The retina consists of a single neuroblastic layer when the optic fissure closes. At 4 to 5 weeks of gestation (WG) in humans, the hyaloid artery and associated mesenchyme invade the area that was previously the fissure.
1 The hyaloid vascular system provides nutrients for subsequent retinal and lens development. The neuroblastic layer rapidly differentiates into two nuclear layers. Although the time course for appearance of the layers and structures was documented many years ago by Mann
2 the interactions and relationships between cell types in humans and the origins of these cell types are not fully understood.
3 4 5 6 For example, the heart (endocardium),
7 aorta and aortic arches,
8 and vasculature in some organ systems, such as lung, pancreas, and spleen, develop by the process of vasculogenesis, differentiation, aggregation of vascular precursors (angioblasts) to form cords, and canalization of the cords.
9 10 11 12 In other organ systems such as the kidney, the vasculature is said to develop by the process of angiogenesis: migration and proliferation of endothelial cells from existing blood vessels.
13 Formation of superficial primordial vessels of the retina in some species are mediated through vasculogenesis,
5 14 although angioblasts have not been identified in human retina; whereas, angiogenesis is responsible for increasing vascular density and vascularization of the peripheral and inner retina in humans
5 and deep plexus in dogs.
14 15 Therefore, the mechanism of retinal vascularization appears similar to that observed during brain development. The primordial vascular bed on the surface of the neuroepithelium is derived from migratory vascular precursor cells,
16 presumably by vasculogenesis. New vessel segments sprout from these preexisting vessels and grow tangentially by angiogenesis into the neuroepithelium.
17 18