Cirvello #4: The Integumentary System Flashcards
Integumentary System Anatomy
The outermost portion is made up of the skin or cutaneous tissue. The skin is composed of two layers composed of different cell types and properties.
Outermost layer is the epidermis, and the lower layer is the dermis. The skin is connected to the underlying tissues by the hypodermis, which isn’t part of the integumentary system.
Integumentary System Physiology
The skin protects the human body against mechanical stresses, chemical stresses, microorganisms, physical stresses, and dehydration.
The skin is a sensory system–an important source of info about the environment, including receptors for heat, cold, touch, pressure, and pain.
The skin plays a key role in the homeostatic regulation of internal body heat by acting as either a radiator (when the body is hot) or an insulator (when the body is cold). It does this by altering blood flow and producing sweat. The presence of fat beneath the skin aids in temperature regulation.
The skin is an endocrine organ that produces a chemical, 7-dehydrocholesterol, which is converted to calcitriol (vitamin D3) in the liver and kdiney. Calcitriol helps to regulate body Ca+2 balance.
The skin is capable of a small amount of excretion of body wastes (though it’s not a physiologically important means of ridding the body of wastes).
Epidermal Strata
In the epidermis, epithelial stem cell mitosis, located at the basement membrane, creates daughter cells that undergo kerantinization (terminal differentiation) as they move to the open surface. Keratinization refers to the process by which keratin protein is incorporated into lengthening keratin filaments. The polymerizing keratin filaments result in the loss of all cellular organelles, cellular metabolism, and eventually cell death. Fully keratinized cells at the skin surface are dead and completely filled with keratin. Even though keratinization is a continual process, distinct transitional stages are visible. These distinct stages can be used to divide the epidermis into strata, or layers. Basale, Spinosum, Granulosum, Lucidum, and Corneum.
Stratum Basale
The deepest layer of the epidermis. Composed of a single layer of cuboidal or columnar epithelium. This layer contains stem cells that create the cells found in all other strata. The cells are attached to the basement membrane through hemidesmosomes and to each other by desmosomes. Keratin intermediate filament proteins provide additional stability by anchoring cells together through desmosomes.
Stem cells undergo division approximately every 3 weeks by asymmetrical mitosis, creating a stem cell and a daughter cell. Further division and differentiation of the daughter cells pushes the cells toward the outer epidermal surface. As the daughter cells differentiate, they fill with keratin filaments, dehydrate (lose their water), and eventually die to form an outer cell layer with excellent resistance to mechanical abrasion, water, detergents, acids, and bases. It takes 6-8 weeks for a daughter cell to reach the outer epidermal layer where it’s eventually sloughed off. It’s estimated a substantial part of the dust in a common house is sloughed off epidermal cells.
Stratum Spinosum
Lies superficial to the stratum basale and consists of 8-10 cell layers. As cells differentiate and are pushed toward the outer surface, they become squamous in shape and the desmosomes rearrange themselves. During fixation for microscopic examination, cells in this layer usually shrink and appear spiny in shape, hence the name. Lamellar bodies, lipid-filled, membrane-bound organelles, are also found in this layer. A limited amount of daughter cell mitosis occurs in this layer. Mitosis does not occur in any of the more superficial strata.
Stratum Granulosum
Consists of two to five layers os squamous-like diamond-shaped cells with long axes parallel to the skin surface. This stratum derives its name from nonmembrane-bound granules of keratohyalin. Karatohyalin is made up of filaggrin and trichohyalin, two major intermediate filament-associated proteins that interact with keratin filaments. These two proteins initially accumulate in cytoplasmic granules called keratohyalin granules, and are a hallmark of this layer of the epidermis. As cells continue to move toward the next layer (stratum lucidum), they begin to lose their cellular orangelles and die, but the keratin fivers and keratohyalin granules do not degenerate. The stratum granulosum is not found in all areas of the skin.
Stratum Lucidum
A thin, clear layer or layers of dead cells superior to the stratum granulosum. In this layer, the keratohyalin granules disperse, making the cell transparent. The stratum lucidum is not found throughout the body; it is found only in areas of thick skin, like the soles of the feet and the palms.
Stratum Corneum
The most superficial stratum in the epidermis and is the layer visible to the eyes. This stratum has approximately 25 layers of dead squamous cells joined tightly be desmosomes. When mechanical stress breaks the desmosomes, the cells are lost from the surface. Dandruff is a visible example of the sloughing off of the stratum corneum from the scalp.
The stratum corneum is composed of cells that have been completely keratinized. The cells are also filled with lipids released from lamellar bodies that essentially make the stratum corneum waterproof. Keratin polymers give the stratum corneum structural strength. The molecular properties of keratin determine the stiffness and structural strength of the surface. Skin spidermis is composed of soft keratin (alpha-keratin); nails and hair shafts have hard keratin (beta-keratin). Cells that contain hard keratin are much more durable than those containing soft keratin, and they do not slough off.
Skin Color
Skin color is due to pigments in the epidermal layer, the thickness of the stratum corneum, and blood flow through skin capillaries. Melanocytes produce a variety of skin pigments. Melanin provides substantial protection against UV radiation, responsible for premature skin aging, as well as more serious problems such as melanoma. Melanin is concentrated in specific skin areas: freckles, moles, nipples, breast areolae, axillae, and genitalia. Other areas such as the lips, palms of the hand, and the soles of the feet contain much less melanin.
Dermis
Inferior to the epidermis and is composed of fibroblasts, collagenous fibers, adipose cells, and macrophages. The dermis is vascularized, but has much less blood flow than the hypodermis. The presence of fibrous connective tissue, adipose, and nervous tissue suggests that this layer of the skin supports the epidermis, and in fact, the dermis is responsible for most of the skin’s structural strength.
The dermis contains nerve cell endings, hair follicles, smooth muscle, sebaceous glands, and lymphatic vessels. The nerve cell endings are sensitive to pain, itch, tickles, and temperature variations. Specialized sensory cells are responsible for light touch (hair follicle cells), depp pressure (Pacinian corpuscles), two-point discrimination (Meissner’s corpuscles), and continuous pressure (Ruffini’s organs).
Dermal Layers
Dermis is divided into two layers: the superficial dermal papillary layer and the inferior reticular later. The reticular layer is the main layer of the dermis, continuous with the hypodermis, and composed of dense irregular connective tissue, made up of mostly irregularly arranged collagen, elastin, and reticular fibers resistant to stretching. There’s some orientation to the placement of collagen and elastin fibers, and these cleavage or tension lines are visible when the skin is stretched. Surgeons typically make incisions parallel to the cleavage lines because those incisions are less likely to pull apart and gap than incisions perpendicular to the lines, minimizing postsurgical infections, and scarring.
If the dermis is stretched to the point that these fibers separate and tear, the damage is visible through the epidermis. These lines, called stretch marks, typically develop int he breasts and abdomen of women during pregnancy or in individuals who have gained large amounts of weight and then lost it.
The superficial papillary layer derives its name from papillae (or porjections) that extend superficially toward the epidermis. Under the finers and feet, the papillae of the dermis are in parallel, curving ridges that shape the overlying epidermis into the whorls known as fingerprints and footprints. The ridges increase friction and improve the grip of the hands and the feet. The papillary layer is less dense than the reticular layer and is composed of loose connective tissue. The papillary layer is rich with capillaries that supply nutrients to the epidermis, remove waste products, and aid int he regulation of core body temperature.
Hypodermis
The structural framework that supports the integumentary system but is not part of it. Acts as a bridge between the integumentary system and the underlying skeletal muscle and bone. The hypodermis is extensively vascularized, extensively innervated, composed of many different cell types, and filled with loose connective tissue rich with collagen, reticular, and elastin fibers, in a ground substance. The hypodermis is also known as the superficial fascia.
The hypodermis is the site of approximately 50% of the body’s adipose tissue and stored fat. The amount and location of this adipose tissue is age, sex, and diet dependent. Fat plays several important roles: efficient energy storage, as an insulating blanket beneath the epidermis to prevent heat loss in the cold or heat gain during warm days, and to provide mechanical protection to the underlying tissues. Babies have large amounts of hypodermal fat, which accounts for their chubby appearance. Women typically have more fat than men do, and it’s distributed in different areas, creating sex-specific differences in body shape.
Because as much as 50% of all body fat can be found within the hypodermis, the thickness of the hypodermis is an indicator of total body fat. Skin calipers can be used to pinch the skin at selected locations, and the thickness of the skin and the underlying hypodermis measured. The thicker the fold, the greater the amount of body fat that exists. Body fat is expressed as a percentage of body weight, and ranges for 4-25% in men and 10-31% in women. Clinical investigations have shown that increases in percentage body fat are associated with an increased risk for heart disease, high blood pressures, gallstones, type 2 diabetes, certain cancers, and bone disorders. Not only does an increase in percentage body fat affect health, but the distribution of fat around the body also affects health. Larger amounts of visceral (belly) fat are associated with health problems.
