NOTE: Lecture notes are intended to help the student organize their notes and facilitate assimilation of the material. They are in no way a substitute for the actual lectures; however, this material will be covered on exams!
Text Reference: CHAPTER 5
The skin and its associated structures (hair, nails, sweat glands, and oil glands) make up a complex set of organs called the INTEGUMENTARY SYSTEM. The skin covers a surface area of 15-22 square feet, ranges in thickness from 0.5 mm (eyelids) to 4.0 mm (heel of the foot), and weighs about 10-11 pounds.
The skin performs many functions, most of which are PROTECTIVE:
1. Skin cushions & insulates the deeper organs.
2. It protects the body from mechanical damage, harmful chemicals and invading bacteria.
3. The skin is waterproof, preventing dehydration.
4. The skin's capillaries & sweat glands regulate heat loss, helping to control body temperature.
5. The skin excretes water, urea and salts in sweat.
6. Skin screens out damaging ultraviolet (UV) radiation from the sun.
7. Skin also uses the UV rays that penetrate the skin to synthesize (make) vitamin D.
8. The skin contains sense organs called sensory receptors that detect touch, pressure, pain, and temperature.
Read more about FUNCTIONS on page 134-135.
The skin is composed of 2 distinct regions: the EPIDERMIS and the DERMIS. The epidermis and the dermis together form the epithelial tissue membrane called the CUTANEOUS MEMBRANE.
EPIDERMIS - Figures 5.1 & 5.3
The epidermis is composed of STRATIFIED SQUAMOUS EPITHELIUM (review Table 3.1, page 65). Most of the cells in the epidermis are KERATINOCYTES. The keratinocytes of the epidermis are organized into 4 or 5 layers. You are only responsible for the layers described below (see FIG. 5.3).
The STRATUM BASALE (basal layer) is the deepest epidermal layer. It is separated from the dermis by a thin BASEMENT MEMBRANE. The stratum basale consists of a single row of cells, mostly COLUMNAR KERATINOCYTES. The keratinocytes are constantly and rapidly dividing.
About 8% of the cells in the stratum basale are MELANOCYTES (FIG. 5.2 & 5.3), which are specialized epithelial cells that synthesize the protein pigment MELANIN. As melanin is formed, it is passed on to the keratinocytes, which ingest the melanin pigments. The melanin accumulates on the superficial side of the keratinocyte nucleus. This serves to form a pigment shield on the sunny side of the keratinocyte nucleus that protects the nucleus and underlying tissues from the destructive effects of ultraviolet (UV) radiation in sunlight. Exposure to UV radiation increases the production of melanin by melanocytes. Since all humans have approximately the same number of melanocytes, racial differences in skin coloring are due to differences in melanocyte activity.
As the KERATINOCYTES are pushed toward the skin surface by the production of new cells beneath them, they begin to produce the fibrous protein KERATIN, which helps waterproof and protect the skin. As the keratinocytes are pushed upward toward the epidermal surface, they move further from the dermis, where the blood vessels are. This means that they move further from their source of oxygen and nutrients. The nuclei of the cells degenerate and the keratinocytes can no longer divide or carry out metabolic activities. Eventually the cells die.
By the time the keratinocytes reach the STRATUM CORNEUM (FIG. 5.3) of the epidermis, they have become dead, flat (SQUAMOUS) cells filled with keratin protein. The stratum corneum is the outermost layer of the epidermis, consisting of 25 to 30 layers of dead, keratin-filled squamous cells. Keratin is a tough, water-repellent protein. Its abundance in the stratum corneum allows that layer to provide a durable "overcoat" for the body, protecting deeper cells from the hostile external environment (physical & chemical damage, bacterial invasion) and from dehydration.
Millions of dead cells from the stratum corneum are worn off by abrasion every day, and we have a totally new epidermis every 4 weeks. In healthy epidermis, the production of new cells balances the cell loss at the skin surface. In body areas regularly subjected to friction, such as hands and feet, both cell production and keratin formation are accelerated.
Other cells are found in the epidermis (see FIG. 5.2 & 5.3). The LANGERHANS CELLS are MACROPHAGES (phagocytic white blood cells) that help activate the immune system. They are easily damaged by UV radiation. MERKEL CELLS function in touch sensation.
DERMIS - Figures 5.1 & 5.3
The DERMIS lies deep to the epidermis and consists of connective tissue. The dermis is thick in the skin of the palms of the hands and the soles of the feet, and thinner in the skin of the eyelids, penis and scrotum.
The dermis is HIGHLY VASCULAR, while the epidermis is AVASCULAR. Epithelial cells in the epidermis receive their oxygen and nutrients from the blood vessels in the dermis.
The dermis is richly supplied with nerve fibers, many of which are sensory receptors: nerve endings that are sensitive to TOUCH (the corpuscles of touch), PRESSURE RECEPTORS (Pacinian corpuscles), and FREE NERVE ENDINGS (pain, heat and cold receptors).
The superficial papillary region of the dermis consists of AREOLAR CONNECTIVE TISSUE. This portion forms projections into the epidermis called DERMAL PAPILLAE (SEE FIGS. 5.1 & 5.3), causing epidermal ridges (for example, fingerprints) in the overlying epidermis. Epidermal ridges increase friction, and therefore grip, of the hands and feet. The dermal papillae contain capillaries and touch receptors (corpuscles of touch).
The deeper, thicker reticular portion of the dermis consists of DENSE IRREGULAR CONNECTIVE TISSUE, which contains collagen and elastic fibers that give skin its strength and elasticity.
SUBCUTANEOUS LAYER - Figure 5.1
The SUBCUTANEOUS LAYER (also called the SUPERFICIAL FASCIA) lies deep to the skin. The subcutaneous layer is made up of AREOLAR & ADIPOSE CONNECTIVE TISSUES. The subcutaneous layer anchors the skin to the underlying muscle. Because of its adipose tissue, the subcutaneous layer also acts as a shock absorber and insulates the deeper body tissues from heat loss.
EPIDERMAL DERIVATIVES
Hair, nails, oil glands, and sweat glands are called epidermal derivatives. This means that they are structures that developed from the embryonic epidermis during prenatal development (see FIG.5.8). Although they are considered to be epidermal structures, some of them are located deep in the dermal layer.
HAIR & HAIR FOLLICLE- Figures 5.1 & 5.5
Hair has protective functions for humans: the hair guards the scalp against physical trauma, heat loss, and sunlight. Eyelashes and eyebrows shield the eyes from sunlight and dust. Nose hairs keep dust and foreign particles out of the respiratory tract.
Hair is distributed on nearly all parts of the body. They are absent from the palms of the hands, soles of the feet, lips, nipples, and portions of the external genitalia. Normal hair loss in an adult scalp is about 70-100 hairs a day.
Hair is a flexible structure that consists of the SHAFT, which projects from the skin, and the ROOT. The hair root is surrounded by the HAIR FOLLICLE and is imbedded in the dermis (& may extend into the subcutaneous layer).
The hair consists of dead, keratinized epithelial cells. The outermost layer of cells is called the CUTICLE. The cuticle is formed from a single layer of flat cells that overlap one another like shingles on a roof. The cuticle is the most heavily keratinized layer of cells in the hair, so it provides strength to the hair.
Hair is produced by the HAIR FOLLICLE (see FIG. 5.5). The hair follicle consists of the internal and external root sheath, which are several layers of epithelial tissue (derived from the epidermis), surrounded by a dermal root sheath made of connective tissue. The hair follicle surrounds the root of the hair and extends down into the dermis. The base of each follicle is expanded into an onion-shaped structure called the BULB. The bulb contains an indentation of areolar connective tissue called the PAPILLA. The papilla of the hair contains capillaries that provide nourishment to the hair follicle. The bulb of the hair follicle also contains a region of cells called the MATRIX. The matrix contains actively dividing epithelial cells that are responsible for hair growth and the production of new hair. As new hair cells are produced by the matrix, the older part of the hair is pushed upward. The hair cells become keratinized and die (like epidermal cells). So the hair shaft is made almost entirely of dead keratin-filled cells.
Hair pigment is made by MELANOCYTES in the MATRIX of the hair bulb of the hair follicle. The melanin is transferred to the cells in the hair shaft. Different proportions of brown-black, yellow and reddish melanin combine to produce all varieties of hair color from blond to black. Gray or white hair results from a decrease in melanin production. The melanin is replaced by air bubbles in the hair shaft.
One of the structures associated with hair follicles is a SMOOTH MUSCLE structure called the ARRECTOR PILI. Hair follicles are normally at a slight angle in the skin. The arrector pili muscles are attached to the hair follicle in such a way that their contraction pulls the hair follicle into an upright position, producing "goosebumps". The arrector pili muscles are regulated by the nervous system and are activated by cold external temperatures or fright.
Around each hair follicle are sensory nerve endings, called the HAIR ROOT PLEXUSES, which are sensitive to touch. They detect if a hair shaft is moved.
NAILS - Figure 5.7
A NAIL consists of a plate of hard, keratinized epidermal cells that form a clear protective covering on the dorsal surface of the distal part of a finger or toe. Nails protect the ends of the digits from trauma. Our fingernails help us to grasp and manipulate small objects.
Each nail has a FREE EDGE, a NAIL BODY (the visible attached portion), and the NAIL ROOT embedded in the skin. The CUTICLE consists of a band of stratum corneum (of the epidermis) at the base of the nail body. The nail body appears pink because of the rich bed of capillaries in the underlying dermis.
The NAIL MATRIX, which is deep to the nail root, is responsible for nail growth. The nail matrix contains rapidly dividing epithelial cells. As the nail cells are produced by the matrix, they become heavily keratinized, and the nail body slides distally over the nail bed. The average growth rate of fingernails is 1 mm per week, but the growth rate is slower in toenails. Longer digits have faster nail growth.
SEBACEOUS GLANDS - Figures 5.1, 5.5, & 5.6
SEBACEOUS (OIL) GLANDS secrete an oily secretion called SEBUM. The sebum contains fats, cholesterol, proteins, salts, and dead cell fragments. Sebum softens and lubricates the skin, making it more pliable. It prevents hair from becoming dry and brittle, and it prevents water loss from the skin when the external humidity is low. The sebum also inhibits bacterial growth on the skin.
The SECRETORY PORTION of the gland lies deep in the dermis. The DUCT of the sebaceous gland usually empties into a hair follicle, or to a pore on the skin surface if no hair is present (like the skin of the lips).
The sebaceous glands are found all over the body, except on the palms of the hands and soles of the feet. They are small on the body trunk and limbs, but quite large on the face, neck and upper chest.
SUDORIFEROUS (SWEAT) GLANDS - Figure 5.1 & 5.6
Humans have more than 3-4 million SUDORIFEROUS (SWEAT) GLANDS distributed over most of the body surface. There are 2 types of sweat glands: eccrine and apocrine.
The ECCRINE SWEAT GLANDS are the most numerous sweat glands and are particularly abundant on the palms of the hands, soles of the feet and in the forehead. The SECRETORY PORTION of the eccrine gland lies coiled deep in the subcutaneous layer; the DUCT extends upward to open in a funnel-shaped pore at the skin surface.
PERSPIRATION or SWEAT is about 99% water, with some salts, traces of metabolic wastes (urea, uric acid, ammonia, lactic acid) and vitamin C, as well as small amounts of drugs currently being taken. The exact composition depends on heredity and diet. Sweating is regulated by the involuntary portion of the nervous system. Its major role is to assist the body in thermoregulation (maintaining normal body temperature). Sweat cools the body by evaporation, which removes heat from the body surface.
The APOCRINE SWEAT GLANDS are largely confined to the AXILLARY (armpit) & INGUINAL (groin or pubic area) regions, as well as the areola (colored area around the nipple) and the bearded area of a man’s face. Apocrine glands are larger than eccrine glands. The DUCT opens into the hair follicles of axillary & pubic hair. Apocrine secretion contains the same basic components as eccrine sweat, but it also contains fatty acids and proteins, giving the sweat a milky or yellowish color. The secretion is odorless, but bacteria on the skin decompose the fatty acids and proteins, giving it its characteristic musky odor. Apocrine glands begin to function at puberty under the influence of androgens (sex hormones). They are responsive to emotional stress.
CERUMINOUS GLANDS
Ceruminous glands are modified sweat glands found in the lining of the external ear canal. The combined secretion of ceruminous and sebaceous glands in the ear canal is called CERUMEN, or ear wax, a sticky substance that apparently repels insects and blocks the entrance of foreign material.
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