The Development of an Embryo Involves Fusion of an Oocyte and Spermatozoon Within the Oviduct
The development of a new individual requires the transfer of male gametes to the female genital tract for fertilization of the female gamete(s). Spermatozoa, which have been concentrated and stored in the epididymis, gradually change from oxidative (aerobic) to glycolytic (anaerobic) metabolism as they progress through the epididymis.
In this situation, spermatozoa are in a state of reduced metabolism. Mature sperm are only able to metabolize a special sugar, fructose, within the reproductive tract. Lactose, glucose, dextrose and fructose have all been used in commercially available semen extenders.Sperm are ejaculated usually into the vagina, although some domestic species (dog, horse, and pig) ejaculate directly into the cervix and uterus. The movement of sperm through the cervix is aided by estrogen-induced changes in cervical mucus, which result in the formation of channels that facilitate movement of sperm. This has been particularly emphasized in primates, in which the thinning of mucus occurs just before ovulation, a factor that can be used to predict the time of ovulation.
The environment of the female genital system is generally inhospitable to the survival of sperm; for example, white blood cells are quickly attracted to the uterine lumen because sperm cells are foreign to the female genital tract. Special reservoirs have evolved in the female tract to aid in the survival of sperm during transport; these include the cervix and oviduct, the latter involving areas at the Uterotubule junction and within the ampulla. The reservoirs are progressively filled (from caudal to cranial in the tract), requiring hours before the oviductal reservoirs are full. Finally, the reservoir within the ampulla is able to release a few sperm on a continuous basis, so that fertilization can occur shortly after the arrival of oocytes within the oviduct.
The first studies in sperm transport emphasized the rapidity of the process, with sperm reported passing from the vagina to the fimbriated end of the oviduct within minutes. It is now known that sperm undergoing so-called fast transport are not involved in fertilization; in fact, they are damaged by the rapid transport.
Sperm need to undergo changes within the female genital tract that are a prerequisite for fertilization; the process is called capacitation. One of the effects of Capacitation is the removal of glycoproteins from the sperm cell surface.
The glycoproteins, perhaps added for protective purposes, interfere with fertilization. This change allows sperm to undergo the acrosome reaction when they come in contact with oocytes. The acrosome reaction involves the release of hydrolytic enzymes from the acrosomal cap; this may be important for penetration of the sperm through the granulosa and zona pellucida to the oocyte plasma membrane. Hyaluronidase causes breakdown of hyaluronic acid, an important component of the intercellular matrix of granulosa cells that surround the oocyte. Acrosin, a proteolytic enzyme, digests the acellular coating around the oocyte. Both enzymatic events allow the sperm to penetrate to the oocyte. The acrosome reaction also changes the surface of the sperm, which allows it to fuse with the oocyte. The acrosomal reaction results in tail movements that feature a flagellar beat that tends to drive sperm in a forward direction.
Because of the changes that spermatozoa must undergo within the female reproductive tract before fertilization, the deposition of sperm before ovulation is the preferred timing for producing maximal fertility. An exception to this takes place when sperm with reduced longevity are used, such as the case with chilled-extended semen or frozen semen. In these cases, deposition of semen into the female reproductive tract should occur close to the time of ova maturation associated with fertilization.
Females are usually sexually receptive for at least 24 hours before ovulation and, in the natural setting (free interaction between genders), insemination usually occurs a number of hours before the occurrence of ovulation. Even with induced ovulators, such as cats, the interval from copulation to ovulation is usually 24 hours or more. In essence, the system has evolved to have ready-to-fertilize sperm at the fertilization site when oocytes arrive. This concurs with the finding that the life span of male gametes tends to be twice that of female gametes.The presentation of male gametes before female gametes in the oviduct implies that oocytes are ready for fertilization on arrival in the ampulla; this is likely true for a majority of animals. A prerequisite for fertilization of the oocyte is that it must undergo the first meiotic division before fertilization. Although this occurs in a number of species before ovulation, in the horse and dog the first meiotic division does not occur until after ovulation (in the dog, not for at least 48 hours). In this situation, spermatozoa often wait for oocytes to mature in the oviduct before fertilization can occur. One means of adaptation to delayed completion of meiosis is that spermatozoa have a longer life span in the dog (6-11 days) and horse compared with other domestic species.
Once fertilization has occurred, the embryo usually develops to the morula, or early blastocyst stage, within the oviduct before moving into the uterus. This period, usually 4 to 5 days, affords the uterus time to finish its inflammatory response involving the removal of spermatozoa. This period also allows the endometrial glands time to secrete nutrients under the influence of progesterone from the developing corpus Iuteum (CL); the nutrients are essential for the development of embryos during their preimplantation stage.
An interesting finding in the mare is her ability to distinguish fertilized from unfertilized oocytes; unfertilized oocytes from previous cycles are retained within the oviduct, whereas recently fertilized oocytes (embryos) move through the oviduct to the uterus. It is likely that all animals recognize pregnancy by the presence of an embryo(s) at the early Oviductal stage. However, this recognition does not necessarily result in prolongation of the CL and the continued production of progesterone, which is essential for the maintenance of pregnancy. In the bitch, despite ovulation and ova maturation spanning several hours, embryonic ages are synchronized by some mechanism inherent to the bitch’s reproductive tract.