Nuclear Degradation in the Lens, circa 1897–1899

The developing lens (progressing left to right in the top line, then left to right in the bottom line) invaginates from the surface and pinches off as a hollow vesicle. The remaining cells then elongate to fill the vesicle and form a solid lens. Finally, the cells in the center of the lens degrade their nuclei and other organelles.

Courtesy of Zeitschrift für wissenschaftliche Zoologie

In 1851, the Swiss anatomist Hermann Meyer dissected the eyes of a newborn dog and noticed that the cells at the center of the lens lacked nuclei. This observation and subsequent similar findings were little more than a curiosity for nearly half a century. Between 1897 and 1899, however, the Austrian anatomist Carl Rabl published a series of seminal papers in which he described the embryonic development and adult morphology of the lens in great detail. Rabl, then working at the German Karl-Ferdinands University in Prague, systematically analyzed the lenses of dozens of vertebrate species, including kiwi birds, chameleons, martens, alligators, chamois, bats, rays, and axolotls, among the more exotic.

In his work, he demonstrated that the nuclei begin to degrade at a precise point in lens cell differentiation. Nuclei first round up and shrink, the chromatin then condenses into large clumps, and, finally, all the nuclei in the center of the lens disappear (as seen here in Rabl’s 1898 drawing of lens development in the sand lizard eye).

Rabl found that nuclear-programmed degradation occurred in all of the species he examined (except, intriguingly, in moles). This work demonstrated that nuclear loss was a consistent feature of vertebrate lens development, which researchers ultimately showed was necessary for the lens to become transparent. Rabl also discovered that, in contrast to most other tissues, lens cells are never replaced—the cells that form in the embryo keep working throughout an animal’s entire life, in some cases for more than a century.

Rabl was nominated three times for the Nobel Prize between 1902 and 1910 for, among other achievements, his work on the structure and development of the lens. On all three occasions, however, the award went to other scientists.