The islands of Calleja (Spanish: [kaˈʎexa]; IC, ISC, or IClj) are a group of neural granule cells located within the ventral striatum in the brains of most animals. This region of the brain is part of the limbic system, where it aids in the reinforcing effects of reward-like activities. Within most species, the islands are specifically located within the olfactory tubercle; however, in primates, these islands are located within the nucleus accumbens, the reward center of the brain, since the olfactory tubercle has practically disappeared in the brains of primates. Both of these structures have been implicated in the processing of incentives as well as addictions to drugs. Projections to and from the islands supplement this knowledge with their involvement in the reward pathways for both cocaine and amphetamines.
Location The islands of Calleja are specifically located within the ventral and medial lining of the ventral striatum in the brain, meaning that they lie towards the front and middle of this region within the temporal lobe. The insula magna, or the major island, of these complexes is located in the medial border of the nucleus accumbens. The ventral group of the islands lies along the pial border of the basal forebrain, a region of the frontal lobe that lies adjacent to the temporal lobe. Due to high concentrations of nitric oxide synthase, an enzyme that makes nitric oxide and includes another enzyme known as NADPH-diaphorase, the islands can be visualized via NADPH-diaphorase staining. Using this method, the islands have been suggested to be a single heterogeneous cell complex. The exact grouping of these structures is unique across species; however, the shaping of the structures is not the same across the hemispheres of the brain.
Etymology The islands of Calleja are named after Julián Calleja y Sánchez, the Spanish anatomist, who studied the structure prior to publishing a paper in 1893 entitled "La región olfatoria del cerebro" ("The olfactory region of the brain"). Although the structure was named after him, Calleja was not the first to study it. Sigbert Ganser, a German psychiatrist, published a paper in 1882 that discussed the region. Furthermore, the current accepted definition of the islands of Calleja is not the same as the region Calleja himself studied. He was examining the thick portions of the cell layer of the olfactory tubercle rather than the granule cells that bear his name today.
Neurogenesis of neurons in the islands of Calleja
Sub-ventricular zone Originating from the lateral ganglionic eminence, one of the three embryonic structures that eventually become specific parts of the brain, the sub-ventricular zone (SVZ) is a group of cells that develop along the surface of the ventricular layer of the brain, following the creation of the cortical plate in embryos. The cells generated from this region migrate either radially along or tangentially to radial glia, the cells that help guide neurons to their targeted destination. These progenitors from the SVZ are best known for their migration down the rostral migratory stream to differentiate into the different cells of the olfactory bulb. However, a separate mass of cells, referred to as the "ventral migratory mass," migrates from the SVZ to the basal forebrain, where it develops into the islands of Calleja.
FOXP2 gene expression The Fox family is a group of genes encoding certain transcription factors that all begin with the same group of nucleotides that attach to a specific promoter on a DNA strand. Most members of the FOX proteins are typically involved in the formation of certain structures of an embryo, where mutations in these genes are evident through the human developmental disorders that result. Of these genes, the FOXP2 variant was the first to be connected to inherited language and speech disorders. Within the islands of Calleja, FOXP2 gene expression has been observed in the developing basal forebrain of monkeys as well as rodents; furthermore, this gene expression has been viewed alongside the expression of two other transcription factors, PBX3 and MEIS2. Developing neurons that express these genes originate from the subventricular zone; for this reason, the involvement of all three of these genes is thought to be responsible for determining the final destination of the neurons in the islands of Calleja.
Structure and neuronal pathways In rodents, the islands of Calleja are composed of seven distinct clusters within the olfactory tubercle, with the major island creating a border between the septum, the nucleus accumbens, and the diagonal band. Some of the islands contain a "core" of neuropil, or unmyelinated axons and dendrites, filled with a large cell in some cases. Projections to and from the islands connect the structures to the piriform cortex, which is responsible for processing smell, as well as areas of the basal forebrain, a region responsible for determining an animal's level of wakefulness. The projections with the piriform cortex align with the rest of the olfactory system, the path beginning in the sensory cells of the nose and then proceeding through the olfactory bulb to regions such as the piriform cortex, olfactory tubercle, and amygdala. However, the projections to the basal forebrain structures actually originate from the smaller outer cells of the island clusters as opposed to the large cells within the "core."
The islands of Calleja receive inputs from the back of the amygdala, which processes emotional memory, as well as the septum, nucleus accumbens, and piriform cortex. The islands also receive information in the form of dopamine from the substantia nigra and ventral tegmental area, located in the midbrain of the brainstem. Information traveling from these midbrain structures is triggered in response to rewarding activities or feelings. The islands project to the back of the thalamus, a region with many responsibilities such as sensory information processing, regulation of wakefulness, and relaying information to the cerebral cortex.
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