Embryology of the Eye (Easy to Understand)

hey guys my name is - and today we're going to be talking about the embryological development of the eye and as usual I've broken it down at the most simple steps possible so that if you have no idea what embryology is you will by the end of this short video and as usual we're going to begin at the beginning with the blastula the blastula is a result of multiplication of a bunch of cells from when a sperm fertilizes an egg now this ball of cells continues to differentiate it goes in from the uterine tube and

it goes into the uterine canal and it plants into the uterine wall and a process of gastrulation will form three germ layers these germ layers are the ectoderm mesoderm and endoderm and you might have seen this pancake structure before this is an oversimplification for this where we have in blue the ectoderm which will become skin nervous tissue and even lies the mesoderm in red which has three paths a paraxial mesoderm which are muscles or thermite's the intermediate mesoderm which forms gonads in kidneys and the lateral plaits in green we have the endoderm and the endoderm

contributes epithelium to the GI T among other things but for today's focus we're only going to be talking about ectoderm because that's whether our eyes derive from okay so if you've seen my previous video on embryology of the CNS you'll know that the CNS is one long tube the central nervous system from ectoderm and it's open at the top and at the bottom and eventually in a normal situation this tube has to close at the top and close at the bottom so when this occurs at around day 22 that's when we first start noticing the

eye being developed so around day 22 if we look at this picture don't let this scare you if we have this fetus or embryo that is curled up like this imagine I'm the embryo and we were looking at it this way and we slice it right here at the optic vesicle and look at it this way this is what we'll have all right wait a second looking at this we have an optic vesical that's where the focus of this video will be because this has to do with embryology of the eye we also have a

otic vesicle not to be confused with the optic vesicle the otic vesicle is a subject for another video development of the ear but let's leave that for now and only focus on the optic vesicle so we slice this embryo here and looking at it this way we will have this so again slice pull and day 22 we have the forebrain in purple which is still ectoderm and we have surfaced ectoderm which is essentially akin to skin let's just think about it as skin surface ectoderm so what happens is that at day 22 at around day

22 optic grooves will start to form so we this is step 1 step 2 step 3 let's keep it simple what happens the optic grooves continue to grow until they reach the surface ectoderm at around day 28 the grooves will become vesicles and when they contact the surface ectoderm it sets off this chain reaction that thickens the surface ectoderm making it the lens placode and triggers the formation of the lens so what happens in the next stage is that the optic vesicle where it was a single wall initially will fold and form two walls at

the same time the lens placode which is surface ectoderm differentiates thickens and digs in to the optic vesicle what does this mean for eye embryology let's have a look over here over here we have the optic stalk and the optic cup what is this from this is from this the optic vesicle will form the optic cup by folding on itself forming a two old structure the lens placode pinches in and eventually will fall the lens let's only look at this week six optic cup for now we will notice that the optic cup where it was

the optic vesicles has two walls and this is let's just say if we pull this out and place it here this is what the optic cup will look like looking at it this way through the middle we have this choroid fissure it's an empty space the purpose of which is to allow the higher loyd artery to enter its destination and reach its destination to continue to provide sustenance to the eye the to the developing I eventually the choroid fissure will close off and fuse and only leave its tip open and that will form the pupil

all right if we were to slice this optic cup right through here and look at it this way and let's pull it here this is what we'll have we will have a choroid fissure here with a highland artery going right through it we will have an inner layer and an outer layer in a layer an outer layer of what inner layer and outer layer of the retina the outer layer is a pigmented layer and the inner layer is the neural layer and we will go into that in a moment first let's have a look at

what happens in eye development in week 7 so we have an eye imagine you take my eye here slice it in half and look at it this way this is what we have if I was the week six week seven embryo in blue we still have ectoderm that's all the surface ectoderm this is the eyelid we notice that the lens where we had this lens placode pinching in digging in into this optic cup you know pinches off and becomes its own structure so this one being week seven let's rewind back to week six focusing here

this view is looking at it like this if I was to take this optic cut cut and slice it this way so let's cut this optic cup that way this way we will have this around week six we notice surface ectoderm and the pinched off or dug in lens vesicle the lens vesicle has its circuits its blood supply sustenance from the hi Lloyd artery we notice in this view that we have an intro retinal space and this is only a temporary space that lies between the inner and outer layer so between the two layers of

the retina the inner and outer layer okay so let's go fast-forward back to week seven we have here the lens and how the lens develops is that from the posterior wall fibers continue to grow anteriorly towards the front until it's filled with fiber essentially breaking it down simply it gets filled and then the lens will be formed what else do we have here we have the retina which has the inner layer the neural layer and the pigment layer on the outside the optic nerve is on the furthermost inside of the of the retina it's not

part of the retina but the when we go through this we will notice how all of the nerves of the whole eye go through the optic nerve over here and if we focus right on this one from here we will notice that this whole eye is encased in this mesenchyme where it's white everywhere that I haven't filled its mesenchyme now what missing chyme is it's a loose connective tissue that can be come anything so it can differentiate into whatever the eye needs and what does that mean so this eye is encased in tissues cells all

around it which will develop into the various important structures that we will see here so again this is a slice looking at it this way we have the conjunctival sack with further development of the surface the cornea in the front which is also continuous with the sclera we will notice that now we have a vitreous body and how that develops is that there is a gelatinous substance that infiltrates the eye into this intricate interstitial network network filling it up and becoming the vitreous body the Highland vessels will eventually degenerate leaving behind a Highland canal in

orange here we have the neural layer this is the neural layer otherwise known as pars optica retina at the most posterior aspect of the neural layer we have the rods and cones these are your light sensing receptors so the light enters this way and the nerves go that way so when the light comes in through this way it activates the rods and cones and then the electrical stimulus is sent back down so we have the first layer which is the rods and cones the second one which is the outer nuclear layer so that is the

nucleus of the rods and cones and then there is the inner nuclear layer with the bipolar cells and then the final layer is that ganglion cells which have all of their nerve fibers going from the inner part of this eye where it's orange going to the optic nerve so the combination of all of these ganglion cell fibers will eventually form the optic nerve okay let's focus over here again let's talk about the Mezen kind there is both a posterior mesenchyme so the white bits like the loose tissue and a anterior mesenchyme the posterior mesenchyme has

an inner layer which is continuous with the pia mater and that is the choroid vessels so the choroid vessels and it has an outer layer which is continuous with the dura mater and that will be the sclera so if we were actually going to make so this is the inner layer the vascular the coid layer would be this one in red and then the outer layer would be the sclera here so for the posterior part of the Mezen kind that's what it differentiates into however for the anterior part the this part of the eye the

loose connective tissue that it was blank here will differentiate into other really important structures so one of them being the anterior chamber and how it was packed full of cells over here the anterior chamber is formed when these cells are hollowed out forming a chamber so this chamber sits in front of the lens and also more mesenchyme differentiates which will become the e-reader pupillary membrane which sits in front right here so it has both anterior and posterior chambers the aqueous humor is produced by the ciliary process now let's talk about the ciliary process ciliary body

just over here let's zoom in and look over here this is the development of the ciliary process the sphincters and the ciliary muscles initially we have in our orange here this neuronal layer and then we have this layer on top and mesenchyme surrounding it the mesenchyme like we said the loose connective tissue the undifferentiated cells will differentiate into a sphincter pupil the dilated pupil a-- and the ciliary muscles the ciliary process is what creates the aqueous humor so the aqueous humor so if we're just looking over here coming back up we notice that the ciliary

Buc process is connected the lens with the suspensory ligament and this is what controls contraction of the lens which will adapt to certain lights among other things notice again let's just go over what we just spoke about let's take this here zoom in we have the ciliary process here which is shown by the folding there is the pigment layer and the it the which is the outer layer of the retina and the inner layer of the retina which will become here on this side in the most anterior aspect the ciliary process with aqueous humor is

formed okay and as we grow we differentiate into the sphincter pupil a-- and the dilator pupil a-- and these are from ectoderm from the mesenchymal intiated cells will produce this type of tissue including the ciliary muscles thanks for watching my video I'm gonna leave it it there there's a lot of high yield points but it's clearly not enough for a high level understanding of embryology of the eye this is just so that you get a great introduction to embryology so that you understand all of your lectures and your textbooks if you have any questions please

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