3 Embryology / Pathology
Optic vesicle
anterolateral outpouching of primitive brain stem; evaginates on day 25 and becomes the globe
Optic cup (Figure 3.2)
develops embryologically as an anterolateral evagination of the forebrain
Potential space between the two becomes the subretinal space (which was the cavity of the neural tube and optic vesicle)
Embryonic fissure
on undersurface of optic cups; closes on day 33 allowing pressurization of globe
Coloboma: failure of closure of embryonic fissure; sporadic or autosomal dominant (AD); typical (located in inferonasal quadrant) or atypical (located elsewhere)
May involve retina and choroid (associated with basal encephalocele, cleft palate, and CHARGE syndrome), iris, and / or optic nerve
Microphthalmos with cyst: small, abnormal eye with cystic expansion extending posteriorly into orbit
Hyaloid artery (Figure 3-3)
enters through embryonic fissure and forms vasa hyaloidea propria (blood supply to primary vitreous)
months; intraneural portion becomes central retinal arteryRetinal vascular development begins during 16th week: mesenchymal cells next to hyaloid artery form capillary network, then form arteries and veins; vessels grow centrifugally from optic disc, reach nasal ora serrata during 8th month and temporal ora 1–2 months later
Remnants of hyaloid vasculature system:
BERGMEISTER’S PAPILLAE: at optic nerve head; glial sheath of Bergmeister envelops posterior third of hyaloid artery and begins to atrophy during 7th month; epipapillary veil results if it does not fully regress
PERIPAPILLARY LOOP: vascular loop extending from optic nerve head; risk of artery obstruction or vitreous hemorrhage
MITTENDORF’S DOT: small opacity on posterior lens capsule at which hyaloid artery is attached to posterior tunica vasculosa lentis
Primitive epithelial papillae
Myelination starts centrally, reaching the chiasm at 
months and lamina cribrosa at birth; complete approximately 1 month after birth
months and lamina cribrosa at birth; complete approximately 1 month after birthON may show deceptively exaggerated cupping because nerve fibers posterior to lamina cribrosa are incompletely myelinated at birth
Vitreous
produced by lens, retina, and walls of hyaloid artery; contains mesenchymal cells
Primary vitreous: formed by hyaloid vascular system (vasa hyaloidea propria, which includes hyaloid canal, hyaloid vessels, and posterior portions of tunica vasculosa lentis); eventually replaced by secondary vitreous; failure to regress causes persistent hyperplastic primary vitreous (PHPV) (Figure 3-4)
Retina
Retinal dysplasia: abnormal proliferation of developing retina produces tubular structures with a rosette-like appearance; represents nonspecific response to disorganizing influence during development; associated with maternal LSD ingestion, Patau’s syndrome (trisomy 13), microphthalmos, congenital glaucoma, Peter’s anomaly, uveal and optic nerve colobomas, cyclopia, and synophthalmia
Cornea
2nd wave grows between epithelium and endothelium; this zone is rich in hyaluronic acid and collagen fibrils
Lens
at 27 days, surface ectoderm adjacent to optic vesicle enlarges to form lens placode (lens plate)
Circular indentation then occurs on lens plate, forming lens pit, which invaginates the wall of the optic vesicle until it closes to form a sphere
Basement membrane of the surface ectoderm forms the surface of the sphere (the lens vesicle) and subsequently becomes the lens capsule
Lens epithelial cells on posterior aspect of this sphere elongate and migrate first (primary lens fibers); these cells fill the core of the lens vesicle at approximately 40 days (embryonal nucleus)
At 7 weeks, anterior cells migrate toward equator and proliferate to form secondary lens fibers that encase the embryonal nucleus and form the Y sutures; Y sutures represent the meeting of embryonal and fetal nuclei (upright anteriorly, inverted posteriorly)
Iris
rim of optic cup grows around lens and forms iris
Epithelial layers (iris pigment epithelium [IPE]; anterior pigmented and posterior nonpigmented) are from inner and outer layers of the optic cup (neuroectoderm); forms part of the blood–aqueous barrier
Newborn iris is usually gray-blue; development of iris color takes weeks to months, as stromal chromatophores (dendritic melanocytes from neural crest) complete their migration into uvea shortly after birth
Ciliary body (CB)
formation begins in 3rd month; fold in optic cup becomes epithelial layers of ciliary processes
In 4th month, filaments from surface cells form zonules; the major arterial circle of the iris (located in CB), the longitudinal ciliary muscle, and the ciliary processes develop
In 5th month, pars plana develops, and ciliary body stroma and ciliary muscle develop from neural crest cells adjacent to cornea
Nasolacrimal system
at 6 weeks, surface ectoderm is buried in mesoderm, between maxillary and lateral nasal processes
Embryologic tissues and their components
Neural ectoderm: sensory retina, RPE, nonpigmented ciliary body epithelium, pigmented CB epithelium (extension of RPE), IPE, iris sphincter and dilator muscle, optic nerve (neural and glial elements), sympathetic ganglion, lateral geniculate body, ocular pigment granules (RPE, CB, IPE), peripheral nerves related to eye function, erector pili muscle associated with hair follicles of the skin
Surface ectoderm: crystalline lens, corneal and conjunctival epithelium, lid epithelium, lacrimal gland, nasolacrimal system
Neural crest cells: corneal stroma and endothelium, iris stroma, TM, chamber angle, Schlemm’s canal, sclera (except temporal portion), sheaths and tendons of extraocular muscles, ciliary muscle (nonpigmented layer of ciliary body), choroidal stroma, melanocytes, meningeal sheaths, orbital bones, connective tissue of orbit, muscular and connective tissue layers of blood vessels
3 WAVES OF NEURAL CREST CELL MIGRATION (during 7th week): corneal and TM endothelium, keratocytes (corneal stroma), iris stroma
Mesoderm: blood vessel endothelium, anterior chamber angle outflow apparatus, sclera (small area temporally), EOM, Schlemm’s canal, portion of vitreous
Mesenchyme: primitive connective tissue; originates from neural crest cells and mesoderm (Figure 3-6)
