Ultrastructural Definitions

Basement Membranes

BMs were first defined by LM as extracellular layers subjacent to epithelia. Because the practical limit of LM resolution is approximately 0.2 mm, these BMs necessarily included considerable quantities of stained ground substance and reticular (small collagen) fibers in addition to the narrow (~100nm) electron-dense basal lamina of TEM. Nevertheless, these early descriptions were accurate, and subsequent TEM studies confirmed their histological locations and demonstrated that basal laminae consistently co-localized with BMs described by LM.

Subsequently, the term BM was taken over by the electron microscopists, who widened the definition considerably to include a ubiquitous network of basal laminae always closely associated with cell surfaces. Moreover, BMs and basal laminae became synonymous terms for extracellular membranes that formed physical boundaries for a large and uncompartmented connective tissue space. BMs separated the space from nonconnective tissue cell types including epithelia, muscle, and nerve, while connective tissue cells (except adipocytes) were free in the tissue space uncovered by BMs. TEM also provided substantial substructural BM detail, including three distinct layers not recognizable by LM. These included a central, dense layer (lamina densa) flanked by two layers that were more electron-lucent (laminae rarae).

Capillary Basement Membranes

TEM analyses of many tissues show that all capillaries are surrounded by CBMs that closely subtend their endothe-lial cell layers. Simple capillaries are surrounded by BMs (basal laminae) composed of a central lamina densa separated from the endothelium by a lamina rara (Figure 1a). Capillaries decorated by perivascular cells (pericytes) are more complex. As in simple capillaries, basal laminae surround the endothelium, but they often split to enclose one or more pericytes as well. In these, CBMs separate endothelial cells from the connective tissue space and subendothelial BMs shared by perictyes, and endothelial cells often fill the narrow space between them (Figures 1b, 1c and 2).

Even more structural sophistication is demonstrated by renal glomerular capillaries, pulmonary alveolar capillaries, and capillaries of the central nervous system (including the retina). In these, both CBM surfaces are demarcated by cell

Figure 1 Transmission electron micrographs of sections through capillaries. (A) Simple capillary from rat skeletal muscle. A continuous capillary basement membrane (CBM) subtends the endothelium (E) and separates it from the connective tissue space. (B) Bovine skeletal muscle capillary surrounded by basement membrane (CBM) and decorated by pericytes (P). A shared basement membrane (arrows) separates pericytes from the endothelium. RBC, red blood cell, (c) Rat retinal capillary near the vitreo-retinal border. A pericyte (P) shares the basement membrane (CBM) with the endothelium for nearly half of the capillary circumference.

Figure 1 Transmission electron micrographs of sections through capillaries. (A) Simple capillary from rat skeletal muscle. A continuous capillary basement membrane (CBM) subtends the endothelium (E) and separates it from the connective tissue space. (B) Bovine skeletal muscle capillary surrounded by basement membrane (CBM) and decorated by pericytes (P). A shared basement membrane (arrows) separates pericytes from the endothelium. RBC, red blood cell, (c) Rat retinal capillary near the vitreo-retinal border. A pericyte (P) shares the basement membrane (CBM) with the endothelium for nearly half of the capillary circumference.

membranes. In the kidney, glomerular BMs are flanked by capillary endothelial cells and podocytes; in the lung, pulmonary alveolar BMs intervene between capillary endothe-lium and the alveolar epithelium, and in the CNS (retina), CBMs are sandwiched between endothelial and glial (Müller) cells in a highly complex microvasculature that often includes single or multiple layers of perictyes (Figures 1c and 2).

With these structural complexities, an important question relates to the definition of CBM. In this regard, most investigators consider the CBM as the BM subjacent to the capillary endothelium, regardless of whether it is shared by other cell types.

CBMs have membranous or sheet-like shapes and are relatively narrow in one dimension (width) compared to the other two (area). Although both dimensions are subject to changes that may have physiological or pathological significance, width (thickness) is by far the simplest to estimate. Accordingly, investigators have applied various

Figure 2 Transmission electron micrograph of section through complex bovine retinal capillary surrounded by numerous pericytes (P). Capillary basement membrane (arrows) surrounds the endothelium and often is shared by pericytes. Within the lumen are a red blood cell (RBC) and a granulocyte (Gc). Nu, endothelial cell nucleus.

Figure 2 Transmission electron micrograph of section through complex bovine retinal capillary surrounded by numerous pericytes (P). Capillary basement membrane (arrows) surrounds the endothelium and often is shared by pericytes. Within the lumen are a red blood cell (RBC) and a granulocyte (Gc). Nu, endothelial cell nucleus.

morphometric techniques in an effort to determine BM thickness in normal and diseased tissues.

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