Simple Cuboidal Epithelium Definition
Simple cuboidal epithelium consists of a monolayer of epithelial cells that appear to be square-shaped in cross section. With large, rounded, centrally located nuclei, all the cells of this epithelium are directly attached to the basement membrane.
Types of Simple Cuboidal Epithelia
This epithelium can be classified based on the location and its corresponding function.
- Cuboidal epithelium of reproductive organs: cells lining the tubuli recti, rete testis and the ovary
- Cuboidal epithelium of the central nervous system: the ependyma
- Cuboidal epithelium of the endocrine system: the follicular cells of the thyroid
- Cuboidal epithelium of the excretory system: lining the proximal and distal convoluted tubules of a nephron
Though these tissues appear histologically similar, in each of these organs, they are specialized for performing certain roles.
Functions of Simple Cuboidal Epithelium
Simple cuboidal epithelium plays specific roles in different organs and has a wide variety of functions. For instance, the epithelium lining the ventricles of the brain and the central canal of the spinal cord is called ependyma and is made of a monolayer of cuboidal epithelial cells. These cells generate cerebrospinal fluid.
In the thyroid, these cells line the thyroid follicles and are called follicular cells. This tissue actively takes up iodine and creates thyroid hormone precursors, which are then processed and secreted into the blood.
A monolayer of cuboidal epithelial cells covers the ovary and forms the ovarian surface epithelium. It plays a role in repairing the damage caused during every ovulation event and may also support the formation of an ovum.
In the kidney, this tissue is found in the proximal and distal convoluted tubules of a nephron, as well as in collecting ducts. Here, these cells are specialized to perform selective secretion and reabsorption in the process of generating urine. Selective reabsorption is also important when these cells are found in the ducts of the male reproductive tract. Tubuli recti and rete testis, structures that connect the seminiferous tubules to the epididymis, are lined with cuboidal cells that begin the process of reabsorbing fluid and concentrating sperm.
Examples of Simple Cuboidal Epithelia
This type of epithelium is an important part of a nephron, where waste products are secreted into the lumen of tubule and essential nutrients such as glucose and amino acids are selectively reabsorbed. Though this epithelium cannot provide protection from mechanical abrasion, its role in selective secretion and absorption contributes to chemical homeostasis and protects the body from corrosion and chemical damage.
Proximal Convoluted Tubule of the Kidney
The kidney functions as a complex filtering device that removes excess water and a variety of waste products, and maintains the ion and pH balance of the body. Cuboidal epithelial cells play an important role in this process, lining the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT) in every nephron.
When blood flows through the glomerulus and comes into close contact with the Bowman’s capsule of the nephron, a large quantity of water, ions and small molecules enter the Bowman’s space. This is a non-specific process to some extent, because every particle that is not negatively charged and below a certain size is removed from the blood. Most of these materials, such as glucose and some amino acids have to be reabsorbed. Some other substances like ammonia or the by-products of drug metabolism need to be secreted into the lumen of the nephron in order to be removed from the body. The cuboidal epithelium of the PCT resorbs nearly two-thirds of the water and sodium chloride that entered the Bowman’s space, in addition to all the glucose and amino acids. A large proportion of potassium, phosphate and citrate ions are also removed from the lumen of the nephron and some urea is also swept back into the PCT. A wide range of medications and their metabolites such as penicillin, quinine (used to treat malaria), digoxin (used in some cardiac conditions) and saccharin (an artificial sweetener), are secreted by this epithelium into the lumen of the nephron.
The extensive secretory and absorptive function of the epithelium in the PCT is aided by the presence of microvilli on the apical surface, vastly increasing the surface area. The cells of this tissue also contain numerous mitochondria in order to accomplish active transport to and from the cell.
The image shows a cross section of the kidney with the glomerulus marked ‘1’, proximal convoluted tubules marked ‘2’ and distal convoluted tubules marked ‘3’.
Ovarian Surface Epithelium
Ovarian surface epithelium (OSE) is an unusual type of simple cuboidal epithelium for a couple of reasons. It begins to form during the tenth week of gestation in humans and slowly differentiates into a stratified multilayer by the fifth month. However, by the time gestation is complete, it reverts to being a cuboidal monolayer and the baby is born with a simple epithelium covering the ovary.
Additionally, while most epithelia of this subtype have a secretory or absorptive function, this tissue plays a major role in repair and regeneration of the ovary. After every ovulation event, the cells of the OSE proliferate in order to heal the wound of ovulatory rupture. A large percentage of ovarian cancers also derive from this tissue. Recently, there has been evidence to suggest that these cells play a role in the development of the ovum as well as in engineering its release from the ovary through proteolytic enzymes.
The OSE gives the ovary a gray color and these cells are derived from the embryonic mesoderm. It is a delicate tissue that is easily damaged, adding to the difficulty in studying its structure and function.