Ultrastructure of blood vessels in the ganglionic eminence of premature rabbits with spontaneous germinal matrix hemorrhages.

Approximately 20% of preterm rabbit pups develop spontaneous germinal matrix hemorrhages (GMH). To understand better the pathogenesis of GMH we studied the ultrastructure of germinal matrix (GM) blood vessels in rabbits delivered at gestational day 28. Regardless of luminal size, the walls of most GM vessels had the structural characteristics associated with a blood-brain barrier (BBB) and consisted of endothelial cells and pericytes, surrounded by GM cell processes. Endothelial cells ranged from voluminous to attenuated, with some cells containing intracytoplasmic, membrane-bound vacuoles, and luminal as well as abluminal cytoplasmic projections. Some short interendothelial junctions had no puncta adherentia, whereas long ones often possessed intermittent pores. In two animals with GMH, intact endothelial cells were separated by narrow and wide gaps filled with luminal contents that occasionally extended beyond the interendothelial opening. The basal lamina (BL) was ill-defined, thin, often discontinuous and of low electron density. Smooth muscle cells and collagen were not present, which precluded any classification into arteries, capillaries and veins. Germinal matrix cell processes lacking both micro- and intermediate filaments were haphazardly disposed around the blood vessel walls in place of astrocytic endplates. Recent reports indicate that an astrocytic environment may be necessary for the development of the interendothelial tight junctions and BL. The presence of "glial foot" processes that lack ultrastructural characteristics of mature astrocytes suggests that interendothelial junctions and basal laminae in the vessels of the ganglionic eminence may not have the necessary structural and functional potential to withstand the transmural pressures or the pathophysiological influence of hypertension, hyperosmolarity, sepsis, and other factors known to open the BBB and to contribute to GMH.