Chapter 5:  The Integument

The skin, glands of the skin, nails, and hair constitute the integumentary system.

I. Skin is composed of three layers: epidermis, dermis, and hypodermis.
The epidermis may further be divided into the following four or five layers (from the top down): the stratum corneum, the stratum lucidum (in the lips and thickened skin of the soles and palms), the stratum granulosum, the stratum spinosum, and the stratum basale.

The function of the epidermis, composed of stratified squamous epithelium, is to provide an outer layer of protection to the body that is resistant to wear and tear and is waterproof. Cells moving from lower layers lose their nuclei and their entire cells fill with the protein keratin.

Identifying characteristics of each layer of epidermis are as follows:
Stratum corneum is 25-30 layers of cells that are thin, flat and filled with keratin. This layer is said to be cornified.

Stratum lucidum is a layer that appears clear when stained because nuclei, organelles and cell membranes are no longer visible.

Stratum granulosum is a thin layer that contains granules of the chemical precursor to keratin called keratohyalin

Stratum spinosum is made of several layers of cells that stain with spine-like extensions; these cells have limited mitotic activity.

Stratum basale is a single layer of cuboidal cells in contact with the basement membrane. It contains keratinocytes (make keratin), melanocytes (make melanin to give skin its color), tactile cells (detect touch), and nonpigmented granular dendrocytes that are actually macrophages.

Skin color is determined by the amount of melanin produced by melanocytes in the stratum basale. Patches of melanin may produce freckles. Benign growths of melanocytes lead to the appearance of seborrheic hyperkeratoses, also known as ‘liver spots’, as we age. A lack of melanocytes leads to vitiligo.

The dermis is thicker than the epidermis and extensively vascularized and innervated. It houses sweat glands, oil-secreting glands, nerve-endings, and follicles. Numerous elastic and collagenous fibers run through the dermis and produce tension lines. It is preferable to make surgical incisions in parallel with these tension lines as they will heal faster and scar less than incisions made perpendicular to them.

The dermis has two layers: the upper stratum papillarosum and the thicker lower layer called the stratum reticularosum. If dermis is stretched to the point it tears, it then heals leaving a stretch mark called a linea albicans.

The hypodermis is not really skin. It is made of loose connective tissue and fat and is important clinically because it is the site for the injection of many water-soluble medicines. It binds the skin to deeper structures.

II. Hair. Humans have three kinds of hair:
Lanugo is very fine hair that comes on in the third trimester
Vellus is a short but still fine hair that replaces the lanugo
Terminal hair is more course and colored and either grows to a length and stops (such as eyelashes and eyebrows) that is called definitive hair or grows continuously (such as is the case on the scalp) and is known as Angora hair

Can be pulled into a more vertical position by the contraction of arrector pili muscles that contract involuntarily due to stimuli such as cold and fear, resulting in ‘goosebumps’.

III. Glands

The glands of the skin reside in the dermis and are referred to as exocrine glands because they release their products directly to the skin surface or transport their secretions through ducts.

Sebaceous glands are holocrine glands that secrete sebum onto the shaft of the hair, where this fatty substance keeps the hair from becoming brittle and waterproofs the stratum corneum. Hyperactivity of these glands is associated with acne.

Sudoriferous glands (sweat glands) secrete sweat onto the surface of the skin. The smaller eccrine sweat glands are formed before birth and active throughout life in helping you keep your cool while the larger apocrine glands found in the axilla and pubic regions become active in puberty
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Mammary glands are specialized sudoriferous glands and produce milk during lactation.

Ceruminous glands are found in the ear canal and secrete ear wax, which is bacteriostatic.

IV. Pathology of the skin

There are several kinds of cancer of the skin to be aware of, including:
• basal cell carcinoma, which appears as a shiny bump that forms a crater that erodes and bleeds. Non-aggressive and easily excised. Most common skin cancer.
• Squamous cell carcinoma presents as a firm, red, keratinzied tumor that is usually well treated with excision and radiation. If left untreated, they may metastasize.
• Malignant melanoma is very aggressive; it often initially appears as mole that enlarges, changes color, and bleeds.

Burns
• First degree burns involve only the epidermis and are followed by desquamation.
• Second degree burns involve the epidermis and dermis. They are painful and involve blistering, but generally recover.
• Third-degree burns go right on through the skin and may damage muscle as well. The nerves have been charred and therefore the person feel no pain but the skin is incapable of preventing fluid loss. Skin grafts may be used as treatment.

Frostbite

• In first-degree frostbite the skin is cyanotic and swollen
• In second-degree frostbite vesicles form and hyperemia are seen; with warming the skin blisters.
• In third-degree frostbite there is edema, bleeding, and numbness followed by pain and necrosis. Gangrene follows.