December (Final Notebook.2)

The modern view of the mammalian bloodbrain barrier (BBB) has shifted from a purely anatomic concept to a more physiological and dynamic definition. Morphologically, the BBB is formed by specialized endothelial cells (ECs) lining the intraluminal portion of brain capillaries. These ECs are characterized by specialized regions of intercellular contact (tight junctions) that prevent leakage of blood-borne substances into the brain parenchyma. The kinetic aspects of the passage of ions and molecules from the blood into the brain and vice versa better approximate the physiological function of the BBB. It is clear, however, that the distinct morphological properties of the BBB, as seen at both the light microscopic and ultrastructural levels, account for the “restraining” nature of brain capillary ECs. In contrast, the often-neglected fact that the BBB does not act as an absolute barrier, together with the asymmetry of its permeation properties, is less evident from a simple morphological investigation of brain microvessels. The failure of BBB structural integrity and function plays a pivotal role in the pathogenesis of many diseases of the central nervous system (CNS) (Table 1). Thus, during ischemia, inflammation, trauma, neoplasia, hypertension, and epilepsy, altered BBB permeability is commonly observed. In barrier pathology, it is useful to take into account not only the endothelial dysfunction but also damage to the basal lamina, pericytes, astrocytes, vascular innervation, and components of the immune system. The extravasation of plasma proteins associated with BBB dysfunction may occur through a number of different transcellular or paracellular routes, including altered tight junctions, induction of fluid-phase or nonspecific pinocytosis and transcytosis, formation of transendothelial channels, or disruption of the endothelial cell membrane. Of course, these pathways may open in combination and are not mutually exclusive. Irrespective of whether the BBB disruption is the main pathogenic factor, or an inevitable consequence of the disease itself, our understanding of the cellular mechanisms that lead to the disruption of the BBB is limited. This may be due, in part, to the lack of available models of BBB. Any such in vitro models must reproduce important features of brain EC (Table 2) while allowing for manipulations aimed at mimicking the disease process itself. For example, the vascular permeability changes associated with neoplasia and inflammation are clearly manifest and of practical importance with regard to the clinical application of diagnostic and therapeutic measures. However, a suitable model to study tumor (or pathogen)-BBB interactions has yet to be developed. Several approaches have been attempted to investigate the unique characteristics of the BBB endothelium in both the normal and disease state. Experimental observations first made by Paul Ehrlich in 1885 and Edwin Goldman in 1901 (that the CNS is not stained by intravascular water-soluble dyes) provided the first demonstration of a BBB to polar compounds. Pioneering studies of the BBB were performed in vivo using intracarotid injection single-pass techniques (11). Further characterization of the BBB at the cellular level has led more recently to the development of in vitro experimental approaches. Isolated brain capillary preparations as well as tissue culture systems using brain ECs have proven to be a promising methodology to define the characteristics of the brain capillary endothelium at the molecular and cellular level. The purpose of this review is to describe the virtues and pitfalls of cell culture-based models of the BBB. In our opinion, none of these models yet fully expresses the unique features of the BBB in situ; it is thus important for physiological, pharmacological, and preclinical studies to compare, whenever possiG. A. Grant and D. Janigro are in the Department of Neurological Surgery, University of Washington School of Medicine, 325 9th Ave., Seattle, WA 98104, USA; N. J. Abbott is in the Biomedical Science Division, King’s College, London, UK. “. . .the BBB is formed by specialized endothelial cells. . . .”