STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 89 



The block to polyspermy is only one of several mechanisms that 

 help to preserve monospermic fertilization, others being the zona 

 reaction (to be discussed shortly), the limitation of the numbers of 

 spermatozoa reaching the site of fertilization (see Austin and Bishop, 

 1957a; Austin and Walton, i960) and possibly also the impedance 

 offered by the cumulus oophorus (also to be discussed shortly). The 

 relative importance of these mechanisms differs in different species 

 but all species appear to possess a block to polyspermy. 



Zona Pellucida 



The zona pellucida is a relatively thick transparent membrane 

 which is best developed in the eggs of placental mammals but is 

 recognizable also in those of marsupials and monotremes (Fig. 10) 

 and even of reptiles, though here the corresponding membrane is 

 perhaps better termed the zona radiata. The zona pellucida is 

 deposited first as an interrupted intercellular structure related to 

 single follicle cells ; in addition, the processes and regions of follicle 

 cells near the egg appear to contain an amorphous substance resem- 

 bling the material of the zona (Chiquoine, 1959, i960; Trujillo- 

 Cenoz and Sotelo, 1959). These two observations support the idea 

 that the zona pellucida is a product of the follicle cells rather than 

 of the egg. As the follicle grows, the layer of new material becomes 

 continuous around the oocyte and increasingly separates the follicle 

 cells from the egg surface. As a result, the follicle-cell processes that 

 maintain contact with the egg surface become extremely attenuated. 

 Initially, the zona pellucida lies in close apposition to the vitellus 

 but becomes separated by the fluid extruded from the vitellus at the 

 time of first-polar-body emission. In the cat, the zona pellucida 

 appears to show further accretion after ovulation, whilst it is passing 

 through the Fallopian tube (Austin and Amoroso, 1959) (compare 

 Figs. 19, 20, 40, 41, 44, 45). The matrix of the zona pellucida is 

 essentially homogeneous, even by electron microscopy. 



The zona pellucida of rat and rabbit eggs has been shown to 

 consist of neutral or weakly acidic mucoprotein ; it is dissolved by 

 strong reducing or oxidizing substances, the rat zona more easily 

 than that of the rabbit, the most effective agent being a mixture of 

 hydrogen peroxide and ascorbic acid ; 2 and 4 per cent urea solutions 

 dissolved only the rat zona (Braden, 1952). Deane (195 2 ) found that 

 in tests on histological sections silver is precipitated in the rat zona 

 pellucida from acid solution and she concluded that the membrane 



