37. NUCLEIC ACID AND PROTEIN SYNTHESIS 355 



dence that the membrane is apparently an active site for concentrating and 

 storing the protein product of the ribosomes. 



A most interesting electron micrographic investigation of the develop- 

 ment of the nematocyst in the hydra by Slautterback and Fawcett 41 sheds 

 light on the possible relationship of the membrane and ribosomes in pro- 

 tein synthesis. The nematocyst, an organ consisting of a dartlike projectile 

 contained within a capsule used by the hydra to obtain its living food, is a 

 complex product of certain specialized cells called "cnidoblasts." The un- 

 differentiated interstitial cells from which the cnidoblasts arise have a 

 cytoplasm filled with ribosomes randomly distributed, and there is no evi- 

 dence of an endoplasmic reticulum. In fact the cytoplasm in this respect 

 resembles that of bacterial and rapidly growing mammalian cells. During 

 the proliferation of these cells the lack of cytoplasmic organization persists, 

 but with the onset of nematocyst construction, there is a striking change 

 in the cytoplasmic organization. This is characterized by the appearance 

 of an endoplasmic membrane system and the organization of the ribosomes 

 along its surface. This system becomes a highly organized network in all 

 respects similar to the endoplasmic reticulum of the cytoplasm of differ- 

 entiated mammalian cells. In the later stages of this process a continuous 

 tubelike structure appears in the cytoplasm in proximity to the endoplasmic 

 membrane. This is later withdrawn into a capsule and appears to be the 

 shaft of the final nematocyst spear. The implication of these studies is 

 that during the early stages of cell proliferation an endoplasmic reticulum 

 is not essential but when the cells undertake the synthesis of a special prod- 

 uct of their differentiated state they require the more complex cytoplasmic 

 organization. It would appear that the product of cytoplasmic synthesis, 

 the nematocyst, accumulates within the membranes to which the synthe- 

 sizing particles are attached and that this product finally coalesces to form 

 the final structure. 



Cytological studies of the regeneration sites of amputated amphibian 

 extremities by Hay 36 show essentially similar results. Here, during the 

 stages of rapid proliferation of cells, there is no evidence of an endoplasmic 

 reticulum but with the onset of differentiation into cartilage cells, with its 

 associated elaboration of extracellular matrix, a typical endoplasmic reticu- 

 lum appears. 



(2) Ribosomal Subunits. To return to a consideration of the ribosomes 

 themselves it may be said that they too are obtained from tissues in a vari- 

 ety of molecular sizes depending on the metabolic activity of the tissue, 

 and perhaps other factors. This particle size distribution is to be distin- 

 guished from the associated phenomenon, discussed above, of separation 

 from the membrane component. 



41 D. B. Slautterback and D. W. Fawcett, J. Biophys. Biochem. Cytol. 5,441 (1959). 



