Yes, epithelial cells divide frequently. They form continuous, tightly packed layers that cover all body surfaces and line internal cavities, acting as a physical and chemical barrier. This tissue type, which includes the outer layer of the skin and the lining of the digestive tract, is highly mitotic. Their primary function is protection, selective absorption, and secretion, requiring rapid and consistent renewal due to constant exposure. Epithelial cells adhere closely and are anchored to an underlying basement membrane, maintaining a cohesive sheet during self-replacement.
The Necessity of Epithelial Cell Turnover
Epithelial cells must divide constantly to maintain tissue homeostasis, the steady state of the tissue. This necessity stems directly from their barrier function, placing them in direct contact with environmental stresses. The high rate of cell turnover is required for replacing cells lost to wear and tear, friction, and chemical exposure.
For example, the epithelial lining of the gastrointestinal tract is constantly bombarded by digestive acids, enzymes, and abrasive food particles. Cells in the skin’s outer layer are continuously shed due to external friction and environmental damage. Cell division is also essential for rapid tissue repair following an injury, allowing the epithelial sheet to quickly close any gap and prevent pathogen entry.
Specific Locations and Rates of Division
The speed at which epithelial cells divide varies dramatically, depending on the tissue’s location and its level of environmental stress. Epithelia in the small intestine and colon exhibit some of the fastest turnover rates in the body, with the entire lining being replaced roughly every three to seven days. The epidermis, the outer layer of the skin, also has a high turnover, with replacement occurring approximately every two to four weeks. Conversely, epithelia in organs like the renal tubules or certain glandular ducts have a much slower rate of division, renewing only when necessary to repair damage.
The Stem Cell Niche
In these high-turnover tissues, division is restricted to a specialized area called the stem cell niche. In the skin, this niche is the basal layer, where stem cells reside in contact with the basement membrane before migrating upward. In the intestine, the niche is the crypt, a pocket-like structure at the base of the lining where stem cells divide and their progeny migrate up the villi to be shed at the tip.
Biological Mechanisms Controlling Division
Epithelial cell division is regulated by a network of internal and external controls to ensure proliferation only occurs when needed for replacement or repair. Internally, the cell cycle is governed by checkpoints, particularly the G1 checkpoint, which serves as a major decision point for a cell to commit to division. These checkpoints monitor for cell size and DNA integrity before allowing the cell to progress to the DNA synthesis phase (S phase) and subsequent mitosis.
External Regulation
External signals play a large role in triggering or halting division. Growth factors, such as epidermal growth factor, are signaling molecules released by neighboring cells that bind to receptors on the epithelial cell surface, instructing them to begin dividing. A counter-mechanism called contact inhibition acts as a brake on proliferation. When epithelial cells become crowded, the physical forces they exert on each other trigger internal signals that halt the cell cycle and prevent further cell growth.
When Epithelial Cell Division Fails
A breakdown in the regulatory mechanisms that control epithelial cell division has severe health consequences. When checks on the cell cycle are lost, or when contact inhibition fails, cells can begin to proliferate uncontrollably, leading to the formation of a tumor. Cancers that arise from epithelial tissues are collectively known as carcinomas, and they are the most common type of cancer, including those of the breast, lung, colon, and skin. This failure to coordinate growth allows a mass of cells to form and potentially invade surrounding tissues, disrupting the barrier function of the epithelium.