P#27  Bax expression in human anagen hair follicle

Silvia Selleri, Cristiano Rumio, Elena Donetti, Elena Boschini, Paolo Castano, Isabella Barajon. Dept of Human Anatomy, Università degli Studi di Milano, Milan, Italy

Keratinocytes of the different layers of the hair follicle and hair shaft (HF/HS) undergo a temporal and spatial complex program of terminal differentiation that leads to keratinization/cell "hardening". As this process has been assimilated to a form of apoptosis, molecules playing a role in apoptotic mechanisms deserve a careful investigation with respect to keratinocyte differentiation. The aim of the present study was to investigate the expression of the pro-apoptotic protein Bax in the different layers of human anagen HF/HS. Bioptic samples from adult females were processed for immunohistochemistry, immunofluorescence and immunogold labelling and observed with confocal laser scanning microscopy and transmission electron microscopy. Bax-positive cells are detectable mostly in the shaft cuticle (SCu) and in the zone of sloughing of the inner root sheath (IRS). In the SCu, Bax immunolabelling has a different subcellular localisation in relation to the differentiation stage. Immunogold TEM reveals that while in undifferentiated cells gold particles are localised close to the plasma membrane on the side where the exo-cuticle will form, in terminally differentiated cells they are present in the cytoplasmic domain corresponding to the endo-cuticle. In the IRS layers, Bax immunoreactivity is virtually absent up to the zone of sloughing i.e. in the region where cornified IRS cells are shed into the pilary canal and the most superficial ORS cells begin to keratinize. At this level sudden and conspicuous appearance of Bax immunolabeling can be observed. Weaker immunoreactivity is also visible in the cortex and medulla. The results of the present study indicate that though terminal differentiation/apoptosis in the various layers of the HF/HS is characterised by common features leading to "dead" cells with a structural role, the molecular mechanisms underlying such process may have layer-specific differences. In particular, changes in subcellular localisation of Bax in SCu cells might be related to cysteine-rich granules asymmetric movements that lead to a compartmentalisation of the cytoplasmic components during the terminal differentiation process.