Skin of Mammals:
The features of mammalian skin are (1) Hair (2) a greater variety of epidermal glands than in any other vertebrate class (3) a highly stratified, cornified epidermis and (4) a dermis, many times thicker than epidermis.
Epidermis: Epidermis is composed of stratified squamous epithelium and consists of several layers of a variety of cells. Rapid cell division in the deepest layer of the epidermis push cells towards the surface of skin. As cells progress toward the surface, they die and become keratinized. Keratinized cells make up the outer skin layer called stratum corneum. Since keratin is virtually insoluble in water, stratum corneum prevents dehydration. Thus mammals colonized the land and are first line of defense against many toxic substances and micro organisms.
Dermis: The thickest portion of mammalian skin is composed of dermis which contains blood vessels, lymphatic vessels, nerve endings, hair follicles, small muscles and glands.
Hypodermis: The hypodermis underneath the dermis is different from that of other vertebrate classes; in that it consists of loose connective tissue, adipose tissue and skeletal muscles. Adipose tissue stores energy in the form of fat and provides insulation in cold environment. Skeletal muscle allows the skin above it to move independently of underlying tissues. Blood vessels thread from the hypodermis to the dermis and are absent from the epidermis. In human and few other animals like horses, the skin regulates body temperature by opening and closing sweet pores and sweating. Skin screens out excessive harmful UV rays from the sun but it also lets in some necessary rays that convert a chemical in the skin into vitamin D.
Skin is sense organ containing sensory receptors for head, cold, touch, pressure and pain. Many nerve endings of skin keep the animal responsible to potentially harmful factors in the environment.
Types of glands:
Sudoriferous glands: They are called sweat glands and are distributed over most of the human body surface. These glands secrete sweat by a process called perspiration. Perspiration helps to regulate body temperature and maintain homeostasis, largely by the cooling effect of evaporation. In some mammals curtain
sweat glands also produce pheromones.
Sebaceous glands: They are simple glands connected to hair (oil) follicles in the dermis. They lubricate and protect by secreting sebum. Sebum is permeability barrier, an emollient and a protective agent against micro organisms. Sebum also acts as a pheromone.
Skin colour: It is either due to pigments or to anatomical structures that absorb or reflect light pigments (e.g. melanin in human skin) are within the cells of the epidermal layer in hair or in specialized cells called chromatophores, some skin colour is due to colour of blood and superficial blood vessels reflected through the epidermis.
Function: Bright skin colour in venomous toxic or bad tasting animals may deter potential predators. Other skin colours may camouflage the animal. Colours serve in social communication helping members of the same species to identify each other their sexes, reproduce status or social rank.
Hair: Hair is composed of keratin filled cells that develop from the epidermis. The portion of hair that protrudes from the skin is the hair shaft and the portion embedded beneath the skin is the root. An arrector pilli muscle(smooth muscle involuntary muscles) attaches to the connective tissue sheath of hair follicle surrounding the bulb of the hair root.
Pili action: When this muscle contracts, it pulls the follicle and its hair to an erect position. In human, this is referred to as a ‘goose bump.’ In other mammals this action helps to warm the animal by producing an insulating layer of warm air between the erect hair and skin. If hair is erect because the animal is frightened instead of cold, the erect hair also makes the animal look larger and less vulnerable to attack.
Nails: Nails, like hair, are modification of epidermis. Nails are flat, horny plates on the dorsal surface of the distal segments of the digits (e.g. finger and toes of primates). Other mammals have claws and hooves. Other keratinized derivatives of mammalian skin are horns and the baleen plates of the tooth less whales.
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