Localization of adhesive proteins in two newly subdivided zones in electron-lucent matrix of human platelet ?-granules

Abstract

Platelet alpha-granules have been reported to consist of two zones, nucleoid and electron-lucent matrix, with different densities under electron microscopy. When washed human platelets were prepared by a rapid freeze-substitution method using liquid helium, we found that the electron-lucent matrix could be further subclassified into two zones having different densities: the intermediate and the light zones. The light zone was located at the periphery opposite the most dense nucleoid and contained several tubular structures with diameters of about 20 nm. The intermediate zone often laid between the nucleoid and light zone. By careful inspection, intermediate and light zones could even be identified in the platelets embedded in Lowicryl K4M, which where then used to localize several adhesive proteins in these two zone by immunocytochemical studies using the respective polyclonal antibodies. Fibrinogen, thrombospondin, and fibronectin were detected only in the intermediate zone. In contrast, von Willebrand factor (vWF) was localized only in the light zone, suggesting an association between vWF and the tubular structures in the light zone. In the nucleoid, none of these adhesive proteins were detected. Glycoprotein IIb/IIIa, a receptor for these adhesive proteins on the platelet surface, was detected not only on the outer surface of the cell membranes but also on the inner surface of the alpha-granule membrane. These data indicate that two zones with different densities in electron-lucent matrix and functions exist in the platelet alpha-granules.

Role of Thrombospondin-1 in Control of von Willebrand Factor Multimer Size in Mice

“…Platelet VWF is localized within tubular structures in the light zone located at one pole of the ␣-granule. TSP-1 along with fibrinogen and fibronectin, on the other hand, appears to be dispersed within the intermediate zone (50). This compartmentalization of VWF has been reported in both human and murine platelets (37).…”

Localization of endothelin A receptors in the rat pituitary TSH cells: light- and electron-microscopic immunohistochemical studies

“…Activation of polymorphonuclear leukocytes induced fusion of specific granules with the cell membrane and expression of laminin receptor and CD18 antigen on the cell surface (Singer et al 1989). In the platelet, glycoprotein IIb/IIIa, a receptor for the adhesive proteins, is detected not only on the surface of the cell membrane, but also on the α-granule membrane (Suzuki et al 1990). Localization of ET A R in the secretory granules of TSH cells would suggest that, as speculated for polymorphonuclear leukocytes and monocytes, ET A R is not only present on the plasma membrane but also densely distributed on the granule membrane, and may be inserted into the plasma membrane via the exocytosis of the secretory granules.…”

The storage lesion of platelets: ultrastructural and functional aspects

“…Within the a-granules, an internal compartmentalization is detectable by different electron densities (Fig. 5) and by immunocytochemical labeling [87], which demonstrates the differential distribution of various substances within the nucleoid and two zones within the matrix. This compartmentalization may be the reason for the differential release of a-granular substances that is observed after in vitro stimulation of platelets [35,79].…”