Terminal differentiation of cultured human epidermal cells.

Three aspects of terminal differentiation of the epidermal keratinocyte have been studied in cell culture—the development of detergent-insoluble cytoplasmic filaments, the formation of a cornified cell envelope and the destruction of the cell nucleus.In the presence of lethally irradiated 3T3 cells, single human epidermal keratinocytes grow into stratified colonies. After the colonies become confluent, the culture enters a steady state in which the upper cells are shed from the surface of the cell layer like stratum corneum cells in vivo and are replaced by the proliferation of dividing cells in the basal layer. The cells shed into the medium are flattened and elongated squames, and are insoluble in solutions of sodium dodecylsulfate. Since the squames usually detach before their nuclei are digested, the cultures behave like some wet-surfaced, stratified squamous epithelia in that they possess little or no anucleate stratum corneum. The rates of proliferation and squame detachment in confluent cultures are increased by the presence of epidermal growth factor.Most of the squames harvested from the medium are permeable to trypan blue. The permeable squames may or may not have a visible nucleus, but squames not permeable to trypan blue nearly always possess a nucleus. When freshly detached squames containing nuclei are incubated in medium containing serum, their nuclei are digested and disappear within a few days. On the other hand, if the squames are washed and incubated in serum-free medium, their nuclei are not digested. This suggests that the permeable cell membrane permits a serum component essential for nuclear digestion to enter the cytoplasm.When growing colonies of epidermal keratinocytes are disaggregated and the cells suspended in medium containing methyl cellulose, they cannot multiply, but within a few days the cells become permeable to trypan blue and insoluble in sodium dodecylsulfate. This insolubility is due to disulfide linking of the proteins of the abundant cytoplasmic filaments, for the filaments are dissolved when β-mercaptoethanol is added as well, leaving the emptied cornified cell envelopes. Nuclear digestion follows some days later. In the absence of serum, cells become permeable and develop detergent-insoluble filaments and a cornified envelope, but, as in the case of spontaneously detached squames of surface cultures, their nuclei are not destroyed. Purified plasminogen supports nuclear destruction, whereas serum depleted of plasminogen does not.Earlier studies on intact skin have suggested that chemical gradients between epidermis and dermis might be responsible for the differentiation of the epidermal cells. In surface culture, basal cells multiply and nonbasal cells undergo terminal differentiation, even though all the cells are bathed in the same medium and the terminally differentiating cells have, if anything, better access to the medium than do the basal cells. Differentiation also begins in virtually all singly suspended cells uniformly exposed to the medium. The program of differentiation is therefore independent of the orientation of any chemical gradients in the cellular environment. Cell-cell contacts are not required for the development of detergent insolubility, the formation of the cornified envelope or the process of nuclear digestion, although they are essential for the formation of flattened squames. Unlike proliferation, which is strongly dependent upon fibroblast products, terminal differentiation proceeds in the absence of fibroblast support.