Page 358 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

340 Veterinary Histology of Domestic Mammals and Birds

lactiferous sinus). The milk exits the teat sinus through in which they are subjected to constant mechanical stress
VetBooks.ir the teat canal (streak canal, ductus papillaris). The wall and wear, necessitates continuous generation of horn-pro-

of the teat contains connective tissue (including elastic fib-
ducing keratinocytes by the stratum germinativum. This is
res) and smooth muscle. Proximally, the teat wall contains supported by specialised microvasculature in the form of
large-calibre, thick-walled veins (venae myotypicae). The capillary networks that facilitate the requisite metabolic
inner surface of the teat is lined with mucosa. Externally, it processes. The digital organs also have sensory functions,
is covered with variably modified skin (see below) (Figures and therefore contain abundant mechanoreceptors and
15.23 to 15.26). nerve fibres.
Within the teat sinus the epithelium transitions from The species-specific differentiation of the dermis into
bi-layered cuboidal epithelium to the keratinised strati- dermal papillae and laminae serves to increase consider-
fied squamous epithelium of the teat canal. In ruminants ably the strength of the horn by enabling the development
and cats this transition is abrupt, while in other species it of tubular and lamellar horn. Mechanical strength is con-
is more gradual. ferred both by the quantity of horn and organisation of
The teat wall contains a network of collagen and the lamellae.
elastic fibres with embedded smooth muscle cells. From At the tip of a papilla, the surface area over which the
inside to outside, the orientation of muscle cells exhibits epidermis receives nutrients by diffusion from the capil-
characteristic variation. Closest to the teat sinus, abundant laries of the underlying dermis (corium) is comparatively
smooth muscle cells are arranged in a circular fashion. In small. The keratinocytes become removed from their
the middle of the teat wall, the muscle fibres are oriented source of nutrition relatively quickly as they are pushed
parallel to the lengthwise axis of the teat. From there they towards the surface. The resulting degree of cornification
radiate towards the teat surface. is limited. Cornified cells originating from the suprapapil-
As the number of muscle cells decreases towards the lar region are large, soft and relatively loosely packed. They
exterior, the amount of elastic fibres markedly increases. form the medulla of the tubular horn.
Near the teat opening (ostium papillare), the radial fibres In contrast, the epidermis overlying the sides of the
give rise to a rosette that projects into the teat canal. At this papillae is amply supplied with nutrients across a relatively
location, muscle fibres concentrate to form the m. sphinc- large surface area. Cornification of these cells is optimised,
ter papillaris that aids in sealing the canal. giving rise to a strong, dense band of cells. This peripa-
At the ostium papillare, the epithelium turns into pillar cylinder of cornified cells forms the cortex of the
the teat lumen to form a stratified keratinised mucosa. tubular horn, and contributes significantly to the overall
Desquamated keratin may temporarily obstruct the teat strength of the cornified tissue.
canal (Figures 15.25 and 15.26). Horn arising from between two papillae is termed
The skin on the external surface of the teat is hairless intertubular horn. It lacks the hardness and strength of
in the cow. In the sheep, the skin may contain hairs and tubular horn.
sebaceous and sweat glands. In the horse, the skin of the
teat bears fine hairs and is pigmented and rich in sebaceous Equine hoof (ungula)
glands. The hoof is a complex epidermal organ in which the lay-
ers of the skin exhibit particularly marked modification.
Digital organs and horn Accumulation of collagen-rich connective tissue and,
In certain body regions, the external skin undergoes ext- in some locations, abundant fatty tissue in the subcu-
reme modification. Massive proliferation of the epidermis tis gives rise to perioplic, coronary and digital cushions
and permanent cornification of this external skin layer (Figures 15.27 to 15.29). Tubular glands, adipose tissue
gives rise to species-specific digital organs (the equine and elastic fibres present in this region support the mecha-
hoof, the ruminant and swine hoof, the claw, or nail, of car- nical function of the hoof and provide shock absorption.
nivores) and the horn of ruminants. In some cases, these There is no subcutis in the wall and sole segments of the
modifications are accompanied by considerable structural hoof.
alteration of the dermis in the form of papillae and dermal In the perioplic and coronary segments, and in the
laminae, and accumulation of subcutaneous tissue to form sole, frog and bulbs, the corium is studded with papillae.
pads and cushions. The structure of the layers of the wall These are particularly long (4–8 mm) in the coronary
of the digital organs varies considerably between species dermis. In the wall segment and in the bars, the dermis
(see Veterinary Anatomy of Domestic Mammals: Textbook and is arranged into longitudinally oriented laminae that give
Colour Atlas). off secondary laminae from their surface. The dermis con-
At the microscopic level the individual layers of the skin tains a dense vascular network (plexus venosus ungulae)
are also modified, reflecting the functional requirements and a large number of sensory nerve fibre plexuses and
of the tissue. The exposed location of the digital organs, mechanoreceptors.

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