The Ultrastnictiin- of a Frog Muscle Spindle 



197 



merit of the pigments in relation to the rod extre- 

 mities. The pigments are all grouped in the interior 

 part of the pigment epithelium. The axes of the 

 pigments remain parallel to the axes of the rods 

 even where the angle of incidence of the latter is 

 very small. 



So far it has been generally accepted (5) that the 

 inner surfaces of the epithelial cells send out proto- 

 plasmic processes around the rods and cones, thus 

 providing each rod and cone with a pigment sheath. 

 Following our observations with the electron micro- 

 scope, it seems at first quite improbable that enough 

 pigments would be present to form such a sheath. 

 Not a single preparation has shown a place where 

 the pigments were farther than the outermost part 

 of the rods. Considering the importance of this 

 problem, work has been started in order to elucidate 

 this physiological aspect. 



RiFERENCES 



1. I RAN^ois, J., Rabaey, M., and Vandermeerssche, G., 



Natuurw. Ti/dschr. 34, 191 (1952). 



2. — Ann. d'Oc. 186, 896 (1953). 



3. — Ophthalmologicii 126. 347 (1953). 



4. — Modem Trends in Ophllialmology — Electron Mi- 



croscopy. 3rd Series. Butlerworili and Co. London, 

 1955. 



5. Maximov, a. a. and Bloom, W., Textbook of Histology. 



6th ed., p. 557. W. B. Saunders Co. 1953. 



6. Newman, S. B., Borysko, E., and Swerdlow, M., /. Re- 



search Natl. Bur. Standards 43. 183 (1949). 



7. Paiade, G. E., /. E.xpil. Med. 95, 285 (1952). 



8. RusKA, H., Stuart, D. C, Jr, and Win.sser, J., Arch. 



ges. Virusforsch. 6, 5 (1956). 



9. SjosTRAND. F. S., /. Appl. Phys. 19. 1188 (1948). 



10. — J. CeUular Comp. Physiol. 33, 383 (1949). 



11. — ibid. 42, 15 (1953). 



12. — ihid. 42, 45 (1953). 



Preliminary Observations on the Ultrastructure of a 



Frog Muscle Spin(dle 



J. D. Robertson 



Department of Anatomy, University College, London 



Trog muscle spindles are sensory proprioceptive 

 end organs consisting of bundles of small muscle 

 fibers with afferent nerve endings encased in a 

 distinctive connective tissue sheath. The spindle 

 system of the m. ext. dig. long. IV of the frog (R. 

 temporaria) has recently been the subject of a detailed 

 histological study by Gray (2). The results of an 

 electron microscope study of this same spindle 

 system is reported here in preliminary form. 



The muscle was fixed in 1 *?o OSO4 at about 5C for 

 about 90 minutes. The spindles were dissected out after 

 fixation, rapidly dehydrated in alcohol, embedded in 

 methacrylate and sectioned with a Porter Blum micro- 

 tome. Successive thin transverse sections were examined 

 with a Siemens electron microscope at levels varying 

 from two to fifty // apart. 



1. The spindle sheath. — The spindle sheath is 

 divided into an outer (figs. 1-2) and an inner 

 (fig. 3) component. The outer consists of several 

 compact cytoplasmic layers about 1-3 // thick 

 with scattered concentrically elongated nuclei and 

 associated bundles of collagen fibrils. The layers are 

 formed by elongated sheet-like cells about 0.2-0.3 /i 

 thick bounded on their free surfaces by a double 

 membrane complex about 300-500 A thick (fig. 2). 

 Several of these thin cells are closely apposed to 

 produce the compact cytoplasmic layers. The ap- 

 posed cells are separated by simple double membranes 

 about 250-300 A thick (fig. 2). In some regions the 

 cells separate to form tissue spaces occupied fre- 

 quently by collagen fibrils. It appears that the indi- 



vidual cells do not split into several layers to form 

 the composite lamellated structures but rather that 

 each cytoplasmic layer belongs to a separate cell; 

 the cells are packed together like the leaves in the 

 center of a head of cabbage. The thin cytoplasm of 

 these cells contains scattered mitochondria and nu- 

 merous round or oval vesicular-appearing structures 

 about 200 500 A in diameter which sometimes 

 appear to originate from the cell surface and some- 

 times appear to form tubular channels across the 

 cytoplasm (fig. 2). These tubular channels drop out 

 in serial sections. 



A space about 5-10 /< wide lies between the outer 

 and inner sheaths (hg. 1). This contains a few 

 scattered collagen fibrils and occasionally a sheath 

 cell is seen traversing the space to connect with the 

 inner sheath. The inner sheath sometimes consists 

 of only one cytoplasmic layer varying from 0.1 // 

 down to 300 A in thickness (tig. 3). Sometimes 

 two or three slightly overlapping layers are present 

 but they arc not closely associated as in the outer 

 sheath. 



Between the inner sheath investing each intrafusal 

 fiber and the surface of the muscle fiber there is 

 present a space about 1-3 microns thick which is 

 occupied by a material of low density and generally 

 amorphous or delicately fibrillar appearance in which 

 terminal nerve fibers are situated (fig. 1). This ma- 

 terial is sometimes partly removed by treatment 

 with phosphotungstic acid (fig. 3) except for a thin 



