Skin is the largest organ of the body and is the mirror of one’s personality. It is the barrier against the environmental insults and prevents loss of water from the body. Skin has important role in regulation of body temperature, perception, synthesis of vitamin D and social communication. We have to understand our skin to not only understand the problems of the skin, but also to adjust to the surrounding environment and to communicate with our fellow humans.
The epidermis is made up of keratinocytes and melanocytes. The keratinocytes are arranged in layers and the top-most layer is called the stratum corneum. This layer is made up of dead cells which are continuously shed. Lower most layer of the epidermis is called stratum germinatum. Cells in this layer divide constantly and give rise to daughter cells that replace the old cells in the stratum corneum. The process of daughter cells maturing and moving to the top layer is called keratinization (cell cycle). This process occurs constantly and continuously and the duration of this cell cycle is 26-42 days. The end product of this process is the stratum corneum. Stratum corneum maintains the water content in the skin. It is an effective barrier to trans-epidermal water loss and to penetration of exogenous substances. When the process of keratinization is disturbed either by disease (genetic or acquired), environmental factors or ageing, the barrier becomes defective. A defective stratum corneum leads to flaking of the skin due to increased trans-epidermal water loss, increased incidence of irritant/allergic dermatitis due to easy entry of obnoxious agents, increased incidence of UV radiation induced changes such as light eruption, pigmentation and growths.
Melanocytes are pigment (melanin) producing cells present in the lower most layer of the epidermis. They produce melanin and disperse it to the neighbouring keratinocytes. Melanin pigment gives colour to the skin and the hair and protects the skin from the sunlight. Each melanocyte is in connection with 32 neighbouring keratinocytes (epidermal melanin unit). Any defect in the process of synthesis and dispersion of the pigment leads to hyper (dark) or hypo (light) pigmentation.
Beneath the epidermis is the layer called dermis. It is made up of ground substance like hyaluronic acid and dermatan sulfate as well as fibres like elastin and collagen that are responsible for the elasticity of the skin. When the collagen and elastin in the dermis get damaged with age, the skin shows wrinkles.
The dermis has three types of glands: the apocrine, eccrine and sebaceous glands. The apocrine glands are found in association with hair follicles, abundantly in the arm pits. They are responsible for the body odour. The eccrine glands are distributed widely over the body; they produce sweat and help in regulation of the body temperature. Normally the sweat is odourless, but colonisation of certain bacteria may impart it a bad odour. The sebaceous glands are present throughout the skin except the palms and soles and are numerous on the scalp and the face. These glands secrete an oily substance called sebum, which lubricates the skin and forms a coating on the hair, keeping it soft and shiny. When the sebum secretion is inadequate, the skin becomes dry and wrinkled and when the secretion is increased, the skin becomes oily and shiny.
In addition to this, the dermis contains a network of blood vessels, lymphatics, nerves and cells of the immune system.
Subcutaneous tissue comprises of fat globules and largely supports the overlying dermis and epidermis.
Hair is essentially a vestigial structure that man has lost during evolution. However, any change in the pattern, quality, quantity or colour of the hair causes a tremendous concern, out of proportion to its physiologic function.
Each hair undergoes a cyclic growth in three phases:
- The growth phase – 3-4 years
- Followed by catagen – 10 days
- Followed by telogen (resting) – 2-3 months
During the resting phase the hair becomes detached from its root and falls out as a new growing hair replaces it in the follicle.
On an average, the scalp contains 100,000 hair. At any given time 10-15% of the scalp hair are in inactive phase, ready to fall out, while the remaining are in various stages of growth. A loss of up to 100 hairs per day is normal and is something one should not worry about as most of the lost hair would be replaced. With age, however, the rate of replacement slows down; so as you grow older, there is a natural tendency for some amount of sparseness of your scalp hair.
The rate of hair growth varies considerably, but on an average it is one cm per month and is more in summer than winter. Growth rate is maximum between 15-30 years. Cutting or shaving does not make the hair grow faster. Neither does trimming affect the growth rate in any way, though it does make you look good by eliminating straggling ends.
Good quality long hair runs in families and so does the male type baldness. A good balanced diet is a must for good growth and texture of hair.
Nails protect the tips of the fingers. The nail unit is composed of nail plate, nail matrix, nail bed and nail folds. The nail plate is surrounded on three sides by the nail fold, the invaginations of the epidermis that provide a guiding groove for nail growth. Cuticle is that part of the nail unit which is between the nail fold and the nail. It helps to seal the matrix and the soft nail from the outside. Nail matrix is a specialised epidermis that produces the nail. As the nail matrix cells differentiate, they keratinise (become hard) and produce the nail plate. The rate of growth of the nail plate is approximately 0.1mm per day.