Abstract
The purpose of this study was to examine the structural and functional maturation of the choriocapillaris (CC) and to determine when fenestrations form, the capillaries are invested with pericytes, and the endothelial cells (ECs) became functional.Immunohistochemistry was performed on cryopreserved sections of embryonic/fetal human eyes from 7 to 22 weeks' gestation (WG), using antibodies against PAL-E, PV-1 (fenestrations), carbonic anhydrase IV (CA IV), eNOS, and alpha-smooth muscle actin (alphaSMA) and NG2 (two pericyte markers) and the EC marker (CD31). Alkaline phosphatase (APase) enzymatic activity was demonstrated by enzyme histochemistry. Transmission electron microscopy (TEM) was performed on eyes at 11, 14, 16, and 22 WG. Adult human eyes were used as the positive control.All EC markers were present in the CC by 7 WG. PAL-E, CA IV, and eNOS immunoreactivities and APase activity were present in the CC by 7 to 9 WG. TEM analysis demonstrated how structurally immature this vasculature was, even at 11 WG: no basement membrane, absence of pericytes, and poorly formed lumens that were filled with filopodia. The few fenestrations that were observed were often present within the luminal space in the filopodia. Contiguous fenestrations and significant PV-1 were not observed until 21 to 22 WG. alphaSMA was prominent at 22 WG, and the maturation of pericytes was confirmed by TEM.It appears that ECs and their precursors express enzymes present in adult CC well before they are structurally mature. Although ECs make tight junctions early in development, contiguous fenestrations and mature pericytes occur much later in development.
Highlights
Human choroid is a thin, highly vascularized and pigmented tissue that forms the posterior portion of the uveal tract
PAL-E, carbonic anhydrase IV (CA IV) and endothelial nitric oxide synthase (eNOS) immunoreactivities and Alkaline phosphatase (APase) activity were present in CC by 7–9 weeks gestation (WG)
Transmission electron microscopy (TEM) analysis demonstrated how structurally immature this vasculature was even at 11 WG: no basement membrane, absence of pericytes, and poorly formed lumens that were filled with filopodia
Summary
Human choroid is a thin, highly vascularized and pigmented tissue that forms the posterior portion of the uveal tract (the iris, ciliary body, and choroid). The vascular system of the choroid consists of three layers in the posterior pole. The capillaries are arranged in a honeycomb-like pattern that appears lobular, especially in the posterior pole. Haller’s layer, the outermost layer of choroidal vasculature, is composed of large arteries measuring 40 to 90 μm and large veins measuring 20 to 100 μm. This distinct three layer choroidal vasculature tapers to a single layer in the periphery that has ladder-like vascular pattern.[1]
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