Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 16

The optic sulci are visible as paired evaginations of the forebrain neural ectoderm on day 13 of gestation in the dog (Figure 1.1). The transformation from optic sulcus to optic vesicle is considered to occur concurrent with the closure of the neural tube (day 15 in the dog).

Figure 1.1 Development of the optic sulci, which are the first sign of eye development. Optic sulci on the inside of the forebrain vesicles consisting of neural ectoderm (shaded cells). The optic sulci evaginate toward the surface ectoderm as the forebrain vesicles simultaneously rotate inward to fuse.

The optic vesicle enlarges and, covered by its own basal lamina, approaches the basal lamina underlying the surface ectoderm. The optic vesicle appears to play a significant role in the induction and size determination of the palpebral fissure and of the orbital and periocular structure. An external bulge indicating the presence of the enlarging optic vesicle can be seen at approximately day 17 in the dog.

The optic vesicle and optic stalk invaginate through differential growth and infolding. Local apical contraction and physiological cell death have been identified during invagination. The surface ectoderm in contact with the optic vesicle thickens to form the lens placode, which then invaginates with the underlying neural ectoderm. The invaginating neural ectoderm folds onto itself as the space within the optic vesicle collapses, thus creating a double layer of neural ectoderm, the optic cup.

This process of optic vesicle/lens placode invagination progresses from inferior to superior, so the sides of the optic cup and stalk meet inferiorly in an area called the optic (choroid/retinal) fissure. Mesenchymal tissue (of primarily neural crest origin) surrounds and fills the optic cup, and by day 25 in the dog, the hyaloid artery develops from mesenchyme in the optic fissure. This artery courses from the optic stalk (i.e., the region of the future optic nerve) to the developing lens. The two edges of the optic fissure meet and initially fuse anterior to the optic stalk, with fusion then progressing anteriorly and posteriorly. This process is mediated by glycosaminoglycan (GAG)‐induced adhesion between the two edges of the fissure. Apoptosis has been identified in the inferior optic cup prior to formation of the optic fissure and, transiently, associated with its closure. Failure of this fissure to close normally may result in inferiorly located defects (i.e., colobomas) in the iris, choroid, or optic nerve. Colobomas other than those in the “typical” six‐o'clock location may occur through a different mechanism and are discussed later. Closure of the optic cup through fusion of the optic fissure allows intraocular pressure (IOP) to be established.