How Do Eyes Form?
Table Of Contents
To better understand how the eye is formed, we will start the process much earlier: with fertilization.
Subsequently, we will learn about the 3 embryonic layers (ectoderm, mesoderm, and endoderm) to better understand the embryonic origins of the eye.
And finally, we will go into where the eye formation begins.
Fertilization
It all begins with the zygote, the cell created from the union of the egg and the sperm.
The zygote begins a process of segmentation, where it starts to divide until it forms a mass of cells. This mass of cells (called blastomeres) is known as the morula.
Once the segmentation process is completed, the blastula is formed. At this point, the embryo is made up of more than 64 cells.
The blastula has a layer of cells, the blastoderm, which in turn, forms 3 layers that develop in the process called gastrulation: the endoderm (inner layer), the mesoderm (intermediate layer), and the ectoderm (outer layer).
The cells that comprise each of these layers have different characteristics, allowing them to develop into different tissues.
These 3 embryonic layers will create all the tissues of the eye and the entire body.
The 3 Embryonic Layers: Ectoderm, Mesoderm and Endoderm.
Layer 1: The Ectoderm
The ectoderm, the outer layer, is the first to form and is responsible for the development of the nervous system, epidermis, and tooth enamel, as well as forming the lining of the mouth, nostrils, anus, hair, nails, and sweat glands.
In vertebrates, the ectoderm consists of 3 parts: the external (or superficial) ectoderm, the neural crest, and the neural tube.
From the superficial ectoderm, epithelial tissues will arise, including the cornea, skin glands and mammary glands, mouth and nasal cavity epithelium, hairs, and nails (as well as hooves, feathers, and horns in the case of animals).
From the neural crest: the peripheral nervous system, teeth, facial cartilage, and melanocytes.
And from the neural tube:
- The retina
- The spinal cord and motor nerves
- The neurohypophysis
- The brain (hindbrain, midbrain, and forebrain)
Some ectoderm cells begin to fold into the blastula, thus creating the endoderm.
Layer 2: The Endoderm
The endoderm is the innermost layer and is responsible for forming the respiratory and digestive tract (except for the mouth, pharynx, and the final part of the rectum); the glands that drain the digestive tract, the epithelium of the ear canal and the tympanic cavity; the bladder, part of the urethra and the epithelium of the thyroid gland and the thymus.
A third layer, the mesoderm, is created through cell division from the ectoderm.
Layer 3: The Mesoderm
The mesoderm is the intermediate embryonic layer formed after the other two layers (ectoderm and endoderm) have formed.
This third embryonic layer is formed between the previous two, and not all animals have it. Animals that have this third layer are called triploblastic.
The mesoderm will be responsible for forming the skeleton, muscles, kidneys, and reproductive system, as well as the rest of the internal tissues, especially those of a connective nature.
As early as the third week of gestation, three parts of the mesoderm can be distinguished: the paraxial mesoderm, the intermediate mesoderm, and the lateral mesoderm.
In each of these parts, a type of connective tissue will form, which has a high capacity to differentiate and will be responsible for creating different tissues of the fetus and organs. This connective tissue is known as mesenchyme.
During the mesoderm development, the following will differentiate in vertebrates: chordate mesoderm, dorsal somitic mesoderm, intermediate mesoderm, lateral plate mesoderm, and prechordal mesoderm.
The neuroectoderm is an ectodermal tissue found in the chordate mesoderm and somitic dorsal mesoderm and is responsible for the neurulation process.
Neurulation occurs when the notochord and mesoderm stimulate the development of the neuroectoderm.
The neuroectoderm is involved in developing the neural plate, neural tube, and neural folds.
What are the Embryonic Origins of the Eye?
We can divide into 3 embryonic origins of the eye:
– First, we have the neuroectoderm that comes from the prosencephalon, the future brain of the embryo, which will form the optic nerve, the retina, the retinal epithelium, the iris, and ciliary body, the vitreous humor (also from the mesoderm) and the sphincter and dilator muscles of the pupil.
– Second, we find the superficial ectoderm, which will develop the crystalline lens and the corneal epithelium (conjunctiva and lacrimal gland).
– Third, we have the mesoderm that will form the vascular covering of the eye, the sclera, the hyaloid system, the coverings of the optic nerve, the extraocular muscles, the connective tissue of the cornea, the ciliary body, the iris, and the choroid.
Where Does Eye Formation Begin?
The eye begins to form around 3 weeks of gestation and will last until week 10.
The first signs of eye development appear at 22-24 days in the region of the diencephalon (secondary vesicle of the prosencephalon) in the neural folds, with the optic sulci.
In the neural area of the embryo, the eye fields begin to form from the cells of the intermediate layer of the blastoderm, the mesoderm, and from the cells of the outer layer of the blastoderm of the blastula, the ectoderm.
The eye formation requires 3 embryonic tissues: the neuroectoderm, the superficial ectoderm, and the mesoderm.
The cells of these three tissues form the eye fields in the neural area of the embryo, and the ocular vesicles are formed in the eye fields.
Formation of the optic vesicles
The optic vesicles form from the optic sulci in the diencephalon by invagination. The optic vesicles are surrounded by neural crest mesenchyme and mesoderm.
Each optic vesicle elongates until it comes into contact with the superficial ectoderm, which it induces until it forms the lens placode. This thickening will later become the lens.
Neuroectoderm
The neuroectoderm will be the future nervous system; the superficial ectoderm will be the skin; the space between them is the mesoderm.
The eye comes from the neuroectoderm part of the prosencephalon.
The superficial ectoderm will induce the neuroectoderm, creating the optic vesicle from the optic sulci around 4 weeks of gestation from the neuroectoderm.
The invagination of the optic vesicle will form the optic dome.
The optic nerve, retina, and iris will be formed from this optic vesicle, among others.
The optic vesicle will move towards the superficial ectoderm and grow until a stalk, the optic stalk, is created.
And around the eighth week of gestation, the optic vesicle will have two structures: the optic vesicle and the optic stalk.
This optic stalk will develop into the optic nerve, which is the nerve responsible for generating images in our brains.
Superficial Ectoderm
When the optic vesicle is close enough to the superficial ectoderm, it will also induce the superficial ectoderm to create another vesicle.
However, this vesicle will no longer be an optic vesicle since its function will be different. It will be called the crystalline vesicle or crystalline lens vesicle. And from this vesicle, the crystalline lens will be formed.
Subsequently, the optic vesicle will envelop the crystalline vesicle, it will change its shape, it will have two layers and a cup shape, and it will be known as an optic cup. At this moment, it will no longer be called optic vesicle but retina.
Of the two layers of the retina, the one closest to the crystalline lens is called the neural retina or retinal nerve layer and will be in charge of receiving the nerve impulses for the optic nerve to decode them into an image for the brain.
The innermost layer of the retina is known as the pigmentary retina.
The neural retina, the layer closest to the lens vesicle, will induce it to separate from the superficial ectoderm and enter the retina. As the lens vesicle separates, the surface ectoderm closes again and forms the corneal epithelium.
And the crystalline lens vesicle (or lens vesicle), once it is completely separated from the lens and has entered the retina, changes its name from crystalline lens vesicle to lens around day 33.
The crystalline lens is the structure that allows the eye to regulate incoming light so that nerve impulses do not directly affect the nervous system.
The separation of the lens from the ectoderm creates the anterior chamber. In turn, once the lens matures, the superficial ectoderm will begin to form the conjunctiva.
The Mesoderm
On the other hand, the blood system comes from the mesoderm that will enter through the lower part of the optic structure and will cross the retina through a groove (the choroidal fissure or optic fissure) until it reaches the crystalline lens, creating a blood vessel, the hyaloid artery.
The hyaloid artery is responsible for carrying the blood supply to the lens and the inner retina.
In the external part of the retina, small vascular structures will be created, which will also provide blood supply.
These vascular structures, which come from the mesoderm and from their union with the neural crest cells forming the mesenchyme, create the choroid in the eye at the end of the fifth week.
The neural crest cells will form a layer on top of the choroid to give it consistency. This upper, outer, fibrous layer will be known as the sclera or sclera.
The retina will continue to develop and will continue to surround the lens more and more (almost completely), which will be completely separated from the superficial ectoderm.
At this point, the retina will coalesce on both sides and form thinner layers. At this point, the iris will have formed on each side, and the space between the iris and the iris will become the pupil.
The ciliary part of the retina, on the outside, will be covered with mesenchyme that will form the ciliary muscle, and on the inside, utilizing the suspensory ligament, it will be connected to the crystalline lens.
Thus, both the iris and the ciliary body have two origins: on the one hand, the mesenchyme of the neural crest and the mesoderm (blood vessels), and on the other hand, from the neuroectoderm, the non-neural retina (two epithelial layers).
The dilator and sphincter muscles of the pupil, by their band, are created between the optic cupula and the superficial epithelium from the ectoderm.
The pupil will be closed by the pupillary membrane (or iridopupillary membrane), a vascular membrane whose vessels come from the edge of the iris and the lens capsule.
This membrane will begin to disappear in the sixth month by absorption from the center to the periphery, allowing communication between the two ocular chambers (anterior and posterior). At birth, only a few fragments of this membrane will remain.
The mesoderm between the crystalline lens and the cornea will divide to create spaces. These spaces will be separated into: the anterior chamber and the posterior chamber from the development of the pupillary membrane and the iris.
Once the pupillary membrane disappears, the anterior chamber, through the pupil, will communicate with the posterior chamber.
The anterior chamber is the part that will be between the cornea and the lens, and the posterior chamber is the area posterior to the iris and in front of the lens.
At a certain point in the process, during the seventh week of embryonic development, the groove through which the hyaloid artery enters the lens will close and increasingly return to the optic stalk, which in turn will become the optic nerve.
The hyaloid artery will change its structure until it is placed in the center of the optic nerve.
From now on, the hyaloid artery changes its name to the central retinal artery.
From now on, the central retinal artery will no longer irrigate the lens and iris, but only the first sections of the retina.
On the other hand, the vitreous body is formed in the posterior part of the lens in the center of the optic cup. It is composed of vitreous humor, which is a kind of gel coming from the mesenchymal cells of the neural crest.
The crystalline lens and the iris will now receive the nutritional contribution of the vitreous humor. This is a liquid in which nutritional substances are deposited and will be absorbed by the crystalline lens to nourish itself.
On its side, the sclera will change its structure until it comes into contact with the last part of the retina, thus forming a capsule, the cornea.
In addition, the cornea has 3 layers: epithelium, stroma, and endothelium. The epithelium is formed from the superficial ectoderm, the stroma comes from the mesenchyme, and the endothelium derives from neural crest cells arriving from the optic layer.
At the point where the sclera becomes cornea, holes appear (the venous sinuses) through which the vitreous humor that is no longer necessary will be filtered.
On the other hand, the eyelids will begin to form around week 6 from the cells of the neural crest and the superficial ectoderm in front of the cornea. At first, they will be two folds of skin over the cornea that are joined together until week 27, at which time they will separate.
Conclusions On How The Eye Is Formed
Recap -Summary:
The neuroectoderm is responsible for forming the retinal layers, the posterior part of the iris, the ciliary body, and the optic nerve.
Thanks to the superficial ectoderm, the eyelids, lacrimal gland, and lacrimal ducts are created, as well as the crystalline lens and the epithelium of the cornea and conjunctiva.
From the mesoderm, the sclera, the choroid, the posterior layers of the cornea, the vitreous, and the anterior part of the iris and the ciliary body are formed. In addition, the ocular muscles and vessels are created.
To recognize the parts of the eye, we recommend you to read first: The Eye- Parts of The Eye And Their Functions.
As already mentioned, the optic cup is created from the invagination of the neuroectoderm. Invagination refers to the internal folding of a cellular layer or membrane that forms a pouch or fold.
The inferior part of the optic cup has a fissure through which the vascular system will enter the interior of the eye and, later, the central retinal artery and vein.
At 6 weeks of gestation, the eye already has its essential structure, and each of its parts will differentiate and develop until the end of pregnancy.
For example, the retina will be differentiating until the ninth month of pregnancy, in its different cell types (cones and rods, ganglion cells…) and in its various layers, except for the macula.
But before we finish with the formation of the eye, let’s see what happens after birth:
After birth:
The macula area will finish developing once the baby is born between the second and third month of life.
At birth, the eye has an anteroposterior diameter of 17 mm and continues to grow to about 22 mm during the first two years, and then continues to grow more slowly to 24 mm in adulthood.
The baby has a thin, elastic sclera, which will become increasingly thicker and lose its elasticity.
The iris has a blue tone due to the lack of pigmentation in the anterior face of the iris. However, the color will change until 6 months of age as it becomes pigmented.
The color will depend on the amount of melanin in the stroma of the iris. The pupil is also smaller during the first months of life.
During the first few weeks, there is a lack of tear secretion because it is common for the tear duct to finish developing and opening during the first months of life.
