The Dog: Nonconformist, Not Only in Maternal Recognition Signaling

Abstract

Although similar at the molecular and cellular levels, endocrine mechanisms governing reproductive function in the domestic dog (Canis familiaris) differ markedly at the regulatory level from those known in other domestic animal species. Some of the events, e.g., the lack of luteolysis in the absence of pregnancy, resulting in similar luteal function and, therefore, hormonal profiles in early pregnant and nonpregnant animals, are species-specific. Consequently, no early gestation marker has so far been identified for the dog. Following implantation, relaxin of fetal placental origin can be detected and used for pregnancy diagnosis. Characterized by the lack of an active luteolytic principle from intra- or extra-luteal sources, the canine reproductive cycle appears to represent a "basic" form of mammalian reproductive function with apparently reduced opportunities for facilitating fecundity and hastening reproduction. Nevertheless, in the dog some kind of mechanism for synchronization between blastocyst development and uterine preparation for pregnancy must have evolved in order to support gestation. Driven by this assumption, studies including our recent investigations have been initiated aimed at characterizing some of the embryo-mediated effects of the preimplantation embryo on the canine uterus. Moreover, the lack of a uterine luteolysin and consequently the absence of a need to develop an antiluteolytic strategy make the dog an interesting model for investigating early evolutionary mechanisms involved in the preparation for implantation and ensuring embryo survival. These mechanisms result in an inverse relationship between the duration of pregnancy and of the nonpregnant cycle in the dog, compared with all other domestic animal species.

Menstruation: science and society

“…All of these animals have a placenta and a gestational period that is longer than their ovarian cycle, socalled trans-cyclic gestation, 38 with the exception of animals that have pseudopregnancy in the absence of fertilization, such as the dog. 39 Despite the substantial similarities, with respect to female reproductive biology, between humans and all other eutherian mammals (eg, compared with reptiles and birds), menstruation only occurs in a small minority of eutherian species. The largest cluster of menstruating species is found among our closest relatives, the primates.…”

“…The best known example is the vaginal bleeding of the dog, which is not a sign of menstruation. 39 The main difference between what is happening in dogs and in menstruating species is that the vaginal bleeding in dogs happens in proestrus, that is, in preparation for mating, rather than after the fertile phase is over, as it is the case in women. The bleeding in dogs is caused by extravasation during the growth of the uterine lining, which can break through the epithelium leading to a vaginal efflux.…”

Uterine responses to early pre-attachment embryos in the domestic dog and comparisons with other domestic animal species†

“…The latter, i.e., biochemical responses of the pre-implantation canine uterus to the presence of embryos, are mirrored in the modulated expression of some genes related to the processes of implantation and placentation, e.g., IGF2, PGR, ER alpha / ESR1 , and PRLR [ 24 ]. Among the members of the PG family system previously implicated in early canine embryo-maternal communication [ 26 ], uterine expression of PGT, PTGES, PGFS/AKR1C3 , and their respective receptors, FP (PTGFR) and PTGER2 (EP2) was increased in the presence of free-floating embryos [ 24 ]. Following implantation, further considerable differences become obvious between the dog and other domestic animal species regarding invasive vs. non-invasive growth of the trophoblast, leading to the formation of the canine endotheliochorial placenta.…”

Canine decidualization <i>in vitro</i>: extracellular matrix modification, progesterone mediated effects and selective blocking of prostaglandin E2 receptors

“…Consequently, in species exhibiting invasive types of placentation, maternal stroma-derived decidual cells develop. This species-specific tissue remodeling is referred to as decidualization, and is one of the most essential events at the beginning of pregnancy in rodents, carnivores and humans [ 3 , 4 , 5 , 6 ]. In rodents [ 6 ] and dogs [ 7 , 8 ] maternal decidual cells of mesenchymal origin are formed in response to the presence of implanting blastocysts, whereas in humans decidua is spontaneously formed during every reproductive cycle in a progesterone (P4)-dependent manner (reviewed in [ 9 ]).…”

“…Despite being devoid of its own steroidogenic activity, due to the presence of PGR in its maternal component the canine placenta can respond to circulating luteal P4. Interfering with its function alters feto-maternal communication, leads to prepartum PGF2α synthesis, and terminates pregnancy in dogs (reviewed in [ 5 , 12 ]). The central role of decidual cells in canine pregnancy becomes apparent when PGR-antagonists are applied, unequivocally leading to termination of pregnancy [ 10 ].…”