Bovine biology series
Part - 34 Skin
We examine the largest organ of the mammalian body, not in surface area as you might expect, but in sheer bulk…the skin.
The word "skin," is derived from the English word "skinn," and that comes from an earlier older English word "scinn." These early derivatives of our present day word described the covering of tissue, hide, or pelt of a wide variety of animals, including man.
Skin has two major functions: one, protect the body from the outside environment, and two, the skin actually communicates or interprets the exterior and the interior (internal body) environment.
Skin may be divided into two components, epidermis and dermis.
The epidermis is the outside layer of skin comprising a very small amount of the total skin mass. As the outside layer of tissue, the epidermis consists of dead keratinized cells called stratum corneum. These dead cells make up the layer on the body’s surface, and therefore, are an effective water barrier for absorption and for loss. The keratinized epidermis provides protection from physical, chemical, and foreign bodies such as bacteria and viruses. These cells have a high level of lipid - fat content facilitating a more effective water barrier.
The thickness of epidermis for an adult cow is 6-8 millimeters or ¼ to 1/3 inch thick.
Just beneath this layer of dead keratinized cells is a very thin layer of viable - live cells called the stratum-basale. The word basale means giving forth, or coming from, so this layer is named because as these cells are made and make their way to the outer skin surface, where they die and become the stratum corneum.
As you might expect, the turnover rate for the epidermis is quick, in cows this layer is constantly undergoing sloughing off of epidermal cells while new ones are built.
The dermis is the fibrous connective tissue of the skin.
The dermis is the layer directly beneath the epidermis, and thus the epidermis is made from cells of the dermis. The dermis is largely connective tissue, which consists of collagen fibers and bundles of elastic fibers so the skin can stretch, remain pliable, and provide for the animal a comfortable layer of protection.
Interestingly, the hooves, lips and snout of domestic animals are skin-like tissues; they are different, however, in that they consist largely of connective fibers (and are more like epidermis than dermis tissue) that serve specific roles of wear resistance in the presence of abrasion, like concrete.
Hair follicles are rooted deeply within the dermis. These follicles receive nourishment from the oxygen-rich arteries. Hair serves the valuable role of filtering debris before harmful substances can reach the skin surface and hair on animals, especially cows, and is a great insulator during low temperatures. Of course hot temperatures create special challenges for body heat regulation. Cows have apocrine sweat glands. Their function is thermoregulation.
A cow cannot get rid of excess body heat, efficiently, however, for the apocrine sweat glands are less efficient than the supremely efficient sweat glands (these sweat glands are called merocrine tubular glands) of the human. Pigs, however, suffer even more as their apocrine sweat glands are almost completely non-functional. So they pant and use saliva to dissipate body heat. Cattle increase their panting too in hot periods as a method of discharging body heat.
The mammary gland may have derived from a sweat gland, as the tissue covering the udder is skin, of course, but more importantly, the way milk is made in the secretory tissue is similar to a sweat glad. The lipids made here are analogous to sweat made in the apocrine gland, whereas the carbohydrates and proteins are made as sweat would be from sweat organs similar to the ones in humans, the merocrine glands.
Repair of the skin is important for the maintenance of integrity as a physical barrier. During an instance of trauma, when the skin is wounded, torn, or scraped away, an immediate response begins whereby the blood-clotting mechanism of the blood chemistry thickens the blood, reducing and stopping the outward flow of blood from disrupted-severed vessels.
Next, inflammatory cells invade the wound in order to engulf and digest the foreign debris, bacteria and soil that threaten the integrity of healthy, neighboring cells. The inflammatory cells, the macrophages and lymphocytes, are rather non-specific, in that they seek out anything that is unrecognizable as belong to the body. And destroy it. This, of course, is the immune system’s first line defenders, the heavy hitters if you will.
During the next stage called the proliferative stage, a scab of dead white blood cells, blood clotting material, and stratum basale cells cover the wound. The scab is a protective mechanism until a newly formed layer of keratinized cells can replace the scab.
Beneath the scar, the blood vessels from neighboring epidermis and dermis proliferate, flooding the region with blood supply, and the recipient of the blood flow are new dermal cells that are held in place with elongated connective and fibrous tissue.
The wound is usually devoid of nerve cells, as the renewal of nerve cell elongation takes much more time.
The final stage of healing or repair is the maturation stage, which is simply the blending together of the various skin layers in such a manner the integrity of the skin is once more accomplished. However, a scar usually exists, for any of us can look at our hands or arms and find a scar of some event that may go back decades of time.
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