Structure of the skin

The skin consists of a superficial layer of ectodermal origin known as the epidermis, and a deeper mesodermal – derived layer known as the dermis(picture a).

Epidermis

The epidermis is a layer of stratified squamous epithelium of varying thickness(0.3 – 1.0 mm), being composed of many layers of cells. The deeper cells are living and actively proliferating, with the cells produced gradually passing toward the surface. As they do so they become cornified(keratinized). They are ultimately shed as the skin rubs against the clothing and other surfaces. The epidermis is avascular but is penetrated by sensory nerve endings. Its deep surface is firmly locked to the underlying dermis by projections into it known as epidermal pegs, with the reciprocal projections from the dermis being known as dermal papillae(figure a).

It is usually convenient to consider the epidermis as being divided into a number of layers, particularly in the so-called thick skin of the palm or sole of the foot. These layers are from within outwards known as the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum(figure a).

The stratum basale consists of a single layer of cells adjacent to the dermis. It is in this layer, as well as in the stratum spinosum that new cells are produced to replace those lost from the surface. The stratum spinosum itself consists of several layers of irregularly shaped cells, which become flattened as they approach the stratum granulosum. The stratum basale and stratum spinosum together are often reffered to as the germinal zone, because of their role in new cell production.

Collectively, the remaining layers of the epidermis(granulosum, lucidium and corneum) are often referred to as the horny layer. In the stratum granulosum the cells become increasingly flattened and the process of keratinization begins. The cells in this layer are in the process of dying. A relatively thin transparent layer(the stratum lucidum) lies between the granulosum and the superficial stratum corneum. It is this latter layer from which the cells are shed, and also which is mainly responsible for the thickness of the skin.

The epidermal melanocytes, which are responsible for the pigmentation of the skin, lie within the deepest layers of the epidermis.

Dermis

The dermis is the deeper interlacing feltwork of collagen and elastic fibres, which generally comprises the greater part of total skin thickness. It can be divided into a superficial finely – textured papillary layer, which, although clearly separated from it, interdigitates with the epidermis, and a deeper coarser reticular layer, which gradually blends into the underlying subcutaneous connective tissue.

The projecting dermal papillae usually contain capillary networks which bring the blood into close association with the epidermis(figure a). The ability to open up or close down these networks is responsible for the regulation of heat loss through the skin, as well as causing the individual to blush in moments of embarrassment. Some of the papillae contain tactile receptors, these obviously being more numerous in regions of high tactile sensitivity(e.g. fingers, lips) and less so in other regions(e.g. back).

The reticular layer of dermis consists of a dense mass of interweaving collagen and elastic connective tissue fibres. It is this layer which gives the skin its toughness and strength. The tissue fibres run in all directions, but are generally tangential to the surface. However, there is a predominant orientation of fibre bundles, with respect to the skin surface, which varies in different regions of the body. It is this orientation which gives rise to the cleavage lines of the skin.

The dermis contains  the numerous blood vessels and lymphatic channels, nerves and sensory nerve endings as well as a small amount of fat. In addition to these it also contains hair follicles, sweat and sebaceous glands, and smooth muscle(arrectores pili). The deep surface of the dermis is invaginated by projections of subcutaneous connective tissue, which serve partly for the entrance of the nerves and blood vessels into the skin(figure b).

Subcutaneous connective tissue

This is a layer of loosely arranged connective tissue containing fat and some elastic fibres. The amount of subcutaneous fat varies in different parts of the body, being completely absent in only a few regions(eyelid, scrotum, penis, nipple and areola). The distribution of subcutaneous fat also differs between men and women, constituting a secondary sexual characteristic in women, e.g. the breast as well as the rounded contour of the hips. The subcutaneous connective tissue contains blood and lymph vessels, the roots of hair follicles, the secretory parts of sweat glands, cutaneous nerves, and sensory endings(particularly Pacinian(pressure) corpuscles) – figure b.

In the subcutaneous tissue overlying joints, subcutaneous bursae exist, which contain a small amount of fluid, and thereby facilitate movement of the skin in these regions.