General features of epithelial tissues
Epithelial tissue is characterized by a number of distinct morphological and functional features that allow it to be identified on a histological slide.
- Cells form a sheet and closely adhere to each other via intercellular junctions: tight junctions, gap junctions, desmosomes, and hemidesmosomes. An exception is found in endocrine gland cells, which are located diffusely.
- Cells rest on a basement membrane (completely or partially, depending on the epithelial type).
- Pronounced polarity: basal and apical poles of the cell are distinguished.
- Absence of blood vessels; nutrition occurs diffusely from the underlying connective tissue.
- Intermediate filaments of the cytoskeleton are represented by keratin.
- High capacity for regeneration: for example, the epithelium of the small intestine is completely renewed in approximately three days.
Epithelia are divided into two major groups: lining (surface) epithelia, which demarcate surfaces and cavities, and glandular epithelia, which perform a secretory function.
Principles of classification of surface epithelium
When describing a specific epithelium, all key characteristics should be indicated sequentially; otherwise, the description will be incomplete or erroneous.
- Attachment of cells to the basement membrane. If all cells contact the membrane, the epithelium is simple; if only the bottom row does, it is stratified. A special variant is pseudostratified epithelium: all cells are connected to the basement membrane, but nuclei are located at different levels, creating the illusion of stratification.
- Shape of surface cells: squamous, cuboidal, columnar (prismatic).
- Presence of specialized structures on the apical surface: cilia (ciliated epithelium) or microvilli (brush-bordered epithelium).
Simple epithelia
Simple squamous epithelium is represented by vascular endothelium, mesothelium of serous membranes (e.g., peritoneum), the epithelium of the renal corpuscle, and the posterior surface of the cornea.
Simple cuboidal epithelium lines gland ducts, renal tubules, and the ovarian surface.
Simple columnar brush-bordered epithelium is characteristic of the mucosa of the small and large intestine; microvilli forming a brush border are present on the apical surface.
Pseudostratified ciliated epithelium lines the respiratory tract. All cells contact the basement membrane but vary in height, so nuclei lie at different levels. Impaired ciliary function is associated with Kartagener syndrome (primary ciliary dyskinesia), accompanied by a susceptibility to respiratory infections.
Stratified epithelia
Transitional epithelium (urothelium) lines the urinary tract — calyces, renal pelvis, ureters, urinary bladder. Surface cells can change shape, ensuring organ distension.
Stratified squamous non-keratinizing epithelium covers the anterior surface of the cornea, oral mucosa, and esophagus. Surface cells retain nuclei and do not accumulate keratin.
Stratified squamous keratinizing epithelium forms the epidermis of the skin. Five layers are distinguished in it: basal, spinous, granular, clear (lucidum), and cornified (horny). The capacity for keratinization is one of the key characteristics of stratified epithelia.
Origin of epithelial tissues
Epithelia belong to a single tissue group based on morphofunctional features, but develop from different embryonic sources: ectoderm, endoderm, and mesoderm. Different types of epithelium can form from the same germ layer, and vice versa.
Special attention is paid to angiodermal epithelia — mesenchymal derivatives, which include vascular endothelium. Despite its mesenchymal origin, this tissue belongs to epithelia based on structural features.
Key points
- Epithelium is a sheet of polar cells on a basement membrane, devoid of vessels and possessing a high regenerative capacity.
- Intermediate filaments of epithelial cells are represented by keratin.
- Classification is based on the connection of cells with the basement membrane, the shape of surface cells, and the presence of specialized structures.
- Pseudostratified epithelium is simple, despite the location of nuclei at different levels.
- Brush-bordered epithelium bears microvilli, while ciliated epithelium bears cilia.
- Transitional epithelium ensures the distension of the urinary tract.
- Stratified squamous keratinizing epithelium of the epidermis includes five layers.
- Vascular endothelium is an angiodermal epithelium of mesenchymal origin.