Skin

The skin is the largest organ of the body. It is made up of three layers, the epidermis, dermis, and
the hypodermis. It protects the body from external factors such as bacteria, chemicals, and
temperature. The skin contains secretions that can kill bacteria and the pigment melanin provides
a chemical pigment defense against ultraviolet light that can damage skin cells. Skin helps in
regulation of body temperature & amount of water loss to environment. The thickness of each
layer of the skin varies depending on body region and categorized based on the thickness of the
epidermal and dermal layers. Skin found in palms of hand and soles of feet are the thickest.
The layers of the epidermis include the stratum basale, stratum spinosum, stratum granulosum,
stratum lucidum, and stratum corneum. Stratum basale is the deepest layer which is separated from
the dermis by the basement membrane. Stratum spinosum contains 8-10 cell layers which consists
of irregular, polyhedral cells with cytoplasmic processes. Stratum granulosum has 3-5 cell layers
which contains diamond shaped cells with keratohyalin granules and lamellar granules. Stratum
lucidum has 2-3 cell layers, present in thicker skin found in the palms and soles. Stratum corneum
has 20-30 cell layers which is the uppermost layer, made up of keratin. Epidermis also contains
Keratinocytes, Melanocytes, Langerhans’cells and Merkel’s cells. Keratinocytes are originated in
the basal layer, produce keratin. Melanocytes are derived from neural crest cells and produce
melanin, which is responsible for the pigment of the skin. Langerhans cells, dendritic cells, are the
skins first line defenders and play a significant role in antigen presentation. Merkel cells serve a
sensory function as mechanoreceptors for light touch.
The dermis is connected to the epidermis at the level of the basement membrane and consists of
two layers as the papillary layer which is the upper layer, thinner, composed of loose connective
tissue and contacts epidermis and the reticular layer which is the deeper layer, thicker, less cellular,
and consists of dense connective tissue/ bundles of collagen fibers.
The hypodermis is deep to the dermis which is the deepest layer of skin and contains adipose
lobules along with some skin appendages like the hair follicles, sensory neurons, and blood vessels.
It serves as a barrier to water, invasion by microorganisms, mechanical and chemical trauma, and
damage from UV light. The skin preserves the body’s homeostasis by regulating temperature and
water loss, while also serving both endocrine and exocrine functions. The endocrine functions
include the production of vitamin D in the keratinocytes which are responsible for converting 7-
dehydrocholesterol in the epidermis to vitamin D, with the assistance of UV light from the
sun. The exocrine functions of the skin are by way of the sweat and sebaceous glands. Also the
skin is a sensation to touch, heat, cold, and pain by the actions of the nociceptors. The general
appearance, turgor, and other qualities also give insight into the general health of the body.