Skin Cell Information

Close up view of skin

The skin, or integument, is made of two types of tissue. The superficial layer is referred to as the epidermis and is composed of stratified squamous epithelium. The deep layer is referred to as the dermis and is composed of connective tissue.

Epidermal Cells

There are five types of cells found in the epidermis. Stem cells are the epidermal precursors of keratinocytes. Stem cells are found in the deepest epidermal layer, the stratum basale. Keratinocytes synthesize a protein called keratin and are the majority of cells in the epidermis. Melanocytes are also found in the stratum basale and give the skin its pigment by synthesizing melanin. Touch sensation receptors called tactile (Merkel) cells are present in the stratum basale as well where they contact dermal nerves. Dendritic (Langerhans) cells serve to protect the skin from pathogens and are found in both the stratum spinosum and stratum granulosum.

Epidermal Layers

The deepest layer of epidermal tissue is the stratum basale, which exists as a layer of simple columnar or simple cuboidal epithelial tissue. The next layer is the stratum spinosum, where keratinocytes are found stacked in layers to form stratified squamous epithelium. Moving superficially, the next layer is the stratum granulosum which is comprised of three to five layers of dead keratinocytes. The stratum lucidum is the next layer. This layer is relatively thin and cosists of densly packed dead keritinocytes bound together by eleidin---a clear protein. Finally, thick skin has an additional layer, the stratum corneum. This durable layer is made up of many layers (up to 30) of scale-like dead keratinocytes. It provides extra protection from water loss and trauma.

Dermal Cells

The dermis is made of collagen-rich dense irregular connective tissue. In addition to collagen fibers, the dermis also contains elastic and reticular fibers. Specialized cells within the dermis called fibroblasts synthesize the three types of fibers. Also in the dermis are macrophages, cells that clear away cellular debris and engulf bacteria. These cells are accompanied by leukocytes---white blood cells that also serve to fight off bacteria and toxins. Along the blood vessels of the dermis, mast cells can be found. Mast cells are often implicated in skin allergies, as they release histamine and stimulate swelling. Fat cells, or adipocytes, are also seen sporadically throughout the dermis.

Dermal Layers

The two layers of the dermis are the papillary layer and the reticular layer. The papillary layer is a thin section of areolar loose connective tissue surrounding finger-like projections of dermal tissue at the epidermal-dermal junction. Rich in blood vessels, this area is the first layer of defense against foreign bodies introduced through breaks in the epidermis. Deeper, the reticular layer can be found. It is a thick layer of dense irregular connective tissue consisting of bundles of collagen. When these collagen bundles tear, as a result of pregnancy or weight gain, they form striae (stretch marks).

Accessory Organs

Cutaneous glands, hair, and nails are accessory organs of the skin. Hair grows from hair follicles that exist within the dermis as thin tubes. These structures house the bulb and root of the hair, supporting its growth. The nails originate from nail roots, which are also found in the dermis. Additionally, the dermis contains sudoriferous (sweat) glands, oil producing sebaceous glands, cerumen (ear-wax) producing ceruminous glands, and milk producing mammary glands.

References

• University of Maryland Medical Center

Resources

• The Association of the British Pharmaceutical Industry - About Skin