| Neural development requires control of processes from neurogenesis and fate choice to fine-tuning of connections. We examined continued neurogenesis in the fish retina to decouple division, fate determination, and differentiation. We studied control of rod photoreceptor production in dedicated progenitors in adult Haplochromis (Astatotilapia ) burtoni. We analyzed cone specification in flounder (Pseudopleuronectes americanus) as the cone mosaic transforms during metamorphosis.; Rod density in the growing eye is maintained by the balance of progenitor division, differentiation, and death. Assaying for DNA laddering, we determined that the embryonic peak in death precedes rod neurogenesis, and that the adult retina contains little death. Thus we conclude that precise control of progenitor division and differentitation determine the number of rods.; To discover genes that regulate rod production, we compared gene expression in dividing progenitors and differentiating rods. We differentially screened cDNA libraries from single cells, then characterized the expression and sequence of candidate clones. This procedure yielded 20 clones whose expression suggests roles in development. Two share homology with novel genes predicted from the human genome.; In addition, we examined the role of estrogen, a modulator of the insulin-like growth factor 1 (IGF-1) pathway, in division. We measured estrogen levels in the fish eye and localized aromatase and estrogen receptor in the retina. Adding estrogen or estrogen antagonists, however, did not alter rod progenitor division.; We examined cone subtype specification during reorganization of the mosaic in flounder. Larval retinas contain hexagonal cone arrays of one opsin type, whereas adult retinas have three cone types arranged in a square mosaic. We cloned flounder opsins then examined expression both in adult retina and in juvenile transitional retinas. We found that cones may change their opsin twice during development and that cone subtype selection occurs before mosaic rearrangement.; These findings underscore the precise regulation of photoreceptor production. Cone mosaic reorganization highlights the production of the correct ratio of cell subtypes, as the mosaic assembles after subtype is determined. Molecular comparisons such as the one employed offer means of determining which sequences in genomic databases are important for the intercellular communication that produces the functional retina. |
