Human Anatomy - Kotsan I. Y. 2009

Sense Organs
Organ of Vision (Eye)
Ontogeny of the Organ of Vision

In humans, the Eyeball develops from multiple sources. The light-sensitive layer (retina) originates from the lateral wall of the Brain vesicle (the future Diencephalon); the lens develops directly from the ectoderm; and the vascular and fibrous tunics are derived from the mesenchyme.

At an early stage of embryonic development (the end of the 1st to the beginning of the 2nd month of intrauterine life), small outpouchings—the optic vesicles—appear on the lateral walls of the primary brain vesicle. The distal ends of these vesicles expand and grow toward the ectoderm, while the stalks connecting them to the brain narrow and subsequently transform into the optic nerves. During development, the anterior wall of the optic vesicle invaginates, turning the vesicle into a double-layered optic cup. The outer wall of the cup later thins and differentiates into the outer pigmented layer of the retina, whereas the inner wall gives rise to the light-perceiving (neural) portion of the retina. At the stage of optic cup formation and the differentiation of its walls, during the 2nd month of prenatal development, the ectoderm adjacent to the anterior part of the optic cup thickens to form the lens pit, which then transforms into the lens vesicle. Detaching from the ectoderm, the vesicle sinks into the optic cup, loses its cavity, and subsequently develops into the lens.

During the 2nd month of prenatal life, mesenchymal Cells penetrate the optic cup through a fissure on its inferior aspect, forming a capillary vascular network within the vitreous body and around the growing lens. Mesenchymal cells adjacent to the inner surface of the optic cup give rise to the vascular tunic (uvea), while those adjacent to the outer surface form the fibrous tunic. The anterior region of the fibrous tunic becomes transparent and transforms into the cornea. In 6- to 8-month fetuses, the Blood Vessels located within the lens capsule and the vitreous body regress, and the membrane covering the pupil (pupillary membrane) dissolves and disappears.

The ectodermal folds that give rise to the eyelids begin to form in the 3rd month of prenatal development. The conjunctival epithelium is also of ectodermal origin, and the lacrimal gland develops from outgrowths of the conjunctival epithelium.