ABSTRACT
In the human, the primitive ophthamolic artery (a branch of the internal carotid) invades the forming cavity of the optic cup creating the hyaloid artery. Branching from this artery supplies both the growing lens and forming retina, until the point when the retina obtains its own vasculature. The hyaloid vessels are a temporary structure which reach their peak of development around 9 weeks. These vessels mostly regress by the end of the third month, but a skeleton hyaloid artery remains in the central Cloquet’s canal until the seventh month when blood flow ceases. Primitive retinal vessels arise in the fourth month of human development from the base of the hyaloid vessels at the optic disc. Between four and eight months a primitive capillary network forms first around the optic disc and spreads towards the periphery. This primitive network is remodeled and a deeper layer of vessels form much later. Between 6-9 months an avascular area forms around arterial vessels leading Ashton to hypothesize that the “vaso-formative” substance produced at earlier stages becomes inactive at higher oxygen levels (Ashton, 1966). This hypothesis has become the cornerstone of vascular development in the retina, especially since it relates to the vaso-formative protein VEGF and to pathogenesis of retinopathy of prematurity (ROP), both will be discussed
in later sections. The final vascular form is only achieved after birth. Pericytes appear in the fifth month, but only in the perinatal period do they associate with vessels in large numbers (Jakobiec, 1982).
