The Structure of Skin

The skin is composed of two main layers: the epidermis and dermis. Below the dermis is the hypodermis, which is composed of loose connective tissue and adipose cells, but is not considered to be part of the skin.

Epidermis:

The epidermal layer is of ectodermal origin. Skin is characterized as either being thick or thin, based on the thickness of the epidermis. Thick skin is present on the palms of the hands and soles of the feet, whereas thin skin is found elsewhere. The epidermis is made up of five sublayers of keratinocytes (keratin-producing cells) , as shown in Figure 2 below [1]:

1) Stratum Corneum:

• The sublayer closest to the external environment. The stratum corneum is composed of stratified squamous epithelial cells, which lack nuclei and are filled with keratin. These cells are metabolically inactive, and are continuously sloughed off. This sublayer forms the main protective barrier of the skin [2].

2) Stratum Lucidum:

• A thin, translucent sublayer, made up of flattened eosinophilic cells. Again, the organelles and nuclei are not visible, as the cells are dead. Thicker in thick skin than in thin skin [1].

3) Stratum Granulosum:

• Flat, hard cells that eventually move up into the stratum corneum sublayer [3]. The cells are polygonal, and filled with basophilic granules [1].

4) Stratum Spinosum:

• A sublayer made up of cuboidal cells, which are known as "prickle cells" due to the presence of thorny cell processes and desmosomes that span the intracellular spaces [4].

5) Stratum Basale:

• The sublayer separating the dermis from the epidermis. These cells may be cubiodal or columnar, and rest on the basement membrane. Most of the mitotic activity of the skin occurs in the stratum basale [1].

Fig. 2: A section of epidermis.

(from http://www.answers.com/topic/stratum-germinativum-1?cat=technology)

Dermis:

The dermal layer is of mesodermal origin. It is much thicker than the epidermis, and is composed mainly of connective tissues. Sweat and sebaceous glands, blood and lymphatic vessels, and hair follicles and nerves are also present here. The dermis layer is divided into two sublayers [1]:

1) Papillary Layer:

• The papillary layer binds the dermis to the epidermis via anchoring fibrils made of collagen. Macrophages, leukocytes, and mast cells, part of the body's immune system, are found within the loose connective tissue composing this layer [1].

2) Reticular Layer:

• Irregular dense connective tissue formed from type I collagen is found in the reticular layer. As well, a network of elastic fibers provide the skin with its elasticity [1]. See Figure 3 below.

Fig. 3: A section of the skin stained to show the elastic fibers of the reticular layer.

(from http://missinglink.ucsf.edu/lm/DermatologyGlossary/dermis.html)