Retinal fissure
The retinal fissure, also known as the optic fissure or embryonic fissure, is a temporary groove that forms on the ventral side of the optic cup and stalk during the embryonic development of the eye. This crucial structure plays a vital role in the formation of the eye by allowing vascular mesenchyme to enter the developing optic cup, giving rise to the hyaloid artery. The hyaloid artery is essential for nourishing the developing lens, retina, and vitreous humor.
Development
The formation of the retinal fissure is an integral part of the complex process of eye morphogenesis. Early in embryonic development, the optic vesicles evaginate from the diencephalon, the posterior part of the forebrain. These vesicles then invaginate to form the double-layered optic cup. This invagination, however, is not symmetrical. It occurs more extensively on the dorsal and lateral aspects of the vesicle than on the ventral side. This uneven folding results in the formation of a linear groove on the inferior aspect of the optic cup and its connection to the brain, the optic stalk. This groove is the retinal fissure.
The presence of the retinal fissure creates a temporary opening that serves as a gateway for mesenchymal tissue, which carries the developing hyaloid artery and vein, to enter the interior of the optic cup. This vascular supply is critical for the development of the internal structures of the eye.
Normally, the retinal fissure begins to close around the fifth week of gestation in humans and is completely fused by the seventh week. The closure process starts in the middle of the fissure and proceeds anteriorly and posteriorly, much like a zipper. The fusion of the fissure margins ensures the continuity of the layers of the optic cup, which will differentiate into the retina and the retinal pigment epithelium.
Clinical Significance: Coloboma
The proper closure of the retinal fissure is essential for the normal development of the eye. Failure of the fissure to close completely results in a congenital defect known as a coloboma, which is a gap or hole in one or more of the structures of the eye. The location of the coloboma depends on which part of the fissure failed to fuse.
Colobomas can affect various parts of the eye, including:
- Iris: This is often the most visible type of coloboma, giving the pupil a "keyhole" or "cat-eye" appearance.
- Retina and Choroid: A coloboma in the posterior part of the eye can affect the retina and the underlying vascular layer, the choroid. These can lead to significant vision impairment, including a scotoma (a blind spot) or an increased risk of retinal detachment.
- Optic Nerve: If the fissure fails to close around the optic stalk, the optic nerve itself can be affected, potentially leading to severe vision loss.
The severity of a coloboma can range from a minor cosmetic issue with no impact on vision to a major structural defect causing blindness. Colobomas can occur in one or both eyes and may be an isolated finding or part of a genetic syndrome affecting other parts of the body.
Terminology: Retinal Fissure vs. Choroid Fissure
While "retinal fissure" and "optic fissure" are the most precise terms for this embryonic structure of the eye, it is sometimes referred to as the choroid fissure. However, the term "choroid fissure" is also used to describe a distinct anatomical structure in the brain—a C-shaped groove in the medial wall of the cerebral hemispheres. This cerebral choroid fissure is involved in the formation of the choroid plexus of the lateral ventricles, which produces cerebrospinal fluid. Therefore, to avoid ambiguity, "retinal fissure" or "optic fissure" are the preferred terms when discussing the embryonic development of the eye.