Three clonal types of keratinocyte with different capacities for multiplication.

Abstract

Colony-forming human epidermal cells are heterogeneous in their capacity for sustained growth. Once a clone has been derived from a single cell, its growth potential can be estimated from the colony types resulting from a single plating, and the clone can be assigned to one of three classes. The holoclone has the greatest reproductive capacity: under standard conditions, fewer than 5% of the colonies formed by the cells of a holoclone abort and terminally differentiate. The paraclone contains exclusively cells with a short replicative lifespan (not more than 15 cell generations), after which they uniformly abort and terminally differentiate. The third type of clone, the meroclone, contains a mixture of cells of different growth potential and is a transitional stage between the holoclone and the paraclone. The incidence of the different clonal types is affected by aging, since cells originating from the epidermis of older donors give rise to a lower proportion of holoclones and a higher proportion of paraclones.The epidermis is a stratified squamous epithelium whose differentiated cells are the progeny of proliferative cells located mainly in the basal cell layer. Although many of the basal cells are capable of multiplication (1), few of them are thought to be self-renewing stem cells (2-4). We have recently shown that the clone-forming ability of a human keratinocyte in culture can be estimated from its size: small keratinocytes give rise to clones with high frequency, larger ones do so with lower frequency, and still larger ones, not at all. But once a colony has formed, its growth potential is not specified by the size of the founding cell (5).We describe here a method of analysis that reveals the growth potential of individual clones. We inoculate a single founding cell, and 7 days later we transfer the progeny, while they are still growing exponentially, to indicator dishes, where they are allowed to grow for a further period of 12 days. According to the growth in the indicator dishes, we can classify the original clone. Holoclones (holo = entire) form large rapidly growing colonies; fewer than 5% of the colonies abort and terminally differentiate. Paraclones (para = beyond) are programmed for limited growth and consequently form uniformly small, terminal colonies on the indicator dishes. Meroclones (mero = partial) form two kinds of colonies on the indicator dishes-growing and terminal. The meroclone therefore contains a proportion of cells that have degraded to paraclone-formers. MATERIALS AND METHODSCell Culture. Human epidermal keratinocytes were cultivated as previously described (6), using lethally irradiated supporting 3T3 cells (7). The epidermal growth factor used to promote multiplication (8) was the cloned human polypeptide kindly provided by Chiron (Emeryville, CA) (9). All experiments were carried out with a single batch of serum tested for its ability to support colony formation. The medium was changed every 4 days. Strains AY and YF 19 were derived from foreskin, strain GMA from thigh,...

Telomerase activity in a specific cell subset co-expressing integrinβ1/EGFR but not p75NGFR/bcl2/integrin β4 in normal human epithelial cells

“…No inhibitory e ect was present in the extract of telomerase-negative cells (see Figure 1B). Since the epithelial cell population, however, constitutes proliferatively heterogeneous cell compartments (Lavker and Sun, 1982;Barrandon and Green, 1987;Jones and Watt, 1993), further analysis was focused on the characteristics of the telomerase-positive and -negative epithelial cell subsets.…”

Attributes of adult stem cells

“…In vitro, stem cells, early TACs and late TACs are thought to generate holoclones, meroclones and paraclones, respectively haematopoietic systems can be broadly equated with such an arrangement. When studied in vitro, colonies formed of tightly packed undifferentiated cells are considered to represent stem cell colonies, named 'holoclones', first described for human epidermal cells [7]. Other colony types, meroclones and paraclones, are considered to represent early and late TACs, with correspondingly reduced replicative capacity.…”

“…The basal layer is contiguous with cells of the outer root sheath that form the outermost layer of hair follicles. Until relatively recently, epidermal stem cell identity was inferred either by the presence of LRCs or by their growth potential in vitro [7]. Of course, neither label retention nor in vitro clonogenicity allow easy isolation of epidermal stem cells, but a number of studies have highlighted that selection of basal keratinocytes with the highest expression of the β1 integrin (integrin bri cells) enriches for cells with a high colony-forming efficiency.…”

Age-Dependent Depletion of Human Skin-Derived Progenitor Cells

“…To our knowledge, evidence for stem/progenitor cell pool depletion in the human body with aging includes peripheral blood stem cells [8,9], satellite cells of skeletal muscle [10], and putative stem cells of the epidermal compartment of the skin [11]. Interestingly, in a retrospective analysis of >6,900 bone marrow transplantations conducted by the National Marrow Donor Program, age was the only donor trait significantly associated with recipient survival [12].…”