At birth, the hoof wall is covered and protected by soft horn (perionychium), which is rapidly worn away as a foal moves within its new environment. The hard outer pigmented part of the foot is gradually exposed to provide support for the newborn foal. Support tissues of the internal hoof wall include the stratum medium and stratum internum, with the latter consisting of the PEL and SEL around the wall perimeter.1–5,a The PEL has long been thought to form a homogeneous population of partially keratinized sheets on the inner surface of the hoof.1,2,4–6,a,b In adult horses, this inner hoof of the stratum internum has approximately 600 PEL distributed as single sheets around the perimeter of the hoof wall with an occasional branched one being interspersed among the laminae.1,5,a Each PEL has approximately 20 to 150 SEL around its surface.1,5,a,b These PEL enmeshed within the highly vascularized corium are joined to the distal phalanx to suspend the bone within the hoof capsule.1,2,4–6,a,b Microscopically, the commonality of shapes and sizes of the PEL and relative numbers of PEL among horses reinforces this notion of homogeneity of the structure of the stratum internum. Once formed at the coronet, the laminar structures move distally along the proximodistal length of the hoof wall.1–3,5,6–8 Although early morphologic evidence has suggested a rather more static structure of the hoof wall laminae during growth, more detailed findings suggest the development of greater architectural changes9,10,b as the overall length of the PEL increases,9,b as does the volume of SEL and associated dermal laminae in a proximodistal direction along the hoof wall dorsum.9
Interestingly, it is evident externally that the hoof wall lacks consistency of structural features from 1 horse to another because the feet within most populations vary greatly in size, shape, and conformation. The varying conformations can range from a rounded and symmetric foot to an asymmetric foot having an oval or elongated shape with a long toe and low heel, to an underrun heel, or to one with an upright hoof wall with high heels.1,11 In addition, a wide range of medial to lateral foot imbalances can be present, creating an asymmetric foot in each of these conformational conditions.12,13 The apparent disparity between the variations of external hoof conformations and the relative constancy of the inner hoof morphology of epidermal laminae and how they relate to each other are not known.
To identify potential factors capable of influencing or changing hoof wall structure is difficult because it is virtually impossible to isolate only a single variable capable of influencing the hoof, especially during its growth. Nutrition, foot disease, and certain husbandry practices, such as an adequate exercise program versus stall confinement, frequent or infrequent farrier care, and variable ground surface conditions, may potentially influence hoof structure. The birth of a foal may represent another situation in which one may be able to examine the hoof wall structure and determine whether certain factors, particularly external environmental influences, may affect the hoof wall morphology as the hoof wall passes from a non–weight-bearing state to a weight-bearing state within moments after birth. At birth, many organ systems undergo specific transformations, which enable the newborn to survive the transition from an in utero environment to a hostile external environment. In a foal, another requisite transition occurs as the precocious newborn rises to its feet and begins to support its weight to extricate itself from potential predators. The rapid transition of the feet from a non–weight-bearing state to full weight support shortly after birth indicates that the musculoskeletal system and the locomotor spinal circuitry are primed and ready to respond to these new environmental conditions. Although we assume that the hoof and foot tissues of a newborn foal do respond to these new weight-bearing conditions, we do not know what these potential changes might be. Thus, the purpose of the study reported here was to examine the inner hoof wall (stratum internum) of the fetus for evidence of structural alterations of the epidermal laminae during the transition period from fetus to newborn foal and compare morphologic changes to the structure of an older foal and yearling.
Primary epidermal laminae
Secondary epidermal laminae
Leach DH. Structure and function of the equine hoof wall. PhD thesis, Department of Veterinary Anatomy, University of Saskatchewan, Saskatchewan, SK, Canada, 1980.
Linford RL. A radiographic, morphometric, histological and ultrastructural investigation of lamellar function, abnormality and the associated radiographic findings for sound and foot sore Thoroughbreds and horses with experimentally induced traumatic and alimentary laminitis. PhD thesis, Department of Comparative Pathology, University of California, Davis, Calif, 1987.
Kunsien L. Uber die Entwicklung des Hornhufes bei eingen Ungulaten. Diss, Dorpat University, Tartu, Estonia, 1882.
Kainer RA. Functional anatomy of equine locomotor organs. In:Stashak TS, ed.Adams' lameness in horses. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2002;1–14.
Schummer A, Witkens H & Wollmerhaus B, et al. The circulatory system: the skin and the cutaneous organs of the domestic mammals. Berlin: Verlag Paul Parey, 1981;541–557.
Pollitt CC. Clinical anatomy and physiology of the normal equine foot. In:Rose RJ, ed.Equine lameness and foot conditions. Sidney, Australia: Massey University Press, 1990;117–217.
Stump JE. Anatomy of the normal equine foot including microscopic features of the laminar region. J Am Vet Med Assoc 1967;151:1588–1597.
Leach DH. Structural changes in intercellular junctions during keratinization of the stratum medium of the equine hoof wall. Acta Anat (Basel) 1993;147:45–55.
Budras KD, Hullinger RL, Sack WO. Light and electron microscopy of keratinization in laminar epidermis of the equine hoof with reference to laminitis. Am J Vet Res 1989;50:1150–1160.
Sarratt SM, Hood DM. Evaluation of architectural changes along the proximal to distal regions of the distal laminar interface in the equine hoof. Am J Vet Res 2005;66:277–283.
Bowker RM. The growth and adaptive capabilities of the hoof wall and sole: functional changes in response to stress, in Proceedings. 49th Annu Conv Am Assoc Equine Pract 2003;103–115.
Stashak TS, Hill C. Conformation and movement. In:Stashak TS, ed.Adams' lameness in horses. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2002;73–111.
Hood DM, Herndon K & Wilkerson K, et al. Conformational symmetry of the equine digit, in Proceedings. 10th Annu Meet Assoc Equine Sports Med 1992;65–67.
Balch OK, Butler D, Collier MA. Balancing the normal foot: hoof preparations, shoe fit and shoe modifications in the performance horse. Equine Vet Educ 1997;9:143–154.
Sheehan DC, Hrapchak BB. Processing of tissue. In:Theory and practice of histotechnology. 2nd ed. Columbus, Ohio: Battele Press, 1980;59–78.
Humason GL. Clearing, infiltrating and embedding: paraffin method. In:Animal tissue techniques. San Francisco: WH Freeman & Co, 1979;37–47.
Wattle O. Cytokeratins of the equine hoof wall, chestnut and skin: bio- and immunohisto-chemistry. Equine Vet J Suppl 1998;26:66–80.
Bragulla H, Budras KD, Reilly JD. Fetal development of the white line (zona alba) of the equine hoof. Equine Vet J Suppl 1998;26:22–26.
Yates RA, Nanney LB & Gates RE, et al. Epidermal growth factor and related growth factors. Int J Dermatol 1991;30:687–694.
Hood DM, Taylor D, Wagner IP. Effects of ground surface deformability, trimming, and shoeing on quasistatic hoof loading patterns in horses. Am J Vet Res 2001;62:895–900.
Douglas J, Thomason JJ. Shape, orientation and spacing of the primary epidermal laminae in the hooves of neonatal and adult horses (Equus caballus). Cells Tissues Organs 2000;166:304–318.
Barry E. Investigation of the vertical hoof force distribution in the equine forelimb with an instrumented horse boot. Equine Vet J Suppl 1990;9:35–38.
Colahan P, Leach D, Muir G. Center of pressure location of the hoof with and without hoof wedges. Equine Exerc Physiol 1991;3:113–119.
Riemersma DJ, Van der Bogert AJ & Jansen MO, et al. Tendon strain in the forelimbs as a function of gait and ground characteristics and in vitro limb loading in ponies. Equine Vet J 1996;28:133–136.
Thomason JJ, McClinchey HL & Faramarzi B, et al. Mechanical behavior and quantitative morphology of the equine laminar junction. Anat Rec A Disco Mol Cell Evol Biol 2005;283:366–379.