Influence of litter size on placental stereological characteristics and its relationship with neonatal outcome in dogs.

Although development of placental structure towards the endometrial layers is determined by effective tissue remodeling, expansion within the uterine surface is limited in polytocous species. We aimed to evaluate the influence of litter size on placental morphological characteristics and puppies' clinical outcome and physical development during neonatal period. Bitches and their neonates were assigned to: Large-Sized Litter Group (n = 8 bitches and n = 16 neonates; litter size exceeding breeds reference) and Small/Normal-Sized Litter Group (n = 7 bitches; n = 12 neonates). Placentas were collected and processed histological and stereologically for systematic structures counting. At birth, 5-10 and 60 min, puppies were evaluated for vitality score, pulse oximetry and lactatemia, as well as biometry subsequently at 10 and 14 days. Histological analysis of placental tissue revealed higher (moderate) intensity of calcification in the lamellar zone (P = 0.02) and hemorrhage in the glandular zone (P = 0.05) in the placenta of the Large-sized Litter Group, while in the Small/Normal-sized Litter Group intensity was mild. Puppies from large-sized litters exhibited lower vitality and muscle tone score, in addition to lower respiratory rate and irritability reflex at birth or 5-10 min. Although a significant increase in neonatal biometry was observed over 15 days, there were no differences over puppies' body measurements. In conclusion, placenta from large litters exhibits distinct features from those originating from small/normal-sized litters, albeit not compromising further puppies' development throughout the neonatal period. Despite the larger number of implantation sites in large-sized litters, there is evident morphological compensatory adaptation in placental structures towards increasing maternal-fetal exchange in order to guarantee normal fetal development.