Part I. General dermatology

After studying the topic, the student should

know:

• anatomy and histology of the skin and its appendages;

be competent in:

• drawing a diagram of the structure of the skin and its appendages.

Human skin is a multifunctional organ interconnected with other body organs and systems; the skin functions as a biological barrier between the human body and the environment.

It comprises the epidermis, dermis, and subcutaneous fat (hypodermis). The skin contains hair, sweat and sebaceous glands, blood vessels, and nerve endings (Fig. 1.1).

Fig. 1.1. Structure of the skin

The epidermis comprises a multilayered keratinizing epithelium, dominated by keratinocytes; they divide constantly and form basal, spiny, granular, clear (present only on the palms and soles), and horny layers (Fig. 1.2).

Fig. 1.2. The structure of the epidermis: 1 — stratum corneum; 2 — stratum germinativum; 3 — Langerhans’ cell; 4 — thorny layer; 5 — melanocyte; 6 — basal layer

The lowest layer: basal (germinative), stratum basale. Two types of cells form the basal layer: basal cells and melanocytes. This layer is separated from the dermis by the basement membrane and comprises a single row of tall cylindrical cells perpendicular to the basement membrane. Cytoplasm of the cells is basophilic. The nuclei are oval or elongated, hyperchromic. The cytoplasm of melanocytes contains melanin.

Above is a spinous layer (stratum spinosum), comprising several (from 5 to 10) rows of larger polygonal cells. Abundant protoplasmic processes are visible between the cells, forming junctions (desmosomes).

A granular layer (stratum germinativum) is above the spinous one, which comprises 3–4 rows of flattened, elongated cells along the surface of the skin. They contain multiple keratohyalin grains, intensely stained with hematoxylin.

Above the granular, a clear (glassy) layer (stratum lucidum) is located, comprising elongated cells. These cells contain substances that refract light (glycogen, lipids, and eleidin), resulting in the shiny strip appearance and invisibility of the boundaries and structure of the layer.

The stratum corneum is above the clear layer. This layer comprises keratinized nuclear-free cells (corneocytes) containing a large amount of keratin and air bubbles resulting from keratinocyte division. Keratinocytes migrating to the skin surface differentiate the features of each layer.

Two layers are anatomically distinguished in the dermis: deep reticular layer and superficial papillary layer (Fig. 1.3). The papillary layer (stratum papillare) has multiple outgrowths (papillae) extending into the epidermis. The layer comprises thin collagen, elastic and reticular fibers, and various connective tissue cells (fibroblasts, mast cells, macrophages, etc.). An amorphous structureless interstitial substance containing acid mucopolysaccharides and various enzymes (hyaluronidase, histaminase, etc.) alternating the connective tissue fibers. The papillary layer is permeated with multiple blood and lymphatic vessels, nerve fibers, and nerve endings. Bundles of smooth muscle cells are also found in this layer.

Fig. 1.3. The structure of the dermis: 1 — epidermis; 2 — papillary layer; 3 — reticular layer; 4 — hypodermis

Below the papillary layer, the reticular layer (stratum reticulare) is located, comprising loose fibrous connective tissue parallel to the skin surface. A significant part of the reticular layer is represented by bundles of collagen and elastic fibers. Blood vessels, single lymphocytes, and leukocytes are visible in the reticular layer. The dermis transforms into the subcutaneous tissue, which comprises fibrous connective tissue and multiple slices of adipose tissue between the fibers.

Appendages of the skin include hair (Fig. 1.4), sebaceous and sweat glands, and nails.

Fig. 1.4. Diagram of the structure and location of the hair in the skin: 1 — the hair shaft; 2 — the hair funnel; 3 — the sebaceous gland; 4 — piloerector (the muscle that lifts the hair); 5 — the bulb; 6 — the papilla of the hair follicle; 7 — the cuticle; 8 — the outer root sheath; 9 — the inner root sheath; 10 — the medulla

In the hair, a shaft and a root are distinguished. The hair root ends with a hair bulb situated in the hair sheath, which has two layers: the inner is epithelial, and the outer is connective tissue. The excretory ducts of the sebaceous glands open in the upper part (hair funnel) of the hair sheath lumen.

The sebaceous glands have an alveolar structure and open with excretory ducts both at the ostium of the hair sac and in the skin (Fig. 1.5).

Fig. 1.5. The structure and location of sebaceous glands in the skin: 1 — hair; 2 — epidermis; 3 — sebaceous gland; 4 — dermis; 5 — hypodermis; 6 — vellus hair

Sweat glands are divided into eccrine and apocrine (Fig. 1.6). They are referred to as simple tubular glands. Eccrine sweat glands are dispersed in all areas of the skin, especially on the palms and soles. The exceptions are the vermillion border of the lips, external ear canal, nail beds, glans penis, clitoris, and labia minora. Apocrine sweat glands are localized in the armpits, genital area, perineum, and mammary gland nipples. Apocrine glands are opened typically by excretory ducts into the hair funnel.