Epidermis cell division

FIG. 3. Stopping the cell division cycles. In Drosophila embryos, the timely arrest of cell proliferation in the epidermis requires the transcriptional activation of dacapo (Lane et al 1996, de Nooij et al 1996) and fi y-related (Sigrist Lehner 1997) which occurs during the final division cycle in parallel to down-regulation of cyclin E expression. 

Dyson 1999). Oogenesis in the absence of RBF1 appears to be normal as does the embryonic proliferation. In the case of the embryonic epidermis, cells are also known to exit from the cell cycle at the appropriate developmental stage. But some cells have problems maintaining quiescence. They turn on cyclin E expression, they progress through another S phase, but they do not seem to complete this extra cell cycle with a division. 

Thus the balance between external and internal loading appears to play an important role in skin metabolism and healing. Tension applied to the epidermis appears to lead to cell division (hyperplasia) whereas compression of the dermis leads to scar tissue resorption. These observations appear to fit the general model that external tension promotes tissue augmentation and external compression leads to tissue destruction (catabolism). 

Lorber M, Milobsky SA. A straining of the skin in vivo. A method of influencing cell division and migration in the rat epidermis. / Invest Dermatol. 1968 51 395—402. 

Tuber structure in cross section from exterior to interior can be separated into the epidermis, cortex, outer medulla, inner medulla, and pith (Mazza, 1985). Relatively little is known about the temporal sequence of cell division and differentiation leading up to bulking. Sink capacity is a function of the combined vacuolar volume of the tubers, the location of fructan synthesis and storage within the cells (Darwen and John, 1989 Keller et al., 1988 Pollock, 1986). Vacuolar volume is a function of cell size and number. The size of individual cells within the tuber varies with tissue type cortex (286 cells per 10 mm2), extension zone (145 cells per 10 mm2), storage tissue (85 cells per 10 mm2), and pith (149 cells per 10 mm2) (Schubert and Feuerle, 1997). The extent that cell number and size increases after the initial formation of the tuber has not been adequately documented. 

The stratum basale is the deepest layer of the epidermis and is composed mainly of keratinocytes with melanocytes making up approximately 10% of the cell population. The stratum basale is one cell layer thick and is a layer of rapid cell division where keratinocytes are rapidly dividing and giving rise to the uppermost layers of the epidermis. 

The post-peel mask cream contains retinol microencapsulated in a cyclodextrin. Vitamin A is involved in the processes of cell division and differentiation that help the epidermis regenerate after the peel from the cells of the basal layer. 

Site-of-contact tissues might be preferable for such compounds (Burlinson 1989 Furihata et al. 1984 Furihata and Matsushima 1987 Mori et al. 1999 Sawyer et al. 1988), if sufficiently validated. The main technical limitation of this assay with respect to other tissues is the need to isolate the cells after in vivo treatment and to get them to incorporate tritiated thymidine in vitro. Because UDS measurement does not require cell division, it can potentially be applied to many different tissues, provided that the cells can be isolated and maintained in primary culture for the few hours required for tritiated thymidine incorporation. The literature contains reports of UDS-based studies of stomach, colon, kidney, pancreas, tracheal epithelium, nasal epithelium, epidermis, keratinocytes, and spermatocytes (Burlinson 1989 Furihata et al. 1984 Furihata and Matsushima 1987 Sawyer et al. 1988 Loury et al. 1987 Mori et al. 1999 Latt et al. 1981 Helleday 2003). 

Malpighian layer (stratum germina-tivum) The iimermost layer of the epidermis of mammalian skin, separated from the underlying dermis by a fibrous basement membrane. It is only in this layer of the epidermis that active cell division ( mitosis) occurs. As the cells produced by these divisions age and mature, they migrate upwards through the layers of the epidermis to replace the cells being continuously worn away at the surface. 

Inner bark Periderm Cortex Periderm cells form when epidermis ruptures due to phloem cells division... 

Gillmor et al. 2002). The embryos of nonconditional AtCESAl mutants are radially swollen in appearance, indicating decreased elongation even at early stages. Although the pattern of cell division appears relatively normal, incompletely formed cell walls are observed frequently (Beeckman et al. 2002). The epidermis of the mutants is markedly affected with an apparent complete loss of guard cells and pavement cell crenulation (Beeckman et al. 2002). 

---