The olfactoiy segment is located on part of the dome of each nasal cavity and, to a variable extent, the contiguous lateral and medial nasal walls. It is lined with a specialized olfactoiy mucosa. In living tissue, this mucosa is distinguished by its slight yellowish brown color caused by pigment in the olfactoiy epithelium and the associated olfactory glands. In humans, the total surface area of the olfactory mucosa is only a few square centimeters; in animals with an acute sense of smell, the total surface area of the olfactory mucosa is considerably more extensive.
The lamina propria of the olfactory mucosa is directly contiguous with the periosteum of the underlying bone.
This connective tissue contains numerous blood and lymphatic vessels, unmyelinated olfactory nerves, myelinated nerves, and olfactory glands.
The olfactory epithelium, like the epithelium of the respiratory segment, is also pseudostratified, but it contains very different cell types. Also, it lacks goblet cells (Fig. 18.3). The olfactory epithelium is composed of the following cell types:
• Olfactoiy cells, bipolar neurons that span the thickness of the epithelium
• Supporting or sustentacular cells, columnar cells that provide mechanical and metabolic support to the olfactory cells
• Basal cells, stem cells from which new olfactory cells and supporting cells differentiate
• Brush cells, the same cell type that occurs in the respiratory epithelium
Olfactory cells are bipolar neurons that possess an apical projection bearing cilia
The apical (luminal) pole of each olfactoiy cell is a dendritic process that projects above the epithelial surface as a knob-like structure called the olfactoiy vesicle. A number of cilia (10 to 23) with typical basal bodies arise from the olfactory vesicle and extend radially in a plane parallel to the epithelial surface (see Fig. 18.3). The cilia are usually up to 200 jxm long and may overlap with cilia of adjacent olfactory cells. The cilia are regarded as nonmotile, although some research suggests that they may have limited motility. The plasma membrane of the cilia contains odorant-bind-ing proteins that act as olfactory receptors. Incoming odor-ant molecules are solubilized in the olfactory mucus and interact with the olfactory receptors to generate an action potential. The basal pole of the cell gives rise to an axonal process that leaves the epithelial compartment to enter the connective tissue, where it joins with axons from other olfactory cells to form the olfactoiy nerve (cranial neive I). Autoradiographic studies show that olfactory cells have a lifespan of about 1 month. If injured, they are quickly replaced. Olfactory cells (and some neurons of the enteric division of the autonomic nervous system) appear to be the only neurons that are readily replaced during postnatal life.
Supporting cells provide mechanical and metabolic support for the olfactory cells
Supporting cells are the most numerous cells in the olfactory epithelium. The nuclei of these tall columnar or sustentacular cells occupy a more apical position in the epithelium than do those of the other cell types, thus aiding in their identification in the light microscope (see Fig. 18.3). They have numerous microvilli on their apical surface, and abundant mitochondria. Numerous profiles of smooth endoplasmic reticulum (sER) and, to a more lim
junctional olfactory vesicle jpporting cell microvilli
basal lamina axon basal cell olfactory cell olfactory
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