These cilia are present in a variety of cells and are called primary cilia or monocilia because each cell usually possesses only one such cilium (Fig. 5.10). They are also found in some epithelial cells (e.g., the epithelial cells of the rete testis in the male reproductive tract, epithelial cells lining the biliary tract, epithelial cells of kidney tubules, epithelial-like ependymal cells lining the fluid-filled cavities of the central nervous system, the connecting stalk of photoreceptor cells in the retina, and the vestibular hair cells of the ear. Primary cilia were formerly classified as nonfunctional vestigial developmental abnormalities of 9  2 motile cilia. Experimental studies of the last decade  elevated the status of primary cilia to the level of important  cellular-signaling devices functioning comparably to an antenna on a global positioning system (GPS) receiver. Similar to an antenna that takes information from satellites and allows the GPS receiver to calculate the user’s exact location, primary cilia receive chemical, osmotic, light, and mechanical stimuli from the extracellular environment. In response to these stimuli, primary cilia generate signals that are transmitted into the cell to modify cellular processes in response to changes in the external environment. In many mammalian cells, signaling through the primary cilia seems to be essential for controlled cell division and subsequent gene expression.
Primary cilia containing a 9  0 pattern of microtubules function as signal receptors sensing a flow of fluid in developing organs.
Primary cilia function in secretory organs such as the kidneys, liver, or pancreas as sensors of fluid flow. They extend from the surface of epithelial cells lining secretory ducts into the extracellular lumen (Fig. 5.11). For instance, primary cilia found in the glomerulus and tubular cells of the kidney function as mechanoreceptors; fluid flow through the renal corpuscle and tubules causes them to bend, which initiates an influx of calcium into the cell (Fig. 5.11). In humans, mutations in two genes, ADPKD1 and ADPKD2, appear to affect development of these primary cilia, leading to polycystic kidney disease (PKD). The proteins encoded by these genes, polycystin-1 and polycystin-2, respectively, are essential in the formation of the calcium channels associated with primary cilia (see Fig 5.11b). This autosomal recessive disorder is characterized by multiple expanding cysts in both kidneys, which ultimately destroy the renal cortex and lead to renal failure. However, individuals with PKD often exhibit other pathologies not associated with the kidney that are now attributed to ciliary
FIGURE 5.10 • Primary cilia in the connective tissue and the kidney tubule. a. Electron micrograph shows a fibroblast surrounded by the extracellular matrix from the uterine connective tissue containing a primary cilium. The primary cilium is characterized by a (9  0) pattern of the microtubule arrangement. 45,000. Inset shows higher magnification of the cilium. Note the visible basal body and doublets of microtubules emerging from the basal body. 90,000. b. This scanning electron micrograph shows a single primary cilium projecting into the lumen of the collecting tubule of the kidney. Primary cilia are prominent on the free surface of the collecting tubule cells and function as mechanoreceptors that become activated by fluid flow through the tubules. Passive bending of cilia opens calcium channels and initiates signaling cascades by the influx of calcium into the cell cytoplasm. 65,000. (Courtesy of Dr. Tetyana V. Masyuk.)