Chapter 5 - Integumentary System PDF

Preview

Summary

Notes about the Integumentary System ...

Description

Chapter 5: The Integumentary System The Skin  The skin covers the entire body, and is composed of 2 regions: 1. Epidermis – composed of epithelial cells and is the outermost protective shield of the body. 2. Dermis – makes up a majority of the skin, is tough, and composed of mostly fibrous connective tissue. Only the dermis is vascularized.  The hypodermis is not part of the skin, and is also called the superficial fascia, because it is superficial to the tough connective tissue wrapping of the skeletal muscles, and consists mostly of adipose tissue. o It anchors the skin to the underlying structures, but is loose enough so the skin can slide freely. o It also acts as a shock absorber and an insulator that reduces heat loss. The Epidermis  The epidermis is a keratinized stratified squamous epithelium consisting of 4 different cell types and multiple layers.  The cells of the epidermis include keratinocytes, melanocytes, dendritic cells, and tactile cells. o Keratinocytes: produce keratin, and are located at the deepest part of the epidermis from the stratum basale. These cells undergo almost continuous mitosis in response to prompting by epidermal growth factor, a peptide produced by various cells throughout the body. By the time they reach the skin surface, they are dead. o Melanocytes: are epithelial cells that synthesize melanin, and are found in the deepest layer of the epidermis. The melanin granules accumulate on the superficial side of the nucleus, forming a pigment shield that protects the nucleus from damaging effects of UV rays in sunlight. o Dendritic Cells: arise from bone marrow and migrate to the epidermis. They are also called Langerhan cells, and they ingest foreign substances, and are key activators of our immune system. o Tactile Cells: are present at the epidermal-dermal junction, and function as a sensory receptor for touch.  The layers of the epidermis vary in thickness. Thick skin, which covers areas that are prone to abrasion, like the palms, fingertips, and soles of the feet, are 5 layers deep. These layers include the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and the stratum corneum. In thin skin, which covers the rest of the body, the stratum lucidum is absent, and the other strata are thinner. o Stratum Basale: is the deepest layer of the epidermis, and is attached to the underlying dermis along a wavy borderline. o Stratum Spinosum: is several cell layers thick, and is a web like system of intermediate filaments. The keratinocytes look like they have spines, and are called prickle cells. They are artifacts that arise during tissue preparation when these cells shrink but their numerous desmosomes hold tight.

o Stratum Granulosum: is a thin layer in which keratinocytes appear to have drastic changes, and go through the process of keratinization. Granules in this layer help to form keratin in the upper layers, as well as contain a water-resistant glycolipid that helps to slow water loss across the epidermis. o Stratum Lucidum: is visible on in thick skin, and consists of flat, dead keratinocytes. o Stratum Corneum: is the outermost epidermal layer. Keratin and thickened plasma membranes of the cells protect the skin against abrasion and penetration, and the glycolipid between its cells nearly waterproofs this layer. It protects the deeper cells from a hostile external environment, water loss, and any chemical and physical assaults. The Dermis  The dermis is a strong flexible connective tissue layer, and the cells include fibroblasts, macrophages, mast cells, and white blood cells. Its semifluid matric, embedded with fibers, binds the entire body together. It has a rich supply of nerve fibers, blood vessels, and lymphatic vessels. It contains 2 layers: the papillary and the reticular layers. o Papillary Layer: is a thin, superficial layer that consists of areolar connective tissue with collagen and elastic fibers that form a loosely woven mat. The looseness allows for phagocytes and other defensive cells to move freely in the skin, looking for bacteria that have breached the surface of the skin.  The papillary layer contains dermal papillae, which overlap the dermis.  In thick skin, the larger mounds of papillae are known as dermal ridges, which then in turn form epidermal ridges. These skin ridges collectively are known as friction ridges, and are more commonly known as fingerprints. They enhance the gripping ability of the fingers and feet. o Reticular Layer: accounts for about 80% of the thickness of the dermis, and is coarse, irregular, and contains dense fibrous connective tissue. There are various separations, or less dense regions, between these bundles of skin, that form cleavage lines in the skin.  Flexure lines are dermal folds that occur at or near joints, where the dermis is tightly secured to deeper structures. Skin Color  Three pigments contribute to skin color: melanin, carotene, and hemoglobin. Of which, only melanin is made in the skin. o Melanin: is a polymer made of tyrosine amino acids. Its synthesis depends on an enzyme in melanocytes called tyrosinase, and it passed from melanocytes to the basal keratinocytes. The melanin pigment is found only in the deeper layers of the epidermis. The difference in skin colors reflects the kind and amount of melanin made and retained.

 Melanocytes of black and brown-skinned people produce many more and darker melanosomes than those of fairskinned people, and their keratinocytes retain it longer.  Freckles and moles are local accumulations of melanin. o When we expose our skin to sunlight, keratinocytes secrete chemicals that stimulate melanocytes. Prolonged sun exposure causes a substantial melanin buildup, which helps protect the DNA of viable skin cells from UV radiation by absorbing the rays and dissipating the energy as heat. o Carotene: a yellow-orange pigment that tends to accumulate in the stratum corneum and in fatty tissue of the hypodermis. o Hemoglobin: is the color of oxygenated, pinkish fair skin. It is located in the red blood cells circulating through the dermal capillaries. Because Caucasian skin contains only small amounts of melanin, the epidermis is nearly transparent and allows hemoglobin’s color to show through. Hair and Hair Follicles  The main function of hair in humans is to sense insects on the skin before they bite or sting us. Hair on the scalp guards the head against physical trauma, heat loss, and sunlight. Eyelashes shield the eyes, and nose hairs filter large particles like link and insects we inhale from the air.  Hairs, or pili, are flexible strands produced by hair follicles and consist largely of dead, keratinized cells. The hard keratin that dominates hairs and nails has 2 advantages over the soft keratin found in epidermal cells, including that it is tougher and more durable, and the individual cells do not flake off.  The shaft, is the portion of the hair in which keratinization is complete, and the root, is where keratinization is still ongoing. A hair has 3 concentric layers of keratinized cells: the medulla, the cortex, and the cuticle. o The medulla is the central core, and consists of large cells and air spaces. It is the only part of the hair that contains soft keratin, and is absent in fine hairs. o The cortex is the bulky layer surrounding the medulla, and consists of several layers of flattened cells. o The outermost cuticle is formed from a single layer of cells overlapping one another. This arrangement helps separate neighboring hairs so the hair does not mat. The most heavily keratinized part of the hair, the cuticle, provides strength and helps keep the inner layers tightly compacted.  Hair pigment is made by melanocytes at the base of the hair follicle and transferred to the cortical cells. When melanin production decreases and air bubbles replace melanin in the hair shaft, hair turns gray or white. Structure of a Hair Follicle  The deep end of the follicle expands to form a hair bulb. A knot of sensory nerve endings called a hair follicle, or a root hair plexus, wraps around each hair bulb. Bending the hair stimulates these endings. A hair papilla protrudes into the hair bulb, and contains a knot of capillaries that supplies nutrients to the growing hair and signals it to grow.



 

The epithelial root sheath, which has external and internal parts, things as it approaches the hair bulb, so that only a single layer of epithelial cells covers the papilla. The cells that compose the hair matric originate in a region called the hair bulge, which is located above the hair bulb. With each hair follicle are a bundle of smooth muscle cells called an arrector pili. This muscle is attached in such a way that its contraction pulls the hair follicle upright and dimples the skin surface to produce goose bumps in response to cold temperatures or fear. The most important role is that the contractions of the arrector pili muscle force sebum out of the hair follicles to the skin surface where it acts as a skin lubricant.

Nails  A nail is a scale-like modification of the epidermis that forms a clear protective covering on the dorsal surface of the distal part of the finger or toe. They contain hard keratin, and each nail has a free edge, a nail plate, or body, and a proximal root. o Nail Matrix: the thickened proximal portion of the nail bed. Is responsible for nail growth, and as the nail cells produced by the matrix become heavily keratinized, the nail body slides distally over the nail bed.  Nails normally appear pink because of the rich bed of capillaries in the underlying dermis. However, the region that lies over the thick nail matrix appears as a white crescent called the lunule.  Skin folds, also called nail folds, overlap the proximal and lateral borders of the nail. The proximal nail fold projects onto the nail body as the cuticle, and the thickened region beneath the free edge of the nail where dirt and debris tends to accumulate is the hyponychium. Sweat (Sudoriferous) Glands  Sweat glands, also called sudoriferous glands, are distributed over the entire skin surface. Humans have 2 types of sweat glands: eccrine and apocrine. In both types, the secretory cells are associated with myoepithelial cells, specialized cells that contract when stimulated by the nervous system. Their contraction forces the sweat into and through the gland’s duct system to the skin surface. o Eccrine Sweat Glands: secrete sweat, which is a hypotonic filtrate of the blood that passes through the secretory cells of the sweat glands and is released by exocytosis. It is 99% water. Its major role is to prevent the body from overheating. o Apocrine Sweat Glands: they are merocrine glands, which release their product by exocytosis like the eccrine sweat glands. Larger than eccrine glands, they lie deeper in the dermis or even in the hypodermis, and their ducts empty into hair follicles. Their secretion is odorless, but when bacteria on the skin decompose its organic molecules, it takes on a musky and unpleasant odor.  Apocrine glands begin functioning at puberty under the influence of the male sex hormone (testosterone) and play little role in maintaining a constant body temperature.

 

Ceruminous Glands are modified apocrine glands found in the lining of the external ear canals. Their secretion mixes with sebum that is thought to deter insects and block entry of foreign material. Mammary Glands are another variety of specialized sweat glands, and secrete milk.

Sebaceous (Oil) Glands  The sebaceous glands, or oil glands, are simple branched alveolar glands that are found all over the body. These glands secrete an oily substance called sebum. The central cells of the alveoli accumulate oily lipids until they become so engorged that they burst, so functionally these glands are holocrine glands.  Sebum softens and lubricates the hair and skin, prevents hair from becoming brittle, and slows water loss from the skin when external humidity is low. Functions of the Integumentary System  The functions include protection, body temperature regulation, cutaneous sensation, metabolic functions, blood reservoir, and excretions. The skin protects the body with chemical, physical, and biological barriers. Chemical barriers include skin secretions and melanin. Skin cells also secrete natural antibiotics called defensins that literally punch holes in bacteria. Melanin provides a chemical pigment shield to prevent UV damage to skin cells. o The continuity of skin and the hardness of its keratinized cells provide physical barriers. Biological barriers include the dendritic cells of the epidermis, macrophages in the dermis, and DNA itself. Dendritic cells are active elements of the immune system.  As long as the external temperature is lower than body temperature, the skin surface loses heat to the air, and to cooler objects in its environment. When the external environment is cold, dermal blood vessels constrict. Their constriction causes the warm blood to bypass the skin temporarily and allows skin temperature to drop to that of the external environment.  Tactile corpuscles and tactile discs allow us to become aware of a caress or the feel of our clothing against our skin, whereas lamella corpuscles alert us to bumps or contacts involving deep pressure.  When sunlight bombards the skin, modified cholesterol molecules are converted to vitamin D precursor, which is transported via the blood to other body areas to be ultimately converted to vitamin D, which plays various roles in calcium metabolism.  The body eliminates limited amounts of nitrogen-containing wastes (ammonia, urea, and uric acid) in sweat, although most such wastes are excreted in urine. Burns  First-degree Burns cause damage to only the epidermis. Symptoms include localized redness, swelling, and pain. They tend to heal in two to three days without special attention.  Second-degree Burns injure the epidermis and the upper region of the dermis. Symptoms mimic those of first-degree burns, but blisters also appear. The burned area is red and painful, but skin regeneration occurs with little or no scarring within three to four weeks if care is taken to prevent infection.



Third-degree burns are full-thickness burns, involving the entire thickness of the skin. There is little to no edema, and since the nerve endings have been destroyed, the burned area is not painful. Skin grafting is generally the treatment....