Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 17

Before contact with the optic vesicle, the surface ectoderm first becomes competent to respond to lens inducers. Inductive signals from the anterior neural plate give this area of ectoderm a “lens‐forming bias.” Signals from the optic vesicle are required for complete lens differentiation, and inhibitory signals from the cranial neural crest may suppress any residual lens‐forming bias in head ectoderm adjacent to the lens. Adhesion between the optic vesicle and surface ectoderm exists, but there is no direct cell contact. The basement membranes of the optic vesicle and the surface ectoderm remain separate and intact throughout the contact period.

Thickening of the lens placode can be seen on day 17 in the dog. A tight, extracellular matrix‐mediated adhesion between the optic vesicle and the surface ectoderm has been described. This anchoring effect on the mitotically active ectoderm results in cell crowding and elongation and in formation of a thickened placode. This adhesion between the optic vesicle and lens placode also assures alignment of the lens and retina in the visual axis.

The lens placode invaginates, forming a hollow sphere, now referred to as a lens vesicle (Figures 1.2 and 1.3). The size of the lens vesicle is determined by the contact area of the optic vesicle with the surface ectoderm and by the ability of the latter tissue to respond to induction. Aplasia may result from failure of lens induction or through later involutions of the lens vesicle, either before or after separation from the surface ectoderm.

Figure 1.2 Formation of the lens vesicle and optic cup. Note that the optic fissure is present, because the optic cup is not yet fused inferiorly. (a) Formation of lens vesicle and optic cup with inferior choroidal or optic fissure. Mesenchyme (M) surrounds the invaginating lens vesicle. (b) Surface ectoderm forms the lens vesicle with a hollow interior. Note that the optic cup and optic stalk are of surface ectoderm origin.

Lens vesicle detachment is the initial event leading to formation of the chambers of the ocular anterior segment. This process is accompanied by active migration of epithelial cells out of the keratolenticular stalk, cellular necrosis, apoptosis, and basement membrane breakdown. Induction of a small lens vesicle that fails to undergo normal separation from the surface ectoderm is one of the characteristics of the teratogen‐induced anterior segment dysgenesis described in animal models.

Following detachment from the surface ectoderm (day 25 in the dog), the lens vesicle is lined by a monolayer of cuboidal cells surrounded by a basal lamina, the future lens capsule. The primitive retina promotes primary lens fiber formation in the adjacent lens epithelial cells. Thus, while the retina develops independently of the lens, the lens appears to be dependent on the retinal primordium for its differentiation. The primitive lens filled with primary lens fibers is the embryonic lens nucleus. In the adult, the embryonic nucleus is the central sphere inside the “Y” sutures; there are no sutures within the embryonal nucleus.

Figure 1.3 Cross section through optic cup and optic fissure. The lens vesicle is separated from the surface ectoderm. Mesenchyme (M) surrounds the developing lens vesicle, and the hyaloid artery is seen within the optic fissure.

At birth, the lens consists almost entirely of lens nucleus, with minimal lens cortex. Lens cortex continues to develop from the anterior cuboidal epithelial cells, which remain mitotic throughout life. Differentiation of epithelial cells into secondary lens fibers occurs at the lens equator (i.e., lens bow). Lens fiber elongation is accompanied by a corresponding increase in cell volume and a decrease in intercellular space within the lens.

The zonule fibers are termed the tertiary vitreous, but their origin remains uncertain. The zonules may form from the developing ciliary epithelium or the endothelium of the posterior tunica vasculosa lentis (TVL).