KIT expression reveals a population of precursor melanocytes in human skin.

Human skin is believed to harbor a reservoir population of precursor melanocytes. It has been difficult to identify these putative cells experimentally, because they lack phenotypic features that define mature melanocytes. We have evaluated expression of the KIT tyrosine kinase receptor, which is critical for melanocyte development, as a possible marker of these cells. Sections of human skin were evaluated with single- and double-immunolabeling techniques. KIT-reactive dendritic cells were identified in the basal layer of the epithelia and were most numerous in the follicular infundibula and the rete ridges. These cells were located on the epithelial side of the basement membrane and lacked expression of cytokeratin and mast cell tryptase. The location of the KIT-reactive cells was distinctly different from that of Langerhans cells (identified with anti-CD1a) or Merkel cells (identified with CAM 5.2). Within the epidermis and upper follicular infundibulum the majority of the KIT-reactive dendritic cells also coexpressed TRP-1, a marker present in differentiated melanocytes. In the deeper follicular regions, the coexpression of TRP-1 in the KIT-reactive cells was absent. Throughout the epidermis and follicle, however, the KIT-reactive cells coexpressed BCL-2, a marker known to be increased in melanocytes. Thus, KIT expression reveals a population of intraepithelial cells that have immunophenotypic characteristics of mature melanocytes within the upper epithelial regions, but lack the differentiated melanocytic phenotype within the deeper follicular regions. We propose that these KIT(+), BCL-2(+), and TRP-1(-) cells constitute a precursor melanocyte reservoir of human skin.

Consequences of chirality in second-order non-linear spectroscopy at surfaces.

Surface second-harmonic generation (SHG) is recognized as a second-order non-linear laser-based spectroscopy offering good surface sensitivity and selectivity. With the goal of developing optical methods to study chiral surfaces, a way to detect dichroic behaviour in SHG in analogy to circular dichroism (CD) spectroscopy has been elucidated. The SHG efficiency from a monolayer of oriented chiral molecules [(R)- or (S)-2,2'-dihydroxy-1,1'-binaphthyl (BN)] has a strong dependence upon whether the excitation beam has left- or right-handed circular polarization. The relative differential SHG signal is much larger than in ordinary CD spectroscopy. The large chiroptical effects in SHG are attributed not to R, the rotational strength responsible for traditional CD (R = Im mu.m), but to the electric dipole-allowed second-order non-linear tensor, chi (2). There is a chiral element of chi (2), chi (xyz), that must be included in descriptions of the SHG response from chiral surfaces. The ability to measure chiral properties of an adsorbed molecule separately, with no background from its achiral properties, provides another form of chiral spectroscopy, different from CD and having no analogue in linear spectroscopy, but related in a well understood manner.