Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

surface ectoderm in this area now becomes known as the neural ectoderm.

As the neural folds elevate and approach each other, a specialized population of mesenchymal cells, the neural crest, emigrates from the neural ectoderm at its junction with the surface ectoderm. Migration and differentiation of the neural crest cells are influenced by the hyaluronic acid‐rich extracellular matrix. This acellular matrix is secreted by the surface epithelium as well as by the crest cells, and it forms a space through which the crest cells migrate. The neural crest cells migrate peripherally beneath the surface ectoderm to spread throughout the embryo, populating the region around the optic vesicle and ultimately giving rise to nearly all the connective tissue structures of the eye (Table 1.3).

Table 1.1 Sequence of ocular development in human, mouse, and dog.

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Human (approximate post‐fertilization age)	Mouse (day post‐fertilization)	Dog (day post‐fertilization or P = postnatal day)	Developmental events
Month	Week	Day
1	3	22	8	13	Optic sulci present in forebrain
	4	24	9	15	Optic sulci convert into optic vesicles
			10	17	Optic vesicle contacts surface epithelium Lens placode begins to thicken
		26			Optic vesicle surrounded by neural crest mesenchyme
2	5	28	10.5		Optic vesicle begins to invaginate, forming optic cup Lens pit forms as lens placode invaginates Retinal primordium thickens, marginal zone present
		32	11	19	Optic vesicle invaginated to form optic cup Optic fissure delineated Retinal primordium consists of external limiting membrane, proliferative zone, primitive zone, marginal zone, and internal limiting membrane Oculomotor nerve present
		33	11.5	25	Pigment in outer layer of optic cup Hyaloid artery enters through the optic cup Lens vesicle separated from surface ectoderm Retina: inner marginal and outer nuclear zones
			11.5	29	Basement membrane of surface ectoderm intact Primary lens fibers form Trochlear and abducens nerves appear Lid fold present
	6	37	12		Edges of optic fissure in contact
			12	30	TVL present Lens vesicle cavity obliterated Ciliary ganglion present
		41	12	32	Posterior retina consists of nerve fiber layer, inner neuroblastic layer, transient fiber layer of Chievitz, proliferative zone, outer neuroblastic layer, and external limiting membrane
			17	32	Eyelids fuse (dog)
	7				Anterior chamber beginning to form
			12.5	40	Secondary lens fibers present
		48	14	32	Corneal endothelium differentiated
	8	51			Optic nerve fibers reach the brain Optic stalk cavity is obliterated Lens sutures appear Acellular corneal stroma present
		54		30–35	Scleral condensation present
	9	57	17	40	First indication of ciliary processes and iris
				—	EOMs visible Eyelids fuse (occurs earlier in the dog)
	10			45	Pigment visible in iris stroma Ciliary processes touch lens equator Rudimentary rods and cones appear
				45–1P	Hyaloid artery begins to atrophy to the disc
3	12