Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 18

The hyaloid artery is the termination of the primitive ophthalmic artery, a branch of the internal ophthalmic artery, and it remains within the optic cup following closure of the optic fissure. The hyaloid artery branches around the posterior lens capsule and continues anteriorly to anastomose with the network of vessels in the pupillary membrane (Figure 1.4). The pupillary membrane consists of vessels and mesenchyme overlying the anterior lens capsule. This hyaloid vascular network that forms around the lens is called the anterior and posterior TVL. The hyaloid artery and associated TVL provide nutrition to the lens and anterior segment during its period of rapid differentiation. Venous drainage occurs via a network near the equatorial lens, in the area where the ciliary body will eventually develop. There is no discrete hyaloid vein.

Once the ciliary body begins actively producing aqueous humor, which circulates and nourishes the lens, the hyaloid system is no longer needed. The hyaloid vasculature and TVL reach their maximal development by day 45 in the dog and then begin to regress.

As the peripheral hyaloid vasculature regresses, the retinal vessels develop. Spindle‐shaped mesenchymal cells from the wall of the hyaloid artery at the optic disc form buds (angiogenesis) that invade the nerve fiber layer.

Branches of the hyaloid artery become sporadically occluded by macrophages prior to their gradual atrophy. Placental growth factor and vascular endothelial growth factor appear to be involved in hyaloid regression. Proximal arteriolar vasoconstriction at birth precedes regression of the major hyaloid vasculature. Atrophy of the pupillary membrane, TVL, and hyaloid artery occurs initially through apoptosis and later through cellular necrosis, and is usually complete by the time of eyelid opening 14 days postnatally.

Figure 1.4 The hyaloid vascular system and TVL.

The clinical lens anomaly known as Mittendorf's dot is a small (1 mm) area of fibrosis on the posterior lens capsule, and it is a manifestation of incomplete regression of the hyaloid artery where it was attached to the posterior lens capsule. Bergmeister's papilla represents a remnant of the hyaloid vasculature consisting of a small, fibrous glial tuft of tissue emanating from the center of the optic nerve. Both are frequently observed as incidental clinical findings.