Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

Retinal vessels 180 Tapetal cells 410

Table 1.3 Embryonic origins of ocular tissues.

Neural ectoderm	Neural crest
Neural retina	Stroma of iris, ciliary body, choroid, and sclera
RPE	Ciliary muscles
Posterior iris epithelium	Corneal stroma and endothelium
Pupillary sphincter and dilator muscle (except in avian species)	Perivascular connective tissue and smooth muscle cells
	Striated muscles of iris (avian species only)
Bilayered ciliary epithelium	Meninges of optic nerve
	Orbital cartilage and bone
	Connective tissue of the extrinsic ocular muscles
	Endothelium of trabecular meshwork
Surface ectoderm	Mesoderm
Lens	Extraocular myoblasts
Corneal and conjunctival epithelium	Vascular endothelium
Lacrimal gland	Schlemm's canal (human)
	Posterior sclera (?)

      It is important to note that mesenchyme is a general term for any embryonic connective tissue. Mesenchymal cells generally appear stellate and are actively migrating populations with extensive extracellular space. In contrast, the term mesoderm refers specifically to the middle embryonic germ layer. In the eye, mesoderm probably gives rise only to the striated myocytes of the extraocular muscles (EOMs) and vascular endothelium. Most of the craniofacial mesenchymal tissue comes from neural crest cell.

Formation of the Optic Vesicle and Optic Cup

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.

Lens Formation

      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