The skin covers the entire external surface of the human body, where it serves as
a protective barrier. The skin consists of 2 layers, the epidermis and the dermis,
which rest on a fatty subcutaneous layer.
The outer layer of the skin begins as a single layer of ectodermal cells. As
development continues, the ectoderm becomes multilayered and the differences
in structure become more obvious.
The first stage in epidermal layering is the formation of the periderm. Cells of the
periderm are involved in water, sodium exchange between the amniotic fluid
and the epidermis.
By the third month, the epidermis becomes a three-layered structure, with a
basal layer, which is an intermediate layer of cells.
During the sixth month, the epidermis beneath the periderm differentiates into
the definitive layers characteristic of the postnatal epidermis. In the multilayered
epidermis, unspecialized cells from the stratum basal differentiate as they move
through the various layers toward the surface of the epidermis. The cells
produce increasing amounts of intracellular keratins and filaggrin; where the
latter is involved in the interconnections of the keratinocytes, the final
differentiated form of the epidermal cell.
The epidermis is derived primarily from surface ectoderm but is colonized by
pigment-containing melanocytes of neural crest origin, antigen-processing
Langerhans cells of bone marrow origin, and pressure-sensing Merkel cells of
neural crest origin. The epidermis contains no blood vessels and is entirely
dependent on the dermis for nutrient delivery and waste disposal.
Melanocytes’ primarily function is to produce pigment, melanin, which absorbs
radiant energy from the sun and protects the skin from the harmful effects of UV
radiation. They are found in the basal layer of the epidermis. Everyone has the
same amount of melanocytes, but people have different skin tones because of
what happens to the melanocytes, not because of