Simple cuboidal epithelium (SCE) is a type of tissue characterized by a single layer of cells that are roughly cube-shaped, meaning their height and width are approximately equal. Each cell possesses a large, spherical nucleus situated centrally within the cell. This structure allows simple cuboidal epithelium to execute two primary generalized functions: secretion and absorption. The presence of this tissue throughout the body highlights its versatility, but its precise functional duties depend entirely on the organ it lines.
Primary Role in Renal Function and Filtration
The kidney is one of the most functionally active locations for simple cuboidal epithelium, where it forms the lining of the renal tubules, the microscopic structures responsible for filtering blood and forming urine. Specifically, the cells lining the Proximal Convoluted Tubules (PCT) exhibit a complex structure tailored for large-scale reabsorption. These cells feature a prominent apical “brush border,” which is a dense covering of microvilli that expands the surface area for transport by as much as 30 to 40 times.
This extensive surface area is necessary because the PCT reabsorbs approximately 65% of the glomerular filtrate, including water, glucose, amino acids, and salts. The cuboidal cells here are metabolically active, packed with numerous mitochondria to power the active transport mechanisms required for this massive movement of substances.
Simple cuboidal epithelium is also found lining the Distal Convoluted Tubules (DCT), though the structure is slightly different to reflect a change in function. DCT cells possess far fewer microvilli, resulting in a less pronounced brush border compared to the PCT. Here, the primary role shifts from bulk reabsorption to the fine-tuning of filtrate composition.
The cells in the DCT are involved in ion exchange, regulating the final concentrations of potassium, sodium, and hydrogen ions in the urine. This is often controlled by hormones, such as aldosterone, which influences sodium reabsorption and water balance.
Essential Component of Glandular Systems
Simple cuboidal epithelium is a widespread component in glandular systems, where its role is primarily focused on secretion in endocrine glands and transport within exocrine ducts. In the thyroid gland, this tissue forms the walls of the spherical thyroid follicles, which are the fundamental structural units of the gland. These follicular cells are actively involved in synthesizing the protein thyroglobulin, which is stored as colloid within the follicle lumen.
The cuboidal cells absorb iodine and use it to process the thyroglobulin into the thyroid hormones. Upon stimulation by thyroid-stimulating hormone (TSH), the follicular cells reabsorb the colloid, cleave the active hormones, and release them into the surrounding capillaries for systemic distribution.
The simple cuboidal epithelium lining the smaller ducts of exocrine glands primarily serves a transport and protective role. This tissue is found in the ducts of organs like the salivary glands and the pancreas, guiding the secreted products out of the gland.
In the liver, simple cuboidal cells known as cholangiocytes line the bile ducts, where they are involved in the secretion and modification of bile. While their main function is transport, they can also absorb or secrete bicarbonate and water, modifying the final composition of the bile fluid. This dual role of providing a protective lining and subtly modifying the transported fluid is characteristic of SCE in many duct systems.
Lining and Structural Support in Specialized Organs
Simple cuboidal epithelium provides specialized lining and structural support in several other locations throughout the body. On the surface of the ovary, it forms the outer covering historically known as the germinal epithelium. This layer provides a smooth, uniform boundary over the organ.
While this layer does not produce ova, its cuboidal cells are involved in the repair and replacement of the ovarian surface tissue. The shape of the cells is ideally suited for creating a durable yet flexible covering.
In the eye, simple cuboidal epithelium forms the anterior lining of the capsule of the lens, where it helps regulate the transport of nutrients necessary to keep the lens clear.
Simple cuboidal cells form the pigmented epithelium layer situated at the posterior surface of the retina. This tissue contains melanin pigment, which absorbs stray light to prevent reflections that could interfere with visual clarity. The cells also regulate the exchange of materials between the photoreceptors of the neural retina and the underlying blood vessels.