Anuran and pig egg zona pellucida glycoproteins in fertilization and early development JERRY L. HEDRICK* Department of Animal Science, University of California, Davis, CA, USA ABSTRACT The envelope surrounding the eggs of all animals has many biological functions in fertilization and development. This review focuses on the anuran egg envelope in terms of its biochemistry, biophysics, structural biology and function in sperm-egg interactions and early development (embryo hatching). Egg envelopes from Xenopus laevis are among the most studied of the anurans, and are the central theme of this review. Comparisons of Xenopus laevis envelopes with those of other anurans and with pig egg envelopes are also included. This article presents historical as well as contemporary comparative perspectives on molecular and cellular mecha- nisms of sperm-egg envelope binding, block to polyspermy, envelope hardening, and hatching. KEY WORDS: Xenopus laevis, pig, zona pellucida, fertilization, sperm binding, block to polyspermy Introduction Previous reviews and the nature of this review This review focuses on anuran egg envelopes, their glycopro- tein composition, chemistry, biophysics and structure, and the functional properties of the envelope. Emphasis will be on contri- butions from the author’s laboratory with comparisons to other anuran and vertebrate systems where appropriate. In addition, a comparative approach to the structure and function of egg enve- lopes will involve a direct comparison of the Xenopus laevis and pig egg envelopes. This review will be selective rather than comprehensive. Other reviews encompassing the structures and functions of egg envelopes offer alternative perspectives (Iwao, 2000b; a; Katagiri, 1987; Watanabe and Onitake, 2002; Wong and Wessel, 2006). Discovery of anuran egg envelopes The original observation of sperm penetration and activation of amphibian eggs was made in a series of papers by Newport using the frog Rana temporaria, the toad Bufo vulgaris, and several urodele species (Newport, 1851; 1853; Newport and Ellis, 1854). Regarding anurans, Newport described the transport of eggs from the ovary to the oviduct, egg transit through the oviduct, and acquisition of jelly coats (‘envelopes’). He experimentally demon- strated that jelly coats were required for fertilization, observed the penetration of sperm through the jelly layers and the egg envelope (‘vitelline membrane’). He also observed the response of the egg to the fertilizing sperm i.e., egg activation, with the subsequent formation of the perivitelline space (‘respiratory chamber’). New- Int. J. Dev. Biol. 52: 683-701 (2008) doi: 10.1387/ijdb.082580jh THE INTERNATIONAL JOURNAL OF DEVELOPMENTAL BIOLOGY www.intjdevbiol.com *Address correspondence to: Jerry L. Hedrick. Department of Animal Science, University of California, Davis, CA 95616, USA. e-mail: jlhedrick@ucdavis.edu Published online: 4th July 2008 0214-6282/2008/$35.00 © UBC Press Printed in Spain Abbreviations used in this paper: CE, coelomic envelope; FE, fertilization envelope; VE, vitelline envelope; ZP, zona pellucida. port also noted that many sperm penetrating the jelly coat layers were stopped by the egg envelope. Newport’s seminal observa- tions on the surface reaction of the egg to sperm and the stoppage of sperm at the egg envelope were extended some 30 years later by Kupffer (Kupffer, 1882). In Bufo variabilis eggs, he observed sperm boring their way through the jelly coat but, after the fertilizing sperm made contact with the egg, surfaces changes took place that prevented further entrance of spermatozoa. Kupffer suggested the egg had established a ‘defense’ against extra sperm [“…a counter demonstration of the eggs against those spermatozoa that had not been able to enter” as described in (Morgan, 1897)]. Using contemporary nomenclature and the accumulated knowledge since that time of the fertilization pro- cess, Newport and Kupffer made the original observations of sperm penetration of the amphibian egg through the jelly coat layer, the vitelline envelope, and into the egg cell, the importance of the egg envelope in the fertilization process, the sperm induced release of cortical granule components into the perivitelline space (the cortical reaction), and establishment of a block to sperm penetration at the level of the egg envelope by components released from the egg. Both Newport and Kupffer suggested a possible connection between components released from the egg and the subsequent envelope block to sperm penetration. How- ever, the effect of egg factors in altering the envelope architecture could not be experimentally demonstrated at that time since the