The crystalline lens
The crystalline lens is developed from the lens vesicle, which recedes within the margin of the cup, and becomes separated from the overlying ectoderm by mesoderm. The cells forming the posterior wall of the vesicle lengthen and are converted into the lens fibers, which grow forward and fill up the cavity of the vesicle (866). The cells forming the anterior wall retain their cellular character, and form the epithelium on the anterior surface of the adult lens. By the second month the lens is invested by a vascular mesodermal capsule, the capsula vasculosa lentis; the bloodvessels supplying the posterior part of this capsule are derived from the hyaloid artery; those for the anterior part from the anterior ciliary arteries; the portion of the capsule which covers the front of the lens is named the pupillary membrane. By the sixth month all the vessels of the capsule are atrophied except the hyaloid artery, which disappears during the ninth month; the position of this artery is indicated in the adult by the hyaloid canal, which reaches from the optic disk to the posterior surface of the lens. With the loss of its bloodvessels the capsula vasculosa lentis disappears, but sometimes the pupillary membrane persists at birth, giving rise to the condition termed congenital atresia of the pupil.
The vitreous body is developed between the lens and the optic cup. The lens rudiment and the optic vesicle are at first in contact with each other, but after the closure of the lens vesicle and the formation of the optic cup the former withdraws itself from the retinal layer of the cup; the two, however, remain connected by a network of delicate protoplasmic processes. This network, derived partly from the cells of the lens and partly from those of the retinal layer of the cup, constitutes the primitive vitreous body (867, 868). At first these protoplasmic processes spring from the whole of the retinal layer of the cup, but later are limited to the ciliary region, where by a process of condensation they appear to form the zonula ciliaris. The mesoderm which enters the cup through the choroidal fissure and around the equator of the lens becomes intimately united with this reticular tissue, and contributes to form the vitreous body, which is therefore derived partly from the ectoderm and partly from the mesoderm.Поділитися цим: