Skin histology: necessary knowledge for a permanent master

Histology (from the Greek ἱστός "tissue" + λόγος "knowledge, word, science") is a branch of biology that studies the structure, vital activity and development of tissues of living organisms. This is usually done by dissecting tissue into thin layers and using a microtome. Unlike anatomy, histology studies the structure of the body at the tissue level.

Skin structure

The skin is made up of three layers: epidermis, dermis and hypodermis.

Epidermis - the outer part of the skin, represented by stratified squamous keratinized epithelium. Its thickness varies from 0.05 mm on the eyelids to 1.5 mm on the palms. About 95% of the cells of the epidermis are keratinocytes (derivatives of the ectoderm), which, as they differentiate, move from the basement membrane towards the surface of the skin. The epidermis consists of 5 layers: basal, spiny, granular, shiny and horny.

The basis of the epidermis is its innermost basal layer (stratum basale syn. germinativum) , consisting of 1 row of small cylindrical cells, located in the form of a palisade and called basal keratinocytes. They have large dark-colored basophilic nuclei and a dense cytoplasm containing many ribosomes and bundles of tonofilaments. Cells are connected to each other by intercellular bridges (desmosomes), and are attached to the basement membrane by hemi-desmosomes.

Directly above the basal layer, keratinocytes increase in size and form a spiny layer (stratum spinosum) , consisting of 3-6 (sometimes 15) rows of spiny keratinocytes, gradually flattening towards the skin surface. The cells of this layer have a polygonal shape and are also interconnected by desmosomes. In the cells of this layer, there are more tonofibrils than in basal keratinocytes; they are concentrically and densely located around the nuclei and are woven into desmosomes. In the cytoplasm of spiny cells there are numerous rounded vesicles of various diameters, tubules of the cytoplasmic reticulum, and melanosomes.

The basal and spiny layers are called the Malpighi germ layer, since mitoses are found in them, and in the spiny one - only with extensive damage to the epidermis. Due to this, the formation and regeneration of the epidermis occurs.

The granular layer (stratum granulosum) consists of 2-3 rows of cells that have a cylindrical or cubic shape near the prickly layer, and diamond-shaped closer to the surface of the skin. Cell nuclei are distinguished by noticeable polymorphism, and inclusions are formed in the cytoplasm - keratohyalin grains. In the lower rows of the granular layer, the biosynthesis of filaggrin, the main protein of keratohyalin granules, occurs. It has the ability to cause the aggregation of keratin fibrils, thus forming the keratin of the horny scales. The second feature of the cells of the granular layer is the presence in their cytoplasm of keratinosomes, or Odland bodies, the contents of which (glycolipids, glycoproteins, free sterols, hydrolytic enzymes) are released into the intercellular spaces, where a lamellar cementing substance is formed from it.

The shiny layer (stratum lucidum) is visible in areas of the most developed epidermis, i.e., on the palms and soles, where it consists of 3-4 rows of elongated, poorly contoured cells containing eleidin, from which keratin is subsequently formed. There are no nuclei in the upper layers of cells.

The stratum corneum (stratum corneum) is formed by completely keratinized non-nuclear cells - corneocytes (horny plates), which contain the insoluble protein keratin. Corneocytes are connected to each other with the help of interpenetrating outgrowths of the membrane and keratinizing desmosomes. In the superficial zone of the stratum corneum, the desmosomes are destroyed and the stratum corneum is easily sloughed off. The thickness of the stratum corneum depends on the rate of reproduction and movement of keratinocytes in the vertical direction and the rate of rejection of the stratum corneum. The stratum corneum is most developed where the skin is subjected to the greatest mechanical stress (palms, soles).

The epithelium of the mucous membranes, with the exception of the back of the tongue and the hard palate, is devoid of the granular and stratum corneum.

Among the cells of the basal layer are melanocytes - dendritic cells that migrate in the embryonic period from the neural crest to the epidermis, epithelium of the mucous membranes, hair follicles, dermis, pia mater, inner ear and some other tissues. They synthesize the pigment melanin. Melanocyte processes spread between keratinocytes. Melanin accumulates in basal keratinocytes over the apical part of the nucleus, forming a protective shield against ultraviolet and radioactive radiation. In persons with dark skin, it also penetrates into the cells of the prickly, up to the granular, layer. In humans, there are two main classes of melanins: eumelanins - produced by ellipsoid melanosomes (eumelanosomes), giving the skin and hair brown and black; pheomelanins - produced by spherical melanosomes (pheomelanosomes) and causing hair color from yellow to red-brown. Skin color does not depend on the number of melanocytes, which is approximately constant in people of different races, but on the amount of melanin in one cell. Sunburn after ultraviolet irradiation is due to the acceleration of melanosome synthesis, melanization by melanosomes, transport of melanosomes into processes and transfer of melanosomes to keratinocytes. A decrease in the number and activity of follicular melanocytes with age leads to progressive graying of the hair.

In the lower part of the epidermis, there are white process Langerhans cells - intraepidermal macrophages that perform an antigen-presenting function for T-helpers. The antigen-presenting function of these cells is carried out by capturing antigens from the external environment, processing them and expressing them on their surface. In combination with their own HLA-DR molecules and interleukin (IL-1), antigens are presented to epidermal lymphocytes, mainly T-helpers, which produce IL-2, which in turn induces the proliferation of T-lymphocytes. Thus activated T cells are involved in the immune response.

In the basal and spiny layers of the epidermis, there are Greenstein cells - a type of tissue macrophages, which are antigen-presenting cells for T-suppressors. The epidermis is separated from the dermis by a basement membrane, 40–50 nm thick, with uneven contours that follow the relief of the epidermal cords penetrating into the dermis. The basement membrane is an elastic support that not only firmly binds the epithelium to the collagen fibers of the dermis, but also prevents the growth of the epidermis into the dermis. It is formed from filaments and hemidesmosomes, as well as plexuses of reticular fibers that are part of the dermis, performs barrier, exchange and other functions, and consists of three layers.

The dermis is the connective tissue part of the skin and consists of three components: fibers, ground substance and a few cells. The dermis is a support for the appendages of the skin (hair, nails, sweat and sebaceous glands), blood vessels and nerves. Its thickness varies from 0.3 to 3 mm. There are two layers in the dermis: papillary and reticular.

Thin upper papillary layer (stratum papillare) , consisting of an amorphous structureless substance and thin connective tissue (collagen, elastic and reticular) fibers, forms papillae that lie between the epithelial ridges of spinous cells. A thicker mesh layer (stratum reticulare) extends from the base of the papillary layer to the subcutaneous fatty tissue; its stroma consists mainly of bundles of thick collagen fibers arranged parallel to the surface of the skin. The strength of the skin depends mainly on the structure of the mesh layer, which is different in its power in different parts of the skin. The dermis is relatively poor in cells. In the papillary layer there are cellular elements characteristic of loose connective tissue, and in the reticular layer - fibrocytes. Small lymphohistiocytic infiltrates may occur around vessels and hairs in the dermis. In the dermis there are histiocytes, or sedentary macrophages, accumulating hemosiderin, melanin, and detritus that has arisen during inflammation, as well as mast cells or tissue basophils, localized mainly around blood vessels, synthesizing and releasing histamine and heparin. In some areas of the papillary layer, smooth muscle fibers are located, mainly associated with hair follicles (muscles that lift hair).

Hypodermis - subcutaneous fatty tissue. It consists of a loose network of collagen, elastic and reticular fibers, in the loops of which there are lobules of adipose tissue - accumulations of large fat cells containing large drops of fat. The thickness of the hypodermis varies from 2 mm (on the skull) to 10 cm or more (on the buttocks). The hypodermis is thicker on the dorsal and extensor surfaces, thinner on the ventral and flexor surfaces of the limbs. In places (on the eyelids, under the nail plates, on the foreskin, labia minora and scrotum), it is absent.

We will talk about the blood and lymphatic vessels of the skin in the next part of the article.