Placental development is important for fetal development and nutrient and waste transport. The pig, a litter bearing animal, has an epitheliochorial placenta that forms a noninvasive attachment with the uterine endometrium. Insufficient placental development is one of the primary causes of fetal death and reduced fetal growth after 35 days of gestation. Necrotic tips develop at the distal ends of each allantochorion between Day 22 and 42 of gestation. During this same period, placenta attaches to the uterine endometrium and establish fetal blood supplies and nutrient exchange. The attached placenta is composed of a central highly vascular placental (HVP) region adjacent to the fetus and less vascular placental (LVP) regions on either side to the fetus, the paraplacenta and necrotic tips, which form after 27 days of gestation. The objective of this study was to comprehensively evaluate uterine–placental interactions and necrotic tip development from 22 to 42 days of gestation in the gilt. Gilts (n = 25) were bred by AI at first detection of oestrus (Day 0) and harvested at 22, 27, 32, 37, or 42 days of gestation. Each conceptus was counted and weighed to identify the large, medium, and small fetus in each litter. From these fetuses, HVP and LVP sections of tissue and necrotic tip (no placental attachment) were collected. Intact attached uterus and placenta were collected for histology and preserved in paraformaldehyde. Placentas were then stripped from the endometrium and individually weighed. Hematoxylin and eosin staining of uterine : placental sections was used to identify uterine fold development in the HVP, LVP, and uterine endometrial morphology in the necrotic tip regions. Three folds were measured for depth, width, and area using BQ Nova Prime software (Bioquant Image Analysis, Nashville, TN, USA). Data were analysed using PROC MIXED in SAS (SAS Institute Inc., Cary, NC, USA). Litter size, 12.1 ± 3.4, was similar (P = 0.86) for all days of gestation. Fetal and placental weight increased (P < 0.05) as day of gestation increased. Average fetal weight was similar (P = 0.30) for Day 27 and 32 with a tendency (P = 0.09) to increase by Day 37 before a significant increase at Day 42 (P = 0.002). The placenta increased (P = 0.02) in weight throughout this period of gestation, with the greatest increase in weight between 37 and 42 days of gestation. The LVP zones had no measureable fold formation until Day 32 from most conceptuses, and all LVP zones displayed microfold formation by Day 37. Necrotic tips became apparent after 27 days of gestation. Necrotic tip areas of the uterus had observable modifications from Day 32 to 42 of gestation, developing folds with dramatic changes in endometrial cell size and morphology. This work demonstrated fundamental time points in placental development that correspond to fetal growth and microfold formation. Gestation Day 27 through 32 show limited changes in either fetal growth or increased placental weight; however, significant morphological changes occur throughout the placenta, and even necrotic tips demonstrating the dynamic architecture of the establishing porcine placenta during early gestation.
Fetal maturity and morbidity as related to placental weight and secondary ossification