Basic concepts of the bovine teat canal.

The bovine teat canal is highly specialized in its unique function of preventing both leakage of milk and entry of bacteria and thereby plays a major role in the defence of the udder against mastitis. The teat canal is a longitudinally folded cylinder-shaped body opening, covered with approximately the same type of epithelia as the normal skin and surrounded with a net-like integrated musculoelastic system facilitating its opening and closure. During milking, dead, flattened, enucleated squamae (cellular detritus) are sloughed from the teat canal surface and are continually replaced by inner cells differentiating outwards. The epidermis is characterized by a polarized pattern of epithelial growth and differentiation, with a single layer of proliferating keratinocytes and multiple overlying differentiated layers. Morphologically, the cells transit from the basal layers on the basement membrane of the dermis through stratum corneum before they finally end up as the keratin of the teat canal. The majority of the epidermal protein synthesizing machinery is devoted to making keratin. This is reflected in the fact that keratins are the major structural proteins, constituting up to 85% of a fully differentiated keratinocyte. Epidermal keratin is a 40-70 kDa alpha-helical coiled-coil dimer of the intermediate filament family that, among other marker proteins, characterizes each stage of keratinocyte differentiation. Studies of skin fragility disorders show that the primary role of keratins in epidermal cells is to reinforce them so that they do not lyse upon physical pressure and to provide cells with subtly different properties of resistance and plasticity to equip the epithelial cells for the physical stress of each particular body site. Epithelial cell specialization for function also depends, however, on the lipid composition and organization and on the epidermal architecture. Epidermal architecture depends on epidermal turnover time, which in turn depends on cell number as well as the proliferative condition. Both in vitro and in vivo studies have implicated calcium as a major modulator of epidermal differentiation. Calcium is a factor known to enhance differentiation and promote expression of the differentiation-specific keratin genes. In animals and humans, both topical and systemic retinoids produce acanthosis, hypergranulosis and a relative (but not absolute) decrease in the thickness of the stratum corneum. Despite a high degree of epithelial specialization, we expect a somewhat similar immunological functional importance in the teat canal epithelia as in other stratified squamous keratinized type epithelia.