Development of the Eye

The eye begins to form in the embryo's fourth week of development.  The last 12 weeks of a full-term delivery, from 28-40 weeks gestation, are particularly active for the growth of the eye of the fetus.  At term a baby's eye is almost half of the adult size and continues to develop over the next two years.

The anterior segment, or front of the eye is almost full size at term so most of the continued growth takes place in the posterior segment, just behind the lens in the periphery of the retina.  Retinal surface area doubles between 6 months (26 weeks) of gestation and full term with a further 50% increase over the next two years.

The retina (the light-sensitive inner layer of the eye), the iris and the optic nerve (which transmits impulses received by the eye to the brain) all develop from the primitive forebrain.  The lens and cornea are derived from the original surface "skin" of the head.  (Both help to focus the incoming light rays on the light-sensitive retina).  The embryonic layer between the brain and head gives rise to the vascular (blood vessel-containing) layer, the choroid and the outer fibrous layer, the sclera.

From about the sixth week of gestation a temporary network of blood vessels supplies the front of the eye via the hyaloid artery, which originates in the back of the eye, passes through the middle of the vitreous humor (the gelatin that fills the back segment of the eye) and wraps vessels around both surfaces of the lens and iris.  The hyaloid artery later incorporates into the optic nerve behind the eye and disappears from inside the eye by seven months of gestation.  The vessels around the lens usually disappear by 34 weeks gestation.

There are no blood vessels in the retina before the 16th week of gestation.  From then on primitive cells extend out from the optic disc (where the optic nerve enters the eye) and stimulate the growth of normal blood vessels.  Production of the new blood vessels is usually complete on the nasal side (towards the nose) of the eye by 8 months (35 weeks) gestation.  The blood vessel networks on the temporal side of the eye (towards the side of the head or temples) are not mature until 2-3 months after normal term birth.