Vitreous

The primary vitreous first appears at approximately 5 weeks gestation and consists of the hyaloid vessels surrounded by mesenchymal cells, collagenous fibrillar material, and macrophages (see Fig. 1-12). Most of the mesenchymal cells are of neural crest origin. The secondary vitreous forms at approximately 8 weeks at the time of fetal fissure closure (see Fig. 1-13).9 It circumfer-entially surrounds the primary vitreous containing the hyaloid vessels. The secondary vitreous consists of a gel containing compact fibrillar network, primitive hyalocytes, monocytes, and a small amount of hyaluronic acid.19 Primitive hyalocytes produce collagen fibrils that expand the volume of the secondary vitreous. At the end of the third month, the tertiary vitreous forms as a thick accumulation of collagen fibers between the lens and optic cup (Fig. 1-19). These fibers are called the marginal bundle of Drualt. Drualt's bundle has a strong attachment to the inner layer of the optic cup and is the precursor to the vitreous base and lens zonules. The early lens zonular fibers appear

Drualt Marginal

FIGURE 1-19. Drawing of cross section of a 10-week-old eye shows the primary vitreous, secondary vitreous, and tertiary vitreous. The primary vitreous includes the hyaloid artery and associated matrix; it extends centrally from the optic nerve to the retrolental space. The secondary vitreous surrounds the primary vitreous; it has less vasculature and is clearer than the primary vitreous. The tertiary vitreous forms between the lens equator and the area of the ciliary body; the lens zonules develop within the fibrillar matrix in this. Note that eyelids are fused at this stage.

FIGURE 1-19. Drawing of cross section of a 10-week-old eye shows the primary vitreous, secondary vitreous, and tertiary vitreous. The primary vitreous includes the hyaloid artery and associated matrix; it extends centrally from the optic nerve to the retrolental space. The secondary vitreous surrounds the primary vitreous; it has less vasculature and is clearer than the primary vitreous. The tertiary vitreous forms between the lens equator and the area of the ciliary body; the lens zonules develop within the fibrillar matrix in this. Note that eyelids are fused at this stage.

to be continuous with the inner limiting membrane of the non-pigmented epithelial layer covering the ciliary muscle. Toward the end of the fourth month of gestation, the primary vitreous and hyaloid vasculature atrophies to a clear, narrow central zone, Cloquet's canal. Apoptosis occurs during hyaloid vessel regression.51 Persistence of the primary vitreous and failure of the posterior tunica vasculosa lentis to regress can result in persistent hyperplastic vitreous (PHPV). PHPV consists of a fibrovascular membrane that extends from the optic nerve along the hyaloid remnant and covers the posterior capsule of the lens. During the fifth month of gestation, an attachment forms between the ciliary body and the lens (Weiger's ligament, or capsulohyaloidal ligament). Later in development, at approximately 5 to 6 months, the hyaloid system completely regresses and the hyaloid artery blood flow ceases. At birth, Cloquet's canal persists as an optically clear zone emanating from the optic nerve to the back of the lens. Cloquet's canal is a remnant of primary vitreous. Most of the posterior vitreous gel at birth is secondary vitreous with the vitreous base and zonules representing tertiary vitreous.

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