Chapter summary

It is easy to overlook the significance of the integu­mentary system and forget that it is the largest organ system. However, its physiological significance is profound and can be summarized in the attributes listed here:

• physical protection: barrier against the outside

• prevention from dehydration

• body temperature regulation

• sensory information via cutaneous receptors

• metabolic actions

• excretion of wastes.

It is also increasingly clear that the skin and espe­cially specialized dendritic cells are important to the immune system for their capacity to essentially "test" the environment so that when necessary, immune cells can be stimulated to respond and fight or prevent infections.

The epidermis and dermis are the two primary layers of the skin, but the relative size (thickness) varies based on location and species. In areas where friction and wear are prominent, the outer layers of the epidermis are expanded to provide additional protection. The discrete layers of epithelial cells listed as follows (outside to inside) are present:

• stratum corneum

• stratum granulosum

• stratum spinosum

• stratum basale.

Other cells suspended with in the epidermis include melanocytes, which are important in protection from UV radiation as well as development of skin color, as well as Langerhans or dendritic cells.

The deeper epidermis has scattered fibroblasts and adipocytes, extracellular fibers (collagen, elas­tin), and proteoglycans, as well as capillaries and blood vessels. There are also nerve endings and spe­cialized receptor cells that occur as either encapsu­lated (Krause's, Meissner's, Pacinian, or Ruffin's corpuscles) or unencapsulated (free nerve endings, modified nerve endings)—Merkel discs, or root hair plexuses that appear around hair follicles. Two re­gions of the dermis include the papillary and reticu­lar layers.

There are two types of sweat glands. Eccrine glands are the most common. They have a simple coiled structure and open via pores on the skin surface. They produce a hypotonic watery secretion that aids in control of body temperature. Control of secretion depends on the sympathetic nervous system. In hu­mans, sweating can also be emotionally induced (fright, embarrassment, or nervousness). In these cases, sweating begins on the palms, soles, and armpits. Apocrine sweat glands make up a smaller propor­tion of the total sweat glands. These glands are larger and open up onto hair follicles. The secretion is also watery and also contains fatty acids and proteins. In primates, these glands are affected by sex steroids and become prominent with the onset of puberty.

Sebaceous or oil glands release their contents (holo­crine mechanism) on the hair follicles. The secretions provide some protection to the hair and are familiar in animal agriculture by the lanolin in sheep wool.

Other skin glands include anal glands, often fa­miliar to dog or cat owners, as well as the infraorbital glands (sheep), scent or horn glands (goats), sub­mental glands of cats, and the Uropygial gland of birds, familiar through observation of birds preen­ing and grooming their feathers. The mammary gland (covered in detail in Chapter 18) is also a skin- derived gland essential in many aspects of animal agriculture.

Hair is probably the most apparent skin adapta­tion. Functions include insulation, physical protec­tion, and sensory reception (the root plexuses around the follicles or the specialized whiskers of cats), as well as secondary sex characteristics and behavioral responses. The hair shaft has three regions: outer cuticle, inner cortex, and central medulla. The outer cuticle is essentially a layer of cornified squamous epithelial cells, the bulk of the hair shaft is the cortex composed of epithelial cells that have accumulated a specialized "hard" keratin as well as pigments, and medullary cells are more cuboidal.

The hair growth cycle has two primary phases: anagen and catagen. Late anagen represents the point when a new hair bulb is created and the older hair moves toward being shed. Control of hair growth is typically closely tied to season and the secretion of prolactin and epidermal growth factor. The distribu­tion and density of hair also varies between species. Consider the dense, luxurious fur of the chinchilla and its value as a fashion item.

Control of hair color is biochemically complex but in humans, for example, melanin and carotene are especially important. The rate and degree of pigment synthesis and accumulation in hair cells and skin cells determine both skin and hair color. In more detail, amounts of eumelanin (brown-black) versus pheomelanin (yellow-red) interact to determine sub­tleties in hair and skin color. The genetics of color in mice is well-studied, in part based on husbandry of genetic mutations of mice that were raised and studied by European fanciers in the late 1800s. More than IOO genes are known to be involved. Some of these act on the melanocytes, and others directly impact the biochemistry of pigmentation. Hormonal control is also apparent. Consider the dramatic sea­sonal changes in the plumage or pelage of Arctic animals. Melanocortin and its related family members are produced from POMC, which is abundant in the pars intermedia of the pituitary gland. Some of these mutations and associated phenotypes are listed here:

• Albino, lack of tyrosinase needed for melanin synthesis, absence of color.

• Agouti, altered signaling protein, increase = yellow; decrease = black.

• Brown, altered tyrosinase related protein 1, loss of function = brown instead of black hair.

• Piebald spotting, altered endothelin receptor, patches without color, deafness.

• Slaty, altered tyrosinase-related protein 2, dilu­tion of black color.

Vitamin metabolism is also an important skin prop­erty in many animals. Specifically, vitamin D₃ can be produced by the effect of UV irradiation to convert 7-dehydrocholesterol into D₃. Subsequent steps in­volving the liver and kidneys are needed to produce the biologically potent 1,25-dihydroxy vitamin D₃.

Finally, products from the integumentary system ( wool and hair, leather, feathers, and horns) pro­vide economic incentive to producers and goods for consumers.

Review questions and answers are available a online.