The IG is a very small portion of the brain; Tubbs et al. (
2013) described the human IG as a glial membrane above the CC without neuronal cells, nor connections with the hippocampus. Recent developmental studies indicate that also human IG contains neurons (Bobić Rasonja et al.
2021), in agreement with a previous Golgi study in rat (Wyss and Sripanidkulchai
1983). Recent studies demonstrated the presence of nNOS-positive neuronal-like cells in human IG, suggesting that it is not a merely rudimentary tissue (Lorenzi et al.
2019; Sanders et al.
2021). The present data confirm earlier findings in adult human IG tissue; in addition, they report, for the first time, a significant presence of nNOS-immunopositive neurons in the IG, suggesting a possible physiological role. In this study, the presence of several nNOS neurons in close proximity to the pial arteries penetrating into the CC was shown, as well as a great number of astrocytes, often very close to the same vessels. The hypothesis proposed is that nNOS neurons and astrocytes in close proximity of arterioles could constitute the IG neurovascular units, participating in the regulation of the CC blood flow. In general, the cerebral blood flow modifies according to the functional activity of the different brain regions (functional hyperemia), so that it increases when the neural activity increases, to guarantee substrate and oxygen delivery, and to remove metabolism by-products, thus maintaining the homeostasis of the cerebral microenvironment (Lassen et al.
1978; Raichle and Mintun
2006; Iadecola
2017). The activity-induced hemodynamic response occurs when neurons, together with astrocytes and vascular cells, communicate through a complex signaling mechanism. These cells act as an integrated unit, termed the neurovascular unit, able to generate and transduce the molecular signals responsible of the changes in blood flow. Brain activation leads to the production of many vasoactive mediators (K
+, H
+, neurotransmitters and neuromodulators) which originate from neurons with processes in close contact with blood vessels (Iadecola
2017). Astrocytes are also involved in neurovascular regulation since they have processes in direct contact with both synapses and contractile cells of the vascular wall (Iadecola and Nedergaard
2007). Since a peculiar feature of cerebral circulation is that large cerebral arteries and pial arteries are responsible for two-thirds of the vascular resistance and are therefore the main site of flow control (Faraci and Heistad
1990), IG might have a crucial role in coupling local increases of blood flow in pial branches with metabolic changes related to neuronal function of the underlying CC (Jovanov-Milosevic et al.
2010; Sagrati et al.
2018,
2019). The prevalence of nNOS-positive neurons in the IG overlying the body of CC, which is crossed by sensory-motor fibers where information need to travel fast, further supports the notion that these neurons are involved in modulating the blood flow to face high energy demands. Not by chance, the long callosal artery, one of the main components of the vascular network supplying blood to the CC, gives rise to multiple perforating branches, especially at the level of the body (Kahilogullari et al.
2008). These vessels enter the CC at the midline (Kahilogullari et al.
2008): this observation is consistent with the higher number of nNOS-positive neurons, along the medio-lateral extension, about 1 mm from the midline of the whole IG. The distribution of nNOS neurons in IG seems to be strictly related to the vascular anatomy of CC. The presence of nNOS neurons at the IG/CC boundary points to a role of these neurons in the IG/CC communication, as already hypothesized by recent immunohistochemical findings in rat IG (Barbaresi
2018). These neurons could be similar to those recently observed in layer III of IG which were positive to calbindin (Bobić Rasonja et al.
2021; Sanders et al.
2021), a protein that has a threefold function as buffer, transporter and likely as a non-canonical sensor of Ca
2+ (Schmidt
2012). It is plausible that these neurons act as neural mediators of signaling between IG and CC, reciprocally enabling to be activated by other brain regions, as indicated by the presence of numerous nNOS-positive fibers in the two structures. Present findings allow to hypothesize that IG is not only an ancillary tissue for the activity of CC, but it is also a real morpho-functional element of the nervous system. This hypothesis is supported by the relevant number of astrocytes observed in the whole IG. Historically considered as merely supporting neurons, recent research has shown that astrocytes actively participate in a large spectrum of central nervous system (CNS) functions including formation, maturation and elimination of synapses, neuronal transmission and modulation of synaptic plasticity (Dallérac and Rouach
2016). The abundance of such multifunction glial cells in IG suggests it is a very active tissue, corroborating the importance of IG in human adult brain.
Other hypotheses have been advanced in recent years suggesting an active functional role for IG. Some studies propose that IG, as a structure containing a heterogeneous mix of both neuronal and glial cells, could have a role in the guidance of the callosal axons during their development (Shu and Richards
2001; Morcom et al.
2016). A very recent study (Izzo et al.
2021) supports the hypothesis of a close interplay between the CC and the IG development. The study describes the hyperplasia of the IG, a midline glial structure, in two rare early gestation fetal cases who also display an abnormally thick and short CC, without any other systemic and/or central nervous system malformation. In addition, in the IG of fetuses with callosal anomalies, NeuN-positive cells were revealed, indicating the presence of neurons, not found in control fetuses; by GFAP immunoreactivity, an increase of glia cells number was also observed respect to the controls, further suggesting that an abnormal IG aspect could be associated to an altered CC embryological development.
Another recent study proves that IG has its own distinct histogenetic differentation pattern (Bobić Rasonja et al.
2021), and it does not shows signs of regression during the fetal period; these observations suggest that IG is not a rudimentary tissue, but it plays a functional role in the adult brain. In line with this hypothesis, NeuN-positive cells, i.e. neurons, have been found in the IG (Bobić Rasonja et al.
2021), both during fetal development and in adults, in accordance with present research.