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T. Baba, R. Grebe, D. S. McLeod, I. A. Bhutto, T. Hasegawa, G. A. Lutty; Maturation of the Fetal Human Choriocapillaris. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5907.
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We have recently demonstrated that the human choriocapillaris (CC) develops by hemovasculogenesis: differentiation of endothelial, hematopoietic and erythropoietic cells from a common precursor, the hemangioblast (Hasegawa et al., Dev Dyn, 236:2089, 2007). The purpose of this study was to document the structural [presence of fenestrations with PAL-E antibody and transmission electron microscopy (TEM)] and functional maturation [carbonic anhydrase IV (CAIV) and alkaline phosphatase (APase)] of this unique capillary system.
Alkaline phosphatase immunohistochemistry was performed on cryopreserved sections of fetal human eyes from 6.5 to 23 weeks gestation (WG) using antibodies against PAL-E, CAIV, and endothelial cell (EC) markers (CD34, CD31, von Willebrand's factor (vWf), and CD-39 (a marker for angioblasts and ECs). APase activity was demonstrated by enzyme histochemistry. TEM was performed on 11 and 14 WG eyes. Adult human eyes were used as a positive control.
Hemovasculogenesis was complete by 10 WG and large choroidal blood vessels were forming. Endothelial cell markers (CD31, CD34, CD39, vWf) were present in CC by 6.5 WG. PAL-E and CAIV immunoreactivities and APase activity were present in CC by 9 WG, yet only a few fenestrations were present at 14 WG by TEM. TEM analysis demonstrated how structurally immature this vasculature was even at 14 WG: no basement membrane, absence of pericytes, and poorly formed lumens that were filled with complex membranous infoldings that resembled filopodia. The few fenestrations that were observed were often present within the lumenal space in the filopodia.
The 14 WG human CC is still structurally immature. It appears that PAL-E is not a marker for fenestrations as reported previously (Niemela et al., Blood 106:3405, 2005). In adult eyes, we observed uniform PAL-E localization even in non-pathologic retinal vessels. Only a few fenestrations were observed at 14 WG by TEM, yet PAL-E in CC was present at all time points examined. Based on CAIV localization and APase activity, the CC cells have some enzymatic characteristics of functional maturity near completion of hemovasculogenesis, yet they are structurally immature.
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