Human skin

Human skin is outer covering of body which performs body`s vital functions of protection, sensation, excretion of sweat through sweat, glands, body  heat regulation (Temperature Homeostasis), storage  of fats and synthesis of Vitamin D.It is the largest sense organ with total area of about 1.5 Sq meters. It weighs 16% of body weight.Skin consists of several tightly arranged dead cells which are impermeable to water and microorganisms.Outer layer of skin consists of several layers  dead cells made of keratin protein. Skin on palms and foot souls is thick and devoid of hair follicles. Finger tips have patterns called finger prints which are highly unique to individual.Skin can be divided into three layers namely Epidermis, Dermis and Hypodermis.

Epidermis

Epidermis is avascular region that is it does not contain any blood vessels.It is composed of about 40 to 50 rows of cells called squamous epithelial cells (keratinocytes). Specialized pigment bearing cells called Melanocytes produce the pigment melanin to protect the skin from ultraviolet radiation.The epidermis is arranged in five distinct layers namely Stratum basale, Stratum spinosum, Stratum granulosum and stratum lucidum and Stratum corneum.The outermost layer of skin is the stratum corneum. 

Stratum corneum 

It is made of many rows of flattened, dead keratinocytes that protect the underlying layers.  Dead keratinocytes are constantly being shed from the surface of the stratum corneum and being replaced by cells arriving from the deeper layers

Stratum lucidum 

It is made of several rows of clear, dead keratinocytes that protect the underlying layers.

Stratum granulosum

In this layer keratinocytes begin to produce waxy lamellar granules to waterproof the skin. The keratinocytes in the stratum granulosum are removed from the dermis that they begin to die from lack of nutrients.The stratum basale and stratum spinosum layers of the epidermis contain a sterol molecule known as 7-dehydrocholesterol. When UV light present in sunlight , it penetrates through the outer layers of the epidermis and activates molecules of 7-dehydrocholesterol, converting it into vitamin D3. Vitamin D3 is converted in the kidneys into calcitriol, the active form of vitamin D.

Stratum spinosum 

In this layer where Langerhans cells are found along with many rows of spiny keratinocytes. The spines found here are cellular projections called desmosomes that form between keratinocytes to hold them together and resist friction.

Stratum basale

It  contains the stem cells that reproduce to form all of the other cells of the epidermis. The cells of the stratum basale include cuboidal keratinocytes, melanocytes, and Merkel cells.

Dermis

The dermis is the  layer of the skin present below the epidermis. The dermis is made of dense irregular connective tissue along with nervous tissue, blood, and blood vessels. The dermis is much thicker than the epidermis and gives the skin its strength and elasticity. Within the dermis there are two regions called the papillary layer and the reticular layer.The papillary layer contains many finger-like extensions called dermal papillae that protrude superficially towards the epidermis.The dermal papillae increase the surface area of the dermis and contain many nerves and blood vessels that are projected toward the surface of the skin. Blood flowing through the dermal papillae provide nutrients and oxygen for the epidermal cells. The nerves of the dermal papillae are used to feel touch, pain, and temperature.

      Pacinian corpuscles are mechanoreceptors present deep with in the dermis which are sensitive to pressure.Ruffini's corpuscles  in the dermis are receptors sensitive to vibrations.Meissner corpuscles are sensitive to sense of touch.The hair follicles and skin glands are derived from the epidermis but are deeply embedded in the dermis. The dermis is richly supplied with blood vessels.Nociceptors sense the unpleasant pain and are responsible for reflex actions.

Hypodermis

The hypodermis serves as the flexible connection between the skin and the underlying muscles and bones and fat storage area. Fatty adipose tissue in the hypodermis stores energy in the form of triglycerides. Adipose also helps to insulate the body by trapping body heat produced by the underlying muscles.

Hair

Hair is  bristle like structures tightly packed dead keratinocytes. Hair helps to protect the body from UV radiation from sun light. Dence hair on legs and hands in some people protects skin from mosquito bites.Hair has three parts namely follicle, root and shaft.

Sweat glands

There are two types of sweat glands namely Eccrine sweat glands distributed almost every where on body producing water and sodium chloride. The second type called Apocrine sweat glands are found mainly under arms and pubic region which produce oily secretions. The digestion of these secretions by bacteria produce characteristic body odor.

Sebaceous Glands
Sebaceous glands  produceoily secretion known as sebum. Sebaceous glands are found in every part of the skin except for the thick skin of the palms of the hands and soles of the feet. Sebum is produced in the sebaceous glands and carried through ducts to the surface of the skin or to hair follicles. Sebum water proofs the skin and protects the hair surface.

Ceruminous glands 

They a waxy secretion known as cerumen to protect and lubricates the ear drum and auditory canal.