Background The limited characterization of equine skin, eye and hoof epithelial stem cell (ESC) and differentiation markers impedes the investigation of the physiology and pathophysiology of these tissues. Findings This is usually the first statement of the characterization of tissue-specific keratin marker manifestation and the localization of putative epithelial progenitor cell populations, including ESCs (high p63 manifestation with low pp63 levels) and TA cells (high manifestation of both p63 and pp63), in the horse. These results will aid further investigation of epidermal and corneal epithelial biology and GMCSF regenerative therapies in horses. Introduction Several aspects of equine body structure predispose horses to epidermal and corneal injury and disease. The elongation of the distal limb and lateral placement of prominent eyes are associated with frequent and hard to treat skin wounds and corneal ulcers.1-4 The equine digital integumentary accessory organ has evolved to form a hoof tablet that is anatomically and functionally integrated with the musculoskeletal system.5 The hoof capsule allows locomotion across hard surfaces, but is also the single most common source of lameness in horses.6 In particular, laminitis is a common and debilitating disease associated with chronic pain and lameness that frequently necessitates euthanasia. 7 In contrast to the body structure of equine haired skin and cornea, which are comparable to those of other mammalian species, the equine hoof tablet has undergone extensive changes and specialization that is usually unique to the equidae. The hoof tablet is usually lined with 550C600 parallel cornified main epidermal 6202-23-9 lamellae (PELs), each of which has 150C200 secondary epidermal lamellae (SELs), giving an estimated total surface area for lamellar attachment to the interdigitating secondary dermal lamellae (SDLs) and main dermal lamellae (PDLs) of approximately 0.8 m2 per foot (observe Determine S1 in Supporting Information).5,8,9 This epidermal-dermal lamellar attachment and dermal connective tissue suspend the distal phalanx within the hoof capsule.8 Laminitis-associated lamellar lesions include necrosis, inflammation and aberrant proliferation, with marked distortion of both epidermal and dermal components that often progress to biomechanical failure of the suspensory apparatus of the distal phalanx.10,11 The epidermis, SELs and the corneal epithelium are stratified epithelia, consisting of a single cell-thick basal layer that rests on the basement membrane, a variable number of suprabasal cell layers and a superficial cell layer 6202-23-9 that is continually shed (skin, cornea), or mechanically exfoliated (hoof wall and PELs).12,13 In contrast to skin and 6202-23-9 cornea, the interdigitated arrays of inner hoof tablet lamellae (e.g. stratum internum) comprise a single layer of columnar basal cells and a 1-2 cell thick layer of fusiform suprabasal cells that transitions abruptly to the central keratinized axis of each PEL, which abaxially merges with the hard keratinized tissues of the hoof wall (e.g. stratum medium) (Figure S1, B-E). However, in spite of the clinical significance of these vital structures, the basic molecular biology and differentiation of equine epithelial tissues are poorly defined. Some explanation of the anatomical nomenclature for equine hoof capsule structures is warranted due to controversy in the literature over the use of epidermal to describe these structures. The current extant nomenclature, both in anatomy references and the laminitis literature, includes the use of epidermal and dermal as adjectives to describe the interdigitating lamellae of the inner hoof capsule and adjacent underlying corium, respectively.5,8,14 Similarly, epidermal is frequently used in reference to integumentary modifications in other species such as the claw of the dog,15,16 epidermal scales of fish and reptiles,17 epidermal scutes of the turtle shell,18 and the feathers of birds.5,16 In all cases, the intention of this nomenclature is to recognize the evolutionary and developmental origin of both skin and the adnexal structures from a common fetal epidermis, which is itself derived from the embryonic surface ectoderm,19,20 and to clearly.