Definition of Simple columnar epithelium
The simple columnar epithelium, which is one type of epithelium, is made up of a single layer consisting of long, elongated cells. These cells are located in areas where absorption or secretion are the primary functions. The cells of the columnar epithelium can also be modified, just like cuboidal epithelium. The columnar epithelium can be either ciliated (or non-ciliated) and microvilli can be found in the apical area of the non-ciliated columnar.
Structure of the simple columnar epithelium
- The simple columnar epithelium consists of one layer of elongated cells that are longer than they are wide. They are attached directly to the basement membrane at their distal ends.
- The nucleus, which is large and oval in shape, is located at the bottom of each cell.
- These cells are tightly arranged next to each other. Goblet cells can also be found in the non-ciliated columnar epithelium.
- The columnar epithelium cells are also equipped with many organelles. They are therefore capable of higher levels of secretion as well as absorption than cuboidal cells.
- The apical epithelium cells are located in the same direction as other epithelial tissues. The junctions and adhesions on the lateral surface facilitate connection between adjacent cells.
- Complexes of adherent junctions, sometimes called “terminal bars”, are found at the apical end of cells.
- The simple cuboidal epithelium cells do not have direct blood supply. They still get their nutrients, water, and gases through diffusion, as they are attached directly to the basement membrane.
- However, the cells are innervated and have a nerve supply.
The simple columnar epithelium can be classified based on whether or not cilia are present.
1. Non-ciliated columnar epithelium
Non-ciliated simple columnar epithelium consists of a single layer consisting of column-like cells that are non-ciliated and have oval nuclei at the base. This layer contains columnar epithelial cells at the apical and goblet cell-like cells. Microvilli, which are tiny projections of cytoplasmic material that look like fingers, can be found on the apical side. They point towards the lumen.
Microvillus average length is approximately 1-mm and width is 0.1-mm. However, these can increase the area by up to 20 or 30 folds as hundreds to thousands are found at the ends of absorptive cells. Each microvillus is composed of bundled actin filaments that are capped and bound by actin-binding protein to the surrounding plasma membrane. The thick glycocalyx that covers microvilli in the intestinal columnar epithelium contains membrane-bound proteins as well as enzymes for specific macromolecule digestion.
Modified columnar epithelial cells, called goblet cells, secrete mucus at their apical surface. The mucus builds up in the cell’s upper part, making it bulge. This causes the entire-cell to look like a wine glass or goblet.
2. Ciliated columnar epithelium
The ciliated simple columnsar epithelium is one layer of ciliated column-like cells. There are oval nuclei at the base of the cells. The goblet cells are often interspersed among the ciliated cells. The cilia measure approximately 5-10mm in length and 0.2mm in diameter. The core structure of each cilium is composed nine microtubule doublets that are arranged around two central microtubules. Cilia have rapid beating patterns that move fluid and suspended matter along the epithelium in one direction. These cilia control the movement of molecules in a specific direction, which aids the function excretion or secretion.
Functions of the simple columnar epithelium
The simple columnar epithelium’s primary functions include secretion, absorption and protection as well as transportation of molecules. The primary functions of the non-ciliated columnar epithelium are absorption and secretion. The ciliated columnar epithelium, on the other hand, aids in the movement or transport of molecules and cells from one location to the next.
1. Absorption
The function of absorption is enhanced by the presence of microvilli on the apical surfaces of the non-ciliated columnsar epithelium. The non-ciliated columnar epithelium, which is located in the gastrointestinal tract and stretches from the stomach to anus, plays a vital role in water absorption. These cells also have accessory membrane-bound protein that aid in active absorption.
2. Secretion
The mucus secreted by the goblet cells between the columnar cells of the tissue lubricates the linings of the digestive, reproductive, and respiratory tracts and most of the urinary system. The mucus in your respiratory tract protects against foreign particles and prevents them from entering the inner respiratory tract through the nasal passage.
3. Protection
The protection provided by the simple columnar epithelium acts as a barrier and protects against non-specific movement luminal substances. Complexes of junctions located at the apical regions of cells restrict the passage of molecules or ions through intracellular spaces. This prevents harmful macromolecules from entering the body. The mucus produced by the goblet cells also protects the linings of the gastrointestinal tract from the damaging effects of acid.
4. Transportation
Some glands, such as the gallbladder, have simple columnar epithelium that helps to move hormones and enzymes to the site of action. The cilia in the columnar epithelium covering the respiratory tract move mucus and other foreign particles in unison towards the throat. They can then be spit out or coughed up. The cilia in the columnar epithelium of the female reproductive system help move the oocytes from the ovaries into the uterus through the fallopian tubes.
Location and examples of simple columnar epithelium
- The simple columnar epithelium can be found in most areas that are responsible for secretion or absorption.
- The ciliated columnar epithelium lines certain bronchioles (small tubes), in the respiratory system, and the uterine tubes (fallopian) and uterus in reproductive system.
- The columnar epithelium lines the same areas as the paranasal sinuses and the central canal of spinal cord.
- However, the non-ciliated columnar epithelium lines the gastrointestinal tract, the ducts for glands, and the gallbladder.
- Some parts of the kidney are lined by the simple columnar epithelium.