Simple Epithelia

Principles of Classification

Within the group of simple tissues, classification is based on nuclear arrangement (stratification tiering), cell shape, and the presence of specialized structures on their apical surface.

By tiering:

• Simple (isomorphic/unilayered): cells of equal height, nuclei forming a single even row.
• Pseudostratified (multitiered): 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 single cell.

By shape (for simple unilayered epithelia):

• Squamous (mesothelium, endothelium).
• Cuboidal.
• Columnar (prismatic).

The presence of specialized organelles on the apical surface is also reflected in the name. For example, striated (brush border) epithelium has microvilli (microvilli), while ciliated epithelium has cilia (cilia).

Types of Simple Unilayered Epithelia

These tissues consist of cells of equal height whose nuclei are aligned in a single row.

1. Simple Squamous Epithelium:
2. Cells are heavily flattened.
3. Examples: mesothelium (lining the pleura, pericardium, peritoneum), endothelium (lining blood vessels and heart chambers), and the epithelium of the parietal layer of Bowman's capsule in the kidneys.
4. On whole-mount silver-impregnated preparations of mesothelium, the tortuous borders of tightly adjacent cells are visible.

1. Simple Cuboidal Epithelium (low columnar):
2. Cell height is roughly equal to width; nuclei are round and slightly displaced toward the basal pole.
3. Found in the proximal and distal convoluted tubules of the kidney.

1. Simple Columnar Epithelium:
2. Cell height exceeds width; nuclei lie in the basal region.
3. Lines the collecting ducts of the kidneys.

1. Simple Columnar Striated (Brush Border) Epithelium:
2. Tall cells with a striated brush border of microvilli on their apical surface.
3. Characteristic of the small intestine, where absorptive enterocytes ensure nutrient uptake, while goblet cells (with clear cytoplasm) secrete mucus.

Pseudostratified Epithelium

Here, cells vary in height and shape, but all without exception are attached to the basement membrane. Due to the varying heights, nuclei align in 3–4 apparent rows.

Its primary representative is the pseudostratified ciliated columnar epithelium, which lines the respiratory passages (trachea, bronchi). Its composition includes:

• Basal (short intercalated) cells: bottom row of nuclei; these are cambial (stem) elements.
• Long intercalated cells: middle row of nuclei; undergoing differentiation.
• Ciliated cells: top row of nuclei; bear cilia at the apical pole.
• Goblet cells: secrete mucus.

Physiological Regeneration

Simple epithelia are constantly renewed. The principle of stationarity applies: the number of dying cells equals the number of new cells generated by mitosis of cambial elements.

The removal of old cells is called extrusion (shedding). Neighboring cells crowd out the dying cell, which maintains its contact with the basement membrane almost until the moment of detachment.

In the small intestine, stem cells reside at the base of the crypts. Following division, they migrate upward along the villus wall, divide a few more times, and mature. Upon reaching the tip of the villus, they lose cell contacts, round up, and shed into the lumen. The entire cycle takes a record 4–6 days.