The ultrastructure of the interfollicular epidermis of the hairless (hr/hr) mouse. I. Basal and granular layer

Morphometric evaluation of dark and clear epidermal basal cells during early 2-stage chemical skin carcinogenesis in the hairless mouse using two different fixation methods

Ultrastructural morphometric characteristics of basal keratinocytes in hairless mouse epidermis were analyzed statistically. The following variables were assessed: (i) low versus physiological osmolality during fixation, (ii) alterations induced by a 2-stage carcinogenesis regimen using DMBA and TPA, (iii) criteria for a cell being dark versus being clear, (iv) inter-observer variation. The results show that with low fixation osmolality most basal cells swell and become electron lucent. The few cells which apparently do not swell stand out as shrunken electron dense dark cells. Morphometrically they are more differentiated than clear cells, but do share many features with the basal cell type which appears after fixation in a buffer of physiological osmolality. Iso-osmolality during fixation seems to induce a homogeneous basal cell population of relatively electron dense cells without typical dark and clear elements. Treatment with DMBA/TPA induces not only intercellular edema and reduced desmosomal contacts, but causes injury to the plasma membrane leading to hydropic changes in the cells. This general intra- and intercellular DMBA/TPA induced hydration might induce secondary compression of some of the cells, leading to an increased number of compressed dark cells. It is, however, only after fixation in low buffer osmolality that these effects of DMBA/TPA are statistically significant and clearly observable. The inter-person variation was, apart from a few instances, either not statistically significant or did not interfere with the other effects. We did not find clear arguments in favor of the view that dark cells are primitive epidermal stem cells. They seem only to reflect non-specific toxic effects of tumor promoters, which appear only under certain fixation conditions, that have been used by most authors. The results suggest that dark and clear cells are mainly a consequence of the degree of cellular hydration.

The morphology of the denuded epidermal basal cell layer of the hairless mouse after different preparation methods. A scanning and transmission electron microscopical study

The denuded basal cell layer of the hairless mouse epidermis is described in the present scanning (SEM) and transmission electron microscopical (TEM) study. The suprabasal layers were removed mechanically after trypsinization or by extracellular calcium depletion. Trypsinization before removal of the suprabasal cells caused the basal cells to shrink. Characteristic surface plication and hemi-desmosomal attachment to the basement membrane were generally preserved. SEM revealed partly maintained intercellular bridging, whereas by TEM such contacts were absent because half desmosomes were internalized. Total calcium depletion induced more serious damage to the basal cell surface, which was smooth with apparent perforations. However, cell bridges, and occasional desmosomes were present. The cell interior demonstrated important cellular injury. If the calcium deprived explants were allowed to recover in calcium-containing medium, the cells acquired an activated "regenerative" morphology, without junctions, similar to that observed in wound healing. Epidermal non-keratinocytes were seen only after trypsinization. Control experiments revealed that they adapted poorly to organ culture conditions. By TEM, we observed several interesting aspects of the differences, between dark and clear basal keratinocytes. This was unexpected because fixation studies had shown, that with the present fixation method, typical dark and clear cells do not occur in untreated epidermis. We believe that membrane injury through mechanical stripping of partly adhering epidermal layers induced "clear cells", whereby the neighboring cells appeared darker. This provides additional evidence as to the origin of the two sub-populations, dark and clear basal cells. The clear cells may be injured cells, caused by cell damage, and not by processes of cellular differentiation. The results of the present investigation supports the view that basal keratinocytes have a polygonal shape with numerous free surface extensions and they are anchored to the basement membrane with "foot pads". Our study also shows that SEM of the epidermal basal layer might be feasible. Various artifacts, however, must be considered, depending on the denudation method used. We prefer trypsinization to calcium depletion because it is less time-consuming and results in a cell morphology which in TEM is comparable to that of basal cells in untreated whole epidermis. Extra-cellular calcium depletion, however, might be useful as a method to prepare single cell suspensions for flow cytometry. Restoration of a normal calcium concentration after stripping, provides an opportunity to mimic wound healing in situ, as an alternative t

Some effects of bleomycin on the proliferation, maturation time and protein synthesis of hairless mouse epidermis

Hairless mice were given 2 mg Bleomycin i.p. in 1-0 ml saline on two successive days. By a stathmokinetic method, by micro-flow fluorometry and by autoradiography certain kinetic parameters were measured during 10 days after the last injection. Cell counts were made and the turnover time of the differentiating cells estimated. Protein synthesis was estimated by the uptake of radioactive histidine, and dry cell mass measured by weighing. Bleomycin affected cell proliferation in the epidermis by depressing biphasically both the number of cells in, and the passage of cells through, the cell cycle phases: S, G2 and M, most probably by directly affecting late G1 cells and cells in mitosis. The time between the two minima of depressed DNA synthesis corresponded to the mean generation time of the basal cells. Histidine uptake and dry cell mass were slightly affected, but the turnover time of the differentiating cells was prolonged. Bleomycin thus had a strong long-lasting inhibitory effect on epidermal cell proliferation and a marked inhibitory effect on epidermal cell maturation in mice.