Page 291 - Anatomy and Physiology of Farm Animals, 8th Edition

276 / Anatomy and Physiology of Farm Animals

the melanocytes rather than from differ- are three main types of hair on domestic
mammals: (1) guard hairs, which form the
ences in numbers of melanocytes or pres-
VetBooks.ir ence of other pigments. This expression smooth outer coat; (2) wool hairs, also
called the undercoat, which are fine and
can be influenced by certain pituitary hor-
mones, notably melanocyte‐stimulating often curly; and (3) tactile hairs, long stiff
hormone (MSH) and adrenocorticotropic hairs with specialized innervation that ren-
hormone (ACTH) (see Chapter 13). ders them effective as organs of touch.
Absence of pigment in the skin (albi- An individual hair arises from a modifi-
nism), which may be partial or total, cation of the epidermis, the hair follicle
arises from a genetic inability of melano- (Fig. 14‐3). The follicle invaginates from
cytes to manufacture pigment. Lack of the surface of the skin as a double‐layered
pigment can render the skin and surface root sheath that surrounds the hair and
mucous membranes more susceptible to terminates in a hair bulb of epidermal ori-
actinic damage (i.e., cellular damage due gin. The hair bulb surrounds a small knob
to ultraviolet light), hence formation of of dermis called the dermal papilla. The
carcinoma (cancer) of the skin or other internal epithelial root sheath intimately
exposed epithelia. Cancer eye, or squa- covers the root of the hair and is continu-
mous cell carcinoma of the conjunctiva ous with the epithelial cells covering the
of the eye, is common in white‐faced cat- dermal papilla. The external epithelial
tle (e.g., Hereford) living at high eleva- root sheath surrounds the internal root
tions, where the ultraviolet component sheath, is continuous with the epidermis,
of sunlight is little attenuated. and gives rise to the sebaceous glands that
are associated with hair follicles. The divi-
Hypodermis sion of the epithelial cells covering the der-
mal papilla generates the hair itself. Growth
In nearly all areas of the body, a layer of and multiplication of these cells extrude
loose connective tissue separates the dermis the hair from the follicle, causing it to grow.
from underlying structures. This areolar An individual hair has a medulla at its
connective tissue, known variously as the center, surrounded by a scaly cortex, out-
superficial fascia, subcutis, or hypoder- side of which is a thin cuticle. All parts of
mis, permits movement of the skin without the hair are composed of compressed,
tearing. Where the skin is tightly attached keratinized epithelial cells. The bulk of the
to underlying bone or muscle, a dimple on hair comprises the cortex, which consists
the body surface may be seen. This is a “tie,” of several layers of cornified cells. The
as is seen where the dermis is attached to amount and type of melanin in cortical
the spinous processes of vertebrae. Variable cells determine whether the hair will be
amounts of fat, the panniculus adiposus, black, brown, or red. The cuticle is a single
are present in the hypodermis, with species‐ layer of thin, clear cells covering the sur-
dependent distribution and relative abun- face of the cortex. The medulla may con-
dance. The panniculus adiposus is an tain pigment, which has little effect on hair
especially notable feature of pigs; on the color, but air spaces between medullary
dorsum of the pig, it is called backfat. cells are believed to give a white or silver
color to the hair if the cortex lacks pig-
ment. Wool hairs lack a medulla or have
Adnexa of the Skin only a very small one, accounting for their
fine, flexible nature.
Hair Tactile hairs, used as probes or feelers,
are also called sinus hairs because a large
Hair is a defining characteristic of mam- blood‐filled sinus surrounds the deep por-
mals. All common domestic mammals tions of the follicle. These hairs are thicker
except the pig have abundant hair. There and usually longer than guard hairs and are