The epidermis is composed of different types of cells that overlap, not randomly but in a well-defined manner. There are four different cell types:
1. Keratinocytes
2. Melanocytes
3. Langerhans cells
4. Merkel cells
The keratinocytes are the predominant cell type and owe their name to the characteristic protein they produce in the course of their life, keratin. This protein is responsible for specific, important skin functions. Keratinocytes are formed, grow, and die rising toward the surface of the epidermis. Gradually as they mature they gain particular morphological characteristics. This mechanism is defined as epidermal cell turnover and is the basis of the continuous and incessant renewal of the epidermis.
Under the microscope the epidermis is an obvious superimposition of cell layers, each clearly different from the others, these being the maturing phases of the keratinocytes.
The basal layer is composed of a single line of more or less cylindrical cells that are densely packed and adherent to the basal membrane. These cells show intense metabolic activity due to their rapid division. They are in fact the parent cells of all the epidermal keratinocytes and thanks to their continuous division they are able to replace all the surface cells, which are continuously lost by exfoliation. Situated above the basal layer is the spinous layer (or Malpighian layer) formed by many layers of polygonal cells (moving upwards, the keratinocytes tend to flatten) that have already begun to produce keratin. Keratin, as previously mentioned, is the structural protein specific to the epidermis. The chemical structure of this protein makes possible certain fundamental functions of the skin: resistance against environmental attack and its impermeability to substances with which it comes into contact. Under the microscope the keratinocytes of the spinous layer are seen to bear thin spines that protrude from the cell membrane. It is from this characteristic that the spinous layer derives its name. The cellular spines are really firm points of contact between one cell and another. Proceeding to the surface one finds the granular layer, where the various lines of largely polygonal, but by now flattened, keratinocytes contain granules in their cytoplasm. The granules are aggregates of keratin, produced in abundant quantities by the keratinocytic cells of the granular layer.
From this point on the keratinocytes mature, changing their function and also their morphology: from polygons they tend to become squashed and to lose their regular shape, and they become less active and accumulate intracellularly large quantities of keratin. So in the next layer, the lucid layer, is formed of one or two strata of keratinocytes. The cells are flat, without either a nucleus or other cytoplasmic organelles, and contain a characteristic homogenous substance called eleidin, This layer is found only in certain areas of the skin, namely the palms of the hand and soles of the feet. This last formation of keratinocytes constitutes the horny layer. In this area the keratinocytes take the name of corneocytes and possess particular characteristics. They are completely without metabolic activity (due to the loss of the nucleus and cytoplasmic organelles), contain large amounts of keratin, and possess a peculiar cell structure, resembling leaves tiled one on top of the other. In the most external layers the corneocytes gradually lose their cohesion and are desquamated, to then be replaced by keratinocytes pushing up from the underlying layers.
Interposed between the keratinocytes of the basal layer there are melanocytes that synthesize melanin, the pigment responsible for skin colour. The number of melanocytes can vary according to body area and are usually present as a ratio of the number of keratinocytes: it has been calculated that one melanocyte is present for every 5-10 keratinocytes. The embryologic origin of the melanocytes is different from that of the keratinocytes, melanocytes being derived from the neural crest, the structure from which the nervous system will develop. During growth of the embryo the melanocytes migrate from the neural crest toward the skin surface. In order to fulfil their function the melanocytes have a very special cell structure. They possess long, thin extensions called dendrites that begin at the cytoplasmic membrane and infiltrate between the keratinocytes of the basal and spinous layers.
On stimulation by the sun's ultraviolet rays, the melanocytes begin to produce melanin, which is absorbed into special cytoplasmic bodies called melanosomes. At this point the melanocytes distribute the melanosomes to all neighbouring keratinocytes using the cellular dendrites as a means of transport. The melanin pigment enclosed in the melanosomes passes out through the melanocyte cell membrane and enters the adjacent keratinocyte cells. In the keratinocytes the melanosomes position themselves around the nucleus, almost as an umbrella, forming a defence against ultraviolet rays. The distribution of melanin from the melanosomes to the keratinocytes is responsible for the phenomenon of tanning on exposure to the sun.
The differences in skin colouring between individuals and between one race and another does not depend on the number of melanocytes but simply on a higher synthesis of melanosomes within the cell. Therefore, an individual of negroid race has the same number of melanocytes as an individual of the caucasian race, but possess melanocytes that synthesize a greater number of melanosornes and, consequently, a larger quantity of melanin.
Melanin is the pigment contained in the structures called melanosomes produced by the melanocytes. It is transferred to the surrounding epidermal keratinocytes, which maintain functional contact forming an epidermal melanin unit. Each melanocyte provides melanosomes to a group of about 36 keratinocytes, melanocytes being responsible for the skin's colouration. There are different types of melanin classifiable into two main groups:
1. Black and brown eumelanins (insoluble in all solvents)
2. Red-brown pheomelanins (soluble in alkaline solutions)
There is also a third group of melanins called trichochromes, which are intermediate between the other two groups and are found in brown-haired individuals. Eumelanins are pigments containing nitrogen groups, whereas pheomelanins, in addition to having nitrogen groups, also contain sulphydril groups. The two groups are derived from tyrosine through processes whose initial steps are common to both. Tyrosine is oxidated to 3,4dihydroxyphenylalanine (DOPA) and subsequently to dopaquinone by the same tyrosinase. The eumelanins are formed by the transformation of dopaquinone to cyclodopa to dopachrome to 5,6-dihydroxyindol through oxidative polymerization processes or to 5,6-dihydroxyindol 2 carboxylic acid.
The pheomelanins' structure is composed of benzothiazolic groups and tetrahydro-isoquinoline groups. The first two reactions in their formation are the same as those for the eumelanins. Dopaquinone is then transformed into 5 cysteinyldopa, which is transformed in its tum to cisdopaquinone to cyclocisdopaquinonimin, benzothiazinylalanine, and so to pheomelanin. It is interesting to note that cysteinyldopa can be found in the plasma and urine in patients with melanoma though its significance remains unknown and controversial. The presence of different types of melanin determines an individual's pigmentation and, consequently, their response to light. As a result subjects can be divided into different phototypes.
These cells are situated above the basal layer and, like the melanocytes, have a dendritic appearance. Unlike the other two cell types in the epidermis, the Langerhans cells are visible under the microscope only when stained. Their function is one of immunocompetence, that is, they belong to the organic defence system that unleashes a rapid response against attack on the human organism. The Langerhans cells unite against exogenous antigens and present them to both skin and lymph-node T lymphocytes. They are also involved in immune surveillance against viral and tumour antigens. They are thought to be involved in the genesis of skin neoplasia caused by the action of ultraviolet rays, which damage the cells and inhibit their immune surveillance functions.
These cells are found mainly in certain areas of the body: the fingertips, the oral mucosa, the lips, and the hair follicles. They are easily visible under the electron microscope and are always associated with a nerve fibre. For this reason Merkel cells are considered as tactile receptors, that is they are the structures responsible for our sense of touch. However, in man this ability has not yet been clearly proven.
Our website is not responsible for the information contained by this article. Articleinput.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
Note: This article was sent to us by: Andrew John Peterson at 03132010
1. Effects on efficacy endpoints
All articles are property of their respective authors. Please read our Privacy Policy!
© 2009 ArticleInput.com.