epithelial cells 



cortex fiber eel Is 



region of active 

 cellular re plicotion 



zone of 

 cellular 

 elongation 



Fig. 1. 



A diagramatlc presentation of the adult vertebrate lens. 

 The lens is surrounded by an external non-cellular 

 capsule. Beneath the capsule are found the lens epithelial 

 cells. The zone of cellular elongation Is found in the 

 peripheral area. This is the region of transition where 

 the epithelial cells begin to elongate into fiber cells. The 

 fiber cells that are newly laid down represent the cortex 

 region; the fiber cells laid down during the early growth 

 period of the lens compose the nucleus region of the 

 adult lens. (Fig. 1, J. Papaconstantinou, Science, in press; 

 Copyright 1966 by the American Association for the 

 Advancement of Science.) 



fiber cell formation represents the final stage 

 of lens cell differentiation and (b) in the adult 

 lens the fiber cells formed during embryonic 

 growth compose the central or nucleus region 

 while the newly formed fiber cells are found 

 in the peripheral or cortex region. 



B. Cytological and cytochemical 

 observations on the process of 

 fiber cell formation 



The lens epithelial cells are characterized 

 by their cuboidal shape, their basophilic stain- 

 ing properties and their ability to replicate (I). 

 In the zone of elongation (Fig. 2), where the 

 epithelial cells begin the process of fiber cell 

 formation the following changes occur in the 

 intracellular structures: (a) the cell sends out 

 cytoplasmic processes anteriorly and poste- 

 riorly beneath the cuboidal epithelial cell layer 

 to form the fiber cell; (b) the nucleus and nu- 

 cleoli enlarge (2); (c) the ribosomal population 

 increases significantly, especially in the cyto- 

 plasm adjacent to the enlarged nucleus (3, 4). 



In the completed fiber cell, (a) the cytoplasm 



loses its basophilic properties and takes on 

 acidophilic properties; (b) the nucleus and 

 nucleoli reduce in size and the endoplasmic 

 reticulum, which has a granular appearance in 

 the epithelial cell, takes on a smoother appear- 

 ance in the fiber cell; (c) through electron 

 microscope studies it has been shown that a 

 significant decrease in the ribosomal population 

 occurs in the differentiated fiber cell (3, 4). 

 These differences in staining properties and 

 changes in intracellular structures indicate 

 that significant macromolecular changes are 

 associated with fiber cell differentiation. The 

 enlargement of the nucleus and nucleoli, for 

 example, as well as the increase in ribosomal 

 population are an indication of increased nucleic 

 acid and protein synthesis during elongation. 

 Keeping these structural changes in mind, I 

 would like to describe a series of biochemical 

 events which are associated with fiber cell 

 formation, and which may be closely linked with 

 the cytological observations just described. 



III. The Biochemistry of Lens Fiber Cell 

 Differentiation 



A. The association of r-crystallin 



synthesis with fiber cell differentiation: 

 gene activation 



I would like to begin this section of my 

 discussion by describing our observations on 

 the appearance of a group of lens proteins, the 

 r-crystallins, during the differentiation of the 

 lens epithelial cell to a fiber cell (5, 6). This 

 presents us with an example of the activation 

 of the synthesis of a specific protein simul- 

 taneously with the initiation of the morphological 

 changes associated with the differentiation of a 

 fiber cell. There are three major groups of 

 proteins synthesized by lens cells; the a-crystal- 

 lins, ^-crystallins and y-crystallins. The crys- 

 tallins were first classified according to their 

 mobility at alkaline pH; the fastest migrating 

 group being the a-crystallins, the intermediate 

 group being the p-crystallins and the slowest 

 migrating group being the r-crystallins (7, 8). 

 More recently, through the efforts of my col- 

 leagues and myself, these structural proteins 

 have been identified according to their elution 

 properties on DEAE-cellulose columns (5, 9). 

 It was essentially through the resolving power 

 of DEAE-cellulose that the qualitative and quan- 

 titative differences in the crystallins of the 

 different lens cells were detected. Typical 

 patterns showing the stepwise elution of a-, ^- 



48 



