172 



R. EKHOLM AND F. S. SJOSTRAND 



cells contains small granules. Predominantly in the 

 middle and basal parts of the cells more or less regu- 

 lar systems of membranes are seen. 



The plasma membrane bounding the cytoplasm 

 of the thyroid cell is in its major course observed 

 as a single membrane having a mean thickness of 

 about 80 A (79 ±3 A). The membranes of two 

 adjacent cells run regularly in close relationship, 

 the mean distance between the lines being about 

 145 A (146 ±7 A). The part of the plasma mem- 

 brane that bounds the villi is thinner, about 60 A 

 (58 ±5 A). Here the membrane, at least in some 

 places, is observed as a double-edged one composed 

 of two dark lines with a less opaque space in be- 

 tween. The maximum length of the microvilli is 

 about 0.4 // and the maximum diameter about 0.08 //. 



77;t' mitochondria, usually rod-shaped, are scat- 

 tered all over the cells apparently at random. The 

 length of the mitochondria varies considerably in 

 the sections but the width was found to be fairly 

 constant with a mean value of 0.25 ±0.016 /*. 



The mitochondria are bordered by an outer 

 double-edged membrane composed of two dark lines 

 separated by a less opaque space. The mean distance 

 between the centers of two dense lines is 1 10 ± 3 A 

 and the total thickness of the membrane measured 

 between the outer limits of the dark lines is 1 65 ± 4 A. 

 The calculated thickness of the individual dark lines 

 is consequently about 55 A and the width of the 

 space between the lines about 55 A. 



Inside the mitochondria a system of inner mem- 

 branes is observed. These membranes, mainly 

 oriented perpendicularly to the long axis of the 

 mitochondrion, also appear double-edged. Usually 

 the inner membranes are in contact with the outer 

 membrane at both ends but sometimes only one 

 end of the inner membrane touches the outer, the 

 other end being free. At the point of contact the 

 central space of the inner membrane continues, in 

 isolated cases, in the corresponding space of the 

 surface membrane, but usually the two spaces are 

 separated by a dark line. 



The measured total thickness of the internal mito- 

 chondria membranes is 180 ± 3 A and the distance 

 between the centers of the two dark components 

 125 +4 A. Consequently, the calculated thickness 

 of the individual dense lines is about 55 A and the 

 width of the space between the lines about 70 A. 

 The intraceUular cytoplasmic membranes, by Sjo- 

 strand (8) also named a-cytomembranes, appear as a 

 thin basic membrane to which at one side small 

 dark particles are attached. These particles are ir- 

 regularly shaped but are usually fairly regularly inter- 

 spaced (150-350 A) along the basic membrane. How- 

 ever, within shorter distances the membranes are 

 completely free from particles. The mean diameter 

 of the particles is 145 ± 9 A and the mean thickness 

 of the basic membranes is 55 ± 3 A. 



The general organization of the a-cytomembranes 

 varies. In restricted areas they show a rather regular. 



parallel arrangement. In such areas the membranes 

 seem to be arranged in pairs in such a way that two 

 adjacent membranes face each other with the particle- 

 covered side, the distance between the two compo- 

 nents of each pair being rather constant (about 

 0.05 //). The smooth surfaces of the membranes are 

 in these areas separated from each other by a wider 

 and more irregular space. If the membranes are 

 followed for a certain distance it is seen that the two 

 nearest membranes from two adjacent pairs join 

 and so close the space which is bounded by the 

 smooth sides of the membranes. 



In other areas this regular arrangement is not 

 obvious because the spaces between the smooth sides 

 of the membranes are irregular in shape and vary 

 very much in size. 



Granules. In the cytoplasm of glands from animals 

 which have lived in normal laboratory conditions 

 only few and small granules are found. But if the 

 gland is slightly stimulated (by low room tempera- 

 ture) the granules increase in number and size and 

 it is possible to distinguish at least two types of 

 granules. One type has a content of irregular aggre- 

 gates of a dense fine granular material. Another type 

 has a denser packed, evenly distributed fine granular 

 content. Both kinds of granules are bounded by a 

 single, about 50 A thick membrane. 



The Golgi apparatus is usually found in close rela- 

 tion to the nucleus. It is made up of short membranes 

 arranged in pairs, vacuoles and small vesicles. 



The cell nucleus has a content of osmium-impreg- 

 nated particles, irregularly arranged but densely 

 accumulated at the nuclear membrane and in some 

 areas in the interior of the nucleus. The nuclear 

 envelope appears in sections cut perpendicularly to 

 the nuclear surface as a double-edged membrane, 

 consisting of two dense layers which are separated 

 from each other by a less opaque space. The dense 

 layers are about 50 A thick and the space in between 

 varies from about 80 to 250 A. The double edging 

 of the membrane is, however, lost at intervals and 

 within these areas the nuclear membrane is rather 

 indistinct. The dimension of these areas is approxi- 

 mately 400 A. 



The capillaries. The basal part of the thyroid cell 

 usually borders on a capillary. Here the plasma 

 membrane runs irregularly and is separated from the 

 capillary endothelium by a space containing two 

 basement membranes, one belonging to the thyroid 

 cell and the other to the endothelium. The thickness 

 of each of these membranes is 300 500 A. 



The thickness of the endothelial wall differs very 

 much from place to place but the major part of the 

 wall is very thin, 0.03-0.05 //. Within these parts a 

 constant feature is the appearance of discontinuities 

 of the endothelial lining (fig. 2). These discontinuities 

 are located at varying distances from each other but 

 never coincide with the boundary between two 

 adjacent endothelial cells. They are, however, no 

 true interruptions for they are always bridged by a 



