Simple epithelium has only one cell layer where every cell is in direct contact with the underlying basement membrane. Generally, this type of epithelium is found inside the body probably due to the fragile nature and forms the lining of the body cavities, blood vessels and heart. Being a thin layer has the physiological advantage of faster absorption and filtration especially when it lines the digestive tract and exchange surfaces of the lungs.
Simple epithelium can be divided into 4 major classes, depending on the shapes of constituent cells.
The cells found in this epithelium type are flat and thin, making simple squamous epithelium ideal for lining areas where passive diffusion of gases occur. Areas where it can be found include: skin, capillary walls, glomeruli, pericardial lining, pleural lining, peritoneal cavity lining, and alveolar lining.
- Hematoxylin and eosin (H&E) stains are one of the most common stains used in histology and viewed under simple light microscopy. Generally, hematoxylin is a dark blue or violet stain that is basic/positive and binds to basophilic substances such as nucleic acids. Therefore, nuclei of cells will appear as round blue structures, and eosin Y is used as a counterstain. Eosin stains eosinophilic structures in shades of red, pink or orange, and will highlight other structures of the cell.
- Endothelium is a type of epithelium found on the inner surface of lymphatic and blood vessels. The endothelial cells that line the vessel are simple squamous cells.
- Mesothelium is composed of specialized mesothelial cells, which are found as a monolayer of squamous epithelial cells lining the basement membrane and supported by dense irregular connective tissue. Mesothelium is therefore found lining body cavities and organs (e.g., the pleura, peritoneum , pericardium, and male and female internal reproductive organs).
This epithelium is composed of cells which are roughly equal in height and breadth with spherical and centrally placed nuclei. There are few cases in which microvilli are found on the free surface of the cuboidal cells which are best seen by electron microscopy. The cytoplasm has more organelles like endoplasmic reticulum and mitochondria compared to simple squamous, which are responsible for several metabolic and functional activities.
These cells offer secretory, absorptive, or excretory functions and therefore often provide protection via active (pumping material into or out of the lumen) or passive mechanisms depending on the physiological location and cellular specialization. Due to its function, simple cuboidal epithelium often differentiates to form secretory or ductal portions of glands.
Areas where this type of epithelium can be found include: the surface of ovaries, renal tubule walls (e.g., the small collecting ducts of the kidney or lining of nephrons), the internal walls of the seminiferous tubules of the male testes, the pancreas, the salivary gland, parts of the eye, and follicles the thyroid.
In this type of epithelium, the height of cells exceeds the width of the cell and seem closely packed narrow columns. The apical surfaces of these cells face the lumen and the opposite surface faces the basement membrane. The ovoid nuclei are usually placed towards basal surface.
This type of epithelium is commonly adapted for secretion, absorption and protection. As a protective epithelium, it lines the minor ducts of many exocrine glands, while as a secretory epithelium it lines the stomach and uterine cervix. As absorptive columnar epithelium is characterised by the presence of striated border on its apical surface. The striated border of the cells is the surface modification of absorptive columnar cells and consist of numerous non-motile minute microvilli which give the striated or brush border appearance on light microscopy and increase the surface area for absorption.
- Glandular goblet cells are a type of simple columnar cell that functions to secrete mucins, a major component of mucin. As described previously, the nucleus of a goblet cell is found polarized at the base of the cell (near the basal surface), as well as the other cellular organelles. The rest of this elongated cell type is composed of cytoplasm occupied by membrane-bound secretory granules that serve to secrete mucin, and are polarized towards the apical surface. Goblet cells are found lining the intestinal tracts, respiratory tracts, and conjunctiva of the eye, where the secreted mucus functions to help protect the membrane through formation of a protective mucinous layer that provides lubrication and protects the organ lining from foreign irritants.
Some of the simple columnar cells are lined with motile cilia and therefore termed as ciliated columnar epithelium. Due to the movement of the cilia in ciliated columnar epithelium, this type generally plays a role of clearing or moving substances or very small foreign bodies. Ciliated columnar epithelium is therefore found in the respiratory tract where mucous and air are pushed away to clear the respiratory tract. Other areas where ciliated columnar epithelium is found are the fallopian tubes, the uterus, and the central canal of the spinal cord. In the female reproductive system, the ciliated columnar epithelium lines the lumen of the uterine tube and the movements generated by the cilia propel the egg towards the uterus.
The nuclei of cells in this type of simple epithelium appear at different levels and therefore give a false impression that the cells are in many layers. In pseudostratified epithelium, all the cells reach the basement membrane even though the nuclei will lie at different levels in the tissue, reflective of the irregular layering of tall and thin cells. However, only the taller cells will reach the free surface. Pseudostratified epithelium is also sometimes referred to as respiratory epithelium, since ciliated pseudostratified columnar epithelia is mainly found in the larger respiratory airways of the nasal cavity, trachea and bronchi.
Pseudostratified columnar epithelium with stereocilia, another sub-classification, can be found in the epididymis, a highly coiled genital duct of the male. Non-ciliated pseudostratified columnar epithelia can be found in the membranous part of male vas deferens. Finally, pseudostratified columnar epithelium can also be found forming the straight, tubular glands of the endometrium in females.
Metaplasia refers to a change in cell type and most commonly involves a change of surface epithelial cells (squamous, columnar, or urothelial) to another cell type. Usually, this happens due to chronic stress and is an adaptation to better handle the stress. A classic example of metaplasia is Barrett’s esophagus. This occurs during prolonged gastroesophageal reflux disease (GERD), where squamous epithelial cells of the esophagus (ideal for handling the friction of a food bolus) are replaced by cell types found in the intestinal lining (columnar non-ciliated mucinous cells and goblet cells) to protect the esophageal lining from reflux.
Neoplasms are a new tissue growth that is unregulated, monoclonal, and irreversible. Benign neoplasms arising from the epithelium include adenomas and papillomas. It is important to note that cancer begins as a single mutated cell. Adenomas and papillomas can become malignant following dysplastic precancerous changes that cause these neoplasms to become adenocarcinomas and papillary carcinomas.
Dysplasia is a disordered cellular growth and is a term used to describe proliferation of precancerous cells that typically arise from long-standing pathologic hyperplasia (e.g., in endometrial hyperplasia). In the precancerous stage, dysplasia is in theory reversible, but failure to alleviate the chronic stress will cause the dysplasia to progress to carcinoma (irreversible). For example, cervical intraepithelial neoplasia (CIN) represents precancerous dysplasia that can progress to cervical cancer.