The Acrosomal Reaction of the Sea Urchin Spermatozoon 



167 



dinal orientation of filaments in the nucleus and 

 made the suggestion that these filaments represented 

 the unwound and elongated chromatids. As the 

 number of filaments is a few thousand and the num- 

 ber of chromatids in the grasshopper approximately 

 20, a large number of turnings of the filaments at 

 their ends should be found if their interpretation 

 is correct. As we have not seen these connections, 

 the idea of the chromosomes existing as extremely 

 long folded threads is not supported by the present 

 material. Another plausible explanation is that each 

 chromosome splits into a great number of subtila- 

 ments. 



References 



1. Ai/nius, B. A., X.Xcllforsc/i. 42. 134(1955). 



2. BuRCJos, M. H. and Fawcktt, D. W., J. Biophys. Bio- 



ciiem. Cytol. 1, 287 (1955). 



3. Depdolla, p., in Schroder, Handbuch der Entomo- 



logie. Jena, 1928. 



4. Fawcett, D. W., Laryngoscope M, 551 {\'-)5A). 



5. Grasse, P-P., Carasso, N., and Favard, P., Compt. 



rend. acad. sci. 141. 1395 (1956). 



6. — Annates Sci. \at. Zool. 18, 339 (1956). 



7. Retzius, G., Biol. Unlers., N. F., 14, 55 (1909). 



8. Sjostrand, F. S., Nature 171, 30(1953). 



9. Wii.KiNS, M. H. F., Piit}f)t. Staz. Zool. Napoli 23, 104 



(1951). 

 10. — personal communication, 1956. 



The Acrosomal Reaction of the Sea Urchin Spermatozoon 



B. A. Afzelius 



The Wenner-Gren Instiliite for Experinwiital Biology and the Laboratory for Biological Ullrastnictiire Research of the 



Department of Anatomy, Karolinska Institiitet. Stockholm 



1 HE acrosomal globule, which in some mammalian 

 spermatozoa flattens to form a "galea capitis" 

 covering the anterior part of the nucleus (6), remains 

 in the primitive or spherical condition in sea urchin 

 spermatozoa. The acrosomal region of this sper- 

 matozoon contains, in addition to the acrosomal 

 globule, a mass of structureless or sometimes fibrous 

 material situated within an apical indentation of the 

 nucleus ( 1 ). This paper deals with the changes the 

 acrosomal region undergoes while the sperm ap- 

 proaches the egg surface or. in most but not all 

 spermatozoa, when treated with egg water. "Egg 

 water" is the term for sea water containing dissolved 

 substances from the jelly coat surrounding the eggs. 



Figure 1 represents a Psammechintis miliaris 

 spermatozoon fixed two minutes after addition of 

 egg water. This spermatozoon has undergone an 

 acrosomal transformation. The most notable feature 

 of this reaction is the position of the acrosomal 

 globule on the outer surface of the cell membrane. 

 The mechanism for this transfer is unknown, since 

 the cell membrane appears not to be ruptured at any 

 point. 



Other changes have been noted in egg-water- 

 treated spermatozoa: (a) The cell membrane is more 

 blackened following fixation in osmium tetroxide 

 than that of untreated spermatozoa, and is often 

 separated from the enclosed structures by an empty 

 space. As a rule the cell membrane is not ruptured. 

 (/)) The middle piece, which appears to be a single 

 huge mitochondrion, may have moved to a position 

 alongside the nucleus and some of its internal double 

 membranes may have swollen to form vesicles, (c) 

 A portion of a tail may also have moved to a posi- 

 tion at the side of the nucleus but inside the cell 

 membrane that ordinarily closely invests the nucleus. 



To a much smaller extent this phenomenon has 

 also been observed in untreated spermatozoa (as in 

 fig. 3, ref. I). The possibility is recognized that some 

 of these changes may represent abnormalities. 



In other experiments spermatozoa have been added 

 to sea urchin eggs that showed a somewhat slow 

 fertilization rate but were otherwise normal. After 

 2-4 minutes the eggs were fixed, dehydrated and em- 

 bedded following the routine scheme (10). In sections 

 of cortex and jelly coat of the eggs, spermatozoa were 

 found in contact with the egg surface (first stage of 

 fertilization). They were often found in a position 

 perpendicular to the egg surface, in which cases 

 the chance of getting a longitudinally sectioned 

 spermatozoa was rather favourable. These spermato- 

 zoa had all undergone an acrosomal reaction; i.e.. 

 the acrosomal globule was always outside the cell 

 membrane of the spermatozoon. Figure 2 is an 

 example of an Echinus csciilentns spermatozoon in 

 the immediate vicinity of an egg. The apical indenta- 

 tion in the nucleus has the same dimensions as in 

 spermatozoa that have not reacted. A fine filament, 

 presumably formed from material originally situated 

 in the apical nuclear indentation, is seen within the 

 cell membrane in the most anterior part of the 

 acrosomal region. This filament, which consists of a 

 rather short rod of longitudinalls oriented fibrils, 

 has apparently pushed the cell membrane before it, 

 resulting in the formation of a pointed apex. The 

 acrosomal globule outside the cell membrane is 

 recognized by its homogeneity and density following 

 osmium tetroxide fixation. It has become ring- 

 shaped, and surrounds the pointed apex mentioned 

 above. 



At a later stage in the acrosomal reaction the cell 

 membrane has ruptured, allowing the ring-shaped 



