Dog pregnancy
Stages of dog pregnancy
The development of a new individual requires the transfer of male gametes to the female genital tract for fertilization of the female gametes. Dog pregnancy stages involve 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 state, spermatozoa are in a situation of reduced metabolism. Mature sperm are only able to metabolize a special sugar, fructose, within the reproductive tract.
Sperm are ejaculated usually 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 first studies in dog pregnancy stages emphasized on sperm transportation. 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, interfer 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.
Acrosin, a proteolytic enzyme plays a role in dog pregnancy stages as well. It 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. Females are usually sexually receptive for at least 24 hours before ovulation and, in the natural setting, insemination usually occurs a number of hours before the occurrence of ovulation.
An interesting finding in the mare is her ability to distinguish fertilize from unfertilized oocytes; unfertilized oocytes from previous cycles are retained within the oviduct, whereas recently fertilized oocytes (embryos) move through the oviduct of the uterus. It is likely that all dogs recognize pregnancy by the presence of an embryo(s) at the early oviductal pregnancy stage. However, this recognition does not necessarily result in prolongation of the corpus luteum and the continued production of progesterone, which is essential for the maintenance of pregnancy in dogs.
