Chapter 166. The Olfactory System - Review of Medical Embryology Book

The primordia of the olfactory system consist of 2 placodes on the right and left of the anteroinferior portion of the frontal prominence, situated above the stomodeum and below and lateral to the forebrain

THE PRIMORDIA appear about day 30, after those of the optic and otic placodes. At this time, too, the neural tube is completely closed
THE PLACODES are primarily induced by the adjacent mesoderm and secondarily by the ventral surface of the prosencephalon
EACH PLACODE INVAGINATES, in the direction of the adjacent brain, to form the olfactory pits

The stratified placodal base of the invagination forms the olfactory epithelium
The lateral walls around the invaginating pits form the surface ectodermal covering of the nasal cavities

THE PLACODAL CELLS of the olfactory epithelium differentiate into neurosensory cells within the thickness of the epithelium and eventually give origin to olfactory nerve fibers

At about 5 months, the deep pole of the superficial cells gives rise to an axon that crosses the epithelium and the mesenchyme and contacts the olfactory area of the cerebral hemisphere (telencephalon)
The arrival of these fibers at the telencephalon induces formation of the olfactory bulb
The axons then connect with the specialized structures of the central nervous system corresponding to the olfactory system, namely, the bulbs

NEAR THE END OF MONTH 3, the mesenchyme between the sensory epithelium and the bulb gives rise to a cartilaginous structure, the lamina cribrosa of the ethmoid bone which is eventually organized around the olfactory nerve networks and separates them into a number of bundles. The cartilage ossifies here to form the cribriform plate of the ethmoid through which the nerves pass to enter the olfactory bulbs
THE OLFACTORY BULB elongates, and eventually the extension of the ventricular cavity into it becomes obliterated

Cells in the bulb, around which the olfactory nerve fibers terminate and synapse, give origin to secondary olfactory fibers which grow centrally and form the olfactory tract

FIBERS OF THE OLFACTORY NERVES are entirely of placodal origin and their cell bodies remain in the olfactory epithelium

In lower mammals, a special part of the olfactory nerve is distributed to Jacobson's organ as the vomeronasal nerv The organ is found in the lower part of the nasal septum

MALFORMATIONS are usually quite serious because they are always accompanied by anomalies of the central nervous system as well as the face. They are classified in the category of the more general malformations. Examples are

Ethmocephalus: in these malformations, the nose is replaced by a proboscis with a single canal, as a result of convergence of both nasal primordia on the midline

The sensory epithelium is much reduced or even totally absent
The olfactory bulbs also may be absent
In some cases, there may be 2 proboscises or even trunks
There are also minor types of this malformation, in which the nose is more or less cylindric and the nasal fossae are totally closed

IN HUMANS, many of the causes of these malformations are apparently genetic in origin