452 



Special Vertebrate Organogenesis 



Thoma that the thickness of the vessel wall 

 is dependent upon blood pressure. 



BLOOD 



Experimental studies on amphibians have 

 shown that the blood islands form the sole 

 source of the primitive erythrocytes (see 

 Fig. 165). Following extirpation of this re- 

 gion, as first performed on anurans by 

 Federici ('26) and on urodeles by Goss ('28), 

 "the blood cells were always reduced in 

 number and in some instances were en- 

 tirely absent." The maximum survival re- 

 ported by Goss for an embryo lacking 

 blood cells was 32 days. An unanswered 

 question is whether definitive erythrocytes 

 would develop in an embryo lacking the 

 primitive blood cells provided the animal 

 could be maintained until the formation 

 of other hematopoietic regions — liver, spleen, 

 etc. 



The progenitors of the red corpuscles are 

 determined considerably in advance of their 

 morphological differentiation in the blood 

 island (considerably before they can be 

 identified by the benzidine technique for 

 hemoglobin as applied by Slonimski, '31). 

 This is evident from the fact that the pre- 

 sumptive blood cells of amphibians can self- 

 differentiate following explantation from 

 neurulae (Slonimski, '31) and from gastru- 

 lae (Fernald, '47). Furthermore, presumptive 

 erythrocytes of an amphibian gastrula are 

 localized to a relatively small zone opposite 

 the blastoporal groove, as shown in Figure 

 158 (Holtfreter, '38; Fernald, '47). The 

 presumptive blood cells of the chick, as in 

 amphibians, are determined in advance 

 of their differentiation but they are dispersed 

 over a relatively large region of the early 

 blastoderm. Erythrocytes will differentiate in 

 chorioallantoic grafts taken from all parts 

 of the blastoderm posterior to the level of 

 the anterior quarter of the primitive streak 

 (Mvirray, '32). 



Goss ('28) has noted that blood cells and 

 endothelial cells differ in their origin be- 

 cause extirpation of the blood island has 

 relatively little effect on the development of 

 the vascular system whereas it eliminates 

 blood formation. Cameron ('41) has observed 

 that the red cell progenitors differ from the 

 endothelial precursors in their susceptibility 

 to x-rays in Amblystoma. 



Considerable disagreement exists concern- 

 ing the capacity of endothelium for erythro- 

 poiesis. It is held generally that the endo- 

 thelium in the blood island region of the 



yolk sac retains erythrogenic potency for 

 a short time after the development of the 

 earliest blood vessels. An exceptional view 

 attributes hemogenic potency to all endo- 

 thelium in amphibians until the stage of 

 metamorphosis (Storti, '31). Experimental 

 evidence has been presented to show that 

 the lining of intersinusoidal "capillaries" of 

 avian and mammalian bone marrow is eryth- 

 rogenic even in the adult (Sabin, '22; and 

 others) but the lining of these channels is 

 probably not a true endothelium (see sum- 



Blood Island 



Fig. 165. A, Ventral view of an axolotl, stage 31, 

 showing blood island (dark) stained by benzidine. 

 B, Amblystoma punciatum, stage 32, showing blood 

 island (stippled) and area excised in experimental 

 production of bloodless embryos. (A, after Slonim- 

 ski, '31; B, after Goss, '28). 



marization by McDonald, '39). On the basis 

 of their defect experiments on amphibians, 

 Federici ('26), Goss ('28), Slonimski ('31) 

 and Fernald ('47) do not attribute any eryth- 

 rogenic potency to endothelium. Their ex- 

 tirpations of blood-forming cells undoubtedly 

 included endothelium-forming cells of the 

 yolk region but approximately normal ves- 

 sels developed within this region, presum- 

 ably from cells migrating from the edge of 

 the wound. Neither the endothelium of the 

 regenerated vessels of the operated field nor 

 that of any other region produced eryth- 

 rocytes. 



Two views exist concerning the nature of 

 the first free blood cells which develop in 

 the blood islands: (1) the primitive cell is 

 a hemocytoblast with potency to form primi- 

 tive erythroblasts and leukocytes; or (2) 

 the earliest free cell is a primitive erythro- 

 blast (megaloblast). The controversy results 



