Embryonic lens development from the top left moving clockwise<\/figcaption><\/figure>\nThe layer of anterior lens epithelial cells beneath the capsule contains lens stem cells and remains metabolically active into adulthood.\u00a0Undifferentiated epithelial cells still contain organelles and regulate homeostatic function and osmotic pressure within the lens.<\/p>\n
All nutrients after embryonic development are derived from the aqueous humor bathing the lens.\u00a0This is because the lens lacks vasculature, lymphatics, and innervation in order to minimize light scatter and promote uniform placement of lens fibers. Differentiated lens fibers lose their organelles and form a homogenous cytoplasm (also to limit light scattering).\u00a0This means fibers must rely on glycolysis for energy (as well as the hexose monophosphate shunt and aldose reductase pathway).\u00a0New fibers are continually produced throughout life, with newer fibers being laid on top of older fibers (primary fibers include those from embryonic development, while secondary fibers are produced from the anterior epithelial stem\u00a0cells after birth).<\/p>\n
The boundaries of the lens nucleus and cortex are not sharply defined and vary\u00a0depending on lens age.\u00a0In young adults, the lens nucleus is said to include all primary fibers produced during embryonic development and secondary fibers produced until sexual maturation, while the cortex includes all secondary fibers produced after sexual maturity.\u00a0In older adults, the lens cortex is considered to include all secondary fibers added since middle age. \u00a0As the lens grows, the\u00a0relative volume of the cortex compared to the nucleus also decreases with age.<\/p>\n
Anterior epithelial cells also generate the lens capsule. The capsule is an elastic, transparent basement membrane composed mostly of type IV collagen (as well as laminin, heparan sulfate proteoglycan, entactin, and fibronectin). The capsule is thickest at the anterior and posterior pre-equatorial zones and thinnest at the central posterior pole. Lens epithelial cells and fibers contribute to the capsule throughout life, making the capsule the thickest basement membrane in the body.<\/p>\n