The Structure of the Skin
The skin is a complex organ that has to contend with a variety of insults from both the external environment and from the interior workings of the body. Knowing a little about the structure of naturally healthy skin can help us understand what is happening when common skin ailments such as acne, rosacea, eczema and psoriasis arise.
On the surface of the skin are dead skin cells that are constantly undergoing a process of sloughing off to reveal newer skin cells beneath. The uppermost layer of living skin cells is called the epidermis (‘epi’ meaning on on over in Latin and ‘dermis’ meaning skin), beneath which is the dermis that is mainly made up of collagen and elastin and which also contains blood vessels, nerve endings, sebaceous glands and sweat glands. Underneath the dermis is a layer of adipose tissue (fat), which cushions the internal organs and bones.
The Horny Layer
Within the epidermis are four layers, the uppermost being the stratum corneum. This is sometimes referred to as the ‘horny layer’ as the cells primarily contain keratin, a strong protein also found in the nails and the hair. Alongside this keratin, produced by cells called keratinocytes, the cells of the stratum corneum also comprise an important mix of lipids (fats) that serve to retain moisture and regulate the passage of substances in and out of the body.
The stratum corneum is thinner than a human hair but these cells, despite being so thin, are exceptionally tough and are an integral part of the body’s barrier to the outside world. The cells forming the stratum corneum flake off or are exfoliated during everyday activities, and are constantly being replaced as new cells form in the deeper basal layer of the epidermis and migrate upwards to the outermost layer.
Skin conditions such as psoriasis arise when new cell formation becomes too rapid and an excess of stratum corneum cells occurs, overwhelming the usual sloughing-off process. In most people the epidermis is completely renewed every six to ten weeks but those with psoriasis may have a process that takes as little as four days, creating the infamous scaly skin effect.
The Granular Layer
Beneath the stratum corneum is the stratum granulosum, with the exception of the skin on the palms and soles where an extra layer, the stratum lucidum, separates this from the uppermost layer of the epidermis. In the granular layer are keratinocytes that have lost their nuclei and whose cytoplasm (the contents of the cell) looks granular. The lipids that are present in these keratin-producing cells are released from within the cells’ lamellar bodies into the space between the cells (extracellular). This process results in a lipid barrier formed by an arrangement of polar and non-polar lipids parallel to the cell surface.
The Spinous Layer
The stratum spinosum is the layer of the epidermis where keratinocytes begin to produce lamellar bodies enriched with polar lipids, glycophospholipids, free sterols, phospholipids and catabolic enzymes. These lamellar bodies later release their lipids once they migrate up into the the granular layer. The keratinocytes in the stratum spinosum are connected through desmosomes, specialised cell structures that aid cell-to-cell adhesion and which prevent the cells shearing.
In the middle of the stratum spinosum are Langerhans cells, which form part of the immune system. These cells take in foreign bodies and present them to other cells of the immune system, such as monocytes, in order to begin an immune system response and production of antibodies against infection.
The Basal Layer
The lowest layer of the epidermis is the basal layer, also known as the germinal layer or stratum germinativum. This layer of the skin is mostly made up of proliferating and non-proliferating keratinocytes. These keratin-producing cells are attached to the basement membrane that separates the epidermis from the dermis by hemidesmosomes.
Amongst the cell types found in the stratum basale are melanocytes that produce melanin (skin pigment) and which are connected to other layers of the epidermis through dendrites (long nerve cells). Merkel cells (nerve cells) are also found in this layer of the skin, and are especially abundant in the fingertips and lips where they are implicated in the sensation of light touch.
Separating the epidermis from the dermis is a basement membrane, a structure which also lines the cavities containing organs, as well as the endothelium that lines the inside of blood vessels.
Next: The Dermis – Sebaceous glands, skin colour, sweat and lymph.