VERTICAL SECTION OF SKIN

 The skin and their associated structure make up the integumentary system. Skin protects terrestrial organisms from desiccation and from heat loss. The largest organ of mammal is skin. It protects the body against physical,chemical, and biological attacks, it helps to regulate body temperature, it is used to communicate to other individuals, and a skin derivative provides nourishment for the young. Mammal skin is composed of two layers, the dermis and the epidermis. Below the dermis there is a layer known as hypodermis, some may consider this as the third layer of skin.

HISTOLOGY OF SKIN :

Skin has four layers of cells is referred to as thin skin. From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Most part of the skin can be classified as thin skin. Thick skin is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, which is located between the stratum corneum and stratum granulosum.

STRATUM BASALE:

The stratum basale is also known as stratum germinativum.  It is the deepest epidermal layer and attaches the epidermis to the basal lamina, below this layer lies the layer of dermis. The cells in the stratum basale are bond to the dermis through the intertwining collagen fibres called the basement membrane. The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are specially abundant on the surfaces of the hands and feet. The second is a Melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its colour, and also helps protect the living cells of the epidermis from UV radiation damage. In a growing fetus, fingerprints form where the cells of stratum basale meet the papillae of the underlying dermal layer (Papillary layer), resulting in the formation of the ridges on your fingers that is recognized as fingerprints. Fingerprints are unique to each individual and are used for  forensic analysis because the patterns do not change with the growth and ageing process.

STRATUM SPINOSUM:

Stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells, The stratum spinosum is composed of 8 to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale. Interspersed among the keratinocytes of this layer is a type of dendritic cell, called Langerhans cells, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer. The keratinocytes in the stratum spinosum synthesizes keratin and releases water-repelling glycolipid that helps to prevent water loss from the body which makes the skin relatively waterproof.

STRATUM GRANULOSUM:

Stratum granulosum has a grainy appearance due to further changes in the keratinocytes as they are pushed further from the stratum spinosum. The cells become fatter, their cell membranes thicken, and they generate large amounts of keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells. These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and giving the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, but keratin, keratohyalin, and cell membranes persists that later form stratum lucidum, the stratum corneum and the accessory structures of hairs and nails.

STRATUM LUCIDUM:

Stratum lucidum is a smooth, translucent layer of the epidermis located just above the stratum granulosum and below stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened. These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent appearance and provides a barrier to water.

STRATUM CORNEUM:

Stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the environment. The increased keratinization of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps to prevent the penetration of microbes and the dehydration of underlying tissues and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (in case of palm and soles of feet it is stratum lucidum). The entire layer is replaced during a period of about 4 weeks.

DERMIS:

Dermis might be considered the core of the integumentary system, as it is distinct from the epidermis and hypodermis. It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. A finger-like projection or fold, known as the dermal papilla is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and the dermis; greater the folding, stronger is the connection made. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagen fibers, produced by fibroblasts.

PAPILLARY LAYER:

Papillary layer is made of loose, areolar connective tissue, therefore collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae. Within the papillary layer are fibroblasts, a small number of adipocytes, and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes and defensive cells that help combat bacteria or other infections that have surpassed the skin. This layer also contains lymphatic capillaries, nerve fibers and touch receptors called the Meissner corpuscles.

RETICULAR LAYER:

Underneath the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated due to a tight meshwork of fibers. Elastin fibers provides some elasticity to the skin, which enables movement. Collagen fibres provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated.

HYPODERMIS:

The hypodermis is a layer directly below the dermis and serves to connect the skin to the underlying fascia of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consist of vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.