Histological structure of the epidermis and dermis
The skin covering consists of two main compartments: the epidermis (of ectodermal origin) and the dermis (connective tissue of mesenchymal origin). Subcutaneous adipose tissue lies deeper.
The epidermis is a stratified squamous keratinizing epithelium. Its base is formed by keratinocytes, which gradually differentiate and migrate from the basement membrane to the surface. In addition to keratinocytes, cells with light cytoplasm, detectable by special staining methods, are present in the epidermis:
- Melanocytes: derived from the neural crest, located in the basal layer (ratio with keratinocytes from 1:4 to 1:9). They lack desmosomes and tonofilaments, and synthesize melanin.
- Langerhans cells: dendritic phagocytes of bone marrow origin. Scattered throughout all layers, expressing MHC class I and II glycoproteins, as well as the langerin protein. Their cytoplasm contains Birbeck granules.
- Merkel cells: neuroendocrine elements with incompletely understood function, located strictly in the basal layer.
The dermis is separated from the epithelium by a basement membrane and includes two layers: papillary and reticular. Blood and lymphatic vessels, nerves, and skin appendages run through it.
Morphology of skin appendages
Appendages play a key role in the physiology of integumentary tissues and often become the source of tumors or targets for inflammatory reactions.
Sweat glands are divided into two types:
- Eccrine: distributed ubiquitously. Their secretory portion lies at the lower border of the dermis and consists of large light (with glycogen) and small basophilic (with mucopolysaccharides) cells. The excretory duct pierces the epidermis in a corkscrew fashion.
- Apocrine: localized in the axillary, perianal, and perigenital areas, and around the nipples. They have a merocrine type of secretion. Unlike eccrine glands, their duct opens into the hair follicle.
Sebaceous glands are alveolar structures with a holocrine type of secretion (the cell is destroyed upon secretion release). They are present everywhere except the palms and soles, and open into the hair infundibulum.
Hair follicles have a complex structure (base with hair bulb and papilla, isthmus, infundibulum). The hair is surrounded by inner (contains trichohyalin, keratinizes) and outer (continuation of the epidermis) root sheaths. The hair life cycle consists of three phases: anagen (growth), catagen (transition), and telogen (rest).
Principles of classification of skin diseases
All pathologies are divided into two massive groups:
- Tumors and tumor-like lesions. They are classified by histogenesis: from the epidermis, melanin-forming tissue, appendages, fibrous, adipose, muscular tissues, and vessels, as well as lymphoproliferative processes.
- Non-tumor diseases. The group is characterized by enormous clinical and morphological diversity. This includes hereditary diseases (genodermatoses), inflammatory (erythematosquamous, vesiculobullous, granulomatous), vasculitis, toxic-allergic reactions. Lesions caused by physical factors (ultraviolet light, radiation, cold), pigmentation disorders, and infectious-parasitic processes are also distinguished.
Pathomorphology of genodermatoses: Ichthyosis
Ichthyosis is a heterogeneous group of genetic pathologies in which the keratinization process is disrupted. Clinically manifested by extensive hyperkeratosis.
Vulgar (common) ichthyosis is the most frequent form (1:3000), inherited in an autosomal dominant manner. The primary cause is a mutation in the filaggrin gene. Patients exhibit dry skin with polygonal scales and pronounced palmar and plantar creases. The disease is frequently associated with atopic dermatitis. Histologically: marked hyperkeratosis (including follicular hyperkeratosis) against the background of thinning or complete absence of the granular layer of the epidermis. Sebaceous glands are atrophic.
X-linked ichthyosis (ichthyosis nigricans) is less common (1:6000) and manifests with the full clinical picture only in males. It is caused by a genetic defect (deletion at the Xp22.32 locus) leading to a deficiency of steroid sulfatase and arylsulfatase enzymes. It is distinguished by a more severe course with brownish scales. Histologically: hyperkeratosis is accompanied by a normal or even thickened granular layer, which is a key distinction from the vulgar form. The dermis remains unchanged.
Briefly: answers to topical questions
How does the biochemical synthesis of melanin occur in melanocytes?
The biochemical synthesis of melanin occurs through the sequential oxidation of the amino acid tyrosine with the participation of enzymes. The key rate-limiting enzyme is tyrosinase (cofactor is a copper ion), and DOPA oxidase is also involved in the process.
Stages of biosynthesis:
- Tyrosine — oxidized to DOPA (dihydroxyphenylalanine / 3,4-dihydroxyphenylalanine).
- DOPA — converted into dopachrome.
- Dopachrome — transformed into 5,6-dihydroxyindole.
- Melanins — formed at the final stage of polymerization of 5,6-dihydroxyindole.
What layers are distinguished in the dermis and what specific receptors are located in each of them?
Two main layers are distinguished in the dermis: the papillary and reticular layers. Specific nerve endings are located in each of them.
- Papillary layer — contains encapsulated Meissner's corpuscles and unencapsulated (non-free) receptors responsible for the perception of temperature and pain.
- Reticular layer — includes deep encapsulated receptors: Vater-Pacinian corpuscles (perception of pressure and vibration), Ruffini corpuscles (sense of stretch), and Krause end bulbs (mechanoreceptors).
What nerve fibers do Merkel cells contact?
Merkel cells contact the dendrite endings of sensory neurons. This contact forms a tactile receptor (Merkel disc) that perceives very light touch. The dendrite extending from the disc of the sensory neuron becomes myelinated immediately beyond the basement membrane of the epidermis. At the same time, the receptor itself belongs to free nerve endings, as it lacks a connective tissue capsule and modified glial cells.
How to remember the histology of ichthyosis: in the vulgar form, the granular layer is "vulgarly erased" (absent), while in the X-linked form, it is preserved or thickened.
Class and exam questions
What is the difference in the localization of sweat gland ducts?
The ducts of eccrine glands pierce the epidermis and open directly onto the skin surface, whereas the ducts of apocrine glands empty into the hair follicle.
What are Langerhans cells?
These are intraepidermal macrophages of bone marrow origin. They participate in the immune response, bear MHC class I and II, and contain characteristic Birbeck granules.
Where are sebaceous glands and hair absent?
They are not found on the palms and soles. Hair is also absent on the dorsum of the terminal phalanges, glans penis, nipples, and inner surfaces of the labia majora.
What else to study on this topic
- Morphological classification of epithelial and melanocytic skin tumors.
- Histological signs of psoriasis and other erythematosquamous dermatoses.
- Pathogenesis and morphology of autoimmune vesiculobullous diseases (pemphigus).
- Histological structure of the connective tissue sheaths and hair bulb.
- Skin manifestations as paraneoplastic syndromes.
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