Coupon Accepted Successfully!


The Cutaneous Membrane: the “Skin”

As noted in the introduction, the skin plays a variety of roles vital to the continuing health of the organism. In previous chapters we examined the excretion of sweat, and the sensory nerve endings located in the skin that help us to gather information about the world around us. Similarly, we noted the role of the skin as a non-specific barrier acting in general defense against infection by pathogens. The skin also plays a major role in thermoregulation, a topic we will consider shortly. Since we have already discussed the many functions associated with the skin, we should now address its structure. The skin can be seen as composed of three layers, each with a unique composition.


The Epidermis: This outermost layer of the skin is composed exclusively of stratified squamous epithelial tissue. Recall from Chapter 13 that stratified epithelium is many layers thick, and this has an interesting consequence for the skin. Since only epithelial cells are present, there is no supply of blood to nourish the epidermis. A single layer of cells that lies close to the dermis is nourished by capillaries found in the thicker lower layer.

The cutaneous membrane (skin)


These cells are able to undergo mitosis and continually produce new cells. As the new cells are pushed outward, further and further from the blood supply, they become deprived of oxygen and nutrients and die. During this process, the dying cells begin to produce and store large amounts of the fibrous protein keratin, which acts as a “sealant”. Thus the outer epidermis consists of many layers of tightly packed dead cells containing large amounts of keratin, making the epithelium impermeable to water. Deeper down in the epidermis lie cells which produce the pigment melanin, giving skin its color. The presence of melanin and keratin in the epidermis allows it to protect underlying skin layers from water gain/loss, potential damage from ultraviolet radiation, and mechanical damage. The epidermis is connected to the underlying dermis by a basement membrane. 
  • The Dermis: The dermis is generally three to four times thicker than the epidermis, and is composed of dense connective tissue containing collagen and elastin fibers. This allows the skin to be both strong and pliable. A major function of the dermis is to anchor the epidermis to underlying structures. The dermis also contains blood vessels, some muscular tissue (smooth muscle associated with involuntary movements of the skin and secretions of glands), and nervous tissue (acting in either a sensory or motor capacity). Also located in the dermis are several types of accessory structures:
    • Hair follicles are composed of epidermal cells protruding into the dermis. As a hair begins to grow, the cells being pushed outward, like other epidermal cells, die and become keratinized. Thus what we perceive as hair is really a shaft of dead, highly proteinaceous cells. Only mammals have true hair, and its function is to act as an insulator. Humans are the only mammalian species in which hair does not cover almost the entire body in large quantities.
    • Sebaceous glands produce and release sebum, an oily substance that is often secreted into hair follicles. Sebum functions to lubricate the hair and skin; in certain individuals, however, bacterial infection of sebaceous glands can result in acne, a condition in which the skin is covered by inflamed, raised lesions.
    • Sweat glands produce sweat, usually to cool the body during conditions of high temperature or physical exertion. As we noted in the last chapter, while sweat is mainly water, it may also contain electrolytes and wastes such as urea, so sweating is also considered an excretory process.
  • The Subcutaneous Layer: The subcutaneous layer is a relatively thin basal portion of the skin. It is composed almost exclusively of loose connective tissue which functions in binding the entire skin to the underlying skeletal muscles. Much of this connective tissue is adipose tissue, which as we noted earlier functions in the storage of fat; its location in the subcutaneous layer allows it to contribute to temperature regulation by acting as an insulator against loss of heat. The subcutaneous layer also contains blood vessels that nourish the skin.

Test Your Skills Now!
Take a Quiz now
Reviewer Name