7 Integumentary System

2 distinct regions of the skin

1. epidermis (composed of epithelium) and an underlying connective tissue, 2. the dermis.

Accessory Organs of the Skin

Accessory organs of the skin- cutaneous glands, hair and nails are all derivatives of the epidermis, but they reside primarily in the dermis. They originate from the stratum basale and grow downward into the deeper skin regions

integument

considered an organ system because it consists of multiple organs, the skin and its accessory organs.

Below the dermis

hypodermis or superfiscial fascia (principally adipose tissue and NOT part of the skin)

Two principal regions of the Dermis

1. Papillary Layer: the more superficial dermal region composed of areolar connective tissue. It is very uneven and has fingerlike projections from its superior surface, the dermal papillae, which attach it to the epidermis above. These projections like on top of the larger dermal ridges. In the palms of the hands and the soles of the feet, they produce the fingerprints, unique patterns of epidermal ridges taht remain unchanged throughout life. Abundant capillary networks in the papillary layer furnish nutrients for the epidermal layers and allow heat to radiate to the skin surface. The pain (free nerve endings) and touch receptors (tactgicle corpuscles in hairless skin) are also found here. 2. Reticular layer: the deepest skin layer is composed of dense irregular connective tissue and contains many arteries and veins, sweat and sebaceous glands, and pressure receptors (lamellar corpuscles).

Skin Color is the result of:

1. relative amount of melanin in skin 2. relative amount of carotene in skin (yellow-orange pigment present in stratum corneum and adipose tissue of the hypodermis) 3. degree of oxygenation of the blood Important diagnostic tools: flushed skin may indicate HYPERTENSION or fever, whereas pale skin is typically seen in ANEMIC individuals; when the blood is inadequately oxygenated, as during asphyxiation and serious lung disease, both blood and the skin take on a bluish cast, a condition called CYANOSIS JUANDICE, in which the tissues become yellowed, is almost always diagnostic for liver disease, whereas a bronzing of the skin hints that a person's adrenal cortex is hypoactive (ADDISON'S DISEASE).

Hair follicle

A structure formed from both epidermal and dermal cells. Its inner epithelial root sheath, with two parts (internal and external), is enclosed by a thickened basement membrane, the glassy membrane, and by a peripheral connective tissue (or fibrous) sheath, which is essentially dermal tissue. A small nipple of dermal tissue protrudes into the hair bulb from the peripheral connective tissue sheath and provides nutrition to the growing hair. It is called the hair papilla. A layer of actively dividing epithelial cells called the hair matrix is located ontop of the hair papilla.

2 types of sweat glands:

Eccrine sweat glands: Also called merocrine sweat glands, these glands are distributed all over the body. They produce clear perspiration consisting primarily of water, salts (mostly NaCl), and urea. Eccrine sweat glands, under the control o the nervous system, are an important part of the body's heat-regulating apparatus. They secrete perspiration when the external temperature or body temperature is high. Apocrine sweat glands: Found predominantly in the auxillary and genital areas, these glands secrete the basic components of eccrine sweat plus proteins and fat-rich substances. Apocrine sweat is an excellent nutrient medium for microorganisms typically found ont he skin. This sweat is initially ordorless, but when bacteria break down its organic components, it begins to smell unpleasant.

Nails

Free edge: portion of the nail growing out away from the body Hyponychium: region beneathe the free edge of the nail Root: the part that is embedded int he skin and adheres to an epithelial nail bed Nail Folds: Skin folds that overlap the borders of the nail Eponychium: Projection of the thick proximal nail fold commonly called the cuticle Nail Bed: Extention of the stratum basale beneath the nail Nail Matrix: The thickened proxmial part of the nail bed containing germinal cells responsible for nail growth. As the matrix produces the nail cells, they become heavily keratinized and die. Thus nails, like hairs, are mostly nonliving material. Lunule: the proximal region of the thickened nail matrix which appears as a white crescent moon. Everywhere else, nails are transparent and nearly colorless, but they appear pink because of the blood supply in the underlying dermis. When someone is cyanotic because of a lack of oxygen in the blood, the nail beds take on a blue cast.

Cells of the Epidermis

Keratinocytes (literally, keratin cells); these are the most abundant epidermal cells, their main function is to produce keratin fibrils. Keratin is a fibrous protein that gives the epidermis its durability and protective capabilities. Keratinocytes are tightly connected to each other by desmosomes. Melanocytes: spidery black cells that produce the brown-to-black pigment called melanin. The skin tans because melanin production increases when the skin is exposed to sunlight. The melanin provides a protective pigment umbrella over the nuclei of the cells in the deeper epidermal layers, thus shielding their genetic material (DNA) from the damaging effects of ultraviolet radiation. A concentration of melanin in one spot is called a freckle. Dendritic cells: Also called Langerhans cells, these cells play a role in immunity by performing phagocytosis. Tacticle (Merkel) cells: Occasional spiky hemispheres that, in combination with sensory nerve endings, form sensitive touch receptors called tactile or Merkel discs located at the epidermal-dermal junction.

Cutaneous Glands 2 categories

Sebaceous glands: found nearly all over the skin except for palms and soles. Their ducts usually empty into a hair follicle, but some open directly on the skin surface. Sebum is the product of sebaceous glands; it is a mixture of oily substances and fragmented cells that acts as a lubricant to keep the skin soft and moist (a natural skin cream) and keeps the hair from becoming brittle. The sebaceous glands become particularly active during puberty, when more male hormones (androgens) begin to be produced for both genders; thus the skin tends to become oilier during this period of life. [Blackheads are accumulations of dried sebum, bacteria, and melanin from epithelial cells in the oil duct. Acne is an active infection of the sebaceous glands. Sweat (Sudoriferous) Glands: exocrine glands that are widely distributed all ove the skin. Outlets of the glands are epithelial openings called pores. Sweat glands are categorized by the composition of their secretions into eccrine sweat glands and apocrine sweat glands.

4 epidermal layers

Stratum corneum- (top layer or most superficial layers, composed of 20-30 layers of dead cells, essentially flat membrane sacs filled with keratin; glycolipids in extracellular space Stratum granulosum- (one to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum- several layers of keratinocytes joined by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale- deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers.

Papillary and Reticular layers are abundant in collagen and elastic fibers...

The elastic fibers give skin its exceptional elasticity in youth, with age, the number of elastic fibers decreases, and the subcutaneous layer loses fat, which leads to wrinkling and inelasticity of skin. Fibroblasts, adipose cells, various macrophages, and other cell types are found throughout the dermis. The abundant dermal blood supply allows the skin to play a role in the regulation of body temperature. When body temperature is high, the arterioles serving the skin dilate, and the capillary network of the dermis becomes engorged with heated blood. Thus body heat is allowed to radiate from the skin surface. Any restrictions of the normal blood supply to the skin results in cell death and, if severe enough, skin ulcers. Bedsores (decubitus ulcers) occur in bedridden patients who are not turned regularly enough. The weight of the body puts pressure on the skin, especially over bony projections (hips, heels, etc.), which leads to restriction of the blood supply and tissue death. The dermis is also richly provided with lymphatic vessels and nerve fibers. Many of the nerve endings bear highly specialized receptor organs that, when stimulated by environmental changes, transmit messages to the central nervous system for interpretation. Some of these receptors- free nerve endings (pain receptors), a lamellar corpuscle, and a hair follicle receptor (also called a root hair plexus)- are shown in Fig. 7.1 (These receptors are discussed in depth in Excercise 22).

Hairs and Associated Structures

Two primary regions: Hair Shaft, Hair Root (follicle and bulb) 2. Hair Root: is beneath the surface of the skin embedded with the hair follicle. The hair bulb is a collection of well-nourished epithelial cells at the base o fhte hair follicle. The hair shaft and teh hair root have three layersof keratinized cells: the MEDULLA in the center, currounded by the CORTEX, adn the protective CUTICLE. Abraision of the cuticle a thte tip fo the hair shaft results in split ends. Hair color depends on the amount and type of melanin pigment found in the hair cortex.

avascular epidermis

a keratinized stratified squamous epithelium consisting of four distinct cell types and four or five distinct layers.

Arrector pili muscle

small bands of smooth muscle cells connect each hair follicle to the papillary layer of the dermis. When these muscles contract (during cold or fright), the slanted hair follicle is pulled upright, dimpling the skin surface with goose bumps. This phenomenon is especially dramatic in a scared cat., whose fur actually stands on end to increase its apparent size.