Olfactory organ

Classification of olfactory organs

The organ of smell is traditionally divided into two anatomical and functional parts:

• Main olfactory organ. It is a section of the olfactory epithelium located in the upper part of the nasal cavity. It occupies an area of about 2 cm² on each side. Visually, this zone differs from the typical pink respiratory mucosa by a more yellowish hue.
• Accessory (vomeronasal, or Jacobson's) organ. Located in the lower part of the nasal septum and appears as two blind epithelial tubules. In comparative physiology, this organ specializes in the perception of pheromones (chemical signals for intraspecies communication). In humans, the structures of the organ are found, but there is yet no convincing scientific evidence of its functioning or the release of pheromones by humans.

Histology of the olfactory epithelium

The main olfactory organ is lined with pseudostratified ciliated epithelium. Unlike the respiratory epithelium of the airways, it has a number of fundamental morphological differences:

1. Presence of neurons. The epithelial layer includes neurosensory cells.
2. 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 types of cells, each of which contacts the basement membrane:

• Receptor (neurosensory) cells;
• Supporting epitheliocytes;
• Basal epitheliocytes.

Characteristics of epithelial cells

Receptor neurons develop from the neural germ layer. Their nucleus-containing parts are located in the middle zone of the epithelial layer. A dendrite extends upward from the cell body (toward the surface) and ends in a thickening—the olfactory vesicle. In the main organ, the vesicles bear 10–12 motile "cilia", whereas in the vomeronasal organ, they bear non-motile microvilli. Reception is only possible when odorant molecules dissolve in the mucus bathing these structures.

Supporting epitheliocytes separate neurons from each other. They have narrow stalks on the basement membrane, and their nuclei form the uppermost row in the epithelium. On the apical surface, they form microvilli. The cytoplasm of these cells contains a yellow pigment and is capable of apocrine-type secretion.

Basal epitheliocytes lie on the basement membrane, forming the lower row of nuclei. These are poorly differentiated (cambial) cells that provide regeneration—they are capable of differentiating into supporting cells.

Neural pathways of the analyzer

Unmyelinated axons depart from the basal part of the receptor neurons (this is the first neuron of the olfactory pathway). They pass through the openings of the cribriform plate and head to the olfactory bulbs on the inferior surface of the brain.

Signal switching takes place in the olfactory bulbs: the cell bodies of mitral cells (second neuron) are located here. The axons of the mitral cells run to the structures of the rhinencephalon—the hippocampus (Ammon's horn), the dentate gyrus, and the fornix gyrus, which form the central part of the analyzer.

Briefly: answers to topical questions

What else to study on this topic

• Structural features of the vomeronasal organ in animals.
• Histological structure of protein-mucous glands of the underlying tissue.
• Mechanism of apocrine secretion of supporting cells.
• Anatomy of the structures of the rhinencephalon (hippocampus and gyri).

Other topics in "Sensory organs: organs of vision and olfaction"

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