Influence of maternal size on placental, fetal and postnatal growth in the horse. I. Development in utero

Abstract

The interacting influences of maternal size and fetal genotype on placental and fetal development in the mare were assessed by comparing conventional within-breed Thoroughbred (Tb-in-Tb, n = 7) and Pony (P-in-P, n = 7) control pregnancies established by artificial insemination (AI) with between-breed (Tb-in-P, n = 8; deprived in utero condition and P-in-Tb, n = 7; luxurious in utero condition) experimental pregnancies established by embryo transfer. All foals were born spontaneously and the mean (+/- SEM) duration of gestation in the two groups of control mares was significantly different (P < 0.001) at 325 +/- 3.0 days for the P-in-P pregnancies and 339 +/- 3.0 days for the Tb-in-Tb pregnancies, whereas the durations of gestation for the two experimental groups were very similar and midway between those of the control pregnancies at 332 +/- 2.8 days for the Tb-in-P and 331 +/- 2.7 days for the P-in-Tb. Mean (+/- SEM) foal birth weight and the mean (+/- SEM) values for the mass, gross area and volume of the allantochorion were all highest in the seven Tb-in-Tb pregnancies (53.1 +/- 2.6 kg, 3.8 +/- 0.3 kg, 12.9 +/- 0.3 x 10(3) cm(2), 3.5 +/- 0.2 l, respectively) and lowest in the seven P-in-P control pregnancies (24.0 +/- 1.3 kg, 1.7 +/- 0.1 kg, 8.3 +/- 0.3 x 10(3) cm(2), 1.8 +/- 0.1 l, respectively). These parameters were higher in the seven P-in-Tb pregnancies (37.9 +/- 2.1 kg, 2.7 +/- 0.1 kg, 10.1 +/- 0.5 x 10(3) cm(2), 2.5 +/- 0.1 l, respectively) than in the eight Tb-in-P (33.0 +/- 2.4 kg, 2.3 +/- 0.2 kg, 9.0 +/- 0.5 x 10(3) cm(2), 2.1 +/- 0.1 l) experimental pregnancies. Foal birth weight was positively correlated with the mass (r = 0.84, P < 0.001), gross area (r = 0.87, P < 0.001) and volume (r = 0.91, P < 0.001) of the allantochorion, and maternal weight was also positively correlated with both the mass and gross area of the allantochorion (r = 0.64 and 0.69, respectively; both P < 0.001). Application of stereology to multiple random biopsies recovered from each placenta produced mean values for the surface density of microcotyledons on the allantochorion (S(v)). Values were higher in Thoroughbred than in Pony mares regardless of the breed of fetus being carried. Multiplication of S(v) by the volume of the allantochorion to give the total microscopic area of fetomaternal contact at the placental interface was also positively correlated with foal birth weight (r = 0.84, P < 0.001). Foal birth weight was determined by the microscopic area of fetomaternal contact of the placenta and there were no differences in foal weight per m(2) of placenta regardless of fetal or maternal genomes. Thus, the results indicate that in equids, maternal size interacts with both the maternal and fetal genotypes to control the rate and extent of fetal growth by influencing the gross area of the diffuse allantochorion, and the density, complexity and depth of the microcotyledons on its surface.

Litter-size-dependent intrauterine growth restriction in sheep

“…At 130 days of gestation, Romanov foetuses gestated in Columbia ewes were 22% heavier than control Romanov foetuses gestated in Romanov ewes. The remarkable plasticity of foetal and placental growth and the ability of the foetus to respond to permissive or restrictive aspects of the maternal environment have also been observed following embryo-transfer experiments involving small and large breeds of horses (Allen et al, 2002), pigs (Wilson et al, 1998) and cattle (Ferrell, 1991).…”

Genetic study of gestation length in andalusian and arabian mares

“…In the horse, there is no strong evidence in support of a major role of the foetus in initiating parturition (Liggins and Thorbum, 1994), even though Allen et al (2002) showed significant effects of both maternal and foetal genome on the duration of gestation. They showed the interacting influences of maternal size and foetal genotype on placental and foetal development in the mare, by comparing conventional within-breed experimental foalings established by embryo transfer.…”

Fetal death: comparative aspects in large domestic animals

“…Where early pregnancy diagnosis and manual twin reduction are not practised, twinning remains a major non-infectious cause of fetal death in the mare. Allen et al (2002) have recently demonstrated the critical influence of placental size on neonatal development in the horse by showing that the birth weight of the foal is primarily determined by the total microscopic surface area of the allantochorionic microvilli. Small foals with signs of intrauterine growth retardation were associated with a placenta with a low total microscopic area of fetomaternal contact.…”