Skin

Skin is the largest organ in the body occupying almost 2m2 of

surface area thickens of 2mm. Skin has 3 main parts. These

are the epidermis, dermis and hypodermis.

Epidermis is the outer layer of the skin that is made of

stratified squamous epithelium. It has no blood supply.

Epidermis contains 4-5 strata. These are stratum cornium,

lucidium, granulosum, spinosum and basale, Stratum corniumis the outer, dead, flat, Keratinized and thicker layer.

Stratum lucidium is next to stratum cornium. It consists of flat,

translucent layers of cells. This stratum found in thick skin

only.Stratum granulosum lies just below stratum lucidium. Thecells in this layer are in the process of keratinization

Stratum spinosum: next down to stratum granulosum. The

cells in this stratum have a poly-hydral shape and they are in

the process of protein synthesis. stratum basale rests on the basement membrane, and it is the last layer of epidermis next to stratum spinosum. Stratum basale together with stratum spinosum constitute stratum germinativum.

Dermis / true skin/ a strong, flexible, connective tissue mesh

work of collagen, reticular and elastic fibers. Most part of the

skin is composed of dermis. Dermis contains papillary and reticular layers. Papillary layer is next to stratum basale of the epidermis. It contains loose connective tissue with in the bundles of collagenous fibers. It also contains loose capillaries that nourish the epidermis. In some areas papillary layer have special nerve endings that serve as touch receptors (meissner’s corpuscles). Indentations of papillary layer in the palms and soles reflected over the epidermis to create ridges. Reticular layer: next to papillary layer. It is made of denseconnective tissue with course of collagenous fiber bundlesthat crisscross to form a storma of elastic network. In thereticular layer many blood and lymphatic vessels, nerves, fatcell, sebaceous (oil) glands and hair roots are embedded.Receptors of deep pressure (pacinian corpuscles) aredistributed through out the dermis.

Hypoderms: it is found beneath the dermis. It is a

subcutaneous layer (under the skin). Hypodermis is

composed of loose, fibrous connective tissue, which is richly

supplied with lymphatic and blood vessels and nerves.

Hypodermis is much thicker than dermis. With in it coils of

ducts of sudoriferous (sweat) glands, and the base of hair

follicles.

Functions of Skin

1. Protection: against harmful microorganisms, foreign

material and it prevents excessive loss of body fluid.

2. Temperature regulation: with the sweat, heat leaves the

body

3. Excretion: Small amount of waste products from the body

such as urea

4. Synthesis: By the action of UV. Vitamin D is synthesized

in the skin. Vitamin D is necessary for absorption calcium

from intestine.

5. Sensory reception: it contains sensory receptors of heat,

cold, touch, pressure, and pain.

Color of the skin

Skin’s color is determined by 3 factors

1. The presence of melanin a dark pigment produced by

specialized cell called melanocyte

2. The accumulation of yellow pigment carotene.

3. The color of blood reflected through the epidermis

* The main function of melanin is to screen out excessive

ultraviolet rays.

* All races have some melanin in their skins although the

darker races have slightly more melanocyte. The personwho is genetically unable to produce any melanin is an

albino.

Glands of the Skin

Glands of the skin are the sudoriferous and

sebaceous glands

.Sudoriferous /sweat/ glands

Types: Eccrine and Apocrine glands

Eccrine glands are small, simple coiled tubular glands

distributed over nearly the entire body, and they are absent

over nail beds, margins of lips of vulva, tips of penis. Eccrine

glands are numerous over the palms and soles. Their

secretary portion is embedded in the hypodermis. The sweat

they secret is colorless, aqueous fluid containing neutral fats,

albumin, urea, lactic acid and sodium chloride. Its excretion

helps body temperature to be regulated.Apocrine glands are odiferous, found at the armpits, in thedark region around nipples, the outer lips of the vulva, and theanal and genital regions. They are larger and deeply situatethan eccrine sweet glands. An apocrine sweet gland becomesactive at puberty. They respond to stress including sexualactivity. The female breasts are apocrine glands that havebecome adapted to secret and release milk instead of sweat.The ceruminous glands in the outer ear canal are also

apocrine skin glands

Sebaceous (Oil) glands

Sebaceous glands are simple branched alveolar glands foundin the dermis. Their main functions are lubrication and

protection. They are connected to hair follicles and secret oily

secretion called sebum. It is a semi fluid substance composedof entirely lipids. It functions as a permeability barrier, anemollient (skin softening) and a protective a gent againstbacteria and fungi. This type of gland found all over the bodyexcept in the palms and soles. Acne vulgaris is a conditionwhen there is over secretion of sebum, which may enlargethe gland and plug the pore.