Skin Disorders Guide

Epidermis

The epidermis is the outermost layer of the skin and is the thinnest of the three layers. In practical terms, it 3 is no thicker than three or four pages of this book. The epidermis is responsible for the impervious nature of the skin. Fluid movement is restricted by the presence of a barrier zone that occurs at the junction of living and nonliving keratinocytes in the outer third of the epidermis. Penetration of ultraviolet light is greatly reduced by the presence of melanin and keratin within the epidermal cells. The epidermis is composed of four cell types: keratinocytes, melanocytes, Langerhans’ cells and Merkel’s cells.

Dermis

Three somewhat-indistinct compartments can be described within the dermis. The uppermost is relatively thin and consists mostly of the loose, areolar connective tissue that occupies the space in and around the rete ridges of the epidermis. This

compartrnent, known as the papillary dermis, is distinguished by its lighter staining and by the vertical orientation of the connective tissue fibers.

Just below the papillary dermis is the thicker, major compartment of the dermis that is known as the reticular dermis. The collagen and elastic fibers found in this section are densely packed and oriented horizontally. It is this tissue that is responsible for the amazing strength and elasticity of the skin.

The deepest of the three compartments consists of dermal and subdermal fat. Fingers and islands of fat cells are found intermingled with sections of connective tissue at the junction between the two layers, but even in the deepest layers of fat, thin septa of connective tissue outline larger globules of fatty tissue. This fatty layer, via its cushioning effect, protects the body from mechanical trauma. Fat cells may also be important as storage and metabolic units.

Dermal-Epidermal Junction of Human Skin

This undulating junction separates the ectodermally derived epidermis from the mesodermally derived dermis . This so-called basement membrane serves to attach the epidermis to the dermis and probably also plays a role in supporting the shape of the plasma membrane of the basal cells. Electron microscopy reveals that, from the top downward, this basement membrane zone is made up of:

1. the plasma membrane of the epidermal basal cells together with their hemidesmosomes,
2. an electron lucent layer (lamina lucida ), and
3. an electron dense band (the basal lamina) to which are attached connective tissue fibrils (anchoring fibrils) from the underlying dermis.

The various proteins that make up these structures also represent the antigens to which antibodies are formed in several of the immunobullous diseases. It is also the site of immunoreactant deposition in lupus erythematosus . Fluids and other small- to meclium - sized molecules pass easily through this zone, but it is not clear to what degree the zone serves as an impediment to the movement of various blood cells in and out of the epidermis.