The olfactory tubercle differs in location and relative size between humans, other primates, rodents, birds, and other animals. In most cases, the olfactory tubercle is identified as a round bulge along the
basal forebrain anterior to the
optic chiasm and posterior to the
olfactory peduncle.
dementia and
depression. The olfactory tubercle has been shown to play a large role in behavior. Unilateral lesions in the olfactory tubercle have been shown to alter attention, social and sensory responsiveness, and even locomotor behavior. This suggests that it may also play a role in odor guided behavior. Thus, it may link perception of odor with action through its connections with attention, reward, and motivation systems of the
basal forebrain. it is also noted for the being innervated by dopaminergic neurons from the ventral tegmental area. The olfactory tubercle also consists of heterogeneous elements, such as medial forebrain bundle, and has a ventral extension of the striatal complex. During the 1970s, the tubercle was found to contain a
striatal component which is composed of
GABAergic medium spiny neurons.
Morphological and neurochemical features The ventral portion of the olfactory tubercle consists of three layers, whereas the dorsal portion contains dense cell clusters and adjoins the
ventral pallidum (within the basal ganglia). The structure of the most ventral and anterior parts of the tubercle can be defined as anatomically defined hills (consisting of gyri and sulci) and clusters of cells. The most common cell types in the olfactory tubercle are medium-size dense spine cells found predominantly in layer II (dense cell layer). The dendrites of these cells are covered by substance p immunoreactive (S.P.I) axons up into layer III (multiform layer). Other medium-size cells reside in layers II and III of the olfactory tubercle as well. These include the spine-poor neurons and spindle cells and they differ from the medium-size dense spine cells because they have sparse dendritic trees. The largest cells, and most striking feature of the olfactory tubercle, are densely packed crescent-shape cell clusters,
Islands of Calleja that reside mostly in the dorsal portion of the olfactory tubercle, layer III, and can also be found in layer II. The olfactory tubercle also contains three classes of small cells found mostly in layers I and II. The first are pial cells (named as such because of location near pial surface), which look like miniature medium-size dense spine cells. The second are radiate cells and are easily identified by numerous multi-directional spineless dendrites. The third, small spine cells, are similar to the pial cells in that they also look like medium-size spine cells except they are not located near the pial surface.
Development Migrating cells from several developmental sites come together to form the olfactory tubercle. This includes the ventral ganglionic eminence (found in ventral part of
telencephalon, where they form bulges in the
ventricles that later become the basal ganglia, present only in
embryonic stages) and the rostromedial telencephalic wall (of the forebrain). Olfactory tubercle neurons originate as early as
embryonic day 13 (E13), and the cell development occurs in a layer specific manner. The emergence of the three main layers of the olfactory tubercle begins almost simultaneously. The large neurons in layer III originate from E13 to E16, while the small and medium originate between E15 and E20. Like the small and medium cells in layer III, the cells of layer II and the striatal bridges also originate between E15 and E20 and develop in a lateral to medial gradient. The
granule cells of the islands of calleja originate between E19 and E22 and continue to migrate into the islands until long after birth. Fibers from the lateral olfactory tract begin branching into the olfactory tubercle around E17. The lateral portion of the olfactory tubercle (which adjoins the olfactory tract) receives the densest fiber input and the medial portion receives light fiber projections. This branching continues until completion about the end of the first week after birth. ==Function==