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The adult equine hoof is subdivided into distinct segments with various keratinization modes. In the periople and bulbs of the heel, the epidermis forms a Stratum granulosum with basophilic keratohyalin granules during soft keratinization, whereas in the coronet, wall proper, sole, and frog, the epidermis undergoes hard keratinization by keratinizing and comifying without forming keratohyalin granules. The present study tests the hypothesis that the presence of specific (profilaggrin-containing) keratohyalin granules in the hoof epidermis is correlated with the water-binding capacity and mechanical properties of the hoof horn. To identify these specific profilaggrin-containing keratohyalin granules, tissue samples of fetal hooves were studied with histochemistry, immunohistochemistry, and transmission electron microscopy. In a fetal hoof, a Stratum granulosum is formed in all hoof segments in the wake of the establishment of a segment-specific papillary body, but at differing developmental stages, starting in the coronet, then in the wall proper, and later in the sole and frog, and disappearing again in the same sequence. In the terminal part of the wall proper (i.e., Zona alba), the Stratum granu/osum is retained at least until three days after birth. In the periople and bulbs of the heel, the Stratum granulosum appears last (and is retained in the adult) when the other segments have not yet completely lost theirs. The basophilic granules in the Stratum granulosum are specific profilaggrin-containing granules that were also described in the human skin. These observations are relevant for a better understanding of certain dyskeratotic processes in the hoof epidermis.

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