ELECTRON-MICROSCOPY OF RABBIT RETINAL TRANSPLANTS

The development of transplants of embryonic rabbit retinal tissue to adult rabbit eyes was examined in the electron microscope. Neuroretinas from mid gestational age (E14-E20) Were transplanted into ten adult hosts and allowed to develop subretinally in the host eye for one to 13 weeks. The transplanted immature retinal cells continued to develop after transplantation, differentiating into different cell types. They developed several of the normal ultrastructural cell characteristics, which made it possible to identify many of the normal retinal cell types. After one week immature rosettes were seen, with zonulae adherentes between the photoreceptor cells and Muller cells, forming an outer limiting membrane. After five weeks a more mature-like structure was revealed. Photoreceptor cells had developed both inner and outer segments and synaptic terminals. The transplants were heterogeneously developed with some parts showing almost normal differentiation and others little. Incompletely differentiated synapses of both the amacrine and bipolar cell types were often observed, especially in the younger transplants. In regions corresponding to the inner plexiform layer, the adult type of synapses was seen, including advanced features like serial synapses as well as a few reciprocal synapses. Ganglion cell processes could not be identified with certainty.