Development of Eye

00:01 Hello. We are now gonna discuss the amazingly intricate process of eye development.

00:05 Initially, the eyes are going to develop as optic vesicles that grow off of the prosencephalon and will later associate with the thalamic region, the diencephalon.

00:16 Now, as these vesicles grow outward, they remain connected to the diencephalon by an optic stalk.

00:23 Now, the optic stalk anchors it but the optic vesicle grows outward and actually reaches the overlying ectoderm.

00:30 As it does so, the ectoderm in that area thickens and forms what´s called the lens placode and it´s going to associate with the hollow space in the optic stalk called the optic vesicle.

00:42 As development proceeds, that vesicle gets deeper and deeper and actually starts to form in a cup.

00:49 So the optic cup surrounds that lens vesicle which is going to move inward from the epidermis or epithelium lining the outside of the body and eventually, detach from it entirely.

01:01 So as the optic cup deepens, it surrounds the developing lens which actually migrates inside the head from the ectoderm.

01:10 As that´s happening, the inside layer of the optic cup specializes to become sensory and it´s gonna become the neural layer of the retina.

01:20 The outside layer is gonna become the pigmented layer of the retina.

01:24 Now, typically, those two layers fuse and there´s really no space present between them but retinal detachment can occur because of the potential space that was once there.

01:34 Now, the neural layer of the retina is connected to the diencephalon through the optic stalk and that is where the optic nerve, cranial nerve two is going to develop and neurons are gonna start spanning the space between the retina and the diencephalon.

01:50 The lens is gonna start developing further once it´s actually reached the inside of the optic cup.

01:56 And what happens with the lens is that the cells on its posterior face are going to start elongating and actually stretch from the front to the back of the lens.

02:07 In the process, the cell nuclei will take up residence along the equator or the central region of the lens until they completely obliterate that vesicle that was inside the lens and we have a solid structure packed full of cells.

02:21 Now, these cells will be essentially start as primary lens fibers but we add secondary lens fibers after a while that create the distinctive shape of the lens.

02:33 It´s not perfectly round, it´s a little more oblong and it can be stretched as these cells are a little bit dynamic and can deform a bit to allow us to focus closer or further away.

02:44 Now, at this point, I´ve made it relatively easy to think of the optic stalk moving out of the brain kind of like a toilet plunger, very round with a cup facing outward.

02:56 But in reality, there´s actually a little groove along its inferior surface and that retinal fissure, that groove has an artery and vein that´s associated with it.

03:08 These are called the hyaloid artery and vein and they are hanging out inside that groove.

03:12 So on the left, we see an inferior view of the optic cup, it´s reaching outward with the hyaloid vessels going towards the lens and on the right, we can see a cross section of it with that groove full of mesenchyme and the artery and vein underneath.

03:28 As the nerves spanning the retina back to the diencephalon develop coming from ganglion cells, they´re gonna wind up surrounding those hyaloid vessels and completely obliterating the groove and once that happens, the hyaloid vessels are now called the central artery and vein of the retina and we have a completely enclosed cranial nerve two.

03:52 But those blood vessels are still supplying the lens and the developing structures of the eye.

03:58 As the optic cup, lens, and vessel are developing, on the front of the body, we have an interesting thing happening.

04:06 The ectoderm that overlies the developing eye is going to become the cornea and the eyelids.

04:13 The cornea starts developing just anterior to the lens and as that´s happening, a small space is going to develop between it and the lens.

04:22 That´s gonna become the anterior chamber of the eye and thereafter, the area between the lens and the retina is called the posterior chamber of the eye.

04:31 Now, the connective tissue around the pigmented layer on the outside of the optic cup is going to grow and thicken, and actually connect to the developing cornea.

04:42 So that connective tissue is gonna form the sclera, the whites of the eyes.

04:47 Whereas the cornea which is confluent with it developed from the overlying ectoderm and is gonna be clear because of the way that the collagen fibers in it are laid down, it´s going to be clear but completely connected to the whitish sclera.

05:01 So here, we can see that the eyelids are starting to develop.

05:05 We´ve got the cornea in front.

05:08 Then, we have the anterior chamber of the eye containing the iris, then the lens, then the posterior chamber of the eye, and then the retina and optic nerve, cranial nerve number two.

05:20 At this stage, the hyaloid artery´s remnants are going to disappear inside the vitreous.

05:27 The artery is still present but it´s going around the retina to get to all the other structures and as it regresses in the vitreous of the eye, it leaves behind a small canal called the hyaloid canal but eventually, that will also disappear although occasionally, it can be noted on optimologic exam.

05:45 The space that´s in there is going to be emptied of cells and filled up with a gelatinous, basically, a goo called the vitreous humor and it is very gelatinous when you ever have a chance to examine it whereas the aqueous humor in the anterior compartment is gonna be a lot more watery.

06:01 So the aqueous humor is produced by an extension of the neural region of the retina called the ciliary body.

06:13 The ciliary body is connected to the iris.

06:15 They both come from the same origin but the iris is gonna be located anterior to the lens whereas the ciliary body is more or less on the same coronal plane as the lens and the ciliary body creates the aqueous humor but also connects to the lens through a series of what are called zonular fibers of suspensory ligaments of the lens.

06:36 And as the ciliary body relaxes, it´s going to stretch the lens, as it contracts, it´s going to allow the lens to round up and focus on things that are a little bit closer.

06:46 And the iris as you may know is also able to contract and relax to allow less or more light into the eye in the first place.

06:56 Now, as all this is happening, the eyelids are forming on the outside of the eye and eventually, they´re going to seal shut during normal development, not firmly, but they will shut and it´s not until close to birth that they actually reopen and the infant is able to open his or her eyes.

07:13 Amongst the things that can go wrong in this process would be coloboma of the iris.

07:19 Essentially, this is a remnant of that groove on the underside of the optic cup.

07:24 If that groove does not completely disappear, it can leave a defect on the inferior side that causes a notch or incomplete kind of circular appearance of the iris.

07:35 This almost always happens on the inferior aspect of it and is called coloboma of the iris.

07:40 It doesn´t tend to cause too many problems but it does tend to make people who have it a bit more sensitive to light since their iris cannot regulate the amount of light coming into the retina as they´d typically be able to.

07:52 Coloboma in and of itself is not terribly bad but it is associated with some other conditions such as Microphthalmia, smaller eyes, and what´s known as CHARGE syndrome and coloboma is the C in charge.

08:06 So the C, coloboma, H, heart defects, A is going to be choanal atresia, atresia of the spaces between the nasal cavity and the nasal pharynx, R for growth retardation, now called growth delay, and then G for genital and urinary problems, and E for ear malformations.

08:28 So that´s gonna be CHARGE syndrome of which coloboma is one member.

08:32 Thank you very much and I´ll see you during the next talk.