Eye Embryology
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Eye Embryology
- Special focus on its development between 4-5 weeks gestational age.
Overview
- Each optic vesicle partitions into the optic cup and the optic stalk.
- The optic cup then becomes the retina.
- The optic stalk becomes the optic nerve.
The Brain at 5 Weeks
- We show the embryo at 5 weeks.
- Brain vesicles from rostral to caudal:
- Telencephalon and Diencephalon, which both derive from the Prosencephalon.
- Mesencephalon
- Metencephalon and Myelencephalon, which both derive from the Rhombencephalon.
- Neural tube is caudally.
The optic vesicle
- The optic vesicle is within the diencephalon.
- We can remember that the optic vesicle develops from the diencephalon because the optic pathway synapses in the lateral geniculate nuclei along the thalamus.
Overview of Eye Embryology
The Retina (Neuroectoderm)
- Innermost layer: the retina, which exits posteriorly as the optic nerve.
- It derives from ectoderm (specifically, neuroectoderm).
- The central retinal artery pierces the optic nerve and runs through its center – we'll see that it derives from the hyaloid artery.
- The macula is the area of highest visual acuity (in the center of it, lies the fovea centralis).
- Internal to the retina, is the vitreous body, which derives from mesenchyme.
The Uvea: Choroid (Mesenchyme) & Ciliary Body (Mesenchyme & Neuroectoderm)
- Middle layer: the uvea.
- In front of the lens, label the pigmented iris, which forms an adjustable diaphragm to funnel light through the pupil.
- Posterior to it, the ciliary body.
- Posterior to it, the choroid, which is a thin, brown, highly vascular layer sandwiched between the sclera and retina.
- Show that the uveal layer derives from the mesenchyme: specifically, the choroid develops from mesenchyme and the ciliary body develops from mesenchyme & neuroectoderm.
Sclera (Mesenchyme)
- The outermost layer of the spherical-shaped eye is the corneoscleral coat.
- The sclera forms from mesenchyme.
Cornea
Outer layer: Surface Ectoderm.
Inner layer: Mesenchyme.
Neural crest cells also contribute.
- The cornea lies anteriorly; it has a pronounced curvature: indicate that its outer layer derives from ectoderm (specficially, surface ectoderm) whereas its inner layer derives from mesenchyme. Note that neural crests cells are contribute to formation of the cornea.
Lens (Surface ectoderm)
- Biconvex lens, which is also transparent and serves to focus a target on the retina.
- It derives from ectoderm (specifically surface ectoderm); we'll see that it forms from the lens placode (pl-aa-code).
Eye Development
- Now, let's focus on the development of the eye from 4-5 weeks gestational age and see how the formation takes place.
Step 1: 4 Weeks Gestation (Starting Point)
- We show a coronal view of the prosencephalon at 4 weeks.
- Inner ectoderm: the neuroectoderm (aka neuroepithelium).
- The middle layer: the mesenchyme.
- The surface ectoderm.
Step 2 (Optic vesicle and the lens placode)
- We show a section of the prosencephalon.
- Again, begin with the neuroectoderm.
- Lateral outpouching of the neuroectoderm as the optic vesicle occurs inferiorly.
- The mesenchyme.
- Next, we show the surface ectoderm, but here in parallel with the optic vesicle, we show the lens placode which forms from thickening of the surface ectoderm; it ultimately invaginates to form the lens vesicle.
Step 3 (Optic vesicle invagination)
- Abutment of the lens placode with the optic vesicle causes it to invaginate.
- Although this is only one of many triggers for optic invagination, it helps us visualize the folding process, so we highlight it.
Step 4 (Optic stalk and optic cup)
- We show that the optic vesicle invaginates into the optic stalk, posteriorly, and the optic cup, anteriorly.
- The mesenchyme surrounds it.
- The surface ectoderm and rounding of the lens placode.
Step 5 (Vitreous body and the lens pit)
- We show further folding of the optic cup and optic stalk.
- The mesenchyme, which forms the vitreous body internal to the optic cup.
- The surface ectoderm and further folding of the lens placode, and we show that this invagination forms the lens pit.
Final (Eye at 5 weeks)
- Now, we show the eye at 5 weeks gestational age: the optic stalk and the C-shaped optic cup.
- The optic stalk forms the optic nerve.
- The optic cup becomes the retina whose sublayers we specify as follows:
- The outer layer forms the pigmented epithelium of the retina, which captures light not picked up by the photoreceptors and helps with photoreceptor regeneration and waste disposal.
- The inner layer forms the neural layer of the retina.
- The intraretinal space between these two layers, anteriorly.
- The lumen of the optic stalk is posterior.
- Internal to the optic cup, we have the vitreous body, which forms from mesenchyme.
- The surface ectoderm.
- Then, we show the lens vesicle, which has pinched off the surface ectoderm.
- Is surrounded by mesenchyme.
Choroid Fissure & Central Retinal Artery
- Lastly, let's take a closer look at the choroid fissure and the development of the central retinal artery.
Coronal View
Optic Stalk
- We show a coronal view of the optic stalk.
- Lumen.
- External surface.
Choroid fissure
- The inferior gap as the choroid fissure.
Hyaloid artery
- Within the choroid fissure, we show the hyaloid artery; that it ultimately derives the central retinal artery.
Optic stalk
- Outer layer
- Inner layer.
- Ganglion cell axons reside within the inner layer; as this layer thickens it compresses the lumen.
Mid-sagittal perspective
- Next, we show a mid-sagittal perspective of the same section.
Optic Stalk
- The empty space is the choroid fissure.
- Surface epithelium.
Lens vesicle
Central retinal artery
- The internal carotid artery ascends into the forebrain.
- The ophthalmic artery branches from the internal carotid.
- The hyaloid artery branches from the ophthalmic.
- Again, pay attention that the central retinal artery derives from the hyaloid artery.