Functional Changes of the Free Cell Surface Membrane 

 of the Intestinal Absorbing Cell 



F. S. Sjostrand and H. Zetterqvist 



The Laboratory for Biological Ultrastriictiire Research of the Department of Anatomy, 



Karolinsha Institiitet, Stockholm 



FvER since it was observed, one hundred years ago, 

 that the free border of the intestinal absorbing cell 

 was cross-striated, the question of how absorption 

 takes place has been eagerly discussed. Many authors 

 have considered that the cross-striation is caused 

 by a system of small canals through which particles 

 with a diameter of less than 0.5 micron can pass, 

 while others advanced the opinion that the striation 

 depends upon rodlets or fibres extending from the 

 cytoplasm in order to increase the absorption area 

 of the cell. In the latter case the absorption of fat, 

 for instance, should occur in an aqueous solution 

 after hydrolysis. 



With the development of electron microscopical 

 techniques and methods of thin sectioning it has been 

 shown in osmium-fixed material that the free border 

 of the cell is built up of small finger-like extensions 



of the cytoplasm (1, 2, 3, 4). At high resolution it is 

 observed that these extensions are covered by a 

 continuous plasma membrane without pores or 

 interruptions either between the bases of the exten- 

 sions or at their tops (4). The membrane is visualized 

 as two opaque lines separated by a less opaque 

 space, the total thickness of the membrane being 

 about 100 A (4). 



The structure of this plasma membrane (fig. 1) 

 has now been investigated at absorption of different 

 food stuffs. The experimental animals — adult white 

 mice weighing between 20 and 30 grams — were fed 

 after at least 12 hours starving and 4 hours without 

 water. In this preliminary investigation the following 

 kinds of food were used: as carbohydrate food, 

 crumbs of white bread; as protein, native hen egg- 

 white; and as fat, heavy cream. The animals ate the 



Figs. 1-2. Cross sections through the brush border of intestinal epithelium of mouse. Fig. 1 . Starving animal. 

 Magnification >< 150,000. Fig. 2. Animal fed with egg white, one hour before killing. Magnification 160,000. 



