BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 479 



1. MORPHOGENETIC GRADIENTS AND PNA DISTRIBUTION 



Experimental embryology, in order to explain morphogenesis, very often 

 resorts to the gradient concept. The application on a large scale of cy to- 

 chemical methods for the detection of PNA (mostly the use of basic dyes) 

 has led to the experimental demonstration of the existence of these pre- 

 viously hypothetical gradients in all vertebrate eggs so far studied. 



In outline, the distribution of PNA during the early stages of develop- 

 ment in the amphibia'^ is as follows: the large oocytes and the unfertilized 

 eggs show a distinct polarity gradient: the amount of PNA decreases 

 steadily and very regularly from the animal to the vegetal pole of the egg. 



Fertilization does not modify conspicuously this distribution of PNA, 

 although there are some indications in favor of the view that there is a slight 

 accumulation of this substance in the dorsal half of the egg. 



There is little change in this primary polarity gradient during cleavage; 

 but, when morphogenetic movements begin at the time of gastrulation, the 

 dorsal lip of the blastopore (the organizer) is the site of a PNA synthesis. 

 A second PNA gradient, decreasing from dorsal to ventral this time, be- 

 comes more and more conspicuous as gastrulation proceeds. 



The interaction of these two PNA gradients, the initial animal-vegetal 

 one and the secondary dorso-ventral one, lead at the time of the formation 

 of the nervous system to well-defined cephalo-caudal (in the nervous sys- 

 tem and the chorda) and dorso-ventral (chorda and mesoderm) gradients. 



Later on, every organ is the site of a PNA accumulation just prior to its 

 differentiation; basophilia, which is an approximate index of the PNA/pro- 

 tein ratio, usually decreases when histological and cytological protein 

 differentiation becomes apparent. It should be added that the existence of 

 the very important animal-vegetal and dorso-ventral PNA gradients at the 

 blastula, gastrula, and neurula stages have been confirmed by direct quanti- 

 tative analysis: the PNA content of different parts of embryos at these 

 stages has been estimated and found to run parallel with the basophilia 

 gradients (Brachet,^^ Steinert'*). 



In particular, it has been shown by Brachet and Chantrenne^^ that PNA 

 synthesis occurs only in the dorsal half of the embryo during gastrulation 

 and neurulation; this half, in contrast to the ventral region, undergoes ex- 

 tensive morphogenesis, including the induction of the neural tube. In fact, 

 Steinert's^^ extensive estimations of the PNA content during amphibian 

 development conclusively show that there is only little synthesis up to the 

 gastrulation stage; but the increase in PNA becomes considerable as soon 

 as induction starts, i.e., as soon as true morphogenetic processes set in. 



'^ J. Brachet, Enzyrnologia 10, 87 (1941). 



'8M. Steinert, Bull. soc. chim. biol. 33, 549 (1951). 



1' J. Brachet and H. Chantrenne, Compt. rend. soc. biol. 140, 892 (1946). 



