The lens capsule defines the lens perimeter. The anterior of the lens comprises a single layer of epithelial cells, which contains putative stem cells (108). At the lens equator, epithelial cells differentiate into fiber cells (i–iii). Those fiber cells found in the very center of the lens, a region called the lens nucleus, are derived from the posterior epithelial cells of the lens vesicle. Thereafter, the lens grows by the addition of secondary lens fiber cells formed by the differentiation of epithelial cells located at the lens equator. In humans, this begins between weeks 7 and 8 (Carnegie stage 22) after fertilization. This process goes on throughout life in all vertebrates. The epithelial cells differentiate into hexagonal lens fiber cells at the lens equator to maximize cell-cell contacts with neighboring cells (10). It also requires dramatic cell elongation. When corresponding apical and basal ends from opposing fiber cells of the same age from a neighboring lens sector make contact, lens sutures are formed. The geometry of the lens and the arrangement of the lens fiber cells in the human lens into three segments results in Y-shaped sutures, but offset by 60° at each pole. One fiber cell profile has been highlighted (dark blue). Lens fiber cells degrade all their organelles, including mitochondria and nuclei (dots) during differentiation (ii). The bulk of the lens thus consists of long, ribbon-like fiber cells devoid of cytoplasmic organelles (iii). In cross-section, cortical fiber cells show a characteristic hexagonal profile. Diagram adapted with permission from Investigative Ophthalmology & Visual Science (109).