The dentate gyrus (DG), is actually a separate structure, a tightly packed layer of small granule cells wrapped around the end of the hippocampus proper, forming a pointed wedge in some cross-sections, a semicircle in others. Next come a series of Cornu Ammonis areas: first CA4 (which underlies the dentate gyrus), then CA3, then a very small zone called CA2, then CA1. The CA areas are all filled with densely packed pyramidal cells similar to those found in the neocortex. After CA1 comes an area called the subiculum. After this comes a pair of ill-defined areas called the presubiculum and parasubiculum, then a transition to the cortex proper (mostly the entorhinal area of the cortex). Most anatomists use the term “hippocampus proper” to refer to the four CA fields, and hippocampal formation to refer to the hippocampus proper plus dentate gyrus and subiculum 1).
Neural stem cells (NSCs) in the adult mouse hippocampal dentate gyrus (DG) are mostly quiescent, and only a few are in the cell cycle at any point in time. DG NSCs become increasingly dormant with age and enter mitosis less frequently, which impinges on neurogenesis. How NSC inactivity is maintained is largely unknown.
Zhang et al. found that Id4 is a downstream target of Notch2 signaling and maintains DG NSC quiescence by blocking cell-cycle entry. Id4 expression is sufficient to promote DG NSC quiescence and Id4 knockdown rescues Notch2-induced inhibition of NSC proliferation. Id4 deletion activates NSC proliferation in the DG without evoking neuron generation, and overexpression increases NSC maintenance while promoting astrogliogenesis at the expense of neurogenesis. Together, this findings indicate that Id4 is a major effector of Notch2 signaling in NSCs and a Notch2-Id4 axis promotes NSC quiescence in the adult DG, uncoupling NSC activation from neuronal differentiation 2).
It is thought to contribute to the formation of new episodic memories, the spontaneous exploration of novel environments, and other functions.
It is notable as being one of a select few brain structures currently known to have high rates of neurogenesis in adult rats (other sites include the olfactory bulb and cerebellum).
The perforant pathway projection from layer II of the entorhinal cortex to the hippocampal dentate gyrus is especially important for long-term memory formation, and is preferentially vulnerable to developing a degenerative tauopathy early in Alzheimer disease (AD) that may spread over time trans-synaptically.
The medial view of the anterior segment of the medial temporal region (MTR). (This illustration correlates with Figures 6(b) and 6©). The anterior choroidal artery (AChA) gives off an anterior uncal artery that irrigates the semilunar gyrus and an uncohippocampal artery that irrigates the uncinate gyrus and band of Giacomini and penetrates the uncal sulcus to vascularize the extraventricular hippocampal head. The internal carotid artery (ICA) gives off an anterior uncal artery that supplies the semilunar gyrus. This branch usually is present when the anterior uncal artery of the AChA is absent. An anterior uncal artery also arises from the M1 segment of the middle cerebral artery (MCA) and supplies the ambient gyrus. An uncoparahippocampal artery arises from the temporopolar artery and irrigates both the ambient gyrus and the anterior parahippocampal area. Branches from the P2a segment of the PCA irrigate the anterior parahippocampal region (anterior parahippocampal artery) or both the anterior parahippocampal gyrus and hippocampal head (anterior hippocampal-parahippocampal artery). (b) The medial surface of the anterior segment of the left MTR. The white arrow points the posterior end of the uncal notch. The anterior part of this segment is irrigated by middle cerebral branches (orange shaded area), the posterosuperior part is supplied by anterior choroidal branches (blue shaded area), and the posteroinferior part is vascularized by posterior cerebral branches (yellow shaded area). The ICA typically supplies the area supplied by the AChA and the MCA if their branches are absent. The branches of the MCA are the anterior uncal artery superiorly and the unco-parahippocampal artery inferiorly. The branches of the AChA are the anterior uncal artery anteriorly, the posterior uncal artery posteriorly, and the unco-hippocampal artery posteroinferiorly. The branches of the PCA are the anterior hippocampal-parahippocampal artery medially and the anterior parahippocampal artery laterally. Areas of vascular anastomosis are typically found at the confluence of vascular territories (curved arrows). © The same view of (a) in a silicon injected anatomic specimen. (d) Inferior view of the anterior segment of the left MTR. The inferior lip of the posterior uncal segment has been removed to expose the extraventricular hippocampal head. The semilunar gyrus has been retracted to expose the branches of the AChA. Two anterior uncal arteries arise from the first one-third of the AChA and irrigate the semilunar gyrus. A posterior uncal artery from the AChA penetrates the uncal sulcus and irrigates the extraventricular hippocampal head. An anterior hippocampal-parahippocampal artery arising from the anteroinferior temporal branch of the PCA gives rise to an anterior hippocampal branch that supplies the extraventricular hippocampal head and anastomoses with the unco-hippocampal branch of the AChA (green arrow). (e) Lower surface of the anterior segment of the right MTR. The entorhinal area is irrigated medially by the parahippocampal branch of the anterior hippocampal-parahippocampal artery that arises from the P2a, and laterally by a large anterior parahippocampal artery that originates from the anterior inferior temporal artery. A.: artery; A.C.A.: anterior cerebral artery; A.Ch.A.: anterior choroidal artery; Amb.: ambient; Ant.: anterior; Car.: carotid; Chor.: choroidal; Dent.: dentate; Entorhin.: entorhinal; Giac.: Giacomini; Hippo.: hippocampus; ICA: internal carotid artery; Inf.: inferior; Intralimb.: intralimbic; Lent.: lenticulo; M.C.A.: middle cerebral artery; M1.: M1 segment of middle cerebral artery; Parahippo.: parahippocampal; P.C.A.: posterior cerebral artery; P2A.: anterior part of the P2 segment of posterior cerebral artery; P2P.: posterior part of the P2 segment of posterior cerebral artery; P.Co.A.: posterior communicating artery; Ped.: peduncle; Pol.: polar; Semianul.: semiannular; Semilun.: semilunar; Str.: striate; Sulc.: sulcus; Temp.: temporal; Tr.: tract; Unc.: uncal; Uncin.: uncinate; V.: vein.
Local interneurons control principal cells within individual brain areas, but anecdotal observations indicate that interneuronal axons sometimes extend beyond strict anatomical boundaries.
Szabo et al. used the case of the dentate gyrus (DG) to show that boundary-crossing interneurons with cell bodies in CA3 and CA1 constitute a numerically significant and diverse population that relays patterns of activity generated within the CA regions back to granule cells. These results reveal the existence of a sophisticated retrograde GABAergic circuit that fundamentally extends the canonical interneuronal network 3).