the olfactory epithelium or olfactory epithelium is the specialized sensory epithelium chemoreception for olfaction. It is one of the three sensory epithelium of the human being, together with the auditory epithelium and the visual epithelium, all of them of ectodermal origin close to the skin and nervous system.
In most mammals and reptiles, there is a main olfactory system and an accessory olfactory system that, together with taste, represent the chemosensory system, since between the two information about the chemical composition of substances is collected.
The olfactory system is responsible for detecting substances transported by the arch on its way through the nasal cavity, interacting directly with the olfactory neurons and nerve cells of the olfactory epithelium.
The axons of these neurons form the olfactory nerve, which connects to the olfactory bulb, a structure that is already part of the brain.
That is, the olfactory nerve or synapses with a nervous ganglion before reaching the brain as occurs with the nerves of the peripheral nervous system.
The same occurs with the visual epithelium and the optic nerve, which is why both sensory epithelia are considered part of the central nervous system and not the peripheral nervous system.
In this sense, the olfactory epithelium is also the only nervous tissue in direct contact with the outside world.
Structure and cells of the olfactory epithelium
It is considered a type of specialized pseudostratified columnar epithelial tissue. It is located in the upper nasal cavity below the cribriform plate or plate of the ethmoid bone. In adults it has an approximate surface of 9 cmtwo.
In the olfactory epithelium, four types of cells can be distinguished and some glands (olfactory glands) that produce serous secretions:Olfactory receptors or neurons supporting cells basal cells brush cells olfactory glands
The receptor cells of the olfactory epithelium are bipolar neurons. The apical pole of these neurons, at the dendritic endings, form immobile cilia that extend into the space of the nasal cavity and in which they express themselves. chemoreceptors capable of interacting with chemicals in the air.
Substances in the air are solubilized in serous secretions produced by the olfactory glands, or Bowman’s glands, and, once dissolved, react with the chemoreceptors on olfactory neurons.
The axons of olfactory neurons pass through the sieve plate of the ethmoid and come together to form the olfactory nerve, generally considered to be the first cranial nerve (CNI). Once they pass through the sieve plate, they synapse with the mitral cells of the olfactory bulb.
Support cells are non-nerve cells that provide structural and metabolic support to olfactory neurons, so they can be considered analogous to the glial cells of the nervous system.
They are located at the apical pole of the olfactory epithelium and can be of two types:sustaincular cells: provide physical and metabolic support cells with microvilli: morphologically and biochemically different from fundacular cells, they present extensions (microvilli) towards the free surface of the olfactory epithelium.
Basal cells are situated close to or in contact with the basal lamina of the olfactory epithelium. They are able to divide and differentiate into new olfactory cells or new support cells. They are also sometimes called olfactory epithelial stem cells.
Basal cell division and differentiation renew the olfactory epithelium every 6-8 weeks.
Olfactory glands or Bowman’s glands
The olfactory glands, also called Bowman’s glands (not to be confused with the Bowman’s capsules of the nephrons), are tubuloalveolar-type glands located in the lamina propria of the nasal mucosa (between the olfactory epithelium and the cribriform lamina of the ethmoid).
These glands produce a serous secretion that they secrete through ducts that cross the olfactory epithelium to its surface.
The olfactory bulb or olfactory bulb is located in the cribriform lamina of the ethmoid bone and is no longer part of the olfactory epithelium, but is already part of the brain, specifically the forebrain.
The olfactory bulb receives the stimuli collected by the olfactory neurons, processes them and integrates them into information that directs them to higher structures of the brain.
Embryonic origin of the olfactory epithelium
The olfactory epithelium originates from the ectoderm and the outermost embryonic germ layer from which the skin and the rest of the nervous system also develop.
The first step is the formation of the olfactory placode or nasal placode, a thickening in the ectoderm that will later remain on the roof of the nostrils and whose cells will differentiate into olfactory neurons.
For a long time it was believed that the olfactory epithelium was formed only from the nasal placode, but it has been shown that neural crest cells also participate.
The olfactory epithelium can be damaged by inhalation of toxic or irritating substances, infections, or a physical injury inside the nose.
However, due to the ability of the olfactory epithelium to regenerate, the damage is usually temporary, except in extreme cases where the damage becomes permanent and produces the syndrome known as anosmia or loss of smell.
Anosmia, in addition to direct damage to the olfactory epithelium, can also be related to certain systemic diseases, such as diabetes or allergies, and damage to brain structures, for example, associated with strokes and strokes.