Classification of Olfactory Organs
The olfactory organ is traditionally divided into two anatomical and functional parts:
- Main olfactory organ. It is a region of olfactory epithelium located in the superior part of the nasal cavity. It covers an area of about 2 cm² on each side. Visually, this zone differs from the typical pink respiratory mucosa by a more yellowish tint.
- Accessory (vomeronasal, or Jacobson's) organ. Located in the lower part of the nasal septum, it appears as two blind epithelial tubes. In comparative physiology, this organ specializes in the perception of pheromones (chemical signals for conspecific communication). While structures of this organ can be identified in humans, there is yet no conclusive scientific proof of its function or of human pheromone release.
Histology of the Olfactory Epithelium
The main olfactory organ is lined by a specialized pseudostratified columnar epithelium. Unlike the respiratory epithelium of the airways, it features several key morphological differences:
- Presence of neurons. The epithelial sheet contains neurosensory cells.
- Absence of goblet cells. Mucus secretion here is provided not by goblet cells, but by protein-mucous glands located deeper in the underlying connective tissue. Their secretion reaches the surface and covers the epithelium.
The cellular composition of the olfactory epithelium includes three cell types, each contacting the basement membrane:
- Receptor (neurosensory) cells;
- Supporting (sustentacular) epithelial cells;
- Basal epithelial cells.
Characteristics of Epithelial Cells
Receptor neurons develop from the neural ectoderm. Their nucleus-containing regions are located in the middle zone of the epithelial layer. A dendrite extends upward (toward the surface) from the cell body, terminating in a swelling called the olfactory vesicle. In the main organ, 10–12 motile cilia extend from the vesicles, whereas the vomeronasal organ bears non-motile microvilli. Reception occurs only when odorant molecules dissolve in the mucus bathing these structures.
Supporting cells separate the neurons from one another. They possess narrow cytoplasmic processes anchored to the basement membrane, and their nuclei form the topmost row in the epithelium. On their apical surface, they form microvilli. The cytoplasm of these cells contains a yellow pigment and is capable of apocrine secretion.
Basal epithelial cells lie on the basement membrane, forming the lowest row of nuclei. These are poorly differentiated (stem/progenitor) cells that provide regeneration—they are capable of differentiating into supporting cells.
Neural Pathways of the Olfactory System
Unmyelinated axons arise from the basal aspect of the receptor neurons (representing the first neuron of the olfactory pathway). They pass through the foramina of the cribriform plate of the ethmoid bone to reach the olfactory bulbs on the inferior surface of the brain.
Signal relay occurs within the olfactory bulbs, which house the cell bodies of mitral cells (the second neuron). Axons of the mitral cells project to the limbic structures—the hippocampus (Ammon's horn), dentate gyrus, and indusium griseum—which form the central processing part of the olfactory system.