General characteristics and classification of epithelium
Epithelium is a border tissue that performs two fundamentally different functions, upon which its basic classification is built. Lining (covering) epithelium lines the external and internal surfaces of the body, creating a barrier. Glandular epithelium specializes in secretion — the synthesis and release of biologically active substances.
From the standpoint of embryonic development, epithelium can originate from five sources: ectoderm, endoderm, mesoderm, neural tube, and mesenchyme. The morphological and functional characteristics of a specific type of epithelium depend on its source of development.
Key morphological features of epithelium
Identification of epithelium in a slide relies on several consistent features reflecting its protective function and border position:
- Cell sheet (layer) — cells are arranged in a continuous layer with no intercellular matrix, which ensures the barrier function.
- Basement membrane — a structure that separates the epithelium from the underlying connective tissue and serves as support; stem (cambial) cells responsible for regeneration are localized in this zone.
- Cell polarity — an apical pole (facing the lumen or external environment) and a basal pole (adjoining the membrane) are distinguished.
- Absence of blood vessels — nutrition occurs diffusely from the basement membrane side; this preserves the density of the cell sheet.
- High regenerative capacity — compensates for the constant mechanical and chemical impact of the external environment.
Classification of surface epithelium
The classification principle is based on the relationship of cells to the basement membrane. Simple (single-layer) epithelium — all cells contact the basement membrane. Stratified (multi-layer) — only the lower (basal) layer is connected to the membrane.
- Simple epithelium is subdivided by the number of nuclear rows into single-row (simple) and multi-row (pseudostratified), as well as by cell shape — squamous, cuboidal, columnar.
- Stratified epithelium is divided into stratified keratinized squamous, stratified non-keratinized squamous, and transitional. Additionally, the shape of the superficial layer cells is indicated.
Slide analysis
When examining a slide, epithelium is sought at the section boundaries — at the upper or lower edge. First, a thin basement membrane is identified, above which the cell sheet is located. The main diagnostic step is to determine whether all cells touch the membrane: if yes, the epithelium is simple. For example, in a trachea slide, pseudostratified ciliated columnar epithelium is identified: the cell sheet, basement membrane, absence of vessels, and clear polarity with an apical pole bearing cilia are visible.
Glandular epithelium
Glandular epithelium forms glands, which are structures specialized for secretion. Based on the number of cells, they are classified into unicellular (e.g., goblet cells) and multicellular glands. Based on the direction of secretion, they are classified as exocrine (secreting products onto a surface via a duct) and endocrine (secreting hormones into the internal environment without ducts).
A multicellular exocrine gland always has two structural components:
- Excretory duct — the pathway for secretion removal;
- Terminal (secretory) portion — the site of secretion synthesis.
Classification of exocrine glands
A gland is described according to three criteria:
- Structure of the excretory duct: simple gland — the duct does not branch; compound — the excretory duct is branched.
- Structure of the terminal portion: unbranched or branched — determined by the presence of multiple secretory portions opening into a single duct.
- Shape of the terminal portion: tubular, alveolar (acinar), or tubulo-alveolar mixed.
Key points
- Epithelium is divided into lining and glandular based on its main function.
- Development sources: ectoderm, endoderm, mesoderm, neural tube, mesenchyme.
- Main features of epithelium: cell sheet, basement membrane, polarity, lack of vessels, high regeneration.
- Simple epithelium — all cells on the basement membrane; stratified — only the lower layer.
- Stratified epithelium can be keratinized squamous, non-keratinized squamous, and transitional.
- Glands are divided into unicellular and multicellular, exocrine and endocrine.
- An exocrine gland consists of an excretory duct and a terminal portion.
- Classification of glands: by duct (simple/compound), by terminal portion (branched/not), and its shape (tubular/alveolar).