Differentiation of germ cells from SCs has the potential of becoming a future source of gametes for research use, although further investigation is needed to understand and develop the appropriate niches and culture conditions. Additionally, if genetic and epigenetic methodological limitations could be solved, therapeutic opportunities could be also considered.

Periglomerular cells (PG) are interneurons of the olfactory bulb (OB) that modulate the first synaptic relay of the olfactory information from the olfactory nerve to the dendrites of the bulbar principal cells. Previous investigations have pointed to the heterogeneity of these interneurons and have demonstrated the presence of two different types of PG. In the rat OB, type 1 PG receive synaptic contacts from the olfactory axons and are gamma-aminobutyric acid (GABA)-ergic, whereas type 2 PG do not receive synaptic contacts from the olfactory axons and are GABA immunonegative. In this study, we analyze and characterize neurochemically a group of PG that has not been previously classified either as type 1 or type 2. These PG are immunoreactive for the neuropeptides somatostatin (SOM) or cholecystokinin (CCK). By using double immunocytochemistry, we demonstrate that neither the SOM- nor the CCK-immunoreactive PG contain GABA immunoreactivity, which is a neurochemical feature of type 1 PG. Moreover, they do not contain the calcium-binding proteins calbindin D-28k and calretinin, which are neurochemical markers of the type 2 PG. Electron microscopy demonstrates that the dendrites of the SOM- and CCK-containing PG are distributed in the synaptic and sensory subcompartments of the glomerular neuropil and receive synaptic contacts from the olfactory axons. Therefore, they should be included in the type 1 group rather than in the type 2. Altogether, these data indicate that the SOM- and the CCK-containing PG may constitute a group of GABA-immunonegative type 1 PG that has not been previously described. These results further extend the high degree of complexity of the glomerular circuitry.

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This study investigates the targets of the population of vasoactive intestinal polypeptide (VIP)-containing deep short-axon cells of the rat olfactory bulb (OB), combining single- and double-immunocytochemical approaches under light and electron microscopy. It has been assumed that deep short-axon cells innervate granule cells in the mammalian OB, but their synaptic connectivity has not been demonstrated to date. Our results indicate that, instead of the accepted scheme of the bulbar circuitry, VIP-containing deep short-axon cells are gamma-aminobutyric acid (GABA)ergic interneurons specialized in the selective innervation of other GABAergic deep short-axon cells. Their axons contact with the perisomatic region and the dendritic portions of subsets of deep short-axon cells that contain VIP, calbindin D-28k and neuropeptide Y. Electron microscopy reveals axo-somatic and axo-dendritic symmetrical synapses from VIP-containing boutons. Taken altogether, our data show that the VIP-containing deep short-axon cells of the rat OB form an interneuronal network that modulates the function of other interneurons different from granule cells. They might be involved indirectly in the inhibition or disinhibition of principal cells or might participate in the generation of oscillatory activity and in the synchronization of populations of interneurons and, then, of principal cells. Present data demonstrate that modulation of the OB by local circuits is more complex than the simple inhibition from periglomerular cells and granule cells, and remark the importance of considering the contribution of other classes of GABAergic interneurons different from periglomerular cells and granule cells to the bulbar circuitry.

Dopamine plays key roles in the processing of the olfactory information that takes place in the olfactory glomeruli. Previous studies using autoradiography demonstrate that, at the glomerular level, these actions are mainly mediated via activation of D2 dopamine receptors. Moreover, it has been suggested that D2 receptors could be present in the olfactory nerve, where they might modulate the entrance of olfactory input into the brain. Nevertheless, the precise subcellular localization of D2 receptors in the glomerular neuropil has not been investigated. In this report, we show the subcellular distribution of D2 receptors in the glomerular circuits of Wistar rats, using pre-embedding immunogold-silver labelling and electron microscopy. Present results demonstrate for the first time the presence of D2 dopamine receptors into the terminals of the olfactory axons. In addition, we demonstrate that D2 receptors are located into presynaptic elements of the glomerular neuropil other than the olfactory axons. These elements include the dendrites of the mitral/tufted cells and the dendrites of a subset of periglomerular cells that are GABAergic and dopaminergic. This distribution pattern provides anatomical support for a wide range of actions of dopamine in the glomerular circuits through presynaptic mechanisms mediated by D2 receptors. These actions would include: (i) modulation of the glutamate release from the olfactory axons to the dendrites of mitral/tufted cells and periglomerular cells; (ii) modulation of glutamatergic synapses from the dendrites of mitral/tufted cells to the dendrites of periglomerular cells and (iii) modulation of the neurotransmission from a subset of GABAergic/dopaminergic periglomerular cells to mitral/tufted cells.