The skin is the largest organ of the body and constitutes 15-20% of body weight. Skin functions as a barrier to protect the rest of the body from microbes, mechanical force, and ultraviolet light. It also acts as a barrier to keep water from entering or leaving the body. Because skin is the outside covering of our bodies, it is used by the immune system as a site for exposing immune cells to antigens from the outside world. Skin is crucial for maintaining proper body temperature through the cooling effects of sweat and by regulating the amount of blood flowing to the surface of the body. In addition, skin insulates the body with hair and adipose tissue (fat). Skin is involved in metabolism through the storage of fat and the production of vitamin D. Finally, skin is a sensory organ that senses touch, temperature, pressure and pain. The sense of touch is so acute that textures of only 5-10 m can be distinguished by the fingertips. This is in contrast to the eye that has a resolution of about 200 m.
Although we will discuss three tissue layers, only the epidermis and the dermis, which are the layers closest to the outside world, constitute skin. Underneath the epidermis and dermis is the hypodermis which contains subcutaneous fat and is the superficial or subcutaneous fascia of gross anatomy. The epidermis is the keratinized stratified squamous epithelium that is the uppermost layer that faces the environment and is derived from ectoderm. Underneath the epidermis is the dermis, a dense irregular connective tissue layer that sits on top of the hypodermis and is derived from mesoderm (Fig. 1).
Figure 1. Layers of skin
Thick vs. Thin skin
There are two main types of skin: thick skin which is hairless (also called glabrous, which means smooth), and thin skin which is usually hairy. Thick skin is found exclusively on the soles of feet, palms of hands, and the surface lining the fingers and toes, where the epidermis is much thicker than anywhere else in the body. Thin skin covers the rest of the body and can be quite variable in thickness mostly due to differences in the thickness of the dermis. The back has the thickest skin because the dermis of the back can be several mm deep. However, it is still classified as thin skin because the epidermis of the back is thin.
The keratinized stratified squamous epithelium consists primarily of a single cell type, keratinocytes, which are present in several states of differentiation. The layers of the epidermis correspond to different stages of keratinocyte differentiation (Fig. 2). Keratinocytes have several characteristics that are important to skin function. They contain keratin, a type of intermediate filament that forms an extensive cytoskeleton within the cells. The keratinocytes also form many desmosome (between cells) and hemidesmosome (with the basal lamina) junctions that are linked to the keratin network. The combination of the adjoined keratin filaments and the desmosome cell junctions give skin its ability to withstand mechanical force.
Layers of the epidermis (Fig. 2):
stratum basale or germinativum
The stratum basale is single layer of cuboidal cells called basal cells that sit on top of the basal lamina. They do not contain much cytoplasm so the nuclei are close together which makes the stratum basale stain more darkly. The cells of the basal layer serve as stem cells for the epithelium and divide to produce keratinocytes that move up to the next layer while at the same time maintaining the population of basal cells in the basal layer.
Above the stratum basale is the stratum spinosum, which is usually several layers of keratinocytes thick with the cells becoming squamous in the uppermost layer. This layer is named for what look like spines coming from the keratinocytes which are really cytoplasmic processes between cells that are attached to one another by desmosomes.
The stratum granulosum is the last non-keratinized layer in the epidermis. It is usually 1-3 layers thick and is distinguishable because the keratinocytes contain keratohyalin granules. The granules contain precursors of the protein filaggrin that will eventually empty into the cytoplasm and aggregate the keratin filaments that are starting to fill the cells.
The stratum corneum is the upper most layer of the epidermis. It consists of keratinized cells (corneocytes or squames), which have no nuclei or cytoplasmic organelles. They are mostly cells or cell debris filled with filaggrin-crosslinked keratin. The bottom layer of keratinocytes in the stratum corneum has a layer of protein on the inside of the plasma membrane to provide strength to the water barrier and layers of lipids on the outside of the plasma membrane to prevent water loss. The lipids are deposited when lamellar bodies from cells in the top layer of the stratum granulosum fuse with the plasma membrane to fill the extracellular space between the stratum granulosum and stratum corneum. The corneocytes gradually make their way to the uppermost layer of the stratum corneum where they are eventually desquamated (the cell detaches from the skin). A keratinocyte can take from 25 to 50 days to mature from a basal cell to a desquamated corneocyte.