Development of the retinal circulation in the pig.

Retinal angiogenesis was studied in 300 eyes of 150 porcine fetuses by means of semithin histologic sections and vascular corrosion casts. In the embryonic and early fetal period, the retina is avascular and nourished via diffusion from the choriocapillaris and the vascular tunic of the lens. The development of the inner vascular plexus of the retina occurs in three different stages. In the first stage (angioblast phase), during gestational weeks 6-7, mesenchymal precursor cells arising from the arterial and venous circle around the optic nerve invade the nerve fiber layer of the retina via the otic disc border. They form a network of angioblasts that gives rise to an immature capillary network in the second stage (angiogenesis phase). This vascular monolayer is located within the nerve fiber layer and reaches the ora serrata around gestational weeks 9-10. Initially, the immature retinal capillaries have an irregular appearance with wide lumina and relatively small intercapillary meshes. Subsequently, the lumina become smaller by involution and atrophy. In the third stage (maturation and remodeling phase) the immature blood vessels differentiate into retinal arterioles, capillaries and venules. From gestational week 11 onwards, the larger retinal arterioles are surrounded by a distinct periarteriolar capillary-free zone. The three stages start at the optic disc and extend centrifugally towards the retinal periphery. The development of the outer vascular plexus is essentially different from the angiogenesis of the inner vascular plexus. The outer retinal vessels that are located in the inner nuclear layer arise from previously developed capillaries and venules located in the inner vascular plexus. Moreover, the development of the outer vascular plexus starts at the macula and proceeds along with the maturation of the neural retina.