Molecular and Cellular Basis of Formation, Hardening, and Breakdown of the Egg Envelope in Fish

This chapter highlights some of the processes from formation to breakdown of the egg envelope (egg membrane) in fish from the perspective of cell and developmental biology. The chapter describes the structure and functions of the egg envelope, compares the egg envelopes of various species, and deduces a biological concept of the egg envelope. The chapter explores the egg envelope as a probe that can be used in the analysis of molecular, cellular, and developmental phenomena in living systems. The egg envelope is an acellular structure enclosing the egg and embryo of all multicellular animals except sponges and some coelenterates. The number of egg envelopes varies from one to several in different animal species. Most of the egg envelopes in fish consist of two or three layers. These layers are different in morphology, ultrastructure, stainability, and chemical properties. The outer one or two layers are thin, while the innermost layer is usually the thickest. The egg envelope of fish has been considered to be synthesized in oocytes or follicle cells and is classified as the primary or secondary egg envelope. The envelopes of fertilized eggs of many fish are hard and tough structures with strong elasticity and are also insoluble in water. The constituent proteins of the egg envelope are, therefore, inconvenient as immunogens to raise antibodies. The hatching enzyme does not break down the egg envelope completely into free amino acids or small peptides but, by limited proteolysis, produces a mixture of water-soluble, high-molecular-weight glycoproteins.