Cell adhesion molecules in human hair follicle morphogenesis
Kaplan, Elizabeth Danford
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Hair follicle formation in fetal development is characterized morphologically by invagination and elongation into the dermis of an epidermal cell collective in association with a follicle-specific population of mesenchymal cells. The process is thought to involve cell proliferation and migration, mediated by molecules such as NCAM and E-cadherin, which govern intercellular adhesion, and the extracellular matrix molecules tenascin-C (TN-C) and chondroitin sulfate proteoglycan, which affect cell-substrate adhesion. In research presented here, immunohistochemical analysis was used to examine their distribution patterns, along with those of the adhesion modulating molecules ICAM-1, alpha-2 beta-1 integrin, hyaluronan, versican and perlecan, in relation to human hair follicle morphogenesis. Alternative splicing of a central domain of TN-C has differential effects on cell adhesion and is associated with increased cell proliferation, so molecular probes were developed and used with isoform-specific antibodies for molecular and biochemical analyses of TN-C expression in relation to cell proliferation. Initiation of hair follicle morphogenesis was distinguished by discrete placodes of a small TN-C isoform which lacked the alternatively spliced domain, and which were present at the dermal-epidermal interface. The alternatively spliced domain was detected later in follicle development, and was distributed in the follicle epithelium, basement membrane, and the extracellular matrix of the follicle-specific mesenchyme. Northern and Western blot analysis resolved three mRNA transcripts of 6, 7, and 8 kb in length, and three protein isoforms of approximately $\rm220\times10\sp3,\ 250\times10\sp3\ and\ 280-300\times10\sp3$ kD. These showed temporally distinct expression patterns, and initiation was marked by maximal levels of the 6 kb mRNA at the same time as TN-C transcript was detected by in situ hybridization in collections of epidermal cells. Mesenchymal cell expression was not detected until later stages of development.Invagination was characterized by the transient presence of ICAM-1 on follicle epithelial cells, and by the overlapping distribution of TN-C, versican and hyaluronan in the basement membrane and follicle-associated mesenchyme, where cells were enriched in NCAM immunostaining. TN-C expression was associated with proliferation of epithelial and mesenchymal cells at the periphery of the developing appendage, while the core and base of the developing structure formed a non-proliferative zone. The distributions of versican and hyaluronan, although more widespread than that of TN-C, overlapped with TN-C in the proliferative region, and the three molecules were diminished or absent from the mitotically inactive regions. Preliminary observations included here suggest that TN-C and versican interact in vitro and may imparting a macromolecular organization to the follicle-specific extracellular matrix in vivo.