Module
22
Icon module 22
Nervous system

Olfactory bulb and tract

The olfactory bulb and tract belong to the rostral telencephalon. From the 4th week, stage 11 the local mesenchyma and the ventral side of the prosencephalon (prospective telencephalon) cause the surface ectoderm to construct the nasal placodes through cell proliferation. Certain cells in the nasal placodes differentiate to become the primary sensory cells of the olfactory epithelium. The nasal placodes become flanked laterally by growths of the cephalic mesenchyme, the medial and lateral nasal processes. The placodes sink into the depths so that then the olfactory grooves (stage 15) and, subsequently, the olfactory pits (stage 16) arise.

Video
Face development: Front view and Side view

Towards the end of the 5th week (stage 16) the axons of the primary sensory cells grow out and come into contact with nerve cells at the rostral end of the telencephalon.
Note that in the induction processes a reciprocal happening is involved: the primary cells induced by the telencephalon cause the formation of the olfactory bulb.

In stage 22 the olfactory bulb isolates itself from the rest of the brain and grows in length. It lies on the lamina cribrosa that arises through ossification of the ethmoid bone. In the olfactory bulb the two sensory neurons differentiate that are in a synaptic connection with the primary sensory cells of the olfactory epithelium. The second sensory neurons (mitral cells) dispatch their axons through the olfactory tract to the olfactory cortex fields of the prepyriform area (palaeocortex) and the subcallosal area of the archicortex. It could be that the conscious perception of a smell in the neocortex takes place after a switching over in the thalamus (medial dorsal nucleus).

Fig. 102 - Situation sketch of the olfactory tract at around the 9th week
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Legend
Fig. 102

Schematic overview for localizing the olfactory tracts in the early fetal phase.

Fig. 103 - Development of the olfactory tract at around the 5th week
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  1. Germinal layer
  2. Wall of the prosencephalon
  3. Differentiating primary sensory cells
  4. Olfactory epithelium

Fig. 104 - Development of the olfactory tract at around the 7th week
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5a
Anlage of the olfactory bulb

Legend
Fig. 103

The olfactory epithelium (yellow) lies on the floor of the sunken olfactory pits in the lower part of the diagram. There, the sensory cells proliferate and their release their axons in the direction of the prosencephalon.

Fig. 104

As soon as the primary sensory cells reach the prosencephalon the development of the olfactory bulb is induced.

Fig. 105 - Development of the olfactory tract at around the 10th week
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3
Axons of the olfactory primary sensory cells (fila olfactoria)
5b
The developing olfactory bulb with a gradual horizontal alignment

Fig. 106 - Development of the olfactory tract at around the 12th week
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3
Axons of the olfactory primary sensory cells (fila olfactoria)
4
Olfactory epithelium
6
Primary olfactory sensory cells (cell bodies)
7
Second sensory neuron
8
Lamina cribrosa of the ethmoid bone

Legend
Fig. 105

The olfactory bulb lengthens and the intrabulbary processes of the lateral ventricle regress until, at the end, they are completely obliterated. In the bulb the second sensory neurons (mitral cells) differentiate.

Fig. 106

The olfactory bulb lies horizontally on the lamina cribrosa of the ethmoid bone. The second sensory neurons now stand in synaptic connection with the primary olfactory sensory cells.