Loricrin immunoreactivity in human skin: localization to specific granules (L-granules) in acrosyringia

Loricrin is a major component of the cornified cell envelope and is also expressed in the granular layer of human epidermis. In newborn mouse epidermis, loricrin accumulates in small round granules (L-granules) in the granular layer before being incorporated into the cornified cell envelope, but the expression of L-granules has not yet been demonstrated in human skin. In the present study we used postembedding immunoelectron microscopy to examine loricrin expression in normal human skin. We observed diffuse loricrin staining in the uppermost granular cell layer in interappendageal epidermis, that was not associated with any granular structures. In the cornified cells, most of the labelling was on the inner face of cornified cell envelopes. By contrast, in the upper segment of acrosyringia there were small granules (L-granules) that specifically labelled for loricrin. In the outer ductal cells, L-granules with a highly osmiophilic electron density were apparent in the nucleus as well as in the cytoplasm. In the inner ductal cells, L-granules were round or oval, less electron-dense and larger than the outer ductal L-granules, and present only in the cytoplasm. Some of the outer ductal L-granules and most of the inner ductal L-granules formed composite granules with filaggrin immunoreactive granules. These results suggest that in human interappendageal epidermis, loricrin might be rapidly incorporated into the cornified cell envelope without prior accumulation in any type of granule. By contrast, in acrosyringia loricrin accumulates in granules, perhaps reflecting increased synthesis or slower processing. The significance of the different morphological features of the L-granules in inner and outer acrosyringial cells remains to be clarified.