Histomorphological and ultrastructural analysis of vibrissal follicle-sinus complexes in the dog (Canis lupus familiaris).

Trimming of vibrissae in dogs (Canis lupus familiaris) for cosmetic reasons remains a common practice, particularly among poodle breeds and show dogs. In discussions surrounding this practice, it has been claimed that domestic dogs would have fewer and structurally less developed sinus hairs than other mammals, and that, especially in poodles, vibrissae would lack functional relevance, although such narratives represent popularized assumptions rather than systematically supported peer-reviewed evidence. The follicle-sinus complexes (FSCs) of various species, including cats (Felis catus), rats (Rattus norvegicus), and marine mammals, have been characterized in detail with respect to their innervation and mechanoreceptor organization. However, comparable histomorphological analyses of canine FSCs have not yet been conducted. Providing an in-depth morphological characterization of canine FSCs, the present study examined facial skin samples from 44 dogs of different ages, breeds (including poodles), and sexes, with assessment of FSC number and size. Regardless of breed, an average of 60 ± 13 FSCs per dog was identified in the present study (with a regional distribution of 28 ± 7 mystacial, 9 ± 2 supraorbital, 16 ± 7 mandibular, and 7 ± 2 buccal FSCs). Mystacial FSCs were the longest, measuring 4.0 ± 1.1 mm in length and 1.7 ± 0.6 mm in width. FSC length and width were significantly positively associated with body weight across all investigated facial regions. FSCs were characterized using histological, immunohistochemical (PGP 9.5, NF200, S100, GFAP), ultrastructural, and quantitative-morphological analyses. Canine FSCs demonstrate a complex mechanosensory architecture that includes Merkel cell-neurite complexes, lanceolate endings, reticular endings, and presumptive free nerve endings located in specific regions of the FSC. Quantitative analyses showed that neuronal tissue accounted for 13.6% ± 3.3% (vol.) of the internal FSC compartments. No evidence was found of a species- or breed-specific reduction in the structural or sensory capacity of canine FSCs.