following major functions.
Sperm transport—The myometrial contraction helps transport the spermatozoa from the site of ejaculation to the site of fertilisation.
Luteolysis and control of ovarian cyclicity—The uterus produces PGF2α, which helps to regress the corpus luteum and control the ovarian cyclicity.
Nourishment of the peri-implantation embryo—Endometrial glands secrete uterine milk to provide nutrition to free- living zygote before implantation.
Contribution to the placenta—The uterus helps form the placenta and aids in nutrition, excretion and exchange of CO2 and O2.
Expulsion of the foetus and foetal membrane—Strong myometrial contraction at the time of parturition helps expel the foetus and foetal membrane.
21.5