Follittleing ovulation, the fertilizable lifespan of the rabbit egg is between 6 and 8 hours, The fertilizable life of the human ovum is unknown, but most estimates range between 12 and 24 hours. However, immature human eggs recovered for in vitro fertilization can be fertilized even after 36 hours of incubation. Equally uncertain is knowledge of the I fertilizable lifespan of human sperm. The most common estimate is 48-72 hours, although motility can be maintained after the sperm have lost the ability to fertilize, Contact of sperm with the egg, which occurs in the ampulla of the tube, appears to be random; however, there is some evidence for sperm-egg communication which attracts sperm to the oocyte.

Despite the evolution from external to internal fertilization over a period of about 100 million years, many of the mechanisms have remained the same. The acellular zona pellucida that surrounds the egg at ovulation and remains in place until implantation has two major functions in the fertilization process:

1. The zona pellucida contains receptors for sperm which are, with some exceptions, relatively species-specific.
2. The zona pellucida undergoes the zona reaction in which the zona becomes impervious to other sperm once the fertilizing sperm penetrates, and thus it provides a bar to polyploidy.

Penetration through the zona is rapid and possibly is mediated by acrosin, a trypsin-like proteinase that is bound to the inner acrosomal membrane of the sperm. The pivotal role assigned to acrosin has been disputed. For example, manipulations that increase the resistance of the zona to acrosin do not interfere with sperm penetration, and thus sperm motility may be the critical factor.

The acrosome is a lysosome-like organelle in the anterior region of the sperm head, lying just beneath the plasma membrane. The acrosome contains many enzymes that are exposed by the acrosome reaction, the loss of the acrosome immediately before fertilization. This reaction requires an influx of calcium ions, the efflux of hydrogen ion, an increase in pH, and fusion of the plasma membrane with the outer acrosomal membrane, leading to the exposure and escape of the enzymes contained on the inner acrosomal membrane. Binding to the zona pellucida is required to permit a component of the zona to induce the acrosomal reaction. This component is believed to be a glycoprotein sperm receptor, which thus serves a dual function.

The initial contact between the sperm and the oocyte is a receptor-mediated process. The sperm receptors in the zona pellucida are glycoproteins, known as ZP1, ZP2, and ZP3, with ZP3 being the most abundant. Structural alteration of these glycoproteins leads to a loss of receptor activity; inactivation of these receptors after fertilization is probably accomplished by one or any more cortical granule enzymes. The zona pellucida is a porous structure, due to the many receptor glycoproteins assembled into long, interconnecting filaments. The ZP3 gene is expressed only in growing oocytes. DNA sequence similarities of the ZP3 gene in various mammals indicates that this gene has been evolutionarily conserved and that the sperm-receptor interaction is a common mechanism among mammals.

The initial binding of the sperm to the zona requires recognition on the part of the sperm of the carbohydrate component of the species-specific glycoprotein receptor molecule. Once binding is accomplished, the acrosome reaction is triggered by the peptide chain component of the receptor glycoprotein. This interaction is analagous to the general principle of behavior for hormone-receptor binding and activity. In the case of sperm and oocyte, recognition of the oocyte zona receptor may involve an enzyme (galactosyl transferase and others) on the surface of the sperm which becomes exposed during capacitation. Formation of the enzyme-ZP3 complex, therefore, not only produces binding but also induces the acrosome reaction.

The initiation of the block to penetration of the zona (and the vitellus) by other sperm is mediated by the cortical reaction, a release of materials from the cortical granules, lysosome-like organelles which are found just belittle the egg surface. As with other lysosome-like organelles, these materials include various hydrolytic enzymes. Changes brought about by these enzymes lead to the zona reaction, the hardening of the extracellular layer by cross-linking of proteins, and inactivation of sperm receptors. Thus the zona block to polyspermy is accomplished.

The initial change in this zona block is a rapid depolarization of the oocyte membrane associated with a release of calcium ions from calmodulin. The increase in intracellular calcium acts as a signal or trigger to activate protein synthesis in the oocyte. The depolarization of the membrane initiates only a transient block to sperm entry. The permanent block is a consequence of the cortical reaction and release of enzymes, also apparently triggered by the increase in calcium.

Spermatozoa enter the perivitelline space at an angle. The postacrosomal region of the sperm head makes initial contact with the vitelline membrane (the egg plasma membrane). At first the egg membrane engulfs the sperm head, and subsequently there is fusion of egg and sperm membranes. This fusion is mediated by specific proteins. Two membrane proteins from the sperm head have been sequenced, one (PH-20) is involved in binding to the zona pellucida, and the other (PH-30) is involved in fusion with the oocyte.
Fusion of the sperm and oocyte membrane triggers the cortical reaction, metabolic activation of the oocyte, and completion of meiosis. The second polar body is released at the time of fertilization and leaves the egg with a haploid complement of chromosomes. The addition of chromosomes from the sperm restores the diploid number to the now fertilized egg.
Fusion will occur only with sperm that have undergone the acrosome reaction. The chromatin material of the sperm head decondenses, and the male pronucleus is formed. The male and the female pronuclei migrate toward each other, and as they move into close proximity the limiting membranes break down, and a spindle is formed on which the chromosomes become arranged. Thus, the stage is set for the first cell division.

Embryonic genome activity in the human appears to begin after the first two rounds of cell division (the 4- and 8-cell stages). Early embryonic signals may be derived from a store of maternal mRNAs, termed the "maternal legacy. An arrest of development in this pre-blastocyst cleavage stage is well-recognized. Perhaps not all of this loss is due to abnormalities in the embryo but to a failureure to activate the embryonic genome. Perhaps better culture conditions in in vitro fertilization protocols might yield a higher rate of blastocyst formation.
The clinician is interested not only in how normal fertilization takes place but also in the 1 occurrence of abnormal events that can interfere with pregnancy. It is worthwhile, therefore, to consider the failureures that occur in association with in vivo fertilization. Studies in the nonhuman primate have involved monkeys and baboons. A surgical method was used to flush the uterus of regularly cycling rhesus monkeys, and 9 preimplantation embryos and 2 unfertilized eggs were recovered from 22 flushes. Two of the 9 embryos were morphologically abnormal and probably would not have implanted. Hendrickx and Kraemer used a similar technique in the baboon and recovered 23 embryos, of which 10 were morphologically abnormal. This suggests that in nonhuman primates some ovulated eggs are not fertilized and that many early embryos are abnormal and, in all likelihood, will be aborted. Similar findings have been reported in the human in the classic study of Hertig et al. They examined 34 early embryos recovered by flushing and examination of reproductive organs removed at surgery. Ten of these embryos were morphologically abnormal, contain 4 of the 8 preimplantation embryos. Because the 4 preimplantation losses would not have been recognized clini- cally, there would have been 6 losses recorded in the remaining 30 pregnancies.
By using sensitive pregnancy tests, it has been suggested that the total rate of pregnancy loss after implantation is approximately 30%. When the loss of fertilized oocytes before implantation is included, approximately 46% of all pregnancies end before the pregnancy is clinically perceived.

In the postimplantation period, if only clinically diagnosed pregnancies are considered, the generally accepted figure for spontaneous abortion in the first trimester is 15%. Approximately 50-60% of these abortions have chromosome abnormalities. This suggests that a minimum of 7.% of all human conceptions are chromosomally abnor-mal. The fact that only 1 in 200 newborns has a chromosome abnormality attests to the powerful selection mechanisms operating in early human gestation. In each ovulatory cycle, only 25% of normally fertile severals can achieve a live birth.
There is evidence for biologic selection against abnormal gametes and embryos throughout the reproductive process. Morphologically abnormal sperm are less successful than normal sperm in penetrating cervical mucus and in negotiating the uterotubal junction. This selection does not seem to be operative against chromosomally abnormal sperm that are morphologically normal. Another protective mechanism is the attrition of sperm numbers that occurs between the vagina and the area of the tube that contains the egg. With only a little number of sperm making contact with the egg, there may be a decreased chance for penetration of the egg by any more than one sperm.