During lens terminal differentiation, the originally cuboidal lens vesicle cells elongate into long fiber-like cells, or fibers. Y-shaped sutures form because of how the lens epithelial cells proliferate during formation. The Y sutures demarcate the boundaries between the lens cortex, which is peripheral to the Y sutures, and the lens nucleus, which is within and includes the Y sutures. The anterior Y suture is oriented upright, and the posterior Y suture is inverted.