Visual Neuroscience 6100 Early Eye Development 2/14/05

Visual Neuroscience 6100 Early Eye Development 2/14/05

The mature vertebrate eye http://webvision.med.utah.edu/

Human brain development Lateral view (A) and midline extended diagram (B) of a 6-week human brain showing secondary bulges of the neural tube. (After Langman, 1969)

Eye field specification and separation • Eye morphogenesis • Anterior segment development: • Lens and ciliary body • Vasculogenesis • Optic vesicle patterning: • Neural retina • Retinal pigmented epithelium (RPE) • Optic stalk

Fate map of the presumptive brain areas of the Xenopus neural plate (stage 15, left) and mouse neural plate (E7, right). Area olfactoria primitiva 1 Primordium hipppocampi 3 Supra chiasmatic nucleus 7 Ventr. hypothalamic nucleus 9 Anterior thalamic nucleus 13 Praetecum 16 Optic tectum 17 Hypophysis 1Cerebellum 19 Epiphysis 20 Tegmentum dorsale 21 Choroid plexus 23 Medulla oblongata 24 E7 = embryonic day 7

Expression of eyeless (Drosophila Pax6) in imaginal discs induces ectopic eyes. - induction of ectopic eyes in structures such as legs, antennae and wings - eye morphogenesis is essentially normal - the photoreceptors were electrically active upon illumination Halder et al. (1995) Science 267: 1788

Ectopic eyes induced by Pax-6 misexpression resemble normal eyes morphologically and histologically. Xenopus embryos were injected with 160 pg of Pax6 RNA in one animal pole blastomere at the 16-cell stage and fixed at stage 48. (A-C) Ectopic eyes from different embryos displaying eye cup (white arrowhead) and lens (black arrowhead). RPE-like extension from eye cup (C, arrow). (G-I) Hematoxylin and eosin staining of coronal sections through (G) normal eye, and (H,I) ectopic eyes. Arrows indicate ciliary margin zone in normal eye and region with similar morphology in ectopic eyes. Chow et al. (1999) Development 126: 4213

The pattern of Pax-6 expression over time in Xenopus embryos supports formation of two retina primordia from a single eye field. Li et al. (1997) Development 124: 603

? Hypothesis: The underlying tissue (ventral diencephalon) provides a signal to suppress retina formation in its median region resulting in the resolution of the retina field into two retina primordia. How does the eye field resolve into two separate domains?

Prechordal plate suppresses Pax6 expression and resolves the eye field into two bilateral domains. Transplantation of an additional prechordal plate underneath the left retinal primordium supresses Pax-6 expression in the left eye. Removal of the prechordal plate in chick embryos leads to holoprosencephaly and formation of a cyclopean eye. Li et al. (1997) Development 124: 603

Otx-2 mRNA expression: wildtype shh-/shh- Brain and eye defects in mouse embryos lacking sonic hedgehog: holoprosencephaly and cyclopia (synophthalmia). In the mutant animal, no midline forms, and there is a single, continuous optic vesicle in the ventral region. Chiang et al. (1996) Nature 383: 407

shh Hedgehog (shh) signaling from the ventral diencephalon regulates separation of the eye field. Modified from Li et al. 1997

Eye field E7 Optic sulci E8.5 Optic vesicle E9 Optic vesicle E9.5 From eye field to optic vesicle (mouse). http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Defects in eye morphogenesis result in microphthalmia or anophthalmia. Anophthalmia caused by a mutation of the transcription factor RAX. Voronina et al. (2004) Human Mol Genetics 13: 315.

Optic vesicle lens placode Morphogenesis of the optic cup E9.5 E10 E11 E13 http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Morphogenesis of the retinal layers The neural retina expands by extensive proliferation and becomes stratified. The RPE remains a single layer of cuboidal cells and becomes pigmented (E11-E14). http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Morphogenesis of the optic stalk Otteson et al. (1998) Dev Biol 193:209.

Defects in morphogenesis of the optic stalk result in coloboma.

Formation of the ciliary body and iris separates the eye into posterior and anterior chambers. http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Defects in anterior segment development can cause congenital glaucoma (e.g. Axenfeld-Rieger syndrome). Developmental disorders of the ocular anterior segment are often associated with elevated intraocular pressure and glaucoma. Known genes that cause anterior segment dysgenesis code for developmentally important transcription factors (PITX2, PITX3, PAX6, FOXE3). Glaucoma can cause damage when the aqueous humor, a fluid that inflates the front of the eye and circulates in a chamber called the anterior chamber, enters the eye but cannot drain properly from the eye. Elevated pressure inside the eye, in turn, can cause damage to the optic nerve or the blood vessels in the eye that nourish the optic nerve. Anterior segment: cornea, iris, lens, ciliary body, and ocular drainage structures (trabecular meshwork and Schlemm’s canal). http://www.nei.nih.gov/health/glaucoma/

Lens placode E10 Lens vesicle E11 Lens vesicle E12.5 Morphogenesis of the lens in the mouse eye. Lens with differentiating lens fibers around E17 http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Different signals control proliferation and differentiation of lens epithelium into lens fibers. Lovicu and McAvoy (2005): Dev Biol.

The Tunica Vasculosa Lentis and Pupillary Membrane are transient vascular structures in the eye. The hyaloid artery develops from mesenchymal tissue in the embryonic fissure (top left) and is the primary source of nutrition in the embryonic retina. It courses from the primitive optic nerve to the posterior lens capsule and forms a capillary network around the lens, the tunica vasculosa lentis. It anastomoses anteriorly with the pupillary membrane (bottom left), which consists of vessels and mesenchyme overlying the anterior lens capsule. Later during development, the hyaloid vasculature and pupillary membrane regress. The choroid and radial intraretinal vessels become the main source of blood supply and nutrition in the eye. Improper oxygen supply in the fetal eye can lead to retinopathy of prematurity (ROP). http://www.med.unc.edu/embryo_images/unit-eye/eye_htms/eyetoc.htm

Origin of ocular and extraocular tissues. Neural ectoderm (optic cup): neural retina, RPE, pupillary sphincter and dilator muscles, posterior iris epithelium, optic nerve. Neural crest (connective tissue): corneal endothelium, trabecular meshwork stroma of cornea, iris and ciliary body, ciliary muscle, choroids and sclera, perivascular connective tissue and smooth muscle cells, meninges of optic nerve, orbital cartilage and bone, connective tissue of the extrinsic ocular muscles, secondary vitreous, zonules. Mesencephalic neural crest cells populate the region around the optic vesicle and ultimately give rise to nearly all the connective tissue structures of the avian eye, and the same can be presumed for the mammalian eye. Surface ectoderm (epithelium): corneal and cojunctival epithelium, lens, lacrimal gland, eyelid epidermis, eyelid cilia, epithelium of adnexa glands, epithelium of nasolacrimal duct. Mesoderm (muscle and vascular endothelium): extraocular muscles, vascular endothelia, Schlemm’s canal endothelium, blood.

Mitf Chx10 The optic vesicle is already patterned into the presumptive neural retina and the presumptive RPE. Fuhrmann et al (2000) Development 127: 4599

head mesenchyme surface ectoderm RPE retina RPE optic stalk dorsal SHH proximal distal ventral ventral diencephalon Neural retina: Pax6, Chx10 RPE: Mitf Optic stalk: Pax2 What regulates patterning of the optic vesicle?

The eye domains are sensitive to sonic hedgehog. Injection of shh leads to formation of reduced optic primordia. shh has opposite effects on Pax-2 and Pax-6 expression in the optic primordia: it reduces Pax-6 expression (A-C) and induces ectopic Pax-2 expression (D-F). Macdonald et al. (1995) Development 121: 3267

head mesenchyme surface ectoderm RPE retina RPE optic stalk dorsal proximal distal SHH ventral ventral diencephalon Neural retina: Pax6, Chx10 RPE: Mitf Optic stalk: Pax2 Sonic hedgehog from the ventral diencephalon promotes optic stalk formation.

FGF in the lens (surface) ectoderm patterns the presumptive neural retina. In the normal optic vesicle, the RPE inducing gene Mitf is expressed first in the whole optic vesicle and becomes then restricted to the presumptive RPE (right), but not after removal of surface ectoderm (below). Nguyen and Arnheiter (2000) Development 127: 3581 FGF is a candidate signal expressed in the surface ectoderm (left) that suppresses RPE development in the distal optic vesicle and promotes differentiation of the retina (right). Pittack et al. (1997) Development 124: 805

head mesenchyme surface ectoderm RPE FGF1/2 retina RPE optic stalk dorsal SHH proximal distal ventral diencephalon ventral Neural retina: Pax6, Chx10 RPE: Mitf Optic stalk: Pax2 FGF from the surface ectoderm induces the neural retina in the distal optic vesicle.

explant + mes explant - mes explant + activin in vivo Mitf Chx10 Extraocular mesenchyme (activin) induces the proximal optic vesicle to develop into the retinal pigmented epithelium. Fuhrmann et al (2000) Development 127: 4599

E11.5 E12.5 Transdifferentiation of the dorsal RPE occurs in mice with a defect in the head mesenchyme (targeted deletion of the transcription factor AP2). J. West-Mays et al. 1999

head mesenchyme activin surface ectoderm RPE FGF1/2 retina RPE optic stalk dorsal SHH proximal distal ventral ventral diencephalon Neural retina: Pax6, Chx10 RPE: Mitf Optic stalk: Pax2 An activin-like (?) signal from the head mesenchyme induces the RPE domain in the optic vesicle.

shh expression patched expression untreated E6 implantationof cells producingblocking shh-antibody Interference with sonic hedgehog signaling causes defects in RPE development in the embryonic chick eye. Zhang and Yang (2001) Dev Biol 233: 271

Ventral RPE formation is dependent upon sonic hedgehog expressed in the ventral diencephalon in mouse. wildtype BF-1 KO BF-1 KO Huh et al. (1999) Dev Biol 211: 53Deletion of the gene encoding for the transcription factor Brain factor-1 results in the loss of sonic hedgehog expression in the ventral forebrain.

head mesenchyme activin surface ectoderm RPE FGF1/2 retina RPE optic stalk dorsal SHH ventral proximal distal diencephalon ventral Neural retina: Pax6, Chx10 RPE: Mitf Optic stalk: Pax2 Extracellular signals regulate patterning of the optic vesicle.

Hedgehog signaling from the ventral diencephalon regulates: separation of the eye field optic stalk formation ventral RPE patterning Modified from Stenkamp and Frey, 2003

Visual Neuroscience 6100 Early Eye Development 2/14/05