Embryonic Eye Development

Early eye development

(IN DRAFT. The following article has not been peer-reviewed.)

In early embryogenesis, there are 3 "germ" layers which give rise to all the structures in the adult organism (4 if you count the highly migrative neural crest cells). The different components of the eye come from a few different sources, head surface ectoderm, the neural tube and neural crest cells. Ectoderm is what gives rise to skin cells, brain cells and pigment cells. The eye derives from the neural tube, also called neuroectoderm. Paired optic vesicles form on both sides, which will eventually grow inwards (like stalks) towards the beginning of the central nervous system.

The lens forms from the surface ectoderm, which becomes thicker, making a lens placode.  The inner optic vesicle will induce the surface ectoderm, forming a lens pit. This is a classic example of induction in developmental biology. After induction, the ectoderm invaginates inwardly, into a cup. A little bit of vasculature is enclosed in it, called angiogenic mesenchyme. The developing eye will need nutrition and gas exchange as it grows larger.

The cup begins to form 2 separate layers. The deeper layer becomes the retinal tunic. The outer layer becomes the pigment epithelium.

Cranial neural crest cells are also required for proper eye development. The iris is derived from neural crest cell derivatives.

Staging

The following are lizard embryos. I stage my embryos according to the publication by Thomas Sanger, Jonathan Losos and Jeremey Gibson-Brown, A Developmental Staging Series for the Lizard Genus Anolis. One question which recently arose in the lab is when do lizard embryos begin retinal development? Is it before the egg is laid, or after? In order to investigate this question, I rifled through my embryo pics, looking for eyes at various stages.

Sanger et al., say that while early embryogenesis begins within the oviduct (following internal fertilization), most eggs are laid at what he calls the Early limb-bud stage or Stage 4.

A close look

Here are some close-ups of the eyes of unstaged embryos. In some cases you can see the lenses clearly. I brought up the contrast in the photos to reveal the vasculature around the eyes, which may indicate development in the area. Some pigmentation can be see localized to the dorsal side of the eye.

Anolis carolinenis embryo

Anole lizard embryo right eye

Anole lizard embryo (left lateral view)

Eye Development throughout the stages

The following embryos have been roughly staged. I'm sure these aren't staged right, as it is difficult to stage the embryos. One strategy would be to count the number of somites and try to stage from that, but the guide itself is based off of incubated embryos, which can be variable from naturally developing embryos. 

Stage 1-2

At the "Late Prelimb-Bud" stage the Lens and optic cup faint and the choroid fissure (the gap at the bottom of the eye) is still open. Soon the cells in the lens will differentiate and pigment production will begin in the retina (by Stage 4).

Green anole embryo (left lateral view)

Anole lizard embryo - optic cup, lens and choroid fissure.

 
Stage 3

In the Forelimb-Bud stage, the lens and optic cup more distinct. Then the choroid fissure begins to narrow.

Anole embryo eye and choroid fissure (right lateral view)

Anole Embryo eye development (left lateral view)

An eyelid is beginning to develop over half of the eye. 
 

Anole lizard embryo (right lateral view)

Anolis carolinensis lizard embryo (right lateral view)

Stage 4

In the Early Limb-Bud stage the eye begins to increase in size. The choroid fissure is still visible, but very slim. The lens begins to differentiate. A dark pigment begins to spread throughout the posterior portion of the retinal ectoderm.

Anole lizard embryo (right lateral view)

Anole lizard embryo (left lateral view)

Stage 5

In the Hindlimb-Bud stage, the surface of the eye begins to noticeably bulge. The choroid fissure is closed, but is still visible. At this point, the pressure of the ocular fluid is critical for shaping the lens. The correct pressure will give it the correct curvature as the tissue continues to grow.

Anole lizard embryo (right lateral view)

Anolis carolinensis embryo (left lateral view)

Stage 7 


The lens begins to visibly bulge.

Anole embryo eye (superior-right lateral view)

Anolis carolinensis lizard embryo (right lateral view)

Here is a dorsal/back view of the embryo. 

Anole lizard embryo (dorsal view)

You can start to see the lens bulge out laterally. 

Anole lizad embryo eye development (right lateral view)

Anole lizard embryo developing eye (left lateral view)

All embryos were ethically collected according to IUCAC protocol.


References: 

Sanger, Thomas. Losos, Jonathan. Gibson-Brown, Jeremy. (2008). A developmental staging series for the lizard genus Anolis: A new system for the integration of evolution, development, and ecology. Journal of Morphology. Vol .269(2):129-137.

Gilbert, Scott. (2010). Developmental Biology. 9^th Edition. Sinauer Associates.

Thomas Caceci. Anatomy and Physiology of the Eye. Associate Professor of Biomedical Sciences & Pathobiology Virginia/Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute & State University, Blacksburg, Virginia USA 24061-0442
Molly Weaver & Brigid Hogan. (2001). Powerful ideas driven by simple tools: lessons from experimental embryology. Nature Cell Biology 3, E165 - E167 (2001). doi:10.1038/35083125


 

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