Oviduct or Fallopian Tube (Synonym: Salpinx or Tunnel of Love)
The oviducts are the paired, narrow, tortuous tubes that connect the ovary to the uterus and suspend in the mesosalpinx. It is the site of fertilisation in many species.
Oviducts are about 20-30 cm long and 3 mm thick.
The oviduct divides into three regions, namely the infundibulum, ampulla and isthmus. The infundibulum is the funnel-shaped structure opening adjacent to the ovary. The opening of the infundibulum (ostium tubae abdominale) contains numerous tiny fingers like projections called fimbriae. They are positioned laterally very close to the ovary and increase the surface area of the infundibulum. The gliding movement of the fimbriae over the ovary’s surface helps capture the ovum into the fallopian tube at the time of ovulation. The ovarian bursa is the additional structure that ensures ovum capture into the infundibulum. It is made up of connective tissues and the ligaments adjoining the ovary. It is not part of the oviduct, instead developed from mesovarium, mesosalpinx, fimbria and infundibulum regions. It is prominent in rodent and sow and poorly developed in cow and ewe. The infundibulum directly terminates at the ampulla, which constitutes about one-half of the oviductal length and merges with the isthmus of the oviduct. The junction between ampulla and isthmus is called the ampullary-isthmus junction, the site of fertilisation in most mammals. The ampulla is highly convoluted; mucosal cells develop profoundly and feel soft in touch. The diameter of the isthmus is smaller than the ampulla with well-developed muscular layer. The opening of the isthmus into the uterus is called ostium tubae uterinum. The junction between the uterus and the isthmus is called the utero-tubal junction. In cows, the utero-tubal junction forms a kink, thus blocking the movement of embryos. The kink straightens with decreasing cortisol levels and allows the embryo to enter the uterus. In pigs, the utero-tubal junction is guarded by a finger-like mucosal process and in cats as a barrier to sperm entry and prevents polyspermy.21.3.1 Histological Features
The oviduct wall has three major layers: mucosa, muscularis and outer serous coat. The oviductal mucosa is made of primary, secondary and tertiary folds, which are highest at the infundibulum and gradually decrease towards the isthmus. The mucosal epithelium contains a single layer of non-ciliated secretory and ciliated columnar epithelial cells together with ‘peg cells’ (depleted secretary cells), more abundant at the ampulla and isthmus. The number and activities of the cells vary during different phases of the reproductive cycle. The proportion of ciliated cells is highest at the fimbriae and declines gradually towards the isthmus. The second muscular layer is composed of inner circular and outer longitudinal smooth muscles. Oviductal muscularis thickness increases from the ovarian to the uterine end. These muscles facilitate peristaltic movement of the oviduct and help in gamete transport. The serosa consists of connective tissues, and an external coat originated from the peritoneum. The oviductal musculature at the isthmus is innervated by adrenergic nerves, making the isthmus act like a sphincter to regulate egg transport.
21.3.2 Functions
The oviduct captures and transports the ovum and spermatozoa to the site of fertilisation in opposite directions almost simultaneously. The oviduct also provides a favourable environment for sperm capacitation, fertilisation and cleavage of the embryo. The oviduct transports the early embryo to the uterus, and it gives a protective and nourishing environment to the sperm, oocyte and early embryo.
21.4