THE INTEGUMENTARY SYSTEM

Prof. Atsma © 2005

The following is a narrative summary of the topic. Click here for the Integumentary System "Classroom Notes" that you can print out and bring to class to save yourself a lot of note-taking.

Introduction: The Integumentary system is divided into two layers, the epidermis and the dermis. The epidermis is made of stratified squamous epithelium, and protects the body from friction and water loss. The dermis is the connective tissue region, better known for all of the important structures found there. The hypodermis is the region of adipose tissue deep to the dermis.

THE EPIDERMIS

The epidermis is made of stratified squamous epithelium, with five "distinct" layers.

Stratum basale: As the base layer of the epidermis, cells in the stratum basale are undergoing almost constant mitosis. Also found here are specialized melanocytes which produce the dark brown pigment melanin. Melanin protects us from UV radiation. Contrary to popular belief, lighter-skinned races to not have drastically fewer melanocytes. The main reason for differences in skin color is the activity of melanocytes, not number. Melanocytes of darker races are always active, while those of lighter races are activated by excess UV radiation. Since UV plays a role in the production of Vitamin D, it makes sense that races evolving in areas having very indirect sunlight through much of the year would reduce melanin production.

Stratum spinosum: The thickest layer of living cells, the stratum spinosum is where cells switch over from a mitotic role, to that of keratin production (actually the precursor molecules to keratin). Keratin is a waterproofing protein which also toughens the outer surface of the skin as it hardens. Macrophages (phagocytes that protect the skin from infection) are also typically sprinkled throughout the spinosum.

Stratum granulosum: As cells flatten, the melanin granules that the skin cells have collected are compressed into a thin layer called the stratum granulosum. This layer acts as a protective shield to the cells below. Because the melanin granules have been compressed into a thin strip, this is often the darkest layer of the epidermis.

Stratum lucidum: The translucent layer of the skin, the stratum lucidum is where keratin production results in a cell loaded with whitish, semi-transparent protein. Since the keratin quickly hardens and "yellows," this layer is too thin to be easily seen in anything except the "thick" skin of the hands and feet.

Stratum corneum: Named for looking like a layer of dried corn kernels, the stratum corneum is the outermost (and toughest) layer of the epidermis. Providing protection from friction or abrasion, these dead cells gradually rub off, and are continuously replaced. Since keratin is a water-proofing protein, this layer also protects us from desication (drying out).

THE DERMIS

As mentioned above, this region is made of connective tissue which provides a "bed" for the important structures listed below. The wavy papillary layer is located right under the epidermis. This wavy attachment surface increases surface area for stronger attachment. The reticular layer is the remainder of the dermis.

Hair follicles: Actually folds of the epidermis that poke down deep into the dermis, hair follicles produce hair via the very same mechanism that produces the epidermal stratum corneum. The difference is that the hair follicle is more or less cylindrical in shape, so instead of a sheet-like stratum corneum, a more "tubular" hair of tightly packed keratinized cells is produced.

Sebaceous glands: Also made of epithelial cells, sebaceous glands are an outpocketing of the hair follicle. They serve to coat the hair with oil in order to prevent the drying-out and brittleness that could damage hair. Sebaceous gland cells are relatively large and light-staining compared to other epithelial cells.

Arrector pili muscles: Also associated with the hair follicles, these small involuntary muscles "raise" the hair. They are vestigial in humans, but important for thermoregulation in other mammals (in humans, they only produce "goose bumps").

Sweat glands: These glands are really a series of coiled ducts that produce a watery secretion. A sweat gland duct leading to the surface via a pore allows water to be deposited on the surface of the epidermis. As the water evaporates, it helps to cool the skin. Eccrine sweat glands (the common variety of sweat glad) cells are relatively small and stain normally. Apocrine sweat glands (found mainly in the groin and axillary areas) produce a secretion containing protein and lipid, and have somewhat larger cells. Waste products of bacteria who live off apocrine sweat gland secretions seem to produce the characteristic human odor.

Meissners corpuscles: These are the nerve endings of light touch, and are located in the superficial papillary layer of the dermis.

Pacinian corpuscles: The large pacinian corpuscles are the nerve endings of deep pressure, and are usually found relatively deep in the dermis. They are easily distinguishable as they look a little like an onion sliced in half.

Free nerve endings: Although you will not see these under the microscope, these fine, branched nerve endings are important for temperature and pain sensation.

Burns/Blisters

Burns are classified by how much damage they do to the skin. Damage or inflammation of the epidermis only is referred to as a 1st degree burn. Localized damage to both epidermis and dermis is a 2nd degree burn. Because blood vessels under the epidermis are often damaged, but surrounding undamaged areas hold the skin in place, 2nd degree burns often produce blisters. Blisters may also be caused by ripping of the epidermis from the dermis by excessive friction. Widespread damage to both epidermis and the dermis results in 3rd degree burns. Without the sheet of keratinized cells to hold in moisture, and with so many blood vessels damaged, death from dehydration can follow extensive 3rd degree burns. Infection is also likely to follow untreated 3rd degree burns.

The rule of nines is often used to estimate the percentage of the body burned. Interestingly, most body parts seem to approximate some multiple or divisible of 9. See text for diagrams illustrating this principle.

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