Page 473 - Atlas of Histology with Functional Correlations

the damaging effects of ultraviolet radiation.

Langerhans cells are monocyte-derived dendritic cells that originate in bone

marrow, migrate via the bloodstream, and reside mainly in the stratum spinosum
of the skin epidermis. Similar to melanocytes, the dendritic processes of
Langerhans cells extend among the cells of stratum spinosum. These dendritic-
type cells participate in the body’s immune responses. Langerhans cells

recognize, phagocytose, and process foreign antigens and then present them to T
lymphocytes for an immune response. Thus, these cells function as antigen-
presenting cells (APCs) and are part of the immunologic defense of the skin.

Merkel cells are found in the stratum basale layer of the epidermis and are

most abundant in the fingertips. These cells are associated with surrounding
keratinocytes and also contact the afferent (sensory) unmyelinated axons. As a
result, Merkel cells function as mechanoreceptors for cutaneous sensation.

MAJOR SKIN FUNCTIONS

The skin comes in direct contact with the external environment and performs
numerous protective functions.

Protection

The keratinized stratified epithelium of the epidermis protects the body
surfaces from mechanical abrasion and forms a physical barrier to pathogens or

foreign microorganisms. Because of a glycolipid layer between the cells of the
stratum granulosum, the epidermis is impermeable to water and prevents the
loss of fluids through dehydration. Increased synthesis of the pigment melanin

by melanocytes further protects the skin against the damaging ultraviolet
radiation.

Temperature Regulation

Physical exercise or a warm environment increases sweating that reduces the
body temperature due to evaporation of sweat from the skin. In addition to
sweating, temperature regulation also involves increased dilation of blood
vessels that brings more blood to the superficial layers of the skin where cooling
of the blood increases heat loss. Conversely, in cold temperatures, body heat is

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