Cell renewal in the oral mucosa and skin of the rat. II. DNA conservation and reutilization during the renewal cycle in vivo and in tissue explants

Nuclear morphology of transitional keratinocytes in normal human epidermis

Transitional keratinocytes (TK) of normal interfollicular human epidermis were studied by electron microscopy to provide more information on the ultrastructural nuclear and nucleolar changes during physiological keratinization. The results revealed that TK contained condensed nuclei in the initial stage, degraded nuclei in the advanced stage, and nuclear remnants in the late stage of their transformation into cornified cells. The condensed nuclei were essentially characterized by ultrastructural changes generally considered to reflect the cessation of RNA synthesis. The focal increase of density and the structural disintegration observed in the degraded nuclei of TK are apparently characteristic morphological features of degradation of nucleoproteins in these cells. Nuclear remnants contained altered nuclear envelope or similar structures and clumps of dense fibrillar material. Similar clumps of dense fibrils were present in the cytoplasm of both the TK with compact nuclear remnants and the TK with structural disintegration. This finding suggests that some dense fibrillar material of structurally disintegrating nuclei of TK is transported into the cytoplasm of these cells.

Availability time of tritium-labeled DNA precursors in newt eyes following intraperitoneal injection of 3H-thymidine

Following intraperitoneal injection of 3H-thymidine into host newts, iris together with a regenerating lens was transplanted from a donor eye into a lentectomized host eye at frequent intervals for 20 hours and then every 1 or 2 days for 14 days. The eyes were fixed 2 hours and 1 or 2 days after implantation and autoradiographs prepared. Following fixation 2 hours after operation, incorporation of 3H-thymidine into DNA, as evidenced by grain counts over nuclei, fell rapidly for 3.5 hours after injection and was no longer apparent after 4.5 hours. However, almost one-half of the implants were lightly labeled when they remained in the host eyes for 1 or 2 days beginning from 1 to 14 days after isotope injection. When these implanted, regenerating lenses were left in the host eyes for longer periods of time, then a light label was found over nuclei in most of the implants remaining in the eye for 3 to 24 days. When 3H-thymidine was injected from 1 to 3 days after extirpation of both lens and neural retina, before DNA synthesis had been initiated in the pigmented retinal epithelium or iris, there were numerous cases of labeled nuclei among depigmenting cells of the pigmented retinal epithelium which was regenerating a new neural retina from 2 to 25 days after isotope injection. Depigmenting cells of the dorsal iris and regenerating lens were similarly labeled. These results provide evidence for the continued availability of small amounts of tritiated DNA-precursor molecules which can be incorporated in DNA of proliferating cells long after the initial injection of 3H-thymidine.

Evidence for discrete cell kinetic subpopulations in mouse epidermis based on mathematical analysis

Continuous (repeated) labelling studies in mouse epidermis indicate that nearly all cells are labelled after about 100 hr. Percentage labelled mitoses studies ([3H]TdR at 15.00 and 03.00 hours) have a first peak that does not reach 100% and has a half-width of about 10 hr. Small second and third peaks can be detected at about 90 and 180 hr. respectively. The changes with time in the number of labelled cells show a difference dependent on the time of day of [3H]TdR administration. Both curves show an early doubling in labelled cells which then decline, forming a peak of labelled cells. A second peak occurs at about 120 hr. This is followed by a progressive decline with no further peak until values of about 1% labelling are obtained at 340 hr. These experiments have been investigated mathematically. A computer programme has been devized that permits all three types of experiments to be analyzed simultaneously. More importantly, it can analyse situations with a heterogeneity in cell cycle parameters in all proliferative subpopulations. Various models for epidermal cell replacement have been considered. The data as a whole can best be explained if the basal layer contains at least two distinct subpopulations of cells and an exponentially decaying post-mitotic population with a half-life of about 30 hr. The proliferative sub-populations must be characterized by near integer differences in the length of cycle, the precursor (stem) compartment having the longer cycle. An inverse relationship is required for the length of S, i.e. the shortest time for the stem cells. A full range of cell kinetic parameters can be calculated and are tabulated for the most appropriate model system which is one involving three transit proliferating subpopulations.

A long-lived thymidine pool in epithelial stem cells

The labelling index (LI) of the individual basal cell positions of the anterior column of mouse tongue filiform papillae was assessed with time after an injection of [3H]TdR at 12.00 hours (the minimum point in the circadian LI rhythm). An initial doubling of the LI in the stem cell zone due to cell division was followed by a second rise of 14-16% 16 hr after injection and this occurred even in the presence of vincristine. Although the uptake of [3H]TdR and the initial LI doubling were largely prevented by a preceding injection of hydroxyurea, the 14-16% LI rise was still observed. The possible explanations are discussed, the favoured one being that an average of one of the six or seven cells (the stem cell) in each stem cell zone can store [3H]TdR in a long-lived precursor pool for at least 16 hr before being utilized for DNA synthesis. This complements previously published work which suggested that one cell in each stem cell zone may selectively segregate DNA at mitosis.

DNA replication in short-time organ cultures of human epidermis. Inhibition by aphidicolin, and detection of DNA polymerases alpha, beta, and gamma

Human epidermis uncontaminated by fibroblasts was isolated by a suction blister method. DNA synthesis in short-time organ cultures of isolated epidermis was strongly inhibited by aphidicolin, suggesting that DNA polymerase alpha is involved in DNA replication in human epidermis. On the basis of their responses to inhibitors, primer-template requirements, and chromatographic properties, DNA polymerases alpha, beta, and gamma were all identified in epidermal extracts.

Epithelial cell kinetics--A review of methods of study and their application to oral mucosa in health and disease. Part A. Methods for studying cell proliferation and some sources of variation

Despite extensive investigation of cell kinetic parameters in wide range of normal and abnormal tissues and their potential value in understanding the pathogenesis of many disease states, as well as in prognosis and treatment planning, their application to oral epithelia remains limited. In part A the methods most commonly applied to the study of epithelial cell kinetics, i.e., isotope labelling, mitotic arrest and flow cytophotorimetry are reviewed with special emphasis on their technical limitations, sources of variation and inapplicability to humans in vivo. The ability of in vitro methods to reliably reproduce in vivo cell kinetic parameters is also considered. The application of cell kinetic studies to normal and abnormal oral tissues will be discussed in part B.

Membrane alterations during cornification of mammalian squamous epithelia: a freeze-fracture, tracer, and thin-section study

Tight junctions (zonulae occludentes) create a pericellular barrier to the diffusion of large molecules in non-keratinizing mammalian epithelia. However, in cornifying epithelia such as the epidermis, the importance of tight-junctional elements versus secreted intercellular lipid for barrier function is uncertain. In an attempt to resolve this question, we compared membrane structure in the stratum granulosum and stratum corneum of epidermis, esophagus, and vagina of newborn and adult humans and mice under both normal and various experimental conditions. We incubated pieces of epidermis in organ culture and infused tissues with lanthanum or horseradish peroxidase in vivo and in vitro. All were processed for electron microscopy of freeze-fracture replicas or thin sections. Lanthanum seeped outward to the stratum granulosum in all tissues examined--further apical migration was halted by lamellar-body contents in skin. A similar pattern of intercellular lamellar lipid deposition and membrane structure occurred in all epithelia studied. Freeze-fracture replicas of these obstructive regions revealed occasional, incomplete junctional strands (particularly in moist epithelia) and abundant lamellar material, but complete zonulae occludentes were never encountered. A possible relationship between moisture and tight junction formation was further suggested by organ culture experiments during which brief incubations stimulated an increase in the number of junctional strands and diminished numbers of lamellar bodies. We conclude that, in the epithelia studied, the deposition of secreted lamellar body contents forms the barrier to water-soluble tracer loss: tight-junctional elements are either absent or too fragmentary to constitute an effective barrier.

Epidermal cell production rates

Labeling and mitotic index data and estimates for the length of the S and M phases of the cell cycle from the literature have been compared with new data obtained over a 24-hr period from various epidermal sites in mouse. It has been found that values obtained at a single time of the day may give misleading results. All data have been interpreted in terms of cell production rates per epidermal proliferative unit (EPU). The final conclusions after consideration of cell production rates, labeling and mitotic indices, epidermal transit times, and epidermal structure are that dorsal and ear skin have rather similar cell production rates while tail and foot rates are 4 to 7 times higher. A comparison has also been made between the mouse results and the available human data. Hairless mouse dorsum appears from structural and proliferative aspects to be the best model for some regions of human skin. A new model has been proposed for epidermal proliferation based on the EPU. The model suggests a role for the Langerhans cells and suggests that there is a program of sequential cellular maturity in the EPU originating at the level of a central basal stem cell through committed proliferative cells and ending with cell loss from the skin surface.