Hello, I am Dr Aizaz from medicovisual. com and in today's visual lecture we will talk about development of eye and we will have a basic overview of development of eye and let's start the discussion from the very basic concept that here we have the trilaminar germ disc and out of the trilaminar I have only shown the ectoderm and endoderm and of course here is the oropharyngeal membrane and here is the cloacal membrane and here is the primitive streak. Now I will not go into details of these structures because you already know these things.
Now we will just hide the endoderm. So if we hide the endoderm we can see that here is the notochord coming from the primitive node starting from the primitive node here is the notochord and this structure here it is called prechordal plate. Prechordal plate is a mesodermal tissue which is wedged between the oropharyngeal membrane and the cranial end of the notochord and if you want to know more about the prechordal plate this structure prechordal plate we have a complete visual lecture only on the prechordal plate.
So what happens actually that this notochord it induces the formation of neural tube. What it will do is that it will induce the thickening of overlying ectoderm. So this is the ectoderm.
So in the midline it will release certain factors certain chemical or certain proteins actually and these proteins will cause the thickening of the central ectoderm. So let's suppose here is this thickened ectoderm in the midline and this thickened ectoderm is called neural plate. It's a thick plate of ectodermal tissue and it is called neural plate.
Now what will happen that at the anterior or cranial end actually it is cranial end but some embryologists like to replace the term cranial with anterior and the reason we have already discussed in the lecture of gastrulation. Let's not go into that. Now what happens that at the cranial end of this neural plate some cells here some cells at the cranial end they will become specialized, and they will be destined to form the eye.
So, they are termed as eye field. So, there will be the formation of eye field here and these cells they will express special proteins. So here are those cells they will produce special proteins and one of the important protein out of these I will not go into details but one of the important protein that is expressed by this eye field it is called eye field.
So, one of the important protein that is expressed that is produced that is transcribed within these cells is called PAX protein PAX6 protein PAX6 protein. So we have two eyes you know but right now we have a single eye field. We have a single eye field.
Now to form the two eyes the single eye field is divided into two. It becomes bilateral structure. Let me tell you how.
Actually this prechordal plate it has only a few functions and one of the important function is that prechordal plate will put fingers on its lips and it will say shh. Well actually I am just joking. The prechordal plate does not have lips and even it does not have hands but it will help you remember one thing.
Let me tell you what is that thing. So what it will do is that it will put its finger on the lips in the center and it will say shh and this will this single eye field will be divided now into two eye fields. It will become bilateral structure and why I said it will say shh.
Actually the reason is that SHH let me write it here SHH. So what it will do is that it will actually release a protein called Sonic Hedgehog. There is a protein called Sonic Hedgehog.
This protein has many functions in embryology but one of the important function is that this protein is being released by this prechordal plate here and as the prechordal plate is lying in the center so what will happen that in the central midline of this eye field, let me show you exactly. So initially here whole this was the eye field right. So what will happen that as there is a huge concentration of Sonic Hedgehog in the central midline here so here in the central midline where a lot of Sonic Hedgehog is expressed these cells they will stop being the part of they will stop being the part of eye field.
So hence this eye field will be divided into two structures. Well you must be knowing that I do not typically teach these molecular biology things in embryology but why I am doing it here the reason is that it has very important clinical implication. Basically, the single eye field is divided into two bilateral structures, two eye field.
This is the normal process but for some reason if it fails to happen then what will happen that rather than two eye fields being formed a single eye field is formed and hence the single eye field will be converted into a single eye, single central eye and that is called cyclopia, that is called cyclopia and that is developmental abnormality. In cyclopia there will be the single midline eye and this is due to the dysfunction, single midline eye. This is due to the dysfunction of division of the eye field, the single eye field must be divided into two eye fields and if it does not happen it leads to cyclopia.
So next what will happen that the process of neurulation will start and we have discussed the neurulation already that neural plate it will invaginate inward there will be a formation of depression here and its folds will meet in the midline. Let me show you the animation then you will be able to understand it easily. Let's watch the animation carefully.
So here you can see these are the folds here these are the folds here they will meet in the midline yeah so they will meet in the midline like this. Are you getting it? And they will appose together in a zip like fashion right but still at the anterior end and at the posterior well not anterior cranial and caudal end at the cranial and caudal end they are by the way also called anterior and posterior end.
So at the cranial and caudal end there will be still the gap within the fold so folds are not clearly not properly closed initially so these are there are the still holes and you know dorsal to it is the cavity let me show you exactly. So here is a cavity actually it is called amniotic cavity. So these holes are connecting the this primitive neural tube with the with what with the amniotic cavity and these holes these openings these pores they are called anterior this is anterior neuropore neuropore and this is posterior posterior neuropore right and by the way these are later closed this is closed at about 24 to 25th day of gestation and this is closed at about 27 to 28 or 29 roughly there is some discrepancy among textbooks but this is roughly the timeline of closure of anterior and posterior neuropores.
So anyhow what will happen that these neuropores will be ultimately closed but the point here that I want to stress that even before these neuropores are closed and even before this neural plate is properly converted into the neural tube even before that process is culminated there will be the formation of depressions bilaterally in the eye field there will be the formation of pits or depression or grooves will be formed. Let me show you so here you can see the grooves will be formed. If we see here the grooves are formed here right here also the grooves are formed and what is the name of these grooves?
These grooves are called optic grooves and they are also called optic sulcus optic grooves or optic sulcus sulci I should write bilaterally we have this optic here is that optic grooves or optic sulci or if you if we talk about one sulcus one groove one pit that is called optic sulcus and the point here that I want to highlight here is that the development of optic grooves or optic sulcus sulci it starts even before the neural tube is formed. This is the reason why in textbooks you must have read that the the optic grooves or optic sulci they develop in the neural folds they do not develop in the neural tube they develop in the neural folds that is very important and you must understand this concept still the neural tube is not formed it is not separated from this ectoderm they will later separate from this ectoderm and even at this folding stage at the neural fold stage the the grooves are formed. Now later what will happen that they will these neuropores will ultimately close and the tube will form tube will separate from the ectoderm the surface ectoderm will then again coalesce in the midline and it will become a continuous layer right it will become a continuous layer which will be forming some important components of the skin let's not go into that and the neural tube will separate.
So here you can see neural tube will separate and again still the neural tube has these optic grooves or optic sulci but the starting of formation of optic groove or optic sulci it begins even before the neural tube is formed it starts with the formation of neural folds. Now here is the same neural tube after the folding has occurred and I will not explain the process of folding in this lecture and along with that of course we have the primitive GI tube and we have the ectoderm. We have cut a part of ectoderm and only the neural tube is shown as a complete model.
Now again at the anterior or cranial end of neural tube we have these optic grooves. Now what will happen that this neural tube will grow and it will form three major swellings. Let me show you here.
So here you can see three major swellings will develop and these swellings are called this superior one is the prosencephalon then we have mesencephalon and the last one is rhombencephalon. Prosencephalon, mesencephalon and rhombencephalon. They are also called forebrain swelling, midbrain swelling and hindbrain swelling respectively.
Now what will happen that if you trace it, this optic groove, it will be present in the cranial most swelling, in the prosencephalon swelling. So here you can see that the optic groove is in the, or optic sulci, they are in the prosencephalon swelling and actually the prosencephalon swelling it is divided into two parts. One is the telencephalon and other one is the diencephalon.
Actually, if you are confused, don't worry, we will discuss the details in the development of central nervous system and this is called diencephalon, here it is mostly the cramming but you must remember this is very important point that if someone asks that where in the neural tube, where this optic groove or optic sulci they are located? You must tell that they are located in the diencephalon part of the prosencephalon, where they are located? They are located in diencephalon part of the prosencephalon and this diencephalon, it will be forming mainly the thalamus of your brain and hence the eye, the optic nerve, it will later form the optic tract and optic nerve and along with that there is also optic tract that we will discuss later but ultimately the optic tract and optic nerves, these structures, the nervous connection, they are first connected with the thalamus.
Of course, they will also develop the connections with the cerebrum. We will not go into details of that but the point is that initially the optic groove, it is connected with the diencephalon which will form the thalamus. Now what will happen that initially as I have told you it's just a groove or depression or pit or sulcus but what will happen that it will grow further, it will grow in the lateral direction outward, it will grow outward like this and the swelling will become much larger.
So, you see this depression, it has become much larger and as it becomes bigger now it is no more called optic groove or optic sulcus, now it is termed as, what it is called? Now it's a vesicular swelling, so now it is called optic vesicle. So, in a nutshell what happened that optic groove grows, and optic groove is converted into optic vesicle.
Now to understand further that what happens next what we will do is that we will just remove a part of this diencephalon swelling and along with that we will remove this optic vesicle and we will also remove of course here outside here there will be the ectoderm as well as here is the ectoderm, so here also there is a ectoderm, so we will also remove a part of ectoderm and then we will observe these things but right now let's just focus on to this part, on to diencephalon and on to this optic vesicle. So let's go to another 3D model to understand these concepts. So here is just a part of diencephalon that has been cut here and here we can see the optic vesicle.
Now as you can see from this 3D model that basically the optic vesicle is actually the continuation, it is direct continuation or it is outward extension of the diencephalon. It is nothing more than an outward extension of the diencephalon. So what will happen with this optic vesicle is that it will grow further and its proximal part, here is this proximal part and here is the distal part.
Now its distal part will grow a bit further but its proximal part it will become narrow like this. Let me show you the animation. So here you can see its proximal part has become narrow and now this narrow part it is no more called optic vesicle.
The distal part is still vesicle. Here we have the optic vesicle. It's like a vesicle.
It's very logical name. This is optic vesicle but the proximal part is like a stalk. So hence it is termed as optic stalk, s-t-a-l-k stalk.
Not that stock, not chicken stock. It is optic stalk. I am not talking about s-t-o-c-k type of stock.
It is optic stalk, s-t-a-l-k. Pardon my pronunciation is not very good. So anyhow these are the structures that form.
Now what will happen that a blood vessel will grow here. A blood vessel will develop here and this blood vessel is called hyaloid artery. What it is called?
It is called hyaloid artery and it is basically a branch of,- hyaloid artery It is basically a branch of ophthalmic artery. So this artery it wants to go inside this optic vesicle and supply the inner structures of the eye. It will later form the central retinal artery.
So what will happen that in the honour of this artery the ventral part of this optic vesicle and some part of this optic stalk as well it will invaginate inwards like this. So you can see this animation that it will invaginate inward and as it invaginates inward this hyaloid artery will also enter inside. Let's watch the animation again.
So initially it was like this but it invaginates inward like this to allow the entry of this hyaloid artery and now here a fissure is formed. So here is a fissure that has been formed and what is the name of this fissure? This fissure is called choroid fissure.
This is called choroid fissure or it is also called optic fissure and some authors they like to put a very generic name to it. They simply call it embryonic fissure. I think it should not be called embryonic fissure because it's a quite generic name.
Embryonic! … everything is embryonic in the embryo so it doesn't give away any details of this fissure. The proper name is choroid fissure or even we can call it optic fissure.
So this optic fissure develops and it allows the entry of this hyaloid artery and now this optic vesicle has been turned into what we call as optic cup. So these are the stages of development that first there is the formation of optic groove then there is formation of optic vesicle and optic stalk and then we have the formation of optic cup. Now what will happen that ultimately this fissure, this choroid fissure or this optic fissure it will close as these folds they will meet in the midline like this.
So it will close like this and still there will be a hole for the entry of this hyaloid artery here and this hyaloid artery will ultimately form the central retinal artery in adults. So to this the eye develops but what about the most important structure one of the most important structure of the eye that is lens of the eye. How does the lens develop?
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How does the lens develop? Actually the lens develops side by side along with the formation of optic cup but I haven't shown that previously so that you do not get confused. So if I show all these structures here, here is the ectoderm.
So here is the part of ectoderm. Now what will happen that as this is developing as this optic vesicle and optic what is this optic stalk is formed now what will happen that some here there will be the formation of a thickening in the ectoderm. So here somewhat here there will be the thickening of ectoderm that will be formed.
This thickening is called what it is called? It is called optic placode. It is called optic placode.
I know I said optic I said optic plakoid in the lecture of ear embryology that is wrong pronunciation please pardon the actual pronunciation is not plakoid it is called placode. So here is the optic placode. Similar to otic placode for the development of ear there is the optic placode.
. right. So here we are talking about optic placode.
So what will happen that within this optic placode some cells will invaginate inwards towards this optic vesicle like this. Let me show you the animation. So they will invaginate like this to form a pit or groove and this is called lens pit.
But what it will form? It will form the lens pit and this lens pit will grow towards this optic vesicle. Now here's something very important that as the optic vesicle grows towards the ectoderm this optic vesicle is actually inducing the formation of this lens pit.
What is happening that this optic vesicle is inducing the formation of lens pit and as the lens pit forms this lens pit in turn induces the formation of optic cup. So it will in turn what it will induce? It will induce the invagination of this ventral part of optic vesicle so that this optic vesicle is converted into optic cup.
So what is happening that optic vesicle is inducing the formation of lens and this lens structure lens primordium it is inducing the formation of optic cup. This is called reciprocal induction. This is called reciprocal induction.
So now what will happen that ultimately this will further invaginate this lens pit will further invaginate and ultimately it will be constricted here its neck will be constricted completely and it will be separated from the surface ectoderm and it will grow towards this it will grow towards the optic cup it will almost go into the optic cup like this. So now this structure is actually called lens vesicle this pit initially it was a pit but as it grows further as it becomes bigger it assumes a shape of a vesicle and this vesicle is not the optic vesicle this is the optic vesicle this is what this is lens vesicle because it is going to form the lens of the eye. These are two similar naming similarly named structures but please do not get confused here is the optic cup which optic vesicle which has now turned into the optic cup but here this is not optic vesicle this is the lens vesicle which is actually the primordium of lens of your eye.
So I hope you are very clear about these structures. Now how to understand it further what I will do is that I will cut a section of it. So here I have cut a section from the side so we are just looking unilaterally of course for example let's suppose this is the right side this is the left side so we have taken a section and now you can observe these structures again.
Again first there is the formation of conversion of this optic vesicle into optic stalk and distal part will still remain optic vesicle. Then of course there will be formation of lens pit here and this lens pit will form the lens vesicle like this and along with that there is the formation of choroid fissure.
