Inhibition of cell proliferation by cytosine-arabinoside and its interference with spatial and temporal differentiation patterns in the chick retina.

Incubation of chick embryos with 200 nmoles/egg of cytosine arabinoside (AraC) completely inhibits cell proliferation in the embryo. At an age older than embryonic day 4 (E4) more than 90% of the embryos survive this treatment. The drug induces various malformations; in particular the retina is heavily affected. This simple method offers the possibility to study the effect of a more or less decreased cell production on processes of further differentiation and histogenesis of retinal tissue. The following results are obtained: In spite of the inhibition of cell proliferation in the retina by AraC an abnormally thick basal lamina is found and the expansion of the eye still proceeds, indicating that eye growth is not only dependent on retinal cell numbers. Stereotyped malformations of retinal histogenesis are induced and categorized into three groups: in addition to areas of normal structure cells are found arranged in rosettes and in half-rosettes sometimes linked by areas of undefined transient cell arrangements. The results point to a strong tendency of a severely diminished cell population to form regularly laminated retinal-like structures as long as a minimal ratio of cell types is given. The spatio-temporal appearance of the type of retinal malformation in a given retinal area is dependent on the time of AraC exposure and thus on the degree of differentiation reached at a "spatio-temporal spot": Full rosettes develop at earlier, and half-rosettes at later stages of AraC interference. Furthermore, deformities first appear on temporal and ventral sides. Thus, the establishment of these malformations follows and reflects the normal sequence of differentiation within the retina. Cells within rosettes organize in specific layers and start to differentiate normally. This shows that earlier formed cells are not dependent on the influence of factors derived from cells that are formed later for their proper differentiation