Covering the limb--formation of the integument

Abstract

An organism's outermost covering, the integument, has evolved to fulfil a diverse range of functions. Skin provides a physical barrier, an environment for immunological surveillance, and also performs a range of sensory, thermoregulatory and biosynthetic functions. Examination of the skin of limb digits reveals a range of skin types including the thickened hairless epidermis of the toe pads (palmar or plantar epidermis) and thinner epidermis between the hair follicles (interfollicular epidermis) of hairy skin. An important developmental function of skin is to give rise to a diverse group of appendages including hair follicles, with associated sebaceous glands (or feathers and scales in chick), eccrine sweat glands and the nail. A key question is how does this morphological variety arise from the single-layered epithelium covering embryonic limb buds? This review will attempt to address this question by linking the extensive morphological/anatomical data on maturation of epidermis and its appendages with (1) current research into the range, plasticity and location of the putative epidermal stems cells; (2) molecular/microenvironmental regulation of epidermal stem cell lineages and lineage choice; and (3) regulation of the differentiation pathways, focusing on differentiation of the interfollicular epidermis.

Fig. 1

Section of mouse toe showing a wide range of skin types and variety of appendages. These include thickened hair-less epidermis of the toe pad (palmar or plantar epidermis, pp), interfollicular epidermis (if), dermis (de), hair follicles (hf), sebaceous glands (sg), eccrine glands (ec) and the nail. The most prominent appendage is the nail, which consists of the nail plate (np), nail bed (nb), nail matrix (nm) and the eponychium (ep). (Haematoxylin and eosin stained).

Fig. 2

Diagrammatic representation of adult skin. The basement membrane separates the dermis (primarily consisting of fibroblasts, elastin and collagen) and the overlying epidermis. The epidermis comprises four distinct layers, basal, spinous, granular and the stratum corneum, all arising from basal stem cells. Cellular integrity and communication is maintained via a variety of intercellular junctions. These comprise tight junctions (1), adherens junctions (2), desmosomes (3) and gap junctions (4), of which desmosomes are the most abundant while tight junctions are confined to the second granular layer. The stratum corneum comprises anucleate keratinocytes which have undergone terminal differentiation. This results in the production of an insoluble protein cellular matrix (the cornified envelope), surrounded by a lipid capsule embedded within a network of non-polar lipid. The cornified envelope comprises numerous proteins including involucrin, envoplakin, periplakin, filaggrin, loricrin, SPRRs and LEPs (see inset, reviewed in Kalinin et al. 2001).

Fig. 3

Epidermal development. (A) Epidermis is derived from single-layered embryonic ectoderm, which undergoes stratification to produce a transitory embryonic cell layer known as the periderm (B). Further stratification of the ectoderm produces an intermediate ectodermal layer (C), the stratum intermedium. Cells of the intermediate layer enter early terminal differentiation (D,E) giving rise to both the spinous and the granular cell layers of postnatal epidermis (F). Late terminal differentiation is marked by the stratum corneum. Developmental times are included of mouse, chick and human. E, embryonic days post conception; Dpi, embryonic days post incubation. Histological sections stained with toludine blue.

Fig. 4

Models for follicle induction. (A) Follicle spacing may be determined by the ‘reaction-diffusion’ principle (Nagorcka & Mooney, 1985; see text). (i) Pro-follicular molecules (e.g. FGFs, Eda – red) localize to the placode and induce follicular fate. Also concentrated at the placode are more diffusible molecules which act to inhibit follicle formation (e.g. BMPs – blue). (ii) Action of follicle activators and inhibitors produces a zone of interfollicular epidermis where follicles are unable to form and results in the characteristic follicle spacing of hexagonal arrays (iii; see Zhou et al. 1995; Headon & Overbeek, 1999). (B) An alternative model suggests that stem cells are programmed down different lineages, depending on microenvironment. For example, multipotent embryonic ectodermal stem cells, which give rise to interfollicular keratinocytes, will first become interfollicular stem cells followed by basal transit amplifying cells. Interfollicular stem cells arise from follicle stem cells in vivo (solid arrows); however, there is experimental evidence (dashed arrows) that stem cells can interconvert and that lineages are plastic (see text).