BIOLOGY OF EGGS AND IMPLANTATION 



831 



forward towards the zona pelliicida. The 

 spermatozoa may move through the cumu- 

 lus with broad sweeps of their flagella and 

 at a rate of forward i^rogression which 

 makes it difficult to conceive of the de- 

 l)olymerization of the matrix to form a 

 tunnel for the sperm. It must be concluded 

 therefore that the role of hyaluronidase in 

 sperm penetration is unknown and that 

 much more critical evaluation needs to be 

 directed into this area. 



Even though the outer layers of the cu- 

 mulus oophorus of ovulated eggs iti vitro 

 may be removed readily by hyaluronidase, 

 the corona radiata may not be dispersed 

 with the same rapidity, especially in eggs 

 treated immediately after ovulation. The 

 basis for this difference lies in the fact that 

 the cells forming the corona radiata send 

 liolar, cytoplasmic extensions into the zona 

 liellucida, thereby anchoring them firmly, 

 although temporarily. In the newly ovulated 

 eggs of the rat, hamster, and mouse the 

 corona cells cannot be removed mechani- 

 cally without breaking the zona pellucida. 

 It is only after the eggs have been in con- 

 tact with spermatozoa or have resided in 

 the oviducts for a number of hours that the 

 corona cells may be either brushed off the 

 zona pellucida or drop away spontaneously. 



Swyer (1947b) and Chang (1951b) sug- 

 gested that the coronal cells are removed 

 mechanically by being more or less brushed 

 off by the ciliary and muscular activity of 

 the oviduct. This may be true for human 

 and rabbit eggs, but in the rat, mouse, and 

 hamster in which the eggs lie in the dilated 

 ampulla, and thus at a distance from the 

 wall of the oviduct, it would seem appropri- 

 ate to assume that factors other than me- 

 chanical are involved in dispersing the 

 corona. If rat eggs are examined approxi- 

 mately 24 hours after ovulation, one can 

 observe that their zonae are completely 

 free of the coronal cells, but that they may 

 be still enclosed in an abundant viscous 

 matrix. It appears that the corona cells 

 gradually retract their cytoplasmic exten- 

 sions from the zonal canaliculi. 



Interesting observations can be made by 

 growing freshly ovulated eggs and their at- 

 tached corona cells in tissue culture. Time- 

 lapse cinematography reveals that the cells 



forming the cumulus and corona, although 

 alive, have lost much of their vitality. The 

 surfaces of the cells undergo peculiar bub- 

 bling movements. This "bubbling" is simi- 

 lar to that described in cells in the late 

 stages of cell division or in cells which are 

 about to die. Changes in the fluidity of the 

 cell surface apparently account for the 

 bubbling which continues for hours in fa- 

 vorable preparations. This i)henomenon 

 accounts for the retraction of the cell proc- 

 esses from the zona and the gradual dis- 

 persal of the cells. That the cumulus and 

 coronal cells lose their vitality rather 

 quickly after ovulation is shown further by 

 their very poor growth in tissue culture 

 compared with that of similar cells removed 

 from young follicles. 



The rate of the dispersal of cumulus cells 

 after ovulation varies in different animals. 

 In mated ral)bits the eggs are completely 

 denuded of cumulus and corona cells 4 to 

 6 hours after ovulation. After sterile mat- 

 ings, however, the cumulus and corona are 

 not dispersed until 7 to 8 hours after ovula- 

 tion (Pincus, 1930; Chang, 1951b; Braden, 

 1952). In the rat there is relatively little 

 change, either in the cumulus mass or in the 

 corona cells for many hours after ovulation 

 and fertilization (Blandau, 1952). Shettles 

 (1953) suggested that in addition to hy- 

 aluronidase there may be a tubal factor 

 which is important in the removal of the 

 cumulus oophorus in the human egg. He 

 found that hyaluronidase had little effect in 

 removing the cumulus cells in ovarian eggs, 

 but, if bits of homologous tubal mucosa 

 were added, the cumulus oophorus was dis- 

 persed readily. 



In spite of the formidable barriers inter- 

 posed by the cumulus and corona, they do 

 not prevent the entrance of sperm into the 

 egg ; in fact, as suggested earlier, their pres- 

 ence seems to be important in some animals 

 if penetration is to be effected (Fig. 14.11). 

 Chang (1952a) demonstrated that, in the 

 rabbit at least, there is a relationship be- 

 tween the loss of the granulosa cells and 

 fertilizability. He counted the spermato- 

 zoa in eggs fixed at different intervals after 

 ovulation and found that the greatest num- 

 ber entered the eggs between the 2nd and 

 4th hours. Once the denudation of the eggs 



