The Integumentary System

The Integumentary System

The word integument means covering. The integumentary system cov-ers the outside of the body. It protects internal structures, prevents the entryof infectious agents, reduces water loss, regulates body temperature, producesvitamin D and detects stimuli such as touch, pain and temperature. Since  
theintegument performs several functions, it is commonly referred to as Jack ofall trades.

The skin or integument rests on layers of cells called hypodermis.The hypodermis attaches the skin to underlying bones and muscles. It suppliesblood vessels and nerves to the skin.

he skin is composed of two major tissues, namely dermis andepidermis. The dermis is mostly formed of connective tissue having fibro-blasts, adipose cells and macrophages. It provides the structural strength tothe skin. The dermis accommodates nerve endings, hair follicles, smooth musclesand glands.

It is divided into two layers, namely the superficial papillary layerand deeper reticular layer. The papillary layer has projections called papil-lae. The reticular layer is the major layer of the dermis. It is dense in nature.It is continuous with the hypodermis.

Epidermis :- The epidermis is made up of stratified squamous epithelium. Itis separated from the dermis by a basement membrane. It contains mel-anocytes giving colour to the skin. Many of the cells of the epidermis producea protein substance called keratin. Hence they are called as keratinocytes.

The deepest layers of the epidermis produce nerve cells by mitosis.As new cells are formed, the older cells are pushed to the surface. The sur-face cells will protect the inner new cells. Gradually the shape and chemicalnature of the surface cells will get altered. Slowly they get filled with keratin.This process is called keratinization. During this process the epidermis getsdivided into five distinct regions or strata. They are the stratum basale, stra-tum spinosum, stratum granulosum, stratum lucidum and stratum cor-neum.


Stratum basale is in the deeper region of the epidermis. It consists ofone layer of columnar cells. Keratinization of cells begins in this region. Abovethis layer stratum spinosum is seen. It has 8-10 layers of polygonal cells.The stratum granulosum is the next upper layer. It has 3-5 layers of flat-tened cells. Above this layer stratum lucidum occurs. It is a thin zone havingseveral layers of dead cells. The top most layer is called the stratum cor-neum. It consists of more than 20 layers of dead cells. These cells get filledwith keratin. They are said to be cornified. The cornified cells are surroundedby a hard protective envelope.

The skin can be either thick or thin. All five epithelial layers are seenin the thick skin. However stratum corneum contains more number of cells.Thick skin is formed in the soles of the feet, the palms of hands and tips offingers. The general body surface has thin skin. In the thin skin each epithe-lial layer inturn has few layers of cells. There are only one or two layers ofcells in stratum granulosum.


Callus :- The regions of skin subjected to constant friction or pressure arethickened to form the callus. The callus has several layers of cells in thestratum corneum.


Skin colour :- The colour of the skin is due to pigments in the skin. Thethickness of the stratum corneum and blood circulation can also cause skincolour. Normally the colour is caused by the pigment melanin. It providescolour to skin, hair and eye. It protects the body from sun’s ultraviolet rays.Melanin is produced by melanocytes. Melanin production is genetically deter-mined. However, hormones and exposure to light can also alter the colour.

Skin dervativesHair :- The hairs are integumentary structures. A hair has a root and a shaft.While the shaft projects above the skin, the root remains well below the sur-face. The base of the root has a hair bulb. It is an expanded region. The shaftand most of the root of the hair are formed of dead keratinized epithelial cells.These are arranged in three concentric layers called the medulla, the cortexand the cuticle. The central axis of the hair is formed of the medulla. Majorpart of the hair is formed of a single layer of cells.

According to the amount and types of melanin, the hair colour mayvary. The colour of the hair is genetically determined. During old age theamount of melanin decreases causing white hair. Grey hair has a mixture offaded, unfaded and white hairs.

The hair growth is due to addition of cells at the base of the hair root.The growth stops at specific stages. After a resting period, new hair replacesold hair. The hairs on the head grow for a period of three years and rest for1-2 years.

The muscle cells found associated with hair follicles are called thearrector pili. Contraction of these muscles cause ‘goose flesh’ making thehairs to ‘stand on end’.

The skin has sebaceous glands and the sweat glands. The seba-ceous glands are located in the dermis. They produce an oily substance calledthe sebum. These glands are connected by a duct to the upper part of the hairfollicles. The mammary glands are the modified sweat glands.

The most common type of sweat gland on the skin are the merocrineglands. They are simple coiled tubular glands. They open directly on to theskin through sweat pores. The gland has two parts. They are the deep coiledportion and the duct which passes to the surface of the skin. The number ofsweat glands are more in the palms of the hands and soles of the feet.

Nails :- Each nail is made up of two parts. They are the nail root and the nailbody. The  nail body is the visible part. The nail root is covered by the skin.The proximal and lateral edges of the nail are covered by nail fold.

The stratum corneum of the nail fold grows onto the nail body as theeponchium. The free edge of the nail body is the hyponchium. The nail isfound placed on the nail matrix and nail bed. A small white region seen at thebase of the nail is the lunula. It contains the nail matrix. The nails grow at anaverage rate of 0.5-1.2 mm per day.