Early postnatal development of the rat lens.

Late prenatal and early postnatal development of the human crystalline lens is described as a process of continued flattening brought on by equatorial mitotic activity. The rat lens is relatively flat at birth, the difference between the axial and equatorial diameters being approximately equal to those of the lens of a newborn human. However, the rat lens rapidly becomes more spherical in shape after birth, with the lens occupying most of the intraocular space, as in adults, by 16 days. Study of the optical quality of excised lenses, from photographs of lens refractive effects on parallel laser beams of varying separation, indicates that the lens of the newborn rat is relatively free of the large negative spherical aberration found in adult lenses. However, large amounts of negative spherical aberration are evident five days after birth. It is suggested that the refractive components of the rat eye are similar in appearance and quality to those of a diurnal mammal at birth and that they assume the characteristics associated with nocturnal vision during an early period of postnatal development.