Development of tyrosine hydroxylase‐like immunoreactive structures in the chick retina: Three‐dimensional analysis

This study was designed to investigate the developmental profile of tyrosine hydroxylase‐like immunoreactive structures in the chick retina in both frozen sections and wholemount preparations. In frozen sections, cells with tyrosine hydroxylase‐like immunoreactivity were first detected in 10 to 15 cell rows from the innermost part of the inner nuclear layer on embryonic or incubation day 11. They were seen in the inner cell rows of the inner nuclear layer during later periods; by embryonic day 18, the immunoreactive cells were located 1 to 3 cell rows outward from the innermost part of the inner nuclear layer where mature immunoreactive cells mainly exist. The immunoreactive cells began to give rise to processes on embryonic day 13. The processes (possibly dendrites) gradually increased in number and intensity in sublayers 1 and 4 of the inner plexiform layer during prenatal life. Several days after hatching, an abrupt increase in immunoreactive processes was noted in sublayer 1 but not in sublayer 4. On the sixth postnatal day, retinal neural elements immunoreactive for tyrosine hydroxylase seemed to exhibit a distribution pattern similar to that of the adult chick. In wholemount retinas, immunoreactive cells were initially detected at the earliest stage of embryonic day 12 in a small circle termed “starting area” occupying the ventral part of the temporal retinal field. The closer to the “starting area” the earlier the retinal area began to express many immunoreactive cells. Thus tyrosine hydroxylase cell density in individual retinal areas, as represented by cell number per square millimeter, peaked in different developmental periods varying from embryonic day 12 to day 14. At this stage, immunoreactive cells were arranged irregularly in the retina. Thereafter, the cell density as well as total cell number gradually declined and reached a plateau around embryonic day 20 when tyrosine hydroxylase‐like immunoreactive cells, like those in the mature retina, showed an even distribution throughout the retina. These findings suggest that: (1) early appearing immunoreactive cells in outer parts of the inner nuclear layer migrate toward inner portions of the inner nuclear layer during development, (2) the developmental profile of tyrosine hydroxylase‐like immunoreactive processes in the inner plexiform layer differs from sublayer to sublayer and might be dependent, in part, on light stimuli at birth, (3) the time of origin of immunoreactive cells in discrete retinal areas depends on the distance from the “starting area” as a possible trigger zone of tyrosine hydroxylase cell differentiation, and (4) immunoreactive cells and processes begin to exhibit a regular arrangement and an even distribution in the chick retina by embryonic day 20, possibly as a result of an intricate organization of tyrosine hydroxylase‐like immunoreactive cell development. Thus the present study shows a complex developmental profile of tyrosine hydroxylase‐containing cells in the chick retina, which is not necessarily identical with the ontogeny of retinal dopamine cells or with that of tyrosine hydroxylase immunoreactive cells in the retinas of other species so far examined.