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IV./1.4.: Microscopical anatomy of the Fallopian tube, and histological changes due to the menstrual cycle
IV./1.4.1.: General features
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The Fallopian tube (oviduct) is a muscular tube, with a lateral free end opening into the abdominal cavity near the ovary, and another end opening into the uterine cavity having penetrated its wall. The wall of the oviduct is trilaminar: the innermost, middle and outer layers being the tunica mucosa, tunica muscularis, and peritoneal covering, respectively.
IV./1.4.2.: Tunica mucosa
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The mucosa is arranged in longitudinal folds, along the entire length of the oviduct. In the distended middle (ampullar) part the folds are tall with multiple branches, forming a labyrinthine meshwork of cavities. The folds tend to become smaller and fewer in number toward the uterine end, as a result of which the labyrinthine structure disappears. The mucosa is covered by simple columnar epithelium with two cell types. The bulk of epithelial cells are columnar and ciliated, and the deflection of cilia is directed toward the uterine cavity. Progressing from the ampulla to the uterus, the number of ciliated cells is decreased, and gradually replaced by another, secretory type of cell, getting more abundant when moving in the direction of the uterus.
The epithelial cells undergo hormone-dependent changes parallelly with the ovarian cycle. They are tallest at the time of ovulation. Formation of cilia is enhanced by estrogens, whereas progesteron promotes secretory activity of the cells. Prior to menstruation, the height of the epithelium is decreased by half, causing the luminal end of secretory epithelial cells to protrude over the surface. At the abdominal end of oviduct and on the tip of fimbriae, the simple columnar epithelial lining of Fallopian tube continues, with a sharp border, into the external peritoneal covering. Similar to that of endometrium, tunica propria of the mucosa of oviduct consists of cell-rich connective tissue, with sizeable amounts of amorphous intercellular substance and reticulin fibers.
IV./1.4.3.: Tunica muscularis
This part is composed of inner and outer layers of myocytes of circular and longitudinal arrangement, respectively. The inner circular layer is more prominent than the outer layer, already in the ampullar part, and its thickness is further increased as one moves toward the uterus. The outer longitudinal muscle layer forms bundles enmeshed with connective tissue fibers. Such reticular arrangement of the layer is loose in the ampullar part but it gets ever more compact in the direction of the uterus, finally merging into its outer muscular layer.
IV./1.4.4.: Tunica subserosa, serosa
Excepting the line of attachment of the mesosalpinx (entry or exit site for blood and lymph vessels and nerves), the outer layer of the oviduct is formed by mesothelial cells of the peritoneum (tunica serosa). The thin connective tissue layer of subserosa continues without sharp border in that connctive tissue which separates the bundles of the longitudinal muscle layer.
IV./1.4.5.: Function and physiology
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The egg (ovum) is driven by ciliary motion and the resulting stream of fluid toward the ampulla and then to the uterus. Passage of the egg is considerably assisted also by peristalsy of the muscular layer of the oviduct. Should the zygote fail to reach the uterine cavity, it might become embedded, irregularly, in the mucosa of the Fallopian tube, which undergoes cyclic changes similarly to those observed in the uterine mucosa. In such cases, the tunica propria displays decidual transformation, similarly to the situation found in the uterine mucosa.
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