Special Histology and Embryology: Practical Guide - V. K. Napkhanyuk 2001

Skin and Its Appendages

The Skin (cutis) is the organ that forms the general covering of the body. It consists of a superficial epithelial part, the epidermis, and an underlying Connective Tissue base, the dermis.

Human Skin Appendages include Hair, Nails, sebaceous, sweat, and Mammary Glands. The mammary glands are functionally associated with the FEMALE Reproductive System.

The primary Functions of the skin include protecting the body from the damaging effects of adverse environmental factors and participating in the maintenance of internal environment Homeostasis. Pain, mechano-, chemo-, and thermoreceptors located in the skin are essential Components of the protective system; they perceive information coming from the external environment. In addition, the skin performs thermoregulatory, metabolic, and Blood reservoir functions. The total surface area of the skin in an adult reaches 1.5–2 m².

The skin consists of two parts: the epidermis, a Cytology/practical/35.html">Stratified squamous keratinized epithelium; and the dermis (or corium), the connective tissue foundation of the skin.

The skin is connected to the underlying PARTS OF THE Organism by a layer of adipose tissue known as the subcutaneous adipose layer (hypodermis).

The epidermis is a stratified squamous keratinized epithelium. Five layers are distinguished in the epidermis: 1) stratum basale; 2) stratum spinosum; 3) stratum granulosum; 4) stratum lucidum; and 5) stratum corneum.

The division of the epidermis into layers reflects The process of keratinization. All epidermal layers are most highly developed in thick skin. The epidermis is a system characterized by a regular direction of differentiation, during which specific Proteins—Keratins—accumulate in the Cytoplasm of keratinocytes; the Histology/5.html">CYTOPLASMIC Organelles AND nuclear structures break down, and the Cells transform into cornified scales that shed from the epidermal surface. Lost cells are continually replenished through the proliferation of cells in the basal layer.

The basal layer is formed by Two Types of cells resting on the basal lamina: basal epitheliocytes and melanocytes (micrograph 39).

Basal epitheliocytes are cylindrical or oval cells with basophilic cytoplasm and a round, Chromatin-rich Nucleus. These cells are interconnected and attached to overlying cells via desmosomes, and to the basement membrane via hemidesmosomes. Stem cells are also present among the basal cells.

Melanocytes are pigment cells containing granules of the black pigment melanin in their cytoplasm. The quantitative ratio between basal epitheliocytes and melanocytes is 10:1.

The spinous layer is formed by polygonal cells arranged in 5–10 layers above the basal cells. These cells possess cytoplasmic projections (spines) through which they interconnect. The cytoplasm of these cells contains numerous tonofibrils, bundles of which terminate in the spines (desmosomes). In addition to these, the spinous layer contains dendritic cells whose cytoplasm features numerous argyrophilic granules (micrograph 40).

The granular layer consists of 3–4 layers of flattened cells whose cytoplasm contains Ribosomes, Cell/35.html">Mitochondria, Lysosomes, and their specialized variants—keratinosomes (in the form of rounded bodies)—as well as bundles of fragmented tonofibrils and large granules (grains) of keratohyalin, the precursors of the cornified substance keratin (micrograph 41).

The clear layer (stratum lucidum) consists of 3–4 layers of flattened cells in which the nuclei undergo karyorrhexis and die, while the cytoplasm is diffusely impregnated with a protein substance, eleidin, which is non-staining and strongly refracts light, giving the entire layer the appearance of a glossy band.

The cornified layer (stratum corneum) is the outermost surface layer, consisting of multiple layers of cornified cells—scales. These scales contain a cornified substance, "soft" keratin, and air bubbles.

The dermis (corium), or true skin, ranges in thickness from 0.5 to 5 mm, being thickest on the back, shoulders, and thighs. The dermis is subdivided into two layers: the papillary layer and the reticular layer.

The papillary layer (stratum papillare) is composed of loose connective tissue rich in cells. It contains encapsulated nerve endings—tactile corpuscles—and small Blood Vessels forming anastomoses.

The reticular layer (stratum reticulare) is represented by dense irregular connective tissue. It is a meshwork of thick Collagen fiber bundles oriented in various directions. The cellular elements of the reticular layer are predominantly fibroblasts.

The subcutaneous adipose tissue (tela subcutanea) is formed by bundles of collagen fibers extending from the dermis. Clusters of fat cells are situated between these collagen bundles, alongside blood vessels, nerve trunks, and lamellar nerve endings—Vater–Pacini corpuscles (baroreceptors).

Slides for Study

Slide 1. Cross-section of human finger skin (Fig. 63).

Low magnification. Examine and sketch the slide.

Locate the epidermis, papillary and reticular layers of the dermis, subcutaneous adipose tissue, and Sweat Glands. The secretory (terminal) portions of the sweat glands are tightly coiled, appearing as clusters in cross-section. The excretory duct within the dermis is lined by a bilayered epithelium whose cells are more basophilic than those of the secretory portion. In the epidermis, the excretory duct appears as a spiral cleft between epidermal cells.

High magnification. It is clearly visible that the basal layer of the epidermis consists of a single layer of low prismatic cells resting on the basement membrane. The cytoplasm of these cells is weakly basophilic, and the nuclei are pale. The basal layer is followed by 7–8 layers of polygonal cells forming the spinous layer, whose cytoplasm is also basophilic. The granular layer consists of 3–4 layers of flattened cells, which stand out prominently due to the presence of dark purple keratohyalin granules in their cytoplasm. The clear layer appears on the slide as a pink, homogeneous, shiny band. The cornified layer consists of hundreds of layers of keratinized scales.

Label the following on the drawing: 1) epidermis, including: a) stratum corneum; b) stratum lucidum; c) stratum granulosum; d) stratum spinosum; e) stratum basale; f) basement membrane; 2) dermis (true skin), including: a) papillary layer; b) reticular layer; c) excretory duct of the sweat gland; d) secretory portion of the sweat gland; 3) subcutaneous adipose tissue; 4) lamellar corpuscle.

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Fig. 63. Section of human finger skin. Hematoxylin and eosin staining. x 56:

1 — epidermis (a — stratum corneum; b — stratum lucidum; c — stratum granulosum; d — stratum spinosum; e — stratum basale; f — basement membrane); 2 — dermis (a — papillary layer; b — reticular layer; c — sweat gland excretory duct; d — secretory portion of a sweat gland); 3 — subcutaneous adipose tissue; 4 — lamellar corpuscle