Outer Retinal (Choroidal) Vasculature Structure and Choroidopathies

The outer retinal vasculature is actually the choroidal vasculature. Its capillary system is the choriocapillaris (CC), which is lobular and fenestrated. The CC is immediately under the retinal pigment epitheLium (RPE) supplying nutrients and removing waste from the RPE, the outermost cell in the retina.The embryonic CC (6-9 weeks' gestation (WG)) forms initially by hemo-vasculogenesis, the process by which blood vessels and blood cells (hematopoiesis (expression of CD34⁺) and erythropoiesis (presence of epsilon chain of hemoglobin or Hb-7u⁺)) differentiate from a common precursor, the hemangioblast. TEM analysis demonstrated that CC is structurally immature even at 11 WG. Angiogenesis (CD34⁺/Ki67⁺) appears to be the mode of intermediate and large choroidal vessel development later in the fetus (10-16 WG). Contiguous fenestrations and mature pericytes occur much later in development (21 WG).There is a mutuaListic symbiotic relationship between the components of the photoreceptor (PR)/RPE/Bruch's membrane (BrMb)/CC complex. In dry age-related macular degeneration (AMD), it appears that large confluent drusen formation and hyperpigmentation (presumably dysfunction in RPE) are the initial insult. The death and dysfunction of PRs and CC appear to be secondary events to loss in RPE. In wet AMD, the loss of choroidal vasculature may be the initial insult to the complex, leaving the hypoxic RPE. These hypoxic cells then produce angiogenic substances Like vascular endotheLial growth factor to stimulate the growth of new vessels from CC, resulting in choroidal neovascularization. Therefore, the mutuaListic symbiotic relationship within the PR/RPE/BrMb/CC complex is lost in both forms of AMD.