CRC TXTBK ASST REPROD TECHNQ VOL 1 PDF

INTRODUCTION

First of all, I must express in limited space my tributes to my teachers, even if inadequately. These include investigators from far-off days when the fundamental facts of reproductive cycles, surgical techniques, endocrinology, and genetics were elicited by many investigators. These fi elds began to move in the 20th century, and if one pioneer of these times should be saluted, it must be Gregory Pincus. Famous for the contraceptive pill, he was a distinguished embryologist, and part of his work dealt with the maturation of mammalian oocytes in vitro. He was the fi rst to show how oocytes aspirated from their follicles would begin their maturation in vitro, and how a number matured and expelled a fi rst polar body. I believe his major work was done in rabbits, where he found that the 10 to 11-hour timings of maturation in vitro accorded exactly with those occurring in vivo after an ovulatory stimulus to the female rabbit.

Pincus et al. also studied human oocytes (1). Extracting oocytes from excised ovaries, they identifi ed chromosomes in a large number of oocytes and interpreted this as evidence of the completion of maturation in vitro. Many oocytes possessed chromosomes after 12 hours, the proportion remaining constant over the next 30 hours and longer. Twelve hours was taken as the period of maturation. Unfortunately, chromosomes were not classifi ed for their meiotic stage. Maturing oocytes would be expected to display diakinesis or metaphase-I chromosome pairs. Fully mature oocytes would display metaphase-II chromosomes, signifying they were fully ripe and ready for fertilization. Nevertheless, it is well known that oocytes can undergo atresia in the ovary involving the formation of metaphase-II chromosomes in many of them. These oocytes complicated Pincus' estimates, even in controls, and were the source of his error which led later workers to inseminate human oocytes 12 hours after collection and culture in vitro (2,3). Work on human fertilization in vitro, and indeed comparable studies in animals, remained in abeyance from then and for many years. Progress in animal IVF had also been slow. After many relatively unsuccessful attempts in several species in the 1950s and 1960s, a virtual dogma arose that spermatozoa had to spend several hours in the female reproductive tract before acquiring the potential to bind to the zona pellucida and achieve fertilization. In the late 1960s, Austin and Chang independently identifi ed the need for sperm capacitation, identifi ed by a delay in fertilization after spermatozoa had entered the female reproductive tract (4,5). This discovery was taken by many investigators as the reason for the failure to achieve fertilization in vitro, and why spermatozoa had to be exposed to secretions of the female reproductive tract. At the same time, Chang reported that rabbit eggs that had fully matured in vitro failed to produce normal blastocysts, none of them implanting normally (6).