Reporter and Curator: Dr. Sudipta Saha, Ph.D.
With the progress in IVF, the contribution of the Fallopian tube towards successful reproduction has been comparatively overlooked. It is clear from the success of IVF, which of course bypasses tubal transport that exposure to the tubal milieu is not an absolute requisite for fertilization or implantation to occur. Thus, the Fallopian tube is often now thought of as little more than a mere conduit. However, in fertilization in vivo, the Fallopian tube plays an essential role in gamete transport, fertilization and the early development of the embryo. It is becoming increasingly evident that the mechanism of tubal transport is much more complex than first thought and can be affected by a wide range of factors and conditions that may impair fertility. The Fallopian tube plays an essential role in tubal transport of both gametes and embryos and in early embryogenesis. The tube undergoes cyclical changes in morphology and ciliary activity in response to ovarian hormones. Whilst the varying contributions to tubal transport of ciliary activity, muscle contractions and secretory activity remain undetermined, there is emerging evidence that muscle contractions may play a role in mixing of secretions rather than in propulsion of gametes and embryos. Ciliary activity is more vigorous in the secretory phase of the menstrual cycle. Many pathological conditions associated with infertility and ectopic pregnancy have been shown either to destroy cilia or to reduce ciliary motion or both. Gonococcal infection produces both destruction of the ciliated cells and reduced ciliary activity, whereas chlamydia also destroys the tubal mucosa. Although the micro-organism itself does not appear to alter ciliary beat, the inflammation and oedema associated with chlamydial salpingitis has been shown to reduce CBF (ciliary beat frequency). Peritoneal fluid from women with mild-and-moderate endometriosis reduces CBF significantly in vitro. An ‘ovum capture inhibitor’ has been described in the peritoneal fluid of women with endometriosis, which covers the fimbrial cilia resulting in a complete but reversible loss of ovum capture ability. The ‘immotile cilia syndrome’ is known to be associated with subfertility. Deciliation is found in Fallopian tubes of women with a past history of ectopic gestation. These women are at increased risk of future tubal pregnancies. This evidence suggests an important role for the tubal cilia in the mechanism of gamete and embryo transport. Further research needs to be undertaken to investigate the functioning of the cilia in vivo. Only one study has measured physiological CBF in vivo, and this needs to be extended to the effect of pathological states on CBF. Direct examination of the effect of conditions such as endometriosis or pelvic inflammatory disease on ovum transport may be possible in animal models using laparoscopy to investigate ovum pick-up and falloposcopy to study ovum transit along the tube. It is only as we begin to understand more about the complex interactions of the effectors of tubal transport that we approach the possibility of being able to improve tubal transport in women afflicted with tubal infertility.
Source References:
http://humupd.oxfordjournals.org/content/12/4/363.long
PUT IT IN CONTEXT OF CANCER CELL MOVEMENT
The contraction of skeletal muscle is triggered by nerve impulses, which stimulate the release of Ca2+ from the sarcoplasmic reticuluma specialized network of internal membranes, similar to the endoplasmic reticulum, that stores high concentrations of Ca2+ ions. The release of Ca2+ from the sarcoplasmic reticulum increases the concentration of Ca2+ in the cytosol from approximately 10-7 to 10-5 M. The increased Ca2+ concentration signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin (Figure 11.25). Tropomyosin is a fibrous protein that binds lengthwise along the groove of actin filaments. In striated muscle, each tropomyosin molecule is bound to troponin, which is a complex of three polypeptides: troponin C (Ca2+-binding), troponin I (inhibitory), and troponin T (tropomyosin-binding). When the concentration of Ca2+ is low, the complex of the troponins with tropomyosin blocks the interaction of actin and myosin, so the muscle does not contract. At high concentrations, Ca2+ binding to troponin C shifts the position of the complex, relieving this inhibition and allowing contraction to proceed.
Figure 11.25
Association of tropomyosin and troponins with actin filaments. (A) Tropomyosin binds lengthwise along actin filaments and, in striated muscle, is associated with a complex of three troponins: troponin I (TnI), troponin C (TnC), and troponin T (TnT). In (more ) Contractile Assemblies of Actin and Myosin in Nonmuscle Cells
Contractile assemblies of actin and myosin, resembling small-scale versions of muscle fibers, are present also in nonmuscle cells. As in muscle, the actin filaments in these contractile assemblies are interdigitated with bipolar filaments of myosin II, consisting of 15 to 20 myosin II molecules, which produce contraction by sliding the actin filaments relative to one another (Figure 11.26). The actin filaments in contractile bundles in nonmuscle cells are also associated with tropomyosin, which facilitates their interaction with myosin II, probably by competing with filamin for binding sites on actin.
Figure 11.26
Contractile assemblies in nonmuscle cells. Bipolar filaments of myosin II produce contraction by sliding actin filaments in opposite directions. Two examples of contractile assemblies in nonmuscle cells, stress fibers and adhesion belts, were discussed earlier with respect to attachment of the actin cytoskeleton to regions of cell-substrate and cell-cell contacts (see Figures 11.13 and 11.14). The contraction of stress fibers produces tension across the cell, allowing the cell to pull on a substrate (e.g., the extracellular matrix) to which it is anchored. The contraction of adhesion belts alters the shape of epithelial cell sheets: a process that is particularly important during embryonic development, when sheets of epithelial cells fold into structures such as tubes.
The most dramatic example of actin-myosin contraction in nonmuscle cells, however, is provided by cytokinesisthe division of a cell into two following mitosis (Figure 11.27). Toward the end of mitosis in animal cells, a contractile ring consisting of actin filaments and myosin II assembles just underneath the plasma membrane. Its contraction pulls the plasma membrane progressively inward, constricting the center of the cell and pinching it in two. Interestingly, the thickness of the contractile ring remains constant as it contracts, implying that actin filaments disassemble as contraction proceeds. The ring then disperses completely following cell division.
Figure 11.27
Cytokinesis. Following completion of mitosis (nuclear division), a contractile ring consisting of actin filaments and myosin II divides the cell in two.
http://www.ncbi.nlm.nih.gov/books/NBK9961/
This is good. I don’t recall seeing it in the original comment. I am very aware of the actin myosin troponin connection in heart and in skeletal muscle, and I did know about the nonmuscle work. I won’t deal with it now, and I have been working with Aviral now online for 2 hours.
I have had a considerable background from way back in atomic orbital theory, physical chemistry, organic chemistry, and the equilibrium necessary for cations and anions. Despite the calcium role in contraction, I would not discount hypomagnesemia in having a disease role because of the intracellular-extracellular connection. The description you pasted reminds me also of a lecture given a few years ago by the Nobel Laureate that year on the mechanism of cell division.