Development of the retinal vasculature

The development of the eye depends on the formation of a vascular system to provide nutrients and oxygen for the ocular tissues at the time of their active differentiation. During embryogenesis, this involves the de novo formation of blood vessels by differentiation of endothelial precursors to form the primary vascular network (vasculogenesis), and the formation of new blood vessels that branch off from existing vessels (angiogenesis) (Carmeliet, 2000). The brain and the eye are thought to be predominantly vascularised by angiogenesis (Fruttiger, 2007). However, there is accumulating evidence that the primary inner vascular plexus in the retina is formed by vasculogenesis (Chan-Ling et al., 2004). During the initial development of the eye, the oxygenation of the retina is ensured by the choroidal vessels and the hyaloid system. The vascularisation of the retina itself occurs only during the late gestation and is restricted to the inner part of the retina, with the outer retina completely avascular to avoid disturbance of light perception.

1.2.2.1 Development of the choroidal vasculature

The choroidal circulation develops at the same stage as the invagination of the optic vesicle and uveal development (Saint-Geniez and D'Amore, 2004). At the fourth week of gestation, the undifferentiated mesoderm surrounding the optic cup starts to differentiate into endothelial cells and a large plexus of primitive vessels adjacent to the RPE. The initial choriocapillaris develops by haemo-vasculogenesis, a process in which blood cells and blood vessels differentiate from a common precursor, the haemangioblast (Luty 2010). By the second

month of development the primitive vascular plexus envelops the entire exterior of the optic cup and the choriocapillary vessels begin to develop a basal lamina which, together with the basement membrane of the RPE, forms the initial Bruch’s membrane, separating the neural retina from the choroid. In the third month of gestation the intermediate choroidal vasculature, the large choroidal vessels and anastomosis betwenn the vascular layers develop by angiogenesis (Luty 2010). The choroidal capillary network becomes almost completely organised by the twelfth week of gestation with connections to the posterior ciliary arteries and to rudimentary vortex veins that develop in all four quadrants of the eye. By the fourth month of gestation most of the choroidal vasculature matures (Anand-Apte B and Hollyfield J K, 2010).

Although the molecular mechanisms underlying the formation of the choroidal vasculature have not been fully defined, it is largely accepted that it depends on the presence of differentiated RPE cells and the production of inductive signal molecules such as VEGF, fibroblast growth factor (FGF)-9 and b-FGF (Saint-Geniez and D'Amore, 2004).

1.2.2.2 Development of the hyaloid vasculature

The process of intraocular vascularisation begins around the first gestational month with the entry of the hyaloid artery into the optic cup through the fetal fissure. The hyaloid artery extends through the primitive vitreous and reachs the posterior pole of the forming lens where it branches intensively to form a dense capillary network over the lens surface, the so called tunica vasculosa lentis (figure 1.4A). The hyaloid system is characterised by the absence of veins and

all hyaloid arterial vessels drain into the choroidal veins. As the lens develops, the tunica vasculosa lentis expands to reach the anterior part of the lens and forms the pupillary membrane. In later stages, in human during the second month of gestation, the hyaloid vasculature regresses and a vascular plexus emerges from the optic nerve head giving rise to the retinal vasculature (figure 1.4.B-C). The occlusion of the regressing capillaries has been suggested to depend on macrophages and on the levels of secreted growth factors such as Vegf and Angiopoietin 2 (Saint-Geniez and D'Amore, 2004).

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Figure 1.4 Development and remodeling of the hyaloid and retinal vasculature. Early in development, the hyaloid artery supplies the hyaloid vasculature, the tunica vasculosa lentis and the pupillary membrane; the venous drain is accomplished by the choroidal veins (A). As the primary plexus grows into the retina, the hyaloid vasculature regresses (B) and the deeper plexus of the retinal vasculature develops from veins in the primary plexus (C). NB the choroidal vasculature is not shown in B and C. Adapted from (Fruttiger, 2007).

1.2.2.3 Development of the inner retinal vasculature

During early development the inner retina is metabolically supported by the hyaloid vasculature and remains avascular until the fourth month of gestation (figure 1.4A). As the retina matures and grows, the demand for oxygen and nutrition increases which necessitates the formation of a retinal vascular bed (Chan-Ling et al., 1995; Stone et al., 1995). Primitive retinal vessels emerge from the optic disc at the base of the hyaloid artery and spread to the periphery of the retina forming a primitive central retinal arterial system (figure 1.4B). The initial intense angiogenic process is followed by a process called pruning which describes the eradication of part of the vessels, in order to adapt the blood flow to the needs and physiologic characteristics of the differentiated tissues. Pruning occurs via selective endothelial cell death (Ishida et al., 2003) or via migration and relocalisation of endothelial cells (Hughes and Chang-Ling, 2000) and is particularly evident in the vicinity of arteries where capillary free zones emerge due to high oxygen tension present in arteries (Riva et al., 1986). Over time, as more vessels are added in the periphery, the primitive plexus is remodeled and starts to mature into a hierarchical vascular tree with clearly defined arteries, capillaries and veins. The primary vessel plexus at the inner retinal surface subsequently remodels into two parallel but inter-connected networks, located in the nerve fiber layer and the inner plexiform layers (figure 1.4C). These vascular beds are further remodelled by proliferation and apoptosis of endothelial cells which is closely controlled by hypoxia and Vegf gradients (Chan-Ling et al., 1995; Stone et al., 1995). The retinal vasculature in humans is fully mature at birth and remains quiescent unless the balance of angiogenic inhibitors is tipped

in favour of angiogenic activators (termed ‘angiogenic switch’) during hypoxia and ischaemia associated retinal disease (Carmeliet, 2000).