The brain is highly metabolically active, yet has no effective way to store oxygen or glucose, thus it depends on a large and stable blood supply. The human brain is approximately 2% of the body weight, but uses approximately 15% of cardiac output and accounts for 25% of oxygen consumption.
The vasculature supplying blood to the brain may be primarily distinguished into two categories, based largely upon structure and function: the arterial and capillary systems. The large arteries, as well as penetrating arterioles with smooth muscle cells, control blood flow and influence global perfusion. The smaller intraparenchymal vessels or microvessels, constituting precapillaries and true capillaries, lack smooth muscle cells and control permeability of water and nutrients. Capillaries and postcapillary venules are associated with pericytes. This association has been suggested to regulate endothelial cell proliferation, survival, migration, differentiation, and vascular branching (Hellström et al., 2001). However, cerebral pericytes have been shown to express contractile proteins (Bandopadhyay et al., 2001), although it is yet to be demonstrated whether pericytes have a prominent role in cerebral blood flow (CBF) regulation.
1.8.1. Hippocampal vasculature in various mammals
The blood supply to the hippocampal formation is rather similar across various mammalian species, concerning the external and internal arteries and the internal arterioles (Nilges, 1944; Goetzen and Sztamska, 1992). The arteries to the hippocampal formation arise from the posterior cerebral artery in a rake-like pattern. However, in the various mammals, the posterior cerebral arteries arise from slightly different maternal trunks. In the primate, vascular arcades have developed before the arteries branch into the internal hippocampal arteries (Nilges, 1944).
The hippocampal vascular system, including the microvasculature, is rather strongly developed in comparison to other cortical structures. Studies in the rat hippocampal formation demonstrated that the strata lacunosum-moleculare and oriens have a more extensive vascularization than the stratum radiatum, where the long axis of capillaries tends to be oriented parallel to the apical dendrites (Coyle, 1978). The capillary
Figure 1.5 lumen (L) side, this layer is called “lamina rara interna”
densa; lre: lamina rara externa; lri: lamina rara interna; mi: mitochondrion; MV: microvessel; N:
nucleus; P: pericyte; tj: tight junction; V: varicosity
density of the rat CA1 region is lower than of CA3 (Coyle, 1978). This was also reported for gerbils (Mossakowski et al., 1994) and man (Marinkovic et al., 1992). The outer portion of the molecular layer of the rat dentate gyrus is more vascularized than the inner third (Coyle, 1978).
1.8.2. Anatomy of the blood-brain barrier
The blood-brain barrier (BBB) is the specialized system of cerebral capillaries that protects the brain from toxic substances in the blood stream, while supplying the brain with the required nutrients for proper functioning. Anatomically, the BBB consists of the endothelial cells with surrounding basement membranes, astrocytes, and pericytes.
More than 90% of the microvascular surface is covered by astrocytic endfeet (Fig. 1.5B). The basic function of astrocytes is to provide a structural framework for the central nervous system and to give metabolic support for neurons. Astrocytic processes reach out to touch neurons, capillary walls, pial surfaces, and other astrocytes (Fig. 1.5A).
There is significant body of evidence, in vitro and in vivo, to indicate that astrocyte interaction with the cerebral endothelium supports BBB function and morphology, and protein expression (Cancilla and DeBault, 1983; Beck et al., 1984; Arthur et al., 1987).
Furthermore, astrocytes serve to guide neurons to their proper place during development and directing cerebral vessels.
Several characteristic anatomical features allow a selective passage of mole-cules into the neuropil. First, the brain capillaries do not have fenestrations, as observed in peripheral capillaries. Second, the endothelial cells of the capillaries contain tight junctions (Fig. 1.5B), which seal cell-to-cell contacts between adjacent endothelial cells forming a continuous blood vessel lining. Then, cerebral endothelial cells have only few pinocytotic vesicles, in which molecules can be transported. Rather, transport across the endothelial cells takes place via transporter proteins. This active transport of nutrients requires increased energy potential. Therefore, cerebral endothelial cells have more mitochondria than do peripheral endothelial cells. The microvascular endothelial cells in the brain are the actual anatomical seat of the BBB.
On the abluminal side, the endothelial cells are surrounded by a basement membrane (BM), which is build up of three layers: the lamina rara interna, facing the endothelial cells, the lamina rara externa, opposite to the astrocytes, and the lamina densa in between these laminae rarae (Fig. 1.5C). The laminae rarae contain laminin, collagen type IV, and heparan sulphate proteoglycan. The lamina densa consists of type IV collagen. The main function of laminin is to connect the BM to the underlying cells.
The negatively charged sites of heparan sulphate proteoglycan block the passage of anionic macromolecules across the BM. The open network of collagen type IV provides elasticity and stability to the endothelial cells. Furthermore, the type IV collagen facilitates the passage of fluids.
1.8.3. Innervation of the cerebrovasculature
Fluctuations in the cerebral perfusion pressure are compensated for by the cerebrovascular autoregulation to maintain a consistent CBF. An intact autoregulation is capable of keeping the CBF independent of perfusion pressure, when the perfusion pressure is within a certain normal range.
The cerebrovasculature has both peripheral (external) and central (internal) innervation. The peripheral (external) innervation has a dual characteristic on large cerebral blood vessels: sympathetic peripheral innervation uses noradrenaline as a neurotransmitter, which causes vasoconstriction and which reduces CBF.
Parasympathetic peripheral fibers that innervate the cerebrovasculature contain acetylcholine, and its release elicits vasodilation, which increases CBF. This dual innervation is also encountered on the level of the central nervous system (internal innervation), not only of large arterial vessels, but also of arterioles (Fig. 1.5A,B).
Vasoconstriction is elicited by the release of noradrenaline and serotonin, and vasodilation is induced by the release of acetylcholine.
Innervation of the larger blood vessels regulates the CBF on a global scale, whereas nerve fibers on the arterioles control the regional CBF (rCBF). Apart from neurogenic control of these fine adjustments, rCBF is regulated also by myogenic, metabolic, and chemical mechanisms. Furthermore, astrocytes also execute intracerebral regulation of vascular tone and CBF indicated by the expression of serotonergic and cholinergic receptors on the perivascular endfeet (Reinhard et al., 1979; Cohen et al., 1995, 1996, 1999; Luiten et al., 1996; Elhusseiny et al., 1999).
1.8.4. Aging-related changes of the blood-brain barrier
There is no striking consensus on the effects of aging on the capillary density within the brain, and the reports on aging-related changes in the microvasculature in the hippocampal formation are scarce. However, the density of the capillaries was decreased in the aged rat’s Ammon’s horn and the dentate gyrus (Amenta et al., 1995). Stereological analysis supports the findings of rarefaction of the microvascular bed for the CA1 region of aged rats (Jucker et al., 1990). Also in various hippocampal subfields of aged humans, a reduction of capillary and arteriolar density was observed (Bell and Ball, 1981).
Aging-related ultrastructural changes of the capillary wall have been observed in rats, monkeys and humans, although these studies sometimes report contrasting results. Diverging findings may to a great extent be explained by the use of different mammalian species or strains, but also by the investigation of different brain regions. Clearly, various brain regions may be differentially affected by the aging process. Nevertheless, thickening of the microvascular BM (BMT) seems to be the most common feature of aging, for it has been demonstrated in the frontoparietal (De Jong et al., 1990, 1992) and frontal (Knox et al., 1980; Hicks et al., 1983) cortex and hippocampal CA1 area (Hicks et al., 1983; Topple et al., 1991) and the dentate gyrus (Topple et al., 1990) of aged rats, the frontal cortex and hippocampal CA3 region of aged marmoset monkeys
(Honavar and Lantos, 1987), the frontal and occipital cortex of aging macaque monkeys (Burns et al., 1979), and the cingulate cortex of aged humans (Farkas et al., 2000).
Further aging-associated morphological alterations include deposition of collagen fibrils in the BM (fibrosis), degeneration of pericytes, loss or thinning of endothelial cells, increased numbers of pinocytotic vesicles and decreased number of mitochondria in the endothelial cells (reviewed in Kalaria (1996); Farkas and Luiten (2001); Riddle et al. (2003)). Although morphological studies on the hippocampal microvasculature are scarce, some of these aging-associated ultrastructural alterations have been reported for the hippocampal formation. The microvessels in the CA1 region of aged rats not only displayed BMT, but also deposition of collagen fibrils in the BM and degenerative pericytes (De Jong et al., 1990). Degenerative pericytes have also been reported in the hippocampus of aged marmoset monkeys (Honavar and Lantos, 1987).
However, in the hippocampal CA3 subfield of aged marmoset monkeys, there was no obvious increase of pinocytotic vesicles, nor was there evidence of fewer endothelial mitochondria (Honavar and Lantos, 1987).
Above-described morphological changes are likely to alter the blood flow within the microvessels and the transport of substances across the capillary wall, which, in turn, may affect cerebral energy metabolism and neuronal function. Indeed, there is evidence for decreased rCBF, regional cerebral metabolic rate for oxygen (rCMRO2), glucose utilization, and BBB permeability (Kalaria, 1996; Farkas and Luiten, 2001). In particular, analysis of rCBF in aged Brown-Norway rats indicated a decreased basal blood flow in CA1, CA3 and dentate gyrus of the hippocampal formation (Lynch et al., 1999). In support of these findings, aged rhesus monkeys were shown to have lower rCBF and lower regional cerebral metabolic rate of glucose in the hippocampus with adjacent cortex compared to young monkeys (Noda et al., 2002). In the human hippocampus, there are conflicting data. An aging-related reduction of rCBF was demonstrated (Larsson et al., 2001), although evidence exists of unchanged rCBF and rCMRO2with advancing age (Marchal et al., 1992).
Aging-associated changes in BBB permeability in the hippocampal formation are not well investigated. Endothelial barrier antigen (EBA) is a rat-specific BBB protein, of which a reduction has been shown to correlate with opening of the BBB. EBA-stained microvessels of the hippocampus were reduced in aged rats (Mooradian et al., 1993).
However, another study demonstrated that the BBB permeability in hippocampus of aged rats is not changed (Rapoport et al., 1979).