Classification principles
Within the group of single-layer tissues, the classification is based on the nuclear arrangement (rowing), cell shape, and the presence of specialized structures at their apex.
Based on the rowing, they are subdivided into:
- Single-row (isomorphic): cells of identical height, nuclei form a single even row.
- Pseudostratified: cells of varying height, nuclei located at different levels, creating the illusion of multiple layers. However, contact with the basement membrane is maintained by every cell.
Based on shape (for single-row):
- Squamous (mesothelium, endothelium).
- Cuboidal.
- Prismatic (columnar).
The presence of special organelles on the apical surface is also reflected in the name. For example, striated epithelium has microvilli (microvilli), while ciliated epithelium has cilia (cilia).
Types of simple single-row epithelia
These tissues consist of cells of identical height whose nuclei are aligned strictly in one line.
- Simple squamous epithelium:
- Cells are strongly flattened.
- Examples: mesothelium (lining the pleura, pericardium, peritoneum), endothelium (lining blood vessels and heart chambers), epithelium of the outer layer of Bowman's capsule in the kidneys.
- On whole-mount film preparations of mesothelium (with silver impregnation), the meandering boundaries of densely packed cells are visible.
- Simple cuboidal epithelium (low prismatic):
- Cell height is approximately equal to width, nuclei are rounded, slightly displaced toward the basal pole.
- Found in the proximal and distal convoluted tubules of the kidney.
- Simple columnar (prismatic) epithelium:
- Cell height exceeds width, nuclei lie in the basal part.
- Lines the collecting ducts of the kidneys.
- Simple columnar striated epithelium:
- Cells are tall, with a brush border of microvilli on the apical surface.
- Characteristic of the small intestine, where striated enterocytes provide absorption, while goblet cells (with light cytoplasm) secrete mucus.
Pseudostratified epithelium
Here cells have varying height and shape, but every single one of them is attached to the basement membrane. Due to varying height, nuclei align into 3–4 visible rows.
The primary representative is pseudostratified ciliated columnar epithelium, lining the airways (trachea, bronchi). It comprises:
- Basal (short intercalated) cells: lower row of nuclei; these are cambial (stem) elements.
- Long intercalated cells: middle row of nuclei; at the stage of differentiation.
- Ciliated cells: upper row of nuclei; bear cilia at the apical pole.
- Goblet cells: secrete mucus.
Physiological regeneration
Single-layer epithelia are constantly renewed. The stationarity principle operates: the number of dead cells equals the number of new ones formed by mitosis of cambial elements.
The removal of old cells is called extrusion (pushing out). Neighboring cells displace the dying one, while it maintains contact with the basement membrane almost until the moment of shedding.
In the small intestine, stem cells lie at the bottom of the crypts. After division, they migrate upward along the villus wall, divide several more times, and mature. Reaching the villus tip, they lose contacts, round up, and desquamate into the lumen. The entire cycle takes a record 4–6 days.
Briefly: answers to topical questions
What functions are performed by the various cell types of pseudostratified ciliated epithelium?
The pseudostratified ciliated epithelium features the following cell types and functions/characteristics specified in the sources:
- Basal (short intercalated) cells — function as cambial (stem) cells.
- Long intercalated cells — represent a transitional form, a stage of differentiation into other types.
- Ciliated cells — bear cilia at the apical pole; the beating of the ciliated epithelium cilia ensures continuous mucus movement in the mucociliary escalator of the respiratory tract.
- Goblet cells — have light cytoplasm containing mucus; goblet cells are assigned the function of mucus secretion in the sources.
- Brush cells — listed among respiratory epithelium cell types, but their function is not described in the cited sources.
- Endocrine cells — listed among respiratory epithelium cell types, but their function is not detailed in the cited sources.
From which embryonic primordia (germ layers) do various types of single-layer epithelia develop?
Single-layer epithelia develop from various embryonic sources:
- Mesoderm (without mesenchyme) (coelonephrodermal type) — source of mesothelium of serous membranes and epithelium of kidney tubules.
- Mesenchyme (angiodermal type) — source of endothelium of blood and lymphatic vessels.
- Endoderm (enterodermal type) — forms the epithelium of the stomach, small and large intestine.
- Prechordal plate (part of endoderm) — source of epithelium of the respiratory organs (trachea, bronchi, lungs).
What specialized intercellular junctions connect cells in the simple striated epithelium of the intestine?
Cells in the simple striated epithelium of the intestine are interconnected via tight junctions (zonulae occludentes). These junctions provide barrier function, cell polarity (division into apical and basolateral membranes), and hinder the absorption of hydrophilic medicinal substances from the intestinal lumen into the blood.
Class and exam questions
How to determine from a mesothelium preparation (top view) that it is a squamous epithelium?
On a whole-mount film preparation, it is impossible to evaluate cell height. We know that mesothelium is squamous from cross-section data, which show that the height of the cells is significantly less than their width.
How do microvilli differ from cilia in appearance under a light microscope?
Microvilli are very small and merge into a continuous oxyphilic strip (brush border) at the cell apex. Cilia are larger and usually distinguishable as separate villus-like structures.
What is extrusion in epithelial tissue?
This is a process of physiological cell death and removal in which an old cell is pushed out (displaced) by neighboring ones, maintaining connection with the basement membrane almost to the very end.
What else to study on this topic
- Ultrastructure of the basement membrane and types of cell junctions.
- Features of the differon of pseudostratified ciliated epithelium.
- Histogenesis of mesothelium and endothelium.
- Structural specifics of the epithelium of the renal proximal tubules.
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