Chapter 7: Integument
Histology of the Digit
The anatomy of the digits of domestic species are varied, and some species, such as horses and ruminants, have highly specialized digits worth additional consideration.
In carnivores, the ventrum of the digit is called the digital pad. The digital pad is the toughest and thickest skin of the body. The digital cushion is an accumulation of abundant fibroadipose tissue (similar to the subcutis) that is also rich in sensory nerves and glands.
The claw (nail) of carnivores is composed of specialized epidermis and dermis that are firmly affixed to the underlying third phalanx (P3). Growth of the claw occurs outward from the epidermis adjacent to the nail fold. The externally apparent outer layer of the claw is composed of abundant layers of compact stratum corneum (anuclear cornified keratinocytes). This stratum corneum is the portion of the claw that is “trimmed”. Beneath the epidermis is the highly vascular and well-innervated dermis, or “quick”. The deep dermis fuses with the periosteum of the ungual process of the distal P3. In this manner, the dermis anchors the claw to the digit.
The equine hoof is an anatomically complex structure. The hoof itself can be compared to the claw of the carnivore, as it represents abundant epidermis with underlying dermis attached to P3. The dorsal and lateral hoof is called the wall, whereas the ventral aspect of the hoof is called the sole. The sole is anatomically similar to that of the digital cushion of carnivores.
The hoof wall is composed of three distinct layers of stratum corneum. The outer two layers, the stratum externum and stratum medium, are generated by and grow downward from the epidermis of the coronary band (or coronet, similar to the cuticle of a fingernail). The stratum externum, or periople, is the thin, outermost layer of the hoof wall. The stratum medium, or tubular horn, is the middle and thickest layer of the hoof wall. This layer also may contain pigment.
The innermost layer of the hoof wall is the stratum internum. This layer is produced by the epithelium of the laminar epidermis and is fused to the stratum medium, forming a relatively contiguous hoof wall. The laminar epidermis sits on top of the laminar dermis, also referred to as the corium. The corium is firmly affixed to P3, similar to that of the carnivore digit.
One of the most critical and specialized structures of the hoof are the laminae (singular: lamina). To understand the anatomy of the hoof, it is beneficial to understand the physiologic roles of the hoof. The forces exerted on the hoof by animals such as the horse are significant. The hoof is required to maintain its structural integrity despite these enormous forces (consider the forces exerted on the hooves of a galloping horse)! Hoof laminae are anatomic adaptations that facilitate this structural integrity. Laminae are interdigitating finger-like projections of dermis (dermal laminae) and epidermis (epidermal laminae). The interdigitation of these laminae generates friction at the interface when force is applied. What’s more, these interdigitations are extensive, and there are both primary laminae as well as smaller projections, called secondary laminae. Secondary laminae greatly increase the surface area of interdigitation between the dermal laminae and epidermal laminae. An increase in surface area of contact between these structures corresponds to increased frictional surface and increased resistance to separation, allowing for optimal structural integrity.
Although structural integrity at the interface of dermal and epidermal laminae is maintained in normal animals, separation at this interface can occur, and is referred to as laminitis. Separation of laminae results in loss of structural integrity and the characteristic radiographic finding of rotation of the third phalanx (P3).