The mysterious human epidermal cell cycle,or an oncogene-induced differentiation checkpoint

Fifteen years ago, we reported that proto-oncogene MYC promoted differentiation of human epidermal stem cells, a finding that was surprising to the MYC and the skin research communities. MYC was one of the first human oncogenes identified, and it had been strongly associated with proliferation. However, it was later shown that MYC could induce apoptosis under low survival conditions. Currently, the notion that MYC promotes epidermal differentiation is widely accepted, but the cell cycle mechanisms that elicit this function remain unresolved. We have recently reported that keratinocytes respond to cell cycle deregulation and DNA damage by triggering terminal differentiation. This mechanism might constitute a homeostatic protection face to cell cycle insults. Here, I discuss recent and not-so-recent evidence suggesting the existence of a largely unexplored oncogene-induced differentiation response (OID) analogous to oncogene-induced apoptosis (OIA) or senescence (OIS). In addition, I propose a model for the role of the cell cycle in skin homeostasis maintenance and for the dual role of MYC in differentiation.     

Structure and maintenance of the epidermis in Friedmaniella sp. (Prolecithophora)

The epidermis of Friedmaniella sp. has been studied using light and electron microscopy. Three main morphological features characterize its cells, namely (1) DNA bodies in the nuclei, (2) an extensive Golgi apparatus with a well-developed system of transport vesicles, (3) clusters of centrioles mainly in the basal cytoplasm and axonemes and rootlets in the middle and apical cell parts. These peculiarities may indicate continuous physiological regeneration within the cell at the level of cell organelles. DNA bodies may prove to be a taxonomic feature distinguishing the Prolecithophora from other turbellarians.     

Systematic and comparative anatomy of Cymbidieae (Orchidaceae)

Cymbideae comprise an assemblage of 28 genera nearly all of which are represented in this study. Their anatomy is relatively homogenous with the exception of Govenia , in which roots lack velamen and pseudobulb vascular bundles lack sclerenchyma, conditions that do not obtain in other genera. Marginal fibre bundles in leaves of Grammatophyllum and Porphyroglottis consist of clusters of thicker-walled, narrower, epidermis-facing fibres as well as thinner-walled, wider, mesophyll-facing fibres. This feature also occurs in some species of Maxillaria . Baculate tilosomes appear in the roots of a majority of genera in Cymbidieae, as they do in species of Maxillaria , confirming DNA analyses showing a close relationship between tribes Cymbidieae and Maxillarieae. Govenia is singled out both on anatomical and molecular grounds as being ill-placed in Cymbidieae. Cladistic analysis produces only a few tentative hypotheses of phylogenetic relationships among the 28 genera, showing that anatomical characters are of limited value in assessing affinities within this tribe. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 139 , 1–27.