CYTOGENESIS IN THE DEVELOPING RETINA OF THE CAT

Cytogenesis in the cat retina was studied using tritiated pH) thymidine autoradiography and nuclear stains. Three zones of cell division were recognized. In the first zone cell cleavage occurs at the outer limiting membrane. The distribution of these mitotic figures is uniform as early as an embryonic age of 29 days (E29) until E50. At about E50 a "cold spot", made apparent by the absence of mitotic figures, is evident at the site of the developing area centralis. This spreads to encompass the whole retina by postnatal day 10 (P10). A second zone of cell division was recognized by the presence in the developing inner nuclear layer of ³H-thymidine labelled nuclei which do not migrate to the outer limiting membrane (ventricular surface) to divide. Some of these labelled nuclei are located in regions of the retina where cytogenesis at the ventricular surface has ceased. A third zone was observed in the optic nerve fibre/ganglion cell layers from about E54 until beyond the first postnatal month. This activity gives rise to vascular endothelial cells in the nerve fibre/ganglion cell layers. Once established, the developing vascular cells invade the inner plexiform and inner nuclear layers to form the deeper capillary net.     

The Importance of Activation of Antihemophilic Globulin and Proaccelerin by Traces of Thrombin in the Generation of Intrinsic Prothrombinase Activity

There are two major time-consuming steps in intrinsic clotting in vitro.The importance of the first step—the triggering of intrinsic clotting throughthe generation of activation product (AP) activity—has been appreciatedfor several years. This paper has been concerned with the delineation of thesecond important time-consuming step—the generation of a trace of thrombin,which by activating both anti-hemophilic globulin (AHG, factor VIII ) andproaccelerin (factor V ), shifts the intrinsic clotting process into high gear.Data have been presented which indicate that when plasma contains AP,activated AHG (AHG'), activated proaccelerin (accelerin ) and free plateletfactor 3-like activity, all of the remaining reactions required to generate powerful intrinsic prothrombinase activity take place within 7 to 12 seconds afterrecalcification. It may well be that AHG and proaccelerin must be activatedby minute traces of thrombin before they can participate effectively in thegeneration of intrinsic prothrombinase activity.

Submitted on May 14, 1962 Accepted on October 24, 1962