A guide to studying human dermal adipocytes in situ.

Dermal white adipose tissue (DWAT) is a main component of human skin, composed of individual lipid-laden mesenchymal cells known as dermal adipocytes (DAs). Besides their well-known role in lipid storage and release, DAs also promote skin immunity, wound healing and hair follicle cycling and are important players in cutaneous neuroendocrinology. The ever-growing insights into DWAT functions, albeit mostly in mice, have invited speculation that it may be involved in multiple skin diseases ranging from fibrosis to alopecia and psoriasis, thus designating human DWAT a clinically relevant, but as yet insufficiently investigated skin compartment. Therefore, this practical, user-friendly guide aims to introduce the techniques available to study human DWAT in situ and ex vivo, including immunohistochemistry, immunofluorescence microscopy and analysis via quantitative immunohistomorphometry. Here, we provide information on a collection of stains comprising pre-adipocyte (Pref1) and mature adipocyte markers (Perilipin1, Caveolin1), as well as various lipid (OilRedO, BODIPY) and histochemical stains (H&E, trichrome) available for use on human DWAT. We offer the reader guidelines on fixing, processing and staining human DAs and highlight caveats and solutions to common problems that one may encounter when studying this fascinating skin compartment. We also suggest standard methods for conducting quantitative immunohistomorphometry on human DWAT and its individual adipocytes to quantify cell size, number, lipid content and fluorescence intensity of adipose-specific markers. Finally, we briefly introduce in situ hybridization, transmission electron microscopy and essentials of magnetic resonance imaging imaging as additional tools for instructively interrogating this largest, but still least-known compartment of human skin.