Dr. Antonio Tan
- Largest and heaviest organ of the body
- 16% of total body weight
- 1.2 to 2.3m2 surface
- A protective barrier and interface with our environment
IT IS DIVIDED INTO THREE LAYERS
a. The epidermis – epithelial layer of ectodermal origin.
b. The dermis – layer of connective tissues of mesodermal origin.
c. The basement membrane – anchors the epidermis to the dermis.
- Dermal projections at the junction of the dermis and epidermis.
- Interdigitate with the epidermal evaginations called epidermal ridges.
Hypodermis or subcutaneous tissue
- Lies beneath the dermis.
- Made up of loose connective tissues that may contain a pad of adipose cells.
c. SEBACEOUS GLANDS
d. SWEAT GLANDS
- Relatively impermeable to water, prevents water loss by evaporation.
- A receptor organ in continuous communication with the environment.
- Protects the organism from impact and friction injuries.
- With the melanin, provides protection against the sun’s ultraviolet rays.
- With the glands of the skin, blood vessels and adipose tissue, it has a major role in thermoregulation, body metabolism and excretion of various substances.
Dermatoglyphics – fingerprints
- Ridges and grooves arranged in distinct patterns found in the palms and soles.
- Unique for each individual.
The Langer’s lines
- Epidermal lines that are oriented perpendicular to the long axis of underlying muscles to allow the greatest degree of stretching and contraction without deformity.
- Mainly stratified, keratinized squamous epithelium.
- Also contains 3 other cell types the melanocyte, the Langerhan’s cell and the Merkel’s cell.
- Glabrous, smooth and non – hairy
- Palms and soles
- 400 – 600 microns
- About 75 – 150 microns
5 layers of the epidermis
a. Stratum Basale ( germinativum )
- Single layer of columnar or cuboidal cells resting on the basal lamina at the dermal – epidermal junction.
- Desmosomes in great quantity binds the cells of this layer in the lateral and upper surfaces.
- Hemidesmosomes – found in the basal plasmalemma help bind the cells to the basal lamina.
- This layer is characterized by intense mitotic activity.
b. Stratum spinosum
- Contains cuboidal, polygonal or slightly flattened cells with central nucleus.
- The epidermis of areas subject to continuous friction and pressure has a thicker stratum spinosum with more abundant tonofibrils and desmosomes.
These two layers together is termed the malpighian layer where all cell divisions of the epidermis take place.
c. Stratum granulosum
- Three to 5 layers of flattened polygonal cells filled with cytoplasmic granules called KERATOHYALINE granules.
- Lamellar granules contain intercellular cement like material that acts as a barrier to penetration by foreign substances.
d. Stratum lucidum
- Translucent thin layer of flattened cells.
- More apparent in thick skin.
e. Stratum corneum
- About 15 to 20 layers of flattened keratinized cells with NO NUCLEUS.
- Filled with keratin.
After keratinization, the cells are called HORNY CELLS.
In the horny layer the keratinocytes senesce, intercellular connection disappears and dead cells are shed.
The transit time of keratinocyte is 40 – 56 days (45 – 60 days).
- Increase in the number of proliferating cells in the malpighian layer
- Increase in the rate of proliferation of cells
- Results in greater epidermal thickness and more rapid renewal of the epidermis (7 days)
- specialized cells that produce the pigment EUMELANIN / melanin, derived from neural crest cells and have dendritic process.
- found beneath or between the cells of stratum basale and the hair follicles.
- release melanosomes by APOCOPATION.
- The melanin granule released are then phagocytized by the keratinocytes.
- once inside the keratinocyte the granules aggregate on the superficial side of the cell in an umbrella- like shape thus protecting the nucleus of the cells from the effects of ultraviolet light.
The number or density of melanocytes is constant between individuals, NOT influences by sex or race. The difference in the number of melanin granules in keratinocytes determines the skin color of the individual - Melanin and Carotene
Tanning after exposure to Ultraviolet Rays is a two step process
1. Darkening of pre-existing melanin and rapid release into the keratinocytes
2. Increase in the rate of melanin synthesis and increase in the amount of the pigment
- Hereditary inability of the melanocyte to synthesize Melanin because of lack of tyrosine activity
- Increased incidence of Basal and squamous cell Ca
- Genetically regulated degeneration and disappearance of melanocytes.
- Star shaped cells found mainly in the stratum spinosum of the epidermis
- Contain the characteristic rod-like or racket-shaped inclusion bodies (Birbeck’s granules)
- came from the bone marrow and function as the skin macrophages
- immunosurveillance against viral infections and neoplasms of the skin
- present in the thick skin of palms and soles
- serve as sensory mechanoreceptors
- Composed of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis).
- Consists mostly of several structural protein – mainly collagen ( 70% ) that is responsible for the skin’s tensile strength.
- Capable of being reversibly stretched to twice their resting length.
- Responsible for the elasticity of the skin.
The surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate projection of the epidermis (the epidermal pegs or ridge).
At the dermal-epidermal junction
- Basal lamina – found between the stratum germinativum and the papillary layer of the dermis follows the contour between these layers.
- Lamina reticularis – delicate network of reticular fibers just under the basal lamina.
Layers in the dermis
a. papillary layer – thin, composed of loose connective tissue fibroblasts and other connective tissue cells and macrophages and mast cells
Anchoring fibrils – binds the dermis and the epidermis
b. Reticular layer
– The deeper layer, thicker composed of irregular connective tissue mainly Collagen Type I.
– More fibers, less cells that are immersed in a dense glycosaminoglycan (dermatan sulfate).
– Also contains a network of elastic fibers.
– A rich network of blood and lymph vessels.
– This vascular network plays a very important role in temperature and blood pressure regulation.
– The Glomus bodies – are tortuos A-V shunts that allow a tremendous increase in blood flow to the skin.
– Consists of loose connective tissue that binds the skin loosely to the subjacent organs allowing it to slide over.
– Contains fat cells that vary in number according to the area of the body, sex and nutritional status.
– Superficial fascia or panniculus adiposus.
- Elongated keratinized structures color, size and disposition vary according to race, age, sex and region of the body.
- Absent on the palms, soles, lips, etc.
- Grow discontinually with periods of growth (ANAGEN) and periods of rest (TELOGEN or CATAGEN).
- The hair has a root or radix and a shaft.
- Arises from epidermal invagination - the Hair follicle.
- Hair bulb is the terminal dilatation of the follicle.
- At the base of the bulb is the dermal papilla with contains a capillary network that sustains the follicle.
Regions of the Hair
a. medulla – central region made up of large vacuolated and moderately keratinized cells
b. hair cortex
c. hair cuticle
d. internal root sheath
e. external root sheath
f. Glassy membrane – basal lamina
Arrector pili muscles
- Bundles of smooth muscle that connects the connective tissue sheath of the follicle to the dermis.
Hair color is caused by the activity of Melanocytes between the papilla and the epithelial cells of the hair root.
- Absence of melanin pigment and the deposition of air bubbles in the medulla and cortex of the hair shaft produce the “GREYING of the HAIR”.
- Plates of keratinized epithelial cells on the dorsal surface of each distal phalanx.
Parts of the hair
a. Nail root – proximal part of the nail hidden in the nail groove by the nail fold.
b. Nail plate - corresponds to the stratum corneum of the skin.
c. nail bed – does not contribute to the formation of the nail
d. lanula – white opque cresent at the proximal end of the nail
EPONYCHIUM OR THE CUTICLE
- Corresponds to the stratum corneum.
GLANDS OF THE SKIN
- Produce sebum that gradually moves up to the surface of the skin then the hair shafts.
- Embedded in the dermis.
- Not found in glabrous skin.
- Begin to function at puberty with TESTOSTERONE as the primary controlling hormone in men.
- Begins to function at puberty with TESTOSTERONE as the primary controlling factor in men.
- In women it is a combination of ovarian and adrenal androgens.
FUNCTIONS of the sebum
a. Preserve flexibility of emerging hair.
b. Have weak antibacterial and anti-fungal properties.
Disturbances of normal flow of the sebum or secretion may lead to the development of ACNE.
- Sudoriferous gland.
- Widely distributed over the surface of the body except lips, nail beds, glans penis.
- Have two classes of glandular structures.
a. Eccrine (merocrine)
- Simple, coiled tubular glands whose ducts open at skin surface.
- Secretion is not viscous containing little protein.
- Main components are water, NaCl, urea, ammonia and uric acid.
- Have two types of cells present; the dark cells which contain secretory granules and the clear cells.
Following the release of eccrine secretion on the surface of the skin, sweat evaporates, cooling the surface and the blood in the underlying dermal capillary network.
b. the apocrine gland
- Present in the axillary, areolar and anal regions, external genitalia.
- Larger than eccrine sweat glands.
- Found in the subcutaneous tissue.
- Ducts open into the hair follicle.
- Thicker secretion.
Vessels and Nerves
- Rich complex network with 2 arterial plexuses (dermis and subcutaneous layer) thin vertical branches leave these plexuses and vascularize the dermal papillae.
- 3 venous plexus – two travel together with the arterial plexuses and 1 in the middle of the dermis.