8.1  Introduction

Overview: The development of the hyaloid artery system and the vitreous both play an integral role in the pathogenesis of PFV. Each system develops from posterior to anterior and ultimately regresses in a similar fashion.

B.M. Buerk, M.C. Sharma, and M.J. Shapiro (*) University of Illinois at Chicago, UIC Eye Center, 1905 West Taylor Avenue, Chicago, IL 60612, USA e-mail: michshap@uic.edu

The primary function of the hyaloid artery system is to nourish the developing lens. It is composed of the hyaloid artery, the vasa hyaloida propria (VHP) and the three components of the tunica vasculosa lentis (TVL): anterior, lateral, and posterior. As the eye grows and the ciliary body develops, aqueous humor production begins and the need for the hyaloid system diminishes [5]. Mann divided the developing vitreous into three stages: the stage of the primary vitreous (up to 13 mm), the

development of the secondary vitreous (13–60 mm), and the development of the tertiary vitreous (60 mm and beyond). Both the hyaloid system and developing vitreous require an orderly sequence of events to assure normal ocular development.

The hyaloid artery is the distal portion of the main trunk of the ophthalmic internal branch of the internal carotid artery [6]. It enters the eye via the fetal fissure at 3 weeks gestation. The artery traverses to the developing lens over the course of the next week. At 4 weeks, the hyaloid artery begins to arborize forming the initial posterior TVL, a meshwork of vessels surrounding the posterior pole of the developing lens [4, 5, 7]. The TVL vessels pass around the equator of the lens as straight vessels without branching, forming the lateral TVL or iridohyaloid vessels. Simultaneously, vessels along the anterior rim of the optic cup begin to form anastamoses creating the annular vessel. Vessels from the hyaloid artery grow forward between the lateral TVL and the anterior rim of the optic cup to connect to the annular vessel [6, 8]. By 5-weeks gestation (13 mm), the hyaloid artery gives off 3–5 buds 3–4 mm behind the lens which develop into the vasa hyaloida propria [5–9].

As the hyaloid artery system develops at this stage, the primary vitreous is also developing. The primary vitreous has two components: a mesenchymnal and an ectodermal component [6, 10]. The mesenchyme enters at two locations: via the fetal fissure with the hyaloid artery and at the anterior rim of the optic cup. The ectodermal fibrils arise from the inner surface of the developing sensory retina and the ectoderm of the lens plate [6, 8]. At 6-weeks gestation, the primary vitreous is well formed and the fibrils are strongly attached to the lens, retina, and future optic disc. The initial fibrils of the secondary vitreous are made by the developing sensory retina and appear at the end of the sixth week. These fibers appear to push the primary vitreous inward and detach it from the sensory retina, except at the periphery and the optic disc. It has been demonstrated however that no compression occurs. The impression that the vessels are being compressed is caused by cessation of growth of the vitreous containing the remaining vessels combined with the rapid growth of the eye [6].

By 7 weeks, the posterior TVL is established and the posterior surface of the lens is completely covered. There are fenestrations on the lenticular side of the endothelium of the posterior TVL. Some authors feel these fenestrations are to provide nourishment for the developing lens [8] while others have questioned this

hypothesis, noting that the number of fenestrations are quite small compared to the choriocapillaris [10].

On the anterior surface of the lens, the annular vessel develops loops with an associated mesoderm to form the anterior TVL [5]. The mesodermal components are destined to become the pupillary membrane as the vascular loops regress [4, 6].

By the eighth week, the hyaloid vasculature is fully developed [8, 9, 11]. The next 3–4 weeks are characterized by the most extensive development of the vascular system [4, 6]. The lens is almost completely enveloped by the TVL [8]. Around the 11th–12th weeks, the VHP begins to atrophy. Simultaneously, the posterior and lateral portions of the TVL begin to atrophy due to the rapidly enlarging lens [6, 9, 11, 12]. Smaller channels of the TVL regress first [9].

While the hyaloid vascular system begins to involute, the secondary vitreous continues to develop and is almost completely formed by the end of the third month [4, 8]. As the secondary vitreous develops, the primary vitreous assumes a cone shape with an apex at the disc and a base at the posterior surface of the lens [8]. The tertiary vitreous begins to grow at the end of the third month from the neural ectoderm of the ciliary region and differentiates into the zonular fibers [6].

During the fourth month, the regression of the VHP and TVL continue. Recent advances in ultrasonography (USG) have shown that no hyaloid artery regression has occurred prior to 18 weeks [13]. Endothelial cells undergo changes leading to collagen formation, vessel occlusion and macrophage-induced phagocytic activity to destroy the fetal vascular system [8]. The lateral TVL (capsulopupillary, iridiohyaloid) vessels cease to be visible [5] allowing the developing zonular fibers to stretch toward the lens capsule [9, 11, 12].

The fifth month is characterized by the continuing development of the pupillary membrane. The anterior TVL disappears and its constituents are incorporated into the anterior border of the iris [14, 15]. The hyaloid system regression continues and the primary vitreous retracts further centrally forming Cloquet’s canal. Zonular fibers of the tertiary vitreous fuse and form bundles which begin to integrate with the lens capsule [8].

By the sixth month, the vessels of the pupillary membrane have reached their maximum. They extend over the entire iris and form the superficial vessels of the iris. By the end of the sixth month, the vessels of the pupillary membrane begin to atrophy and form the collarette or lesser circle of the iris [8]. Ultrasonograpy­