I. FERTILIZATION, IMPLANTATION, AND PLACENTATION

Fertilization, fusion of a mature egg with a mature spermatozoon, takes place in the uterine tube in dogs. This is far from the site of insemination, which is the vaginal vault during natural service or vaginal insemination, or the uterine body during intrauterine insemination. Few spermatozoa actually reach the site of fertilization; most are lost through the cervix and vagina, some are lost by movement through the uterine tube and into the abdominal cavity, and some are taken up by white blood cells. Those that remain must traverse mucus in the female reproductive tract and bind by their head to the lining of the uterus and uterine tube, forming spermatozoal reservoirs. Factors that aid movement of spermatozoa forward in the bitch’s reproductive tract include waves of prostatic fluid ejaculated by the male, contractions of the female reproductive tract, and slow movement of spermatozoa along the surfaces lining the uterus (sperm migration). Normally shaped spermatozoa are more likely to ascend the reproductive tract. Living spermatozoa may persist in the reproductive tract of the bitch for up to 11 days.

The bitch’s ovary contains multiple mature follicles, each of which contains one egg (see Folliculogenesis). At ovulation, luteinizing hormone (LH) released from the pituitary causes thinning of the follicle. The follicle ruptures, and the egg (or ovum) is released into the uterine tube. In dogs the egg is not yet ready to be fertilized and must undergo two more cell divisions before fertilization can take place; this usually takes about 48 hours.

The spermatozoa introduced at breeding or artificial insemination are not capable of fertilizing the egg immediately on introduction to the female’s reproductive tract but first must undergo capacitation. During capacitation proteins are altered or removed from the head of the spermatozoon. Capacitated spermatozoa have enhanced motion, called hypermotility. The acrosome is a bag of enzymes covering the head of the spermatozoon (Figure 7-1). The acrosome reaction, which also occurs during capacitation, is dissolution of the membrane on the outer surface. This allows the enzymes to be exposed that will penetrate the outer layer of the egg during fertilization.

Figure 7-1 A spermatozoon.

Capacitated spermatozoa reach the mature egg in the uterine tube. The ovulated egg is surrounded by a layer of cells from within the ovarian follicle. Within this layer of cells is a thick protein layer, the zona pellucida (Figure 7-2). The enzymes on the head of the spermatozoa and the hypermotile activity allow multiple spermatozoa to work their way through the outer cellular layer and approach the zona pellucida. As soon as one spermatozoon penetrates the zona pellucida completely and the inner membrane is reached, a reaction occurs that prevents any other spermatozoa from fertilizing the egg. The head of the spermatozoon that has penetrated the zona pellucida is drawn into the egg and its genetic material is released. Both the egg and spermatozoon contain half the genetic material of a dog. As the egg and spermatozoon fuse, the genetic material is combined and cell division begins.

Figure 7-2 Ovum surrounded by cumulus cells.

In the dog, the developing embryo moves freely within the uterus for about 17 days after fertilization. At that time, the embryo invades the surface of the uterus, the endometrium; causes edema and swelling; and becomes fixed. This is called implantation. Implantation has been reported to occur 18 to 20 days after ovulation, or 17 to 22 days after breeding.

The placenta is a structure formed by fusion of tissues in the fertilized egg with the uterus of the dam. The embryonic tissues involved are those not required for embryonic development. The placenta is a physiologic exchange site that allows the introduction of oxygen and nutrients to the developing offspring from the dam’s bloodstream.

Placentas are classified by shape, amount of invasion into the uterine tissue, and amount of maternal tissue lost at the time of parturition (whelping). The shape of the canine placenta is zonary; the pup lies in a clear sac called the amnion, which is encircled with a strip of placental tissue that attaches like a ring inside the uterine horn (Figure 7-3). The pup is connected to the placenta by the umbilical cord. The margin of the placenta is a blood-rich band called the marginal hematoma, which is made up of stagnant pools of maternal blood that slowly leached into the space between the maternal and embryonic tissues. This may function as a source of iron for the developing puppies, but its exact function is unknown. The invasion type is endotheliochorial; the outer surface of the tissues arising from the developing embryo invades through the uterine lining and connective tissue to allow the bitch’s small blood vessels to grow directly into the placenta. The tissue-loss type is semideciduate; some maternal tissue is lost as the placentas detach at whelping.

Figure 7-3 Zonary placenta and pup within amniotic sac.

(Courtesy Dr. Shirley Johnston.)