How the lens develops and acquires its distinctive morphology and growth patterns has been a major research focus for developmental biologists. Growth factors are known to play key roles in influencing cell behavior and cell fates during development. In recent years researchers have identified some of the growth factor families involved in regulating the processes of lens induction, morphogenesis, and growth. The aim of this chapter is to review the current state of knowledge in this key area of lens research.
The lens develops from head ectoderm that is associated with an evagination of the developing brain: the optic vesicle (Fig. 11.1). Soon after these two tissues become associated, the presumptive lens ectoderm grows and thickens to form the lens placode. Subsequent invagination of the placode forms the lens pit, which later closes to form the lens vesicle. Cells in the posterior segment of the lens vesicle, next to the optic cup, elongate to form the primary fibers, whereas cells in the anterior segment of the vesicle differentiate into epithelial cells. These divergent fates of embryonic lens cells give the lens its distinctive polarity. From this stage onwards, the lens grows by continued proliferation of epithelial cells and differentiation of fiber cells. Proliferation initially occurs throughout the lens epithelial compartment but during development becomes progressively restricted to a band of cells above the equator, known as the germinative zone. Progeny of divisions that shift below the equator enter the transitional zone and elongate to give rise to secondary fibers.