Integumentary System

I. Overview Diagram

• Two Distinct regions of the integumentary system: epidermis & dermis.
• Only dermis is vascularized
• Nutrients reach epidermis by diffusing through the tissue from the blood vessels
• Underneath dermis is hypodermis (superficial fascia), also called subcutaneous tissue which is composed of loose connective tissue (mostly areolar and adipose).

II. Epidermis

• Thick keratinized stratified squamous epithelium consisting of four distinct cell types and five distinct layers.

A. Cells: Keratinocytes, Melanocytes, Merkel Cells, and Langerhans Cells

• Keratinocytes
  • Cells producing keratin (fibrous protein)
  • Keritinocytes produce keratin continuously until reaching the surface of the skin, then these scale-like structures are sloughed off. The hands and feet have accelerated keratin production
  • Keratinocyte production and keratin formation are regulated by a hormone, epidermal growth factor (EGF).
• Melanocytes
  • Cells synthesizing the pigment melanin
  • These cells are located in deep layers of epidermis and contain branching processes that contact all keratinocytes in basal epidermal layer
  • The melanin formed in melanosomes are transported to branching processes, shed, and engulfed by keratinocytes
  • It must be noted that all races have the same relative number of melanocytes.
• Langerhans cells
  • Star-shaped cells arising from bone marrow that migrate to epidermis
  • They are macrophages (engulf foreign particles by phagocytosis)
  • Their primary role is immunologic, the first line of defense.
• Merkel cells
  • Hemisphere-shaped cells located at epidermal-dermal junction and are associated with a sensory nerve fiber (sensory/touch perception)

B. Layers of Epidermis (strata) Diagram

• Stratum basal (stratum germinativum)
  • Basal layer, separated from dermis by thin basement membrane; single row of cells (mostly columnar keratinocytes) with rapid mitotic division; also contain merkel cells and melanocytes.
• Stratum spinosum
  • Many layers, less mitosis; the keratincoytes contain tononfilaments (keratin filaments); superficial stratum spinosum receives less nutrients, therefore keratin production increases (and cells less viable begin to die).
• Stratum granulosum
  • Three to five layers of flattened cells; cells contain tonofilaments and laminated granules which contain a waterproofing glycolipid that is secreted into the intercellular space to slow water loss across the epidermal layer.
• Stratum lucidum (clear layer)
  • Thin translucent band, few rows of flattened dead keratinocytes.
• Stratum corneum (horny layer)
  • Very thick (25 + layers) with shingle- like dead cells.

III. Dermis

• Flexible connective tissue layer containing elastic, collagen, and reticular fibers; dermal cellular components include fibroblasts, macrophages, mast cells (produce heparin and histamine), and white blood cells; dermis also contains nerve fibers, blood vessels, and lymphatic vessels; there two major dermal layers: the papillary and reticular layers.
  • Papillary layer - loose connective tissue with superior surface arranged into nipple-like projections (dermal papillae) that indent the epidermis above; may contain capillary loops, free (bare) nerve endings (pain receptors), and touch receptors (Meissner's corpuscles); ventral portion of hands and feet have definite patterns (looped and whorled) called epidermal ridges which increase/enhance gripping ability; sweat pores in ridges cause films of sweat (fingerprints).
  • Reticular layer - (80% of dermis) dense connective tissue containing bundles of interlocking collagen fibers that run in various planes parallel to the skin surface; elastic and reticular fibers also present; separations or less dense regions are referred to as lines of cleavage or tension lines in skin; cleavage lines run longitudinally in limbs and circular around neck and trunk; third type of skin marking = flexure lines, dermal folds positioned near or at joints
    result from dermis being tightly secured to deeper structures by the hypodermis - skin cannot slide easily to accommodate joint movement (obvious on wrists, palms, soles of feet, fingers, and toes).

IV. Skin Color

• Three pigments contribute to skin color: melanin, carotene, and hemoglobin
• Racial differences are due to the amount and variations of melanin produced
• Ultraviolet light stimulates production of melanin and excessive exposure causes solar elastosis (clumping of elastin fibers) and DNA damage
• Carotene derived from vitamin A accumulates in stratum corneum and adipose tissue and imparts an orange color to the skin. Lastly hemoglobin in red blood cells give a pinkish hue on fair skin.
• Skin color varies as a result of disease or emotional states:
  • Redness (erythmia) - reddened skin, embarrassment, fever, hypertension, inflammation, or allergy
  • Pallor/blanching - pale skin, emotional distress or anemia, low blood pressure
  • Jaundice - liver disease, bile deposited in tissue
  • Bronzing - bronze coloration (addison's disease) hypofunction of adrenal cortex
  • Black & blue - bruises, escaped blood clots in tissue spaces (clotted blood masses = hematomas)

    NOTE: know dermatomes and the rules of nine!

V. Clinically Related Terms

• Macule - localized change in color (freckle)
• Wheal - accumulated fluid in papillary layer (hives)
• Papule - solid elevated area, epidermal and papillary (insect bite)
• Nodule - large papules extending into subcutaneous layer (cyst)
• Vesicle - papule with fluid core (second degree burn)
• Pustule - papule with pus core (acne)
• Erosion - ruptured vesicle (ulcer)
• Xerosis - "dry skin"
• Hemangiomas - benign tumor in the dermis (capillary and cavernous)
• Sebaceous hyperplasia - enlargement of the sebaceous gland
• Pruritis - irritating itching sensation of the skin
• Seborrheic dermatitis - inflammation around abnormally active sebaceous glands
• Basal cell carcinoma - malignant cancer originating in the germinative layer
• Squamous cell carcinoma - malignant cancer originating in the top layer of the skin
• Malignant melanomas - metastasizing melanocytes