Embryonic epithelium

This layer appears to be unique to H. nana. It forms a layer between the oncospheral membrane and the oncosphere (Fig. 7.11). It is apparently formed by the delamination of the epithelial covering of the oncosphere into two layers, separated from each other by membranes - the outer polar filament layer and the inner embryonic epithelium. It has been suggested that the polar filaments in H. nana are reminiscent of the tendrils of the egg cases of elasmobranchs and may serve to delay expulsion of the oncosphere from the mammalian intestine by becoming entangled amongst the intestinal villi or mucous lining of the gut. This may further serve to bring the oncosphere into close contact with the gut wall for successful penetration to take place (204). 

The cyclophyllidean oncosphere is well supplied with musculature and the general pattern of muscles appears to be similar in most species. So in both Hymenolepis citelli (which penetrates an invertebrate gut) and E. granulosus (which penetrates a vertebrate gut) there are 16 somatic muscle cells (146, 839). However, E. granulosus has 16 hook muscle cells but H. citelli has only 13. In E. granulosus, the hook muscles have been shown to insert at the collar and base of the hooks and at the basal lamina of the embryonic epithelium. Each pair of hooks has three muscle systems associated with it (a) a protractor system, for hook extensions (b) an abductor system, which draws the hooks together and (c) a retractor system which pulls the hooks into the body (839). 

Figure 7. Expression pattern of the mouse tyrosinase gene during embryonic development and its recapitulation in transgenic mice as determined by in situ hybridization (Beermann et al., 1992a). Black box, mouse tyrosinase open box, transgene ptrTyrf striped box, ptrTyr5. Interrupted boxes indicate variations between lines. RPE, retinal pigment epithelium e, days of gestation d0.5, newborn.

Radioactivity from " C-labeled chloroform was detected in the placenta and fetuses of mice shortly after inhalation exposure (Danielsson et al. 1986). In early gestation, accumulation of radioactivity was observed in the embryonic neural tissues, while the respiratory epithelium was more involved in chloroform metabolism in the late fetal period. 

The body s cells are normally subject to strict social control. They only divide until they come into contact with neighboring cells cell division then ceases due to contact inhibition. Exceptions to this rule include embryonic cells, cells of the intestinal epithelium (where the cells are constantly being replaced), cells in the bone marrow (where formation of blood cells takes place), and tumor cells. Uncontrolled cell proliferation is an important indicator of the presence of a tumor. While normal cells in cell culture only divide 20-60 times, tumor cells are potentially immortal and are not subject to contact inhibition. 

As regards the function of this electronegative shield in some cell-types, membrane sialic acids prevent aggregation due to electrostatic repulsion in, for example, blood platelets, erythrocytes, and carcinoma cells in culture,418 whereas, in others, for example, chick, embryonic muscle-cells,422 aggregation is facilitated, most probably by Ca24 bridges. Sialic acid was also shown to be involved in the attachment both of endothelium and epithelium to glomerular basement-membranes of rat kidney.423... 

When the embryo arrives in the uterine cavity, the trophectoderm infiltrates the uterine epithelium by a poorly characterized mechanism. Several maternal and embryonic factors are crucial to implantation. These factors include colony stimulating factor, leukaemia inhibitory factor, interleukin- and several proteolytic enzymes. Before the placenta develops, the implanted embryo is nourished by histotrophic material from degradation of endometrial cells during implantation. The embryo is also nourished by secretions from endometrial glands and by the yolk sac the latter persists for different time periods and plays different roles in different species. 

The blastocyst implants into the maternal endometrial wall on about the 7th day of embryonic life. Trophoblastic cells attach to the uterine mucosa by apposition and adhesion. Under the influence of progesterone and estrogen, the uterine lumen closes, which brings the blastocyst into close contact with the endometrium. Adhesion of the trophoblastto the uterine epithelium occurs with increasing apposition and involves cell surface glycoproteins. The uterine epithelium is penetrated by syncytial growths on the trophoblast into the adjacent uterine epithelial cells. Subsequently, the trophoblastic membranes... 

Much has been said about the positive effects of vitamin E (a-tocopherol) on sexual performance and ability in humans. Unfortunately, there is little scientific rationale to substantiate such claims. The primary reasons for attributing a positive role in sexual performance to vitamin E come from experiments on vitamin E deficiency in laboratory animals. In such experiments the principal manifestation of this deficiency is infertility, although the reasons for this condition differ in males and females. In female rats there is no loss in ability to produce apparently healthy ova, nor is there any defect in the placenta or uterus. However, fetal death occurs shortly after the first week of embryonic life, and fetuses are reabsorbed. This situation can be prevented if vitamin E is administered any time up to day 5 or 6 of embryonic life. In the male rat the earliest observable effect of vitamin E deficiency is immobility of spermatozoa, with subsequent degeneration of the germinal epithelium. Secondary sex organs are not altered and sexual vigor is not diminished, but vigor may decrease if the deficiency continues. 

A population of desired cell types that have the potential to produce new tissues should be generated. The potential of embryonic totipotent stem cells could be exploited in the transplantation of retinal pigment epithelium, myocardial progenitor cells capable of restoring cardiac function and contractility, dopaminergic neurons for the treatment of Parkinson s disease, pancreatic cells for the treatment of diabetes, and others.55... 

Gipson I, Surgrue S (1994) Cell biology of the corneal epithelium. In Albert D, Jakobiec F (eds) Principles and Practice of Ophthalmology. WB Saunders, Philadelphia pp 3-16 Saitou M, Fujimoto K, Doi Y et al. (1998) Occludin-deficient embryonic stem cells can differentiate into polarized epithelial cells bearing tight junctions. J Cell Biol 141 397—408... 

Rothermel A, Willbold E, dcGrip WJ, Layer PG. 1997. Pigmented epithelium induces complete retinal reconstitution from dispersed embryonic chick retinae in reaggregation culture. Proc R Soc Lond B Biol Sci 264 1293-1302. 

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