**3. Results**

### **3.1 In light microscopy**

The histological examination of the H and E stained sections revealed a substantial number of perivenular and pericapillary infiltrations in the brain hemispheres, the brain stem, and the cerebellum of the animals, with the greatest amount of infiltrations seen in animals, which were scored 4 and 5. The spinal cord was seriously involved showing the highest number of perivascular infiltrates in all animals. No infiltrations were observed in animals scored 0.

The semi-thin sections of Araldite embedded tissue, which were studied in light microscopy revealed, besides the perivascular infiltrates, alterations of the myelin sheath of the myelinated axons in the brain hemispheres, the cerebellum, and extensively in the spinal cord in animals scored 4 and 5.

#### **3.2 In electron microscopy**

By electron microscopy, the pericytes were seen in the wall of the brain capillaries, around the endothelial cells (**Figure 1**). They are characterized by their large round or ovoid nucleus, with rough distribution of the chromatin, the plenty of mitochondria and ribosomes perikaryon, and the basal lamina, which surrounds the cell body. They interacted with the endothelial cells, which create gap junction, surrounding them.

A substantial proliferation of pericytes was noticed in the spinal cord and the cerebellum around the capillaries and the venules, escaping the basal lamina (**Figure 2**) particularly in animals scored 4 or 5. All of them extend long processes, on the one hand surrounding the wall of the blood capillaries and on the other approaching the astrocytes in the perivascular space.

Morphologically, the majority of the pericytes and the endothelial cells demonstrated aggregations of many small mitochondria around the nucleus, dilatation of the cisternae of Golgi apparatus, and large lysosomes (**Figures 2** and **3**). The nucleus of the activated perivascular pericytes was mostly round or ovoid, distinguished clearly from the very elongated nuclei of the endothelial cells (**Figure 4**). The nucleus of the perivascular pericytes demonstrated, as a rule, a rough distribution of heterochromatin in the periphery. The perikaryon included large number of small round mitochondria, with fragmentation of the cristae in the majority of

*Pericytes of the Brain in Demyelinating Conditions DOI: http://dx.doi.org/10.5772/intechopen.103167*

**Figure 1.** *Pericyte (P) in the wall of a brain capillary near the endothelial cell (E). Electron micrograph (mag. 35,000×).*

#### **Figure 2.**

*Pericyte escaping the basal membrane (bm) around a brain capillary. The mitochondrial alterations are obvious. Electron micrograph (mag.128,000×).*

them. A substantial number of endothelial cells demonstrated dilatation or disruption of the tight junctions (**Figure 5**).

Many capillaries showed marked perivascular edema and accumulation of lymphocytes and monocytes. It was noticed that pericytes in the neuropile space were intermixed with astrocytic processes (**Figure 6**).

A large number of pericytes demonstrated an increased number of pinocytotic vesicles, large lipid granules, and mitochondrial alterations, and marked dilatation of the cisternae of the smooth endoplasmic reticulum (**Figures 2** and **3**).

#### **Figure 3.**

*Alterations of the mitochondria and dilatation of the smooth endoplasmic reticulum (ser) in an endothelial cell of the wall of a brain capillary. Electron micrograph (mag. 128,000×).*

#### **Figure 4.**

*An endothelial cell of a brain capillary, showing abundant peripheral accumulation of heterochromatin and disruption of the tight junction (Tj). Electron micrograph (mag. 35,000×).*

*Pericytes of the Brain in Demyelinating Conditions DOI: http://dx.doi.org/10.5772/intechopen.103167*

**Figure 5.** *Disruption of the tight junctions (Tj) of a brain capillary. Electron micrograph (mag. 128,000×).*

#### **Figure 6.**

*Pericytes (P) in the neuropile space around the endothelial cell (E) of a dilated brain capillary. There is a marked perivascular edema. Electron micrograph (mag. 35,000×).*

## **4. Discussion**

Pericytes are polymorphic perivascular cells, which collaborate with the endothelial cells for the regulation of the blood–brain barrier's permeability [39]. A substantial body of evidence, derived from morphometric observations in light

#### *Demyelination Disorders*

and electron microscope, revealed that the ratio of pericytes to endothelial cells in the majority of the structures of the central nervous system is approximately 1:1 [39]. However, not all of the perivascular cells are pericytes, given that some of them are macrophages or adventitia cells [40], presumably derived from the pericytes, which as pluripotent cells can generate other cell types, to maintain the brain homeostatic equilibrium [22].

In MS, the proliferation or the degeneration of the pericytes associated with dysfunction or disruption of the blood-brain barrier is one of the initial neuropathological phenomena [41], triggering a cascade of inflammatory reactions and cellular interactions.

In the model of the experimental allergic encephalomyelitis, alterations of the blood-brain barrier have been described in electron microscopy by many authors [42–44]. The role of the pericytes in inducing those alterations may be crucial, given that perivascular pericytes regulate endothelial transcytosis, which would increase the permeability of the blood-brain barrier [45].

In the model of pericyte-deficient mice, an increased expression of leukocyte adhesion molecules has been described in association with polarization defect of astrocyte end feet in the vessels of the brain, underlining the importance of the pericytes for the integrity of the blood-brain barrier [46].

On the contrary, the proliferation of the pericytes suggests that they participate in the immune reactions of the brain, a fact that is noticed and described also in multiple sclerosis [21]. It was noticed that the pathological alterations in experimental allergic encephalomyelitis mimic to some degree, in many aspects, the morphological alterations, which occur in multiple sclerosis [47, 48].

Many histological observations revealed that the morphology of the pericytes varies considerably in the various structures of the brain in normal and pathological conditions. Among other conditions, proliferation of pericytes was described in early cases of Alzheimer's disease, associated with disruption of the BBB [49] as well as in traumatic brain injuries [50].

Although many markers have been used for the identification of pericytes in various conditions, none is unanimously accepted as the precise and definite one, given that pericytes retain the multipotential properties of stem cells [51] or express a macrophage-like function [52].

The proliferation of the pericytes around the capillaries and the venules in the central nervous system has been observed mostly at the initial stages of the inflammatory conditions, autoimmune reactions, and degeneration of the brain, given that as the process advances, the pericytes further migrate into the neuropile space, and the ratio between them and the endothelial cells declines consequently [53].

In many pathological conditions, the pericytes contribute to the restoration of the BBB substantially, either by their contact with the endothelial cells or through proper signaling [54, 55], a fact that is beneficial for the establishment of the brain homeostasis.

A reasonable therapeutic approach to multiple sclerosis may be attempted by enforcing the interactions between pericytes, endothelial cells, and astrocytes, which may result in the restoration of the blood-brain barrier [56, 57].

It would also be emphasized that the observation that activated pericytes may contribute substantially to the differentiation of the oligodendrocyte progenitors, enabling consequently the restoration of the myelin sheath, and the protection of the axons is of substantial importance for finding an escape from the labyrinth of the disease [58]. This novel role of pericytes may open new therapeutic horizons in the field of demyelinating conditions [59, 60], as a catharsis from the drama of multiple sclerosis.
