Elsevier

Placenta

Volume 29, Issue 6, June 2008, Pages 539-548
Placenta

Effect of Variable Long-Term Maternal Feed Allowance on the Development of the Ovine Placenta and Fetus

https://doi.org/10.1016/j.placenta.2008.02.014Get rights and content

Abstract

Maternal feed allowance during pregnancy can affect the development of the ovine placenta and fetus. The impact of variations in feed allowance prior to as well as throughout pregnancy has received less attention. Ewes were offered 0.6 (R), 1.2 (C) or 1.8 (AL) maintenance requirements from 89 days before conception until day 133 of pregnancy. Ewes were euthanased on days 50, 92 and 133 of pregnancy. Ewe live weight and body condition score, maternal and fetal metabolic and hormonal profiles, fetal body dimensions and organ weights, and the number, weight and morphology of placentomes were measured. Maternal live weight and condition score were lower in R compared to AL ewes at all stages of pregnancy (P < 0.05). Plasma glucose and albumin concentrations of R ewes were significantly reduced (P < 0.05) at mid and late gestation, respectively. Placental components were generally unresponsive to long term variations in maternal feed allowance. However, placental weight was significantly (P < 0.05) correlated with fetal weight at days 50 (r = 0.59) and 133 (r = 0.69) of gestation. By late gestation growth-retarded singleton fetuses from R ewes were 19% lighter (P < 0.05), with reduced abdominal (9%) and thoracic (10%) girths (P<0.05) but of similar crown-rump length compared with fetuses from AL ewes. These differences were associated with significantly reduced IGF-I concentrations in fetal plasma (P < 0.05). In conclusion, maternal, placental and fetal adaptations to long established planes of variable maternal feed allowance were able to maintain fetal growth during early and mid-pregnancy while fetal growth restriction, associated with reduced fetal IGF-I levels, became apparent in late pregnancy.

Introduction

Perturbations of the pre-natal environment can affect the development of the fetus and placenta, subsequently impacting on post-natal viability and productivity. These latter aspects have implications for the fields of agriculture and medicine.

The effects of intrauterine growth retardation (IUGR) via artificial methods (e.g. carunclectomy) [1], maternal under-nutrition [2], [3], maternal nutrient partitioning (e.g. adolescent ewe model) [4], [5] and litter size (e.g. prolific ewe model) [6] on ovine fetal development are widely reported. Severe restrictions of substrate supply may result in maternal, placental and fetal adaptations to the altered pre-natal environment, which allows survival of offspring, often at the expense of normal fetal development. There is increasing evidence from epidemiological and experimental studies that the programming of fetal development may be particularly sensitive to maternal nutrient status during the peri-conception period. Changes in maternal live weight [7] and differences in ewe body condition score [8] prior to mating have been related to altered utero-placental weights at days 53–56 and 65 of gestation, respectively, in sheep. However, no studies appear to have investigated if these differences persist throughout pregnancy.

Perturbed fetal development may be apparent as altered fetal/birth weight, length, girth and disproportionate tissue growth. There is a tendency for substrates to be directed to maintain essential tissues and organs, such as the brain, at the expense of less vital ones. As a result, size at birth, muscle development and growth [9], [10], wool follicle formation [11], adiposity [6], [12] and gastrointestinal tract development [13] may be altered in growth restricted fetuses and newborns. Such perturbations in development may affect neonatal survival or influence post-natal productivity of livestock.

Adaptations of the fetus to restricted nutrient supply not only alter post-natal performance but are also considered likely precursors to adulthood disease in humans. These fetal adaptations are often associated with fetal hypoglycaemia, hypoxaemia and modifications to the insulin-like growth factor (IGF) and hypothalmo-pituitary-adrenal axes. A number of epidemiology studies have implicated these fetal adaptations to placental insufficiency with diseases later in life [14].

The following experiment examined the hypothesis that long-term pre- and post-conception feed allowance of the ewe induces adaptations in feto-placental and maternal characteristics and affects the development of the ovine fetus. Some of the mechanisms responsible for these adaptations were investigated.

Section snippets

Materials and methods

The experimental protocol was reviewed and approved by the Primary Industries and Resources South Australia, Animal Ethics Committee in accordance with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes [15].

Maternal live weight and condition score

Live weights and condition scores of R and AL ewes differed significantly from 50 days prior to insemination, at insemination and on allocated days of slaughter (P < 0.05, Table 1). Ewe live weight increased significantly between days 92 and 133 of pregnancy (P < 0.05) in all treatment groups, even though feed allowances remained fixed at initial treatment levels and condition scores of treatment groups continued to diverge. Nevertheless, AL ewes were 30–40% heavier than their R counterparts at late

Discussion

The present study is unique in the timing of onset and duration of the maternal feed restriction imposed. It is the only study in which fetal, placental and maternal characteristics were quantified at three stages of gestation in ewes that were already significantly divergent in live weight and body condition score from approximately 50 days prior to conception as well as throughout gestation.

Differential maternal feed allowance coupled with variable litter size, resulted in compensatory

Acknowledgements

The assistance of Karen Kind, Kathy Gatford and Melissa Bradbury (Research Centre for Research Centre for Early Origins of Adult Disease, University of Adelaide) and the Turretfield Farm staff in fetal and placental collections is gratefully acknowledged. Simon Humphrys (Primegro LTD) conducted the IGF assays on fetal plasma samples. This work was supported by SARDI Livestock Systems.

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