As the primary vitreous and hyaloid vasculature regress, they are succeeded by the acellular, avascular secondary vitreous, which ultimately forms the bulk of what is thought of as the mature vitreous. The first phase, termed the primary vitreous function to house the hyaloid vasculature as it provides nourishment to the developing anterior segment (Figure 6). ![]() The developmental progression of the vitreous can be thought of as a succession of three distinct phases. ![]() The vitreous is primarily composed of a gel-like substance called vitreous humor, and is a mesenchymal derivative. Neural crest cells also produce the stroma of the iris, which, based on its eventual concentration of melanocytes, is the largest determinant of the mature iris color. The intrinsic muscles of the iris (sphincter pupillae and dilator pupillae) are derived from the surrounding neuroectoderm, whereas the ciliary muscle, the structure eventually responsible for changing the shape of the mature lens, is derived from invading neural crest cells. The inner and outer layers of the anterior rim, induced by surrounding mesenchymal development, give rise to the posterior and anterior portions of iris epithelium respectively. The iris and ciliary body are a product of the anterior rim of the optic cup (Figure 5). Later, after hyaloid artery regression, lens nourishment is supplied via the aqueous humor. Lens development obtains nutrition via derivatives of the hyaloid artery. Lens formation begins when the optic vesicle induces inward invagination and budding of the overlying surface ectoderm to form the lens vesicle (Figure 3). Notably, while the lacrimal gland develops acini and the lacrimal ducts canalized while still in-utero, in a significant minority of infants, reflex tear production does not begin until weeks 1-3 of life. Each gland is derived from a set of 15-20 neural crest-derived glandular buds at the superolateral angle of the conjunctival sac. ![]() The lacrimal gland begins its development in the seventh week of gestation. In the mid-second trimester, a gradual re-separation of the lids commences, forming the mature palpebral fissure. Later, mesenchyme begins to infiltrate the lids to and begin to form the palpebral musculature. During week eight, the upper and lower lids proceed to refuse via epithelial cell migration and proliferation. ![]() By week seven, two distinct eyelid folds are apparent, and epithelial cells begin to invaginate in the area of the medial lid margins, forming the precursors to the future lacrimal puncta and canaliculi. As the optic vesicles extend outward towards the surface ectoderm, the proximal portion of the vesicle constricts to form the optic stalk (future optic nerve), and the distal portion invaginates to form the optic cup (future retina, iris, and ciliary body).įigure 3: Formation of the optic cup & lens vesicle through the interaction of the optic vesicle and the overlying surface ectodermĭuring the fifth week of gestation, the eyelids begin to form from contributions of both neural crest-infiltrated mesenchyme and nearby surface ectoderm. On day 23 of gestation, the optic pits appear on the neural tube, which then develop into optic vesicles on day 25. During neurulation, a flat sheet of cells called the neural plate folds in on itself to form the neural tube. Soon after gastrulation, the embryo then undergoes neurulation. By the third week of development, the three germ layers are arranged in a flat disc-like structure called the trilaminar disc, which marks a key milestone towards the formation of the developing embryo. These three germ layers give rise to all the major tissues and organs in the body, including those necessary for ocular development. This critical event transforms the blastula into a gastrula, which is a three-layered structure comprised of the endoderm, mesoderm, and ectoderm layers. The development of the eye begins with a process called gastrulation.
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