Guideline for self-study of students for practical lesson

Module I . Dermatology and Venereology

Thematic module 1. General Dermatology

LESSON 2

Anatomy, histology and physiology of the normal skin. Histomorphological changes in the skin.

Skin anatomy and histology

Skin anatomy. The human skin (cutis) is the outer covering of the body and is continuous with the mucous membrane in the region of the mouth, nose, urogenital organs, and the anus. In an adult, the skin surface measures 1.5 to 2 m² while the thickness of the skin (without the subcutaneous fat) varies from fractions of a millimetre (on the eyelids, the external acoustic meatus) to 4 mm (on the palms and soles). The thickness of the epidermis varies from 0.06-0.09 mm (on the eyelids) to 0.5-0.8 mm (on the palms and soles). The thickness of the subcutaneous fat varies considerably: some areas are devoid of fat while in others (on the abdomen and buttocks of obese persons) it is several centimetres thick. The mass of skin in an adult accounts for approximately 5 per cent while together with the subcutaneous fat for about 16 to 17.7 per cent of the total body mass.

There are numerous furrows, folds and depressions on the skin surface which form a complicated pattern of triangular or rhomboid fields. The wrinkles on the face and the folds on the palms, soles and scrotum are the coarse furrows of the skin. The ridges and furrows running parallel to each other on the skin of the palms and the plantar surface of the fingers form diverse figures the pattern of which is very individual and is an authentic distinctive mark of a person (dactyloscopy).

The skin has a mat tinge and a peculiar colour due to the colour of its component tissues, the thickness of the granular and horny layers, the blood vessels visible through the skin, and the presence of the pigment melanin. The colour of the skin may change because the amount of the pigment in it varies under the effect of external and internal factors.

The skin surface is covered with hairs over a great area. The areas devoid of hairs are the lips (vermilion border), the palms and soles, the palmar surface of the fingers and the plantar surface of the toes, the glans penis, the inner surface of the prepuce, and the inner surface of the large and small pudendal lips.

There are hardly noticeable pores on the skin surface which are openings of the sweat and sebaceous glands. In some diseases (e.g. seborrhoea) these pores are seen with the naked eye.

The distal phalanges of the fingers and toes have nails on their dorsal surfaces.

Skin histology. In ontogenesis the skin develops from two germinative zones: the ectoderm (the outermost (external) embryonal layer) which is represented by the epidermis (the most superficial skin layer) and the mesoderm (the middle embryonal layer) represented by two layers, namely the true skin, or dermis (the middle layer) and the subcutaneous fat, or hypoderm (the deepest skin layer).

The boundary between the epidermis and dermis forms a wavy line because of the presence of skin papillae (special outgrowths on the surface of the true skin) the spaces between which are filled with epithelial processes.

Epidermis, the outer skin layer

The epidermis is stratified epithelium undergoing keratinization, it consists of the following layers: (1) germinative layer, or stratum basale, or stratum germinativum; (2) prickle-cell layer, or stratum spinosum; (3) granular layer, or stratum granulosum; (4) stratum lucidum; (5) horny layer, or stratum corneum. All these layers are pronounced well in the skin of the palms and soles; the stratum lucidum is not found on the face, chest or the flexor surface of the limbs, whereas the stratum granulosum in these areas is formed of a single, sometimes interrupted row of cells.

There are many nerve endings in the epidermis but no blood vessels; the cells are supplied with nutrients by the lymph flowing in the intercellular slits.

The germinative layer (stratum basale or stratum germinativum) is the innermost layer of the epidermis and borders directly upon the dermis, or true skin. It consists of a single layer of prismatic (columnar) cells arranged like a palisade; between these cells there are slit-like spaces called intercellular bridges. Large round or oval nuclei are seen mostly in the upper part of the cells. These nuclei are rich in chromatin.

With regard to function, the cells of the germinative layer have two features. First, they are the main sprouting (cambium) elements of the epidermis, from which cells of all the overlying epidermal layers form. The columnar cells arranged perpendicular to the basement membrane divide by mitosis. Second, the protoplasm of the cells of the germinative layer contains a pigment, melanin, in the form of brown granules of various size.

In addition to the prismatic cells, the germinative layer contains a few peculiar branching (dendritic) cells melanocytes with small dark nuclei and a light protoplasm. The bodies of these cells lie on a level with the prismatic cells while their numerous processes entwine the neighbouring cells and penetrate between the cells of the overlying layers. Melanocytes, derived from neural crest cells, primarily function to produce a pigment, melanin, which absorbs radiant energy from the sun and protects the skin from the harmful effects of ultraviolet radiation. Melanin accumulates in organelles termed melanosomes that are incorporated into dendrites anchoring the melanosome to the surrounding keratinocytes. Ultimately, the melanosomes are transferred to the adjacent keratinocytes where they remain as granules. Melanocytes are found in the basal layer of the epidermis as well as in hair follicles, the retina, uveal tract, and leptomeninges. These cells are the sites of origin of melanoma.

In areas exposed to the sun, the ratio of melanocytes to keratinocytes is approximately 1:4. In areas not exposed to solar radiation, the ratio may be as small as 1:30. Absolute numbers of melanosomes are the same among the sexes and various races. Differing pigmentation among individuals is related to melanosome size rather than cell number. Sun exposure, melanocyte-stimulating hormone (MSH), adrenocorticotrophic hormone (ACTH), estrogens, and progesterones stimulate melanin production. Melanin forms in the melanocyte cytoplasm through polymerization of the products of tyrosine oxidation under the effect of the enzyme tyrosinase the activity of which depends on the presence of copper ions. Stimulation of the sympathetic nerve inhibits the production of the pigment, while ultraviolet rays, ionizing radiation, and some chemical substances stimulate it. Vitamins, vitamin С in particular, play an important role in melanin formation.

With aging, a decline is observed in the number of melanocytes populating the skin of an individual. Since these cells are of neural crest origin, they have no ability to reproduce.

The prickle-cell layer (stratum spinosum) overlies the germinative layer and consists of five to ten rows of cells which are cuboid in the deep parts of the layer but become flatter gradually as they approach the next layer, the granular layer. The cells of the prickle-cell layer like the cells of the germinative layer are separated from each other by intercellular bridges and come in contact by means of protoplasmic processes. The nuclei of these cells are spherical and large and contain one or two nucleoli. Special Langerhans cells are demonstrated in this layer. They have a poorly staining nucleus and numerous branching processes stretching between other cells. These cells are devoid of pigment and are always located above the germinative layer. Langerhans cells originate from the bone marrow and are found in the basal, spinous, and granular layers of the epidermis. They serve as antigen-presenting cells. They are capable of ingesting foreign antigens, processing them into small peptide fragments, binding them with major histocompatibility complexes, and subsequently presenting them to lymphocytes for activation of the immune system. An example of activation of this component of the immune system is contact hypersensitivity. The cells of the prickle-cell layer are marked by the presence of specific tonofibrils in their cytoplasm. The tonofibrils do not pass from cell to cell but terminate in the protoplasmic processes; in the cytoplasm of the prismatic cells of the germinative layer they are demonstrated less clearly. As we approach the next layer, the stratum granulosum, the cells of the stratum spinosum become flatter gradually and elongate parallel to the surface of the epidermis and blend with the overlying layer without forming a distinct boundary.

The granular layer (stratum granulosum) contains one to two or four (on the palms and soles) rows of cells elongated parallel to the epidermis; the nuclei of these cells gradually grow smaller and numerous granules which take a deep stain with the main dyes appear in the protoplasm. Some authors believe these granules to be the products of nuclear degeneration, others think that they are the result of fragmentation of the tonofibrils. It was considered previously that they were formed of a special substance called keratohyalin; it proved, however, that this substance is neither keratin nor hyalin but is related to DNA in structure. The presence of the 'keratohyalin' granules is the first visible stage of the beginning of the process of keratinization of the epidermal cells.

As keratinocytes migrate out through the outermost layers, their keratohyalin granules break up and their contents are dispersed throughout the cytoplasm, leading to keratinization and the formation of a thick and tough peripheral protein coating called the horny envelope.

The epidermal germinative, prickle-cell, and granular layers are sometimes embraced under the name of Malpighian layer.

The lucid layer (stratum lucidum) overlies the granular layer and is composed of elongated cells containing a special protein substance which refracts light strongly. This substance resembles drops of oil and is called eleidin (Gr. elaia olive tree). Besides its main component, eleidin, the stratum lucidum contains glycogen and fatty substances (lipoids, oleic acid).

With the commonly used staining methods, the stratum lucidum of skin areas that have a thick epithelial layer (e.g. on the palms and soles) is seen as a colourless strip. It is also demonstrated well in some pathological processes (ichthyosis, porokeratosis). A substantiated opinion has been advanced to the effect that impermeability of the epidermis to water and electrolytes is associated with the stratum lucidum and that it consists of two layers, the upper one has an acid and the lower an alkaline reaction. Therefore, this stratum is a very complex epidermal layer.

The horny layer (stratum corneum) is the outermost layer of the epidermis, it comes in direct contact with the external environment and is distinguished by resistance to a variety of external factors. It is composed of fine, anuclear keratinized elongated cells. They are firmly attached to one another and are filled with a horny substance (keratin) the chemical structure of which has still not been finally determined. It is believed that this is an albunoid substance poor in water and rich in sulphur and contains fats and polysaccharides.

The outer part of stratum corneum is less compact and occasional lamina separate from the main bulk, i.e. the process of physiological desquamation occurs. The various areas of the skin differ in the thickness of the stratum corneum which is especially thick on the palms and soles and very thin on the eyelids and the external male genitals.