The olfactory system develops from the olfactory placode and nasal ectoderm, establishing a unique regenerative sensory system. Its neurons induce and connect with the forebrain, forming direct links to limbic structures important for emotion and memory.
It begins early in embryogenesis and continues postnatally, involving interactions between ectoderm, neural crest cells, and the forebrain. Here’s a structured overview:
The olfactory system develops primarily from the olfactory placode, a thickening of ectoderm on the ventrolateral surface of the head (around the 4th week of human gestation).
The placode invaginates to form the olfactory pit, which later gives rise to the olfactory epithelium.
Neural crest cells contribute supporting cells and some mesenchymal elements.
The olfactory pit deepens and becomes continuous with the primitive nasal cavity.
The lining epithelium differentiates into:
Olfactory receptor neurons (ORNs) – bipolar neurons with apical cilia that detect odorants.
Supporting (sustentacular) cells – provide metabolic and structural support.
Basal cells – stem/progenitor cells that continually regenerate olfactory receptor neurons (unique for a sensory system).
By the 7th week, axons from ORNs project toward the telencephalon.
ORN axons extend through the cribriform plate and form the olfactory nerve (CN I).
These axons induce the formation of the olfactory bulb from the rostral telencephalon.
In the bulb, axons synapse in glomeruli with mitral and tufted cells (secondary neurons of the olfactory pathway).
This target-dependent maturation means the bulb and ORNs co-develop: ORN axons need the bulb for guidance, and the bulb requires ORN innervation for proper organization.
Mitral and tufted cell axons form the olfactory tract, projecting to:
Piriform cortex
Amygdala
Entorhinal cortex
Orbitofrontal cortex (indirectly via thalamus)
Unlike most sensory systems, the olfactory pathway bypasses the thalamus initially and projects directly to the limbic system.
Olfactory receptor neurons are continually replaced throughout life (every 30–60 days).
This regenerative capacity is sustained by basal stem cells in the olfactory epithelium.
The system remains plastic, allowing adaptation to new odorants and recovery after injury.
Kallmann syndrome – failure of olfactory axons and GnRH neurons to migrate properly → anosmia + hypogonadotropic hypogonadism.
Congenital anosmia – isolated absence of olfactory bulb/tract.
Neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s) – early olfactory deficits due to degeneration of olfactory structures.
✅ In summary:
Would you like me to create a developmental timeline (week-by-week) for the olfactory system, so you can see when each key event happens?
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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