skin anatomy and physiology

If you are looking for the detailed information about skin anatomy and physiology, you have landed at a right place. In this article, you will find the information about skin anatomy and physiology, skin layers, dermis, hair, nails and skin system.

ANATOMY OF THE SKIN

The skin is the largest organ of the body. It consists of specialized epithelial and connective tissue cells and has many protective and synthetic functions.

DIFFERENT LAYERS OF THE SKIN

The skin is composed of two layers, the epidermis and the dermis. Thesubcutaneous tissue, also referred to as the subcutis or panniculus, is located underneath the dermis.

1. EPIDERMIS

The epidermis is mainly composed of cells called keratinocytes. It is divided into four layers:

a. Stratum Basale (Stratum Germinativum or Basal layer).

b. Stratum spinosum.

c. Stratum granulosum.

d. Stratum corneum.

The basal layer is composed of undifferentiated proliferating stem cells. Newly formed cells from this layer migrate upward and begin the process of differentiation.

The stratum spinosum lies above the basal layer and is composed ofkeratinocytes, also known as squamous cells. These cells produce keratin, which is a fibrous protein. This stratum spinosum derives its name from the "spines" or intercellular bridges that extend between cells.

The stratum granulosum, or granular layer, is composed of cells that containkeratohyaline granules, which are visible with light microscopy.

The stratum corneum is composed oflarge, flat, keratin-filledcells. They are vertically stacked in layers that range in thickness from 15 layers on most surfaces, to as many as 100 layers on the palms and soles.

In summary, the epidermis is composed ofcells that divide in the basal layer, keratinize in the stratum spinosum and granular cell layers, and ultimately differentiate into "dead" cells in the stratum corneum. It takes approximately 4 weeks for migration of a cell from the basal layer to the stratum corneum, where it will be shed.

Melanocytesare pigment-producing cells with long, squidlike extensions calleddendrites. They are located in the basal cell layer. The dendrites facilitate the transfer of pigment granules, called melanosomes, to neighboringkeratinocytes. The number of melanocytes in the epidermis is the same regardless of race or color. The number and size of the melanosomes account for racial differences in skin color. Sunlight stimulates melanocytic activity and transfer of melanosomes.

Langerhans’ cells are also dendritic cells that have an immunologic function. They are located between keratinocytes. On electron microscopy, diagnostic tennis racket-shaped organelles called Bribeck’s granules are seen.

Indeterminate dendritic cells lack melanosomes andBribeck’s granules. In some ways they are similar to Langerhans’ cells; however, their exact function is not known.

Merkel’s cells are located directly above the basement membrane. They probably enhance touch sensation.

The junction between the epidermis and dermis is referred to as the basement membrane zone. It permits selective exchange of cells and fluid between the epidermis and dermis. Furthermore, it provides structural support for the epidermis and "glues" the epidermis to the dermis.

2. DERMIS

Skin Anatomy

Skin Layers

The dermiscontains blood vessels, nerves and cutaneous appendages.  It is much thicker than the epidermis.  The principal components of the dermis arecollagen and elastic fibers andground substance, which are synthesized by dermal fibroblast cells.  Collagen and elastic fibers are fibrous proteins that provide structural support to the dermis. The ground substance fills the spaces between fibers.

Beneath the dermis is thesubcutaneous tissue. It is composed of fat cells or lipocytes, which are separated by islands (septa) of collagen and blood vessels.

VARIOUS SKIN APPENDAGES

The skinappendages include theeccrine and apocrine sweat glands, hair follicles, sebaceousglands, and nails.

Eccrine sweat glands help regulate body temperature by releasing sweat onto the surface of the skin.  The sweat evaporates, thereby facilitating the cooling process.  There are 2 to 3 million eccrine sweat glands on the body, which can secrete 10 liters of sweat per day.

Apocrine sweat glands are responsible for body odor.  The odor results when bacteria act on odorless apocrine sweat.  Apocrine glands are most numerous in the axillae (armpits) and anogenitalregion.

HAIR

Hairs grow out of tubular invaginations of the epidermis known as follicles, and a hair follicle and its associated sebaceous glands are referred to as apilosebaceous unit. 

(An electron micrograph of the base of hair follicle. The epidermal papilla, shown at the base, controls the growth rate of the hair follicle)

Hair follicles (located over the entire body surface with the exception of the palms and soles) extend into the dermis at an angle.

A small bundle of smooth muscle fibers, the arrector pili muscle, extends from just beneath the epidermis and is attached to the side of the follicle at an angle. Arrector pili muscles are supplied by adrenergic nerves, and are responsible for the erection of hair during cold or emotional stress ('goose flesh'). The sebaceous gland is attached to the follicle just above the point of attachment 0of the arrector pili.

At the lower end of the follicle is the hair bulb, part of which, the hair matrix, is a zone of rapidly dividing cells which is responsible for the formation of the hair shaft.  Hair pigment is produced by melanocytes in the hair bulb.  Cells produced in the hair bulb become densely packed, elongated and arranged parallel to the long axis of the hair shaft.  They gradually become keratinized as they ascend in the hair follicle.

Similar to skin and nails, actively dividing matrix cells differentiate and ultimately form a keratinous structure, the hair shaft.  Hair growth is cyclic and is composed of three phases.

Anagen (growing)

Catagen (transition)

Telogen (resting)

The length of the phases varies from one site to another.

NAILS

Nails similar to skin and hair, are made of keratin that is produced by a matrix of dividing and differentiating cell.  The nail unit has four components:

i.          Proximals nail fold

ii.         Matrix

iii.       Nail bed

iv.        Hyponychium

The proximal nail fold protects the matrix and forms the cuticle.  The matrix produces the cells that will ultimately become the nail plate.  The nail bed is the surface on which the nail plate lies.  The pink color of the nail bed is due to blood vessels in the dermis.  The hyponychium is located beneath the distal free edge of the nail.

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