Animal skin is composed of three basic layers. From outside to inside these layers may be called the
The epidermis is formed by layers of flat cells composing a stratified squamous epithelium. New cells originate in the lowest layer and become keratinized as they are pushed to the surface. Keratin is a fibrous protein that also forms the substance of hair, horns and hoofs. At the ultrastructural level it is deposited in a fibrillar form which then may be incorporated into a granular form.
Each hair follicle develops from an inpushing of the epidermis down into the dermis (the indentations in the image above). Hair is formed by epithelial cells of a papilla at the base of the follicle. There is considerable variation in the rate of hair growth in meat animals. For example, the average length of bovine hair may reach a maximum between 6 and 24 months, and then may decrease. The underlying sequence of events in hair growth is due to the periodic shedding of hairs from their follicles. The bulb at the base of the hair eventually becomes hard and clublike. This holds the hair in its follicle for some time, but no further growth is possible. Eventually the hair is released when a new hair starts to form in the base of the follicle. This cycle determines the average external hair length and is influenced by factors such as climate, age, nutrition and breed. Chemical analysis of animal hairs may be used to measure the nutritional status of an animal, but the method is not very precise.
Most mammalian hairs and bristles have three layers that appear as concentric rings in a cross section through the hair shaft. From outside to inside these are:
In meat animals, most of the sweat glands open near the entrance of hair follicles. Although less conspicuous than the sweat glands of human skin, they still make an important contribution to thermoregulation in meat animals. It has been suggested that hair follicles exert some control over the development of surrounding adipose tissue.
Feathers are also formed in follicles, they don't just drop out of pillows. The follicles are grouped in feather tracts that are readily visible on the skin of the eviscerated carcass. In the spaces between the tracts, the follicles produce only filoplumes with a rudimentary feather vane at the end of a hair-like shaft. The arrangement of feather follicles is governed by waves of morphogenetic activity that move across the skin of the embryonic chick like ripples on a pond. The large feathers of the wings are called remiges while those of the tail are called retrices. The contour feathers provide the main covering of the body and are interspersed with filoplumes. Young birds have large numbers of down feathers. The structure of the vane of a typical feather resembles a hollow quill that has been obliquely sliced and unrolled. Thus, when it is formed within the follicle it is like a hollow cylinder. The lateral branches or barbs of the vane are held together by hooked anterior barbules that catch on the saw-like edges of adjacent posterior barbules. The skin of poultry is dry and does not produce its own oil. In poultry, there is an oil gland located dorsally to the stumpy tail of the bird. The oil is distributed when the feathers are kept in order as the bird preens itself.
Pigment cells or melanocytes are located in the deepest layers of the epidermis or in the underlying dermis. Melanin is a pigment formed in organelles called melanosomes. Melanin is passed from melanocytes to skin cells by cytocrine secretion. Melanin is formed from the oxidation of tyrosine by tyrosinase. Absence of this enzyme results in an albino animal. Variation in the color of farm animals is caused by variations in the amount and distribution of melanin. Melanin may be extracted with an aqueous solution of sodium hydroxide and then recovered by acidification. Melanin is a polymer based on indole monomers, but there is also a protein component involved that makes precise determination of its structure difficult. The distribution of melanin over the animals' skin is determined prenatally by an interaction between the migration patterns of melanocytes and the diffusion patterns of the messenger substances that either activate or suppress the synthesis of melanin. A single dominant gene determines the belt pattern marking that runs over the shoulders and forelimbs of some breeds of pigs.
The epidermis is supported on the ridged surface of the underlying dermis. The upper region of the dermis, often called the papillary layer of the dermis, is a tightly woven network of collagen fibers with some elastin fibers. After the tanning of a hide to make leather, the papillary layer becomes the top surface of the leather. With a hand lens, the openings where the hair follicles once penetrated the dermis are easily visible. When the leather is turned over, the much looser coarse fibrous weave of the lower dermis is evident. In pigskin, the follicles of the strong bristles are rooted at the lowest level of the dermis so that many of the follicles almost perforate the leather.
When the hide is removed from the carcass, the separation is made through the deepest layer of the integument - the hypodermis. Fat is often deposited in the hypodermis and, particularly in sheep, may even infiltrate the dermis. Numerous blood vessels run through the hypodermis to reach the extensive vascular bed (for heat dissipation) in the dermis. The hide weight of a typical lean steer is about 7% of the live weight, but there is considerable seasonal variation with colder climates inducing heavier hides and there are also differences between types of cattle.
Beef hides are graded on their cleanliness and degree of damage due to branding or warble fly larvae. If beef hides have been processed with a high standard of hygiene, the collagen of the inner layer of the hide may be used for processed food products such as sausage casings. Green hides (those from recently slaughtered animals) are treated with sodium chloride prior to tanning. The hides are trimmed, split into left and right sides, and soaked for several days in water. Then the hides are dehaired in a calcium hydroxide solution that contains sodium or calcium hydrosulfide. The conversion of a hide to leather occurs when it is TANNED, originally with a tree bark extract but now usually with sodium dichromate. Hair remnants are physically forced from the hair follicles (scudding) prior to deliming in sulfuric acid. Elastin fibers are removed enzymatically before the hides are pickled in sodium chloride acidified with sulfuric acid.