Placentomal differentiation may compensate for maternal nutrient
restriction in ewes adapted to harsh range conditions1
K. A. Vonnahme,*2 B. W. Hess,*† M. J. Nijland,*‡ P. W. Nathanielsz,*‡ and S. P. Ford*†3
*The Center for the Study of Fetal Programming, Laramie, WY 82071; †Department of Animal Science,
University of Wyoming, Laramie 82071; ‡Department of Obstetrics and Gynecology,
University of Texas Health Sciences Center, San Antonio 78229
ABSTRACT: Maternal nutrient restriction from
early to midgestation can lead to fetal growth retardation, with long-term impacts on offspring growth, physiology, and metabolism. We hypothesized that ewes from
flocks managed under markedly different environmental conditions and levels of nutrition might differ in
their ability to protect their own fetus from a bout of
maternal nutrient restriction. We utilized multiparous
ewes of similar breeding, age, and parity from 2 flocks
managed as 1) ewes adapted to a nomadic existence
and year-long, limited nutrition near Baggs, WY (Baggs
ewes), and 2) University of Wyoming ewes with a sedentary lifestyle and continuous provision of more than
adequate nutrition (UW ewes). Groups of Baggs ewes
and UW ewes were fed 50 (nutrient restricted) or 100%
(control fed) of National Research Council recommendations from d 28 to 78 of gestation, then necropsied, and
fetal and placental data were obtained. Although there
was a marked decrease (P < 0.05) in fetal weight and
blood glucose concentrations in nutrient-restricted vs.
control fed UW ewes, there was no difference in these
fetal measurements between nutrient-restricted and
control-fed Baggs ewes. Nutrient-restricted and con-
trol-fed UW ewes exhibited predominantly type A placentomes on d 78, but there were fewer (P < 0.05) type
A and greater (P < 0.05) numbers of type B, C, and
D placentomes in nutrient-restricted than control-fed
Baggs ewes. Placental efficiency (fetal weight/placentomal weight) was reduced (P = 0.04) in d 78 nutrientrestricted UW ewes when compared with control-fed
UW ewes. In contrast, nutrient-restricted and controlfed Baggs ewes exhibited similar placental efficiencies
on d 78. This is the first report of different placental
responses to a nutritional challenge during pregnancy
when ewes were selected under different management
systems. These data are consistent with the concept
that Baggs ewes or their conceptuses, which were
adapted to both harsh environments and limited nutrition, initiated conversion of type A placentomes to other
placentomal types when subjected to an early to midgestational nutrient restriction, whereas this conversion failed to occur in UW ewes. This early placentomal
conversion in the Baggs ewes may function to maintain
normal nutrient delivery to their developing fetuses
during maternal nutrient restriction.
Key words: fetal growth, gestational nutrient restriction, placentomal conversion, sheep
©2006 American Society of Animal Science. All rights reserved.
INTRODUCTION
Rangelands of the High Plains and Intermountain
West of the United States experience yearly fluctuation
1
The authors thank Carole Hertz for her technical assistance, Robert Stobart for confirming pregnancy status via ultrasonography, and
Mark Drumhiller, Britney Burt, and Hyung Chul Han for help with
sample collection and processing. Supported, in part, by grants from
the University of Wyoming BRIN P20 RR16474 and INBRE P20
RR016474-04, NIH HD 21350, and USDA/NRI 2004-35203-14949.
2
Current address: Department of Animal and Range Sciences,
North Dakota State University, Fargo 58105.
3
Corresponding author: spford@uwyo.edu
Received March 6, 2006.
Accepted June 26, 2006.
J. Anim. Sci. 2006. 84:3451–3459
doi:10.2527/jas.2006-132
in quality and quantity of forages (Anderson, 1993;
DelCurto et al., 1999; DelCurto et al., 2000). Paterson et
al. (2002) reported that forage digestibility of Montana
pastures declined from May to September. Because of
low protein and high fiber content of poor quality forages, livestock consumption of forages may decline
(Krysl and Hess, 1993). Pregnant ewes on rangeland
with no supplementation often experience bouts of nutrient restriction of less than 50% of National Research
Council (NRC, 1985) recommendations during early
gestation (Thomas and Kott, 1995).
The fetal origins hypothesis proposes that alterations
in fetal nutrition during critical periods of gestation
can affect growth, physiology, and metabolism of offspring in postnatal life (Barker and Clark, 1997;
3451
3452
Vonnahme et al.
Hawkins et al., 1997; McMillen et al., 2001). Impacts
of nutrient restriction during the first half of gestation
in ewes include reduced fetal growth (Vonnahme et al.,
2003), poor wool and carcass quality (Black, 1983; Bell,
1992; Kelley et al., 1996), and increased blood pressure
and reduced kidney glomerulus number (Gilbert et al.,
2005) of their offspring.
Factors that influence placental growth and vascular
development affect fetal growth and thus fetal and neonatal mortality and morbidity (Mellor, 1983; Reynolds
and Redmer, 1995; Torry et al., 2004). In sheep, cotyledons on the chorioallantoic membrane attach by fingerlike projections to discrete caruncles on the uterine wall
(see Ford, 2000, for review). The cotyledonary-caruncular unit (placentome) is the area of hemotrophic, as
opposed to histotrophic, nutrient, and waste product
exchange between the mother and fetus.
This study compared the effects of early to midgestational nutrient restriction on maternal and fetal
weights and blood glucose concentrations, as well as
placentomal growth and differentiation, in ewes derived from 2 flocks selected under markedly different
production environments and nutrient availabilities.
MATERIALS AND METHODS
Animals
All animal procedures were approved by the University of Wyoming Animal Care and Use Committee.
Ewes of similar age (4- to 5-yr-old) and parity (2 to 3)
were obtained from 2 flocks of Rambouillet/Columbia
cross ewes. The first flock located near Baggs, Wyoming
(Baggs ewes) was adapted over approximately 30 yr to
a nomadic existence, traveling approximately 402 km/
yr to graze a land mass that ranged from desert terrain
(annual rainfall 18 to 23 cm; Ostresh et al., 1990) to
high mountain pastures with limited nutritional supplementation. The second flock was also maintained for
approximately 30 yr by the University of Wyoming (UW
ewes), and in contrast to the Baggs ewes, had a relatively sedentary lifestyle and consumed a diet from
birth that always met or exceeded NRC recommendations.
For 1 mo before breeding, Baggs and UW ewes were
maintained in separate, group pens in an open-fronted
pole barn located at the Animal Science Livestock Center in Laramie, Wyoming. Beginning on September 1,
2004, all ewes were observed for estrus twice daily and
bred to the same intact ram (Rambouillet/Columbia
cross) at first exhibition of estrus and 12 h later (first
day of mating = d 0). Animals were fed alfalfa hay at
2% of BW from 30 d before mating to d 20 postmating.
A BCS of 1 (emaciated) to 9 (obese) was assigned by 2
trained observers after palpation of the transverse and
vertical processes of the lumbar vertebrae (L2 through
L5) and the region around the tail head (Sanson et al.,
1993). Sanson et al. (1993) demonstrated that BCS is
highly related to carcass lipids and can be used to esti-
mate energy reserves available to ewes. On d 20 postmating, 40 Baggs ewes and 37 UW ewes were weighed
and body condition scored, and then placed in an indoor
facility in individual pens containing feeders and waterers. The diet consisted of a pelleted beet pulp and a
mineral-vitamin mixture as previously described (Vonnahme et al., 2003). Rations were delivered on a DM
basis to meet the TDN required for an early pregnant
ewe (NRC, 1985). Individual diets were calculated on
a metabolic BW basis (BW0.75).
On d 28, ewes within the UW or Baggs flocks were
paired by BW and BCS, and 1 member of each pair was
assigned to a control-fed group (100% NRC recommendations); the other member of each pair was assigned
to a nutrient-restricted group, which received 50% of
the pelleted beet pulp and mineral-vitamin mixture
provided to the control-fed group. Beginning on d 28 of
gestation, and continuing at 7-d intervals, all ewes were
weighed and their rations were individually adjusted
up for BW gain or down for BW loss. Body condition
scores were obtained again on d 49 of gestation and
again on the day of necropsy (d 78 of gestation). On
approximately d 45, pregnancy was confirmed and the
number of fetuses carried by each ewe was determined
by ultrasonography (Ausonics Microimager 1000 sector
scanning instrument, Ausonics Pty. Ltd., Sydney, Australia). A subset of ewes in each group carrying singleton and twin fetuses was selected to be necropsied on
d 78 of gestation, and included 13 control-fed UW ewes
(7 singleton and 6 twin pregnancies), 10 nutrient-restricted UW ewes (6 singleton and 4 twin pregnancies),
10 control-fed Baggs ewes (5 singleton and 5 twin pregnancies), and 8 nutrient-restricted Baggs ewes (4 singleton and 4 twin pregnancies). Beginning on d 79, the
remainder of the nutrient-restricted ewes in each group
was realimented to 100% of NRC requirements until
delivery and allowed to lamb.
Immediately before necropsy, each ewe was weighed
and a blood sample was collected via jugular venipuncture into a 10-mL heparinized vacuum tube containing
2.5 mg of sodium fluoride per mL of blood (Sigma, St.
Louis, MO) for glucose determination. Ewes were then
given an overdose of sodium pentabarbitol (Abbott Laboratories, Abbott Park, IL) and exsanguinated, and the
gravid uterus was carefully opened to expose the umbilical cord of each fetus. Umbilical venous blood was then
collected into a heparinized vacuum tube containing
2.5 mg of sodium fluoride per mL of blood, as previously
described for maternal blood. Tubes containing maternal and umbilical blood were cooled on ice and then
centrifuged at 3,000 × g for 10 min and stored at −80°C
until analyzed for glucose. After removing the fetus(es)
from the gravid uterus, weight(s) and crown-rump
length(s) (CRL) were recorded. Crown-rump length
was measured with a cloth tape measure as the distance
from the crown of the skull to the base of the tail. The
fetal heart was removed, and the weights of right and
left ventricular walls were recorded, as previously described (Vonnahme et al., 2003).
3453
Placental conversion and fetal growth in ewes
Figure 1. Placentomal types (types A, B, C, and D) recovered from a single, d 78, nutrient-restricted Baggs, WY
(Baggs) ewe, carrying a singleton fetus. Arrows depict cotyledonary (COT) and caruncular (CAR) tissues.
Placentomal Evaluation
Each placentome was dissected from the uterine wall,
weighed, and its morphologic type was recorded (Figure
1). Morphologic type was based on the classification
scheme of Vatnick et al. (1991) and was based on the
placentome appearance, as follows: 1) caruncular tissue
completely surrounding the cotyledonary tissue (type
A), 2) cotyledonary tissue beginning to grow over the
surrounding caruncular tissue (type B), 3) flat placentomes with cotyledonary tissue on 1 surface and caruncular tissue on the other (type C), and 4) everted placentomes resembling bovine placentomes (type D). All
placentomes were pooled to determine total placentomal weight.
Glucose Analysis
Glucose was analyzed using the Infinity colorimetric
assay (Cat. # R15498, ThermoTrace Ltd., Melbourne,
Australia) modified in the following manner: plasma
samples were diluted 1:5 in PBS, and 10 ␮L of diluted
plasma was added to 300 ␮L of reagent mix. All samples
were run in triplicate, and sample analysis was completed using multiple assays. The intra- and interassay
CV were 5 and 7%, respectively.
Statistical Analysis
Data were analyzed by ANOVA appropriate for a
completely random design having a 2 (ewe type) × 2
(diet) factorial arrangement of treatments. Analyses
were completed using the GLM procedure (SAS Inst.
Inc., Cary, NC). Model statements included the effects
of diet, ewe type, fetal number, and their interactions on
fetal weight, fetal CRL, fetal right and left ventricular
weight divided by the weight of the fetus, placentomal
number, placentomal weight, and percentage of each
morphologic type of placentome. Percentage morphological type was calculated by taking the number of
each placentomal type (i.e., A, B, C, or D) and dividing
by the total number of placentomes for that conceptus.
Further, the model statements included effects of diet
and ewe type on ewe BCS, percentage BW loss or gain,
and eviscerated carcass weight. Because ewe type × diet
interactions were detected for some variables (P < 0.05),
simple effect means are presented throughout the
manuscript, and means were separated using PDIFF
of SAS. Data are presented as means ± SEM and are
considered different when P < 0.05, unless otherwise
stated.
RESULTS
Body condition scores of UW and Baggs ewes were
similar at d 28 of gestation (Table 1) averaging
3454
Vonnahme et al.
Table 1. Body condition scores for University of Wyoming (UW) ewes and ewes located near Baggs, WY
(Baggs)1,2
Dietary treatment3
Day4
UW ewes6
28
49
78
Baggs ewes7
28
49
78
Control
fed
Nutrient
restricted
20
5.4a
5.6a
4.9a
18
5.6a
5.6a
5.4a
20
5.4a
4.5b
3.8b
19
5.7a
5.5a
4.9b
SE5
0.2
0.2
0.2
0.2
0.2
0.2
Row means with different superscripts differ (P < 0.05).
BCS was used to estimate energy reserves available to ewes.
Ewe type × dietary treatment means.
3
Groups of Baggs ewes and UW ewes were fed either 100 (control
fed) or 50% (nutrient restricted) of NRC recommendations from d 28
to 78 of gestation.
4
Day of gestation.
5
Standard error based on 20 (UW) ewes and 18 (Baggs) ewes.
6
Selected to a sedentary lifestyle and more than adequate nutrition.
7
Selected to a nomadic lifestyle and limited nutrition.
a,b
1
2
5.4 ± 0.1 overall. The BCS of both control-fed UW and
control-fed Baggs ewes remained relatively constant
throughout the 50-d experimental period. The BCS of
nutrient-restricted UW ewes declined from d 28 to 49
(P = 0.03) and again between d 49 and 78 (P = 0.04).
In contrast, the BCS of nutrient-restricted Baggs ewes
remained relatively constant between d 28 and 49, before declining by d 78 (P = 0.04). As a result, BCS of
nutrient-restricted Baggs ewes were greater (P = 0.03)
than those of nutrient-restricted UW ewes on d 78 of
gestation.
On d 28 of gestation, whereas there were no BW
differences between control-fed and nutrient-restricted
UW ewes or between control-fed and nutrient-restricted
Baggs ewes, UW ewes were heavier (P = 0.03) than
Baggs ewes (93 ± 5 vs. 78 ± 3 kg, respectively). As
depicted in Figure 2, both control-fed UW and controlfed Baggs ewes exhibited similar and progressive increases in BW change from d 28 through 78, averaging
7.5 ± 0.6%. In contrast, whereas nutrient-restricted
UW and nutrient-restricted Baggs ewes lost BW
throughout the 50-d nutrient restriction period, nutrient-restricted UW ewes lost a greater percentage of
their d-28 BW than nutrient-restricted Baggs ewes
(7.4 ± 0.4 vs. 4.7 ± 0.5%, respectively, P = 0.03). Nutrient-restricted UW and nutrient-restricted Baggs
ewes had lighter eviscerated carcass weights than their
control-fed groups (57.4 ± 4.4 vs. 66.0 ± 3.3 kg and
47.1 ± 1.4 vs. 55.2 ± 1.3 kg, respectively; P < 0.01).
Within ewe type (UW or Baggs) and dietary treatment (control fed and nutrient restricted), there was
no significant difference in the weight or CRL of singleton and twin fetuses (Table 2). In contrast, fetuses from
nutrient-restricted UW ewes were markedly lighter
(P = 0.02) than fetuses from control-fed UW ewes.
Figure 2. Percentage weight change of Baggs, WY
(Baggs) ewes selected to a nomadic lifestyle and limited
nutrition and University of Wyoming (UW) ewes selected
to a sedentary lifestyle and more than adequate nutrition
when fed 100 (control fed) or 50% (nutrient restricted) of
NRC (1985) recommendations from d 28 to 78 of gestation.
Whereas fetuses from nutrient-restricted UW ewes
were also numerically shorter than fetuses from control-fed UW fetuses, only the CRL of twin fetuses
reached significance (P < 0.05). No difference in weight
or CRL was observed between fetuses from control-fed
and nutrient-restricted Baggs ewes. When fetal weight
was divided by ewe live weight on d 78 of gestation,
control-fed and nutrient-restricted UW ewes averaged
0.32 and 0.26%, respectively, whereas control-fed and
nutrient-restricted Baggs ewes averaged 0.32 and
0.38%, respectively.
When fetal ventricular weight was divided by fetal
BW, singleton fetuses from nutrient-restricted UW
ewes had greater right ventricular/fetal weight ratios
than singleton or twin fetuses from control-fed UW ewes
(P < 0.05, Table 2). Singleton and twin fetuses from
nutrient-restricted UW ewes also had greater (P < 0.05)
left ventricular weight/fetal weight ratios than fetuses
from control-fed UW ewes. There were no differences
in right ventricular weight/fetal weight ratios in the
fetuses of control-fed and nutrient-restricted Baggs
ewes, and increased (P < 0.05) left ventricular/fetal
weight ratios were only observed in twin fetuses of nutrient-restricted Baggs ewes when compared with singleton and twin fetuses from control-fed Baggs ewes.
There were no differences in total placentome number per gravid uterus between control-fed and nutrientrestricted UW ewes regardless of whether they were
gestating singleton or twin fetuses (Table 2). In contrast, the total placentome number of singleton or twin
pregnancies of nutrient-restricted Baggs ewes tended
to be reduced when compared with that of singleton or
twin pregnancies of control-fed Baggs ewes, but only
3455
12a
0.6a
0.02a
0.02b
8a
145b
±
±
±
±
±
±
273
20.6
0.23
0.36
79
1,179
14a
0.4a
0.02a
0.02ab
18b
123a
±
±
±
±
±
±
285
21.0
0.23
0.34
59
592
10a
0.3a
0.01a
0.01a
3a
15b
±
±
±
±
±
±
260
21.1
0.23
0.33
93
1,000
6a
0.4a
0.02a
0.02a
2a
35a
±
±
±
±
±
±
1
a–c
Within ewe type (UW or Baggs ewes), row means with different superscripts differ (P < 0.05).
Selected to a sedentary lifestyle and more than adequate nutrition.
2
Selected to a nomadic lifestyle and limited nutrition.
3
Fed 100% of NRC (1985) recommendations from d 28 to 78 of gestation.
4
Fed 50% of NRC (1985) recommendations from d 28 to 78 of gestation.
5
Values for twin fetuses were first averaged, and this mean was then used to generate the group averages.
6
Crown rump length.
7
Measurements divided by fetal weight are expressed as a percentage (i.e., they were multiplied by 100).
283
21.1
0.24
0.30
93
729
6b
0.3b
0.01ab
0.01b
2a
36c
±
±
±
±
±
±
215
20.1
0.22
0.37
81
936
±
±
±
±
±
±
19a
0.4a
0.01a
0.02a
3a
89b
307
23.4
0.20
0.29
84
1,241
22a
0.6a
0.01a
0.04a
3a
29a
±
±
±
±
±
±
309
23.1
0.20
0.32
86
696
Fetal wt, g
CRL,6 cm
Right ventricle wt/fetal wt7
Left ventricle wt/fetal wt7
Placentomes, No.
Total placentome wt, g
Item5
Single,
n=7
Control fed3
235
21.7
0.26
0.42
78
821
±
±
±
±
±
±
4b
0.8ab
0.04b
0.03b
4a
92ac
Twin,
n=4
Single,
n=4
Twin,
n=4
Single,
n=6
Twin,
n=6
Nutrient restricted4
Single,
n=5
Control fed3
Twin,
n=5
Baggs ewes2
UW ewes1
Table 2. Fetal measurements for University of Wyoming (UW) ewes and ewes located near Baggs, WY (Baggs) on d 78 of gestation
Nutrient restricted4
Placental conversion and fetal growth in ewes
placentome numbers of singleton pregnancies reached
statistical significance (P < 0.05). Total placentome
weight per gravid uterus of control-fed UW ewes with
twin pregnancies was markedly greater (P = 0.04) than
that of singletons, whereas there was no difference in
total placentomal weight between singleton and twin
pregnancies of nutrient-restricted UW ewes (Table 2).
In contrast, total placentome weight per gravid uterus
of Baggs ewes with twin pregnancies was greater (P <
0.05) than that of singleton pregnancies regardless of
dietary group. Placental efficiency (fetal weight/total
placentomal weight) was reduced (P = 0.04) in nutrientrestricted UW ewes when compared with control-fed
UW ewes on d 78 (0.37 ± 0.05 vs. 0.53 ± 0.05) but was
similar (P > 0.50) for nutrient-restricted and control-fed
Baggs ewes (0.50 ± 0.04 vs. 0.47 ± 0.02, respectively).
In singleton and twin pregnancies, placentomal morphology was similar for control-fed and nutrient-restricted UW ewes, which were predominantly type A
(Figure 3). In contrast, there was a marked reduction
(P < 0.05) in the percentage of type A placentomes and
increased percentages of type B, type C, and type D
placentomes in singleton and twin-bearing nutrientrestricted Baggs ewes compared with singleton and
twin-bearing control-fed Baggs ewes (Figure 3).
On d 78, blood glucose concentration of nutrient-restricted UW ewes and nutrient-restricted Baggs ewes
were markedly lower (P < 0.05) than those of controlfed UW ewes and control-fed Baggs ewes (Table 3).
Blood glucose concentrations were lower (P = 0.04) in
fetuses from control-fed Baggs ewes when compared
with those of fetuses from control-fed UW ewes on d
78. Whereas glucose concentrations in fetal blood of
nutrient-restricted UW ewes were reduced (P = 0.04)
when compared with glucose concentrations in the
blood of fetuses from control-fed UW ewes, concentrations of glucose in the blood of fetuses from nutrientrestricted and control-fed Baggs ewes were similar on
d 78. No differences were observed between glucose
concentration in the blood of singleton and twin fetuses
in any dietary group (data not shown).
DISCUSSION
To our knowledge, this is the first study to demonstrate that the environment, nutrition, or both under
which ewes are selected can alter physiological responses to a bout of maternal nutrient restriction. Further, although these ewes were of similar breeding
(Rambouillet/Columbia), considerable genetic differences may exist between the UW and Baggs ewes and
contribute to the differences observed. Fetal weights
were unaffected when Baggs ewes, selected under limited nutritional inputs and harsh environments, were
exposed to a bout of nutrient restriction from early to
midgestation. In contrast, UW ewes, selected with more
than adequate nutrition and optimal environments, experienced a 30% decrease in fetal weight to the same
bout of nutrient restriction. In the past few years, sev-
3456
Vonnahme et al.
Table 3. Maternal and fetal glucose concentration (mg/
dL) in University of Wyoming (UW) ewes and ewes located near Baggs, WY (Baggs) on d 78 of gestation
Dietary treatment1
Item
UW ewes3
Maternal glucose
Fetal glucose
Baggs ewes4
Maternal glucose
Fetal glucose
Control
fed
Nutrient
restricted
13
50a
17a
10
54a
13b
10
42b
13b
8
47b
14b
SE2
2
1
2
2
Row means with different superscripts differ (P < 0.05).
Groups of Baggs ewes and UW ewes were fed 100% (control fed)
or 50% (nutrient restricted) of NRC recommendations from d 28 to
78 of gestation.
2
Standard error based on 10 (UW) ewes and 8 (Baggs) ewes.
3
Selected to a sedentary lifestyle and more than adequate nutrition.
4
Selected to a nomadic lifestyle and limited nutrition.
a,b
1
Figure 3. Percentages of type A, B, C, and D placentomes on d 78 of gestation from (panel A) University of
Wyoming (UW) ewes selected to a sedentary lifestyle and
more than adequate nutrition and which were controlfed [100% of NRC (1985) recommendations, n = 13] or
nutrient-restricted [50% of NRC (1985) recommendations,
n = 10] from d 28 to 78 of gestation; or from (panel B)
Baggs, WY (Baggs) ewes selected to a nomadic lifestyle
and limited nutrition and which were control-fed [100%
of NRC (1985) recommendations, n = 10] or nutrientrestricted [50% of NRC (1985) recommendations, n = 8]
from d 28 to 78 of gestation. *Means ± SEM of nutrientrestricted ewes differ from those of control-fed ewes (P
< 0.05).
eral laboratories have evaluated the impact of maternal
nutrient restriction from early to midgestation on fetal
and placental development in the sheep (Clarke et al.,
1998; Osgerby et al., 1999; Vonnahme et al., 2003) and
have reported markedly different results. The reasons
for these differences may relate to differences in ewe
breed, age, or parity but may also relate to differences
in nutritional or environmental exposures of their experimental ewes during fetal or postnatal life. An understanding of how the 2 flocks of ewes utilized in this
study evolved in approximately 30 yr to be nutrient
responsive or nutrient resistant could be critically rele-
vant to our understanding of factors dictating nutrient
requirements during gestation.
Buhimschi et al. (2001) reported that the levels of
hypertension, intrauterine growth restriction, and fetal
death to nitric oxide inhibition during gestation were
significantly different in the same strain of rat purchased from different commercial suppliers. They concluded that these differences may represent subtle genetic or environmental differences between the 2 outbred colonies resulting in dramatic variations in
response to nitric oxide inhibition. One difference reported by Buhimschi et al. (2001) of possible relevance
to this study was that the diet fed to these 2 colonies
differed in relative amounts of several AA, minerals,
and vitamins. In this regard, Knight et al. (2005) reported different impacts of a 30% maternal nutrient
restriction from d 6.5 to 18.5 of gestation in 2 strains
of mice on fetal and placental size and offspring growth.
Further, differences in the susceptability of 2 outbred
rat strains to 2,3,7,8-teterachlorodibenzo-p-dioxin-induced fetal death and placental dysfunction have been
observed, despite the fact that they exhibited identical
primary structure of their aryl hydrocarbon receptor
(Kawakami et al., 2005). These studies suggest that
within or between breed (or strain) differences may
exist in the susceptibility of the conceptus to a variety
of maternal stressors.
Whereas nutrient-restricted UW and nutrient-restricted Baggs ewes lost BW and BCS throughout the
50-d nutrient restriction period, nutrient-restricted
Baggs ewes lost less BW and BCS than nutrient-restricted UW ewes. Although BW of UW ewes were
greater than Baggs ewes at the start of the study, this
difference appeared to be related to frame size because
BCS were similar across the 2 groups. It is speculated
that this difference in BW loss during nutrient restriction may be due to the Baggs ewes being more efficient
in feed conversion, utilization, or both when compared
with UW ewes. An increased efficiency of nutrient deliv-
Placental conversion and fetal growth in ewes
ery to the fetus by nutrient-restricted Baggs ewes when
compared with nutrient-restricted UW ewes is also suggested by the differential impacts of a prolonged maternal nutrient restriction on the weights of fetuses recovered on d 78 of gestation. Although fetuses recovered
from nutrient-restricted UW ewes were approximately
30% lighter than fetuses recovered from control-fed UW
ewes, there were no differences in fetal weight between
nutrient-restricted and control-fed Baggs ewes. This
ability to maintain fetal size in the face of restricted
nutrient supply does not result from differences in the
fetal/maternal weight ratio because fetal size on d 78 in
control-fed UW and control-fed Baggs ewes was directly
proportional to group differences in ewe BW averaging
0.32% of ewe BW in both groups. Because of the marked
decrease in fetal weight in the nutrient-restricted UW
ewes, the fetal/maternal weight ratio had decreased to
0.26% by d 78, whereas nutrient-restricted Baggs ewes,
which exhibited d 78 fetal weights similar to that of
their respective controls, averaged 0.38%.
Although major placentomal growth is complete by
midgestation (Ehrhardt and Bell, 1995; Reynolds et al.,
2005a,b), individual placentomes may undergo morphologic and functional transformations as fetal demand
for nutrients increases in the second half of gestation
(Hoet and Hanson, 1999; Osgerby et al., 2004). Results
from several laboratories demonstrate an increased
conversion of type A placentomes to B, C, or D placentomal types in undernourished ewes when compared with
their well-fed counterparts; however, this conversion
has been thought to occur only in late gestation in association with an exponentially growing fetus (Hoet and
Hanson, 1999; Osgerby et al., 2002; Osgerby et al.,
2004). Osgerby et al. (2004) reported that the conversion of type A placentomes to more advanced placentomal types was associated with a flattening of the placentome and an increased ratio in the area of interdigitated
maternal and fetal villi to unattached fetal allantochorion, which may enhance nutrient delivery to the fetus.
This conversion from type A through more advanced
placentomal types is also generally associated with an
increase in placentomal weight (Ford et al., 2004; Osgerby et al., 2004).
The apparent ability of the nutrient-restricted Baggs
ewes to supply the fetus with adequate nutrients for
normal growth and development may be facilitated by
the early conversion of type A placentomes to more
advanced types (B, C, and D). This concept is supported
by the observation that the failure of nutrient-restricted
UW ewes to convert their type A placentomes to more
advanced types was associated with significant fetal
growth restriction by d 78 of gestation. Similar to the
results obtained with nutrient-restricted UW ewes in
our study, Osgerby et al. (2002) reported that a less
severe 30% global nutrient restriction from d 26 of gestation in a group of multiparous Welsh mountain ewes,
which failed to affect placentomal growth or type by d
90, led to alterations in fetal organ growth. Interestingly, when these researchers allowed a group of these
3457
undernourished ewes to gestate their fetuses through
d 135 of gestation, they exhibited greater numbers of
type C and D placentomes than adequately fed control
ewes, but fetal BW and fetal organ weights remained
growth retarded. These data are consistent with the
concept that early placentomal conversion may be necessary to alleviate the impact of maternal nutrient restriction on the fetus.
Hoet and Hanson (1999) suggested that the advancement in placentomal morphologic type may be associated with a compensatory increase in placentomal vascularization in response to nutrient restriction. Our
laboratory has confirmed that as ovine placentomes
progress from type A through types B, C, and D, they
increase in size and vascularity (Ford et al., 2004), as
well as blood flow (Ford et al., 2006). On the basis of a
number of studies, increased blood flow at the fetalmaternal interface appears to be a primary determinant of increased transplacental exchange during the
later half of gestation in livestock species including the
sheep (Ford, 1995; Reynolds and Redmer, 1995; Reynolds et al., 2005a,b). The fact that placentome number
was reduced (P < 0.05) in nutrient-restricted Baggs
ewes carrying normal weight singleton fetuses and
tended to be reduced (P < 0.09) in nutrient-restricted
Baggs ewes carrying normal weight twins suggests that
the efficiency of individual placentomes must be increased with their conversion of type A placentomes to
types B, C, and D. Evidence suggests that placentome
number in the sheep is established by d 40 of gestation
and remains constant thereafter (Schneider, 1996;
Heasman et al., 1999). Further, the fact that both ewe
strains had a similar eviscerated BW in response to
nutrient restriction on a percentage basis (nutrientrestricted UW ewes were 86% of control-fed UW ewes
and nutrient-restricted Baggs ewes were 85% of controlfed Baggs ewes), suggests that the body stores of 2
strains were not affected differently by the nutrient
restriction. These data are again consistent with the
concept that the ability of nutrient-restricted Baggs
ewes to maintain a fetal size similar to their controlfed group is a function of their ability to convert type
A placentomes to more efficient forms.
Substrates such as glucose, which is a major energy
substrate for the fetus, can directly promote fetal
growth and development (Fowden, 1997). In the current
study, blood glucose concentrations were significantly
reduced in the blood of fetuses from nutrient-restricted
UW ewes when compared with fetuses from control-fed
UW ewes. In contrast, blood concentrations of glucose
were similar for fetuses of control-fed and nutrientrestricted Baggs ewes. The authors have no explanation
for the reduced glucose concentration in the blood of
fetuses gestated by control-fed Baggs ewes when compared with fetuses gestated by control-fed UW ewes.
This may be related to strain differences in fetal metabolic rate and thus glucose utilization, which were not
measured in this study but will be the subject of future
research. It should be noted, however, that because
3458
Vonnahme et al.
Baggs fetuses were 30% larger on d 78 than UW fetuses
from nutrient-restricted ewes, the total amount of glucose in fetal circulation of Baggs fetuses would actually
be higher than that of UW fetuses in the nutrientrestricted group. We have previously reported that essential AA concentrations were also reduced in fetal
blood of nutrient-restricted UW vs. control-fed UW
pregnancies on d 78 (Kwon et al., 2004), whereas no
differences in fetal blood AA concentrations were observed between nutrient-restricted and control-fed
Baggs pregnancies (Wu et al., 2005). In contrast, these
researchers reported that maternal concentrations of
essential AA were reduced on d 78 in nutrient-restricted
UW and Baggs ewes compared with their respective
control-fed groups. Together, these data are consistent
with the concept that the earlier conversion of type A
placentomes to more advanced placentomal types in
nutrient-restricted Baggs ewes helped to maintain normal concentrations of glucose and AA in fetal blood in
the face of a maternal decline. This is supported by the
fact that placental efficiency (fetal weight/total placentomal weight) was reduced in nutrient-restricted UW
ewes when compared with control-fed UW ewes on d 78
but was similar for control-fed and nutrient-restricted
Baggs ewes. Further, Osgerby et al. (2002) reported
that a 30% nutrient restriction from d 26 failed to alter
placentomal growth trajectory or type by d 90 of gestation, in association with a significant decrease in fetal
blood glucose in undernourished vs. control-fed ewes.
Again, these data support an association between the
ability of a ewe to alter placentomal growth and development during early gestation in response to early gestational nutrient restriction, and increased nutrient delivery to the fetus.
Evidence has linked reduced fetal growth with increased risk of cardiovascular disease in later life
(Barker et al., 1990). Whereas growth-retarded fetuses
from nutrient-restricted UW ewes in the current study
exhibited bilateral ventricular hypertrophy when compared with control-fed UW ewes, the normal size fetuses
from nutrient-restricted Baggs ewes did not. We have
previously demonstrated upregulation of genes inducing and regulating cellular growth and remodeling in
the hypertrophied ventricles of UW ewes subjected to
an identical bout of nutrient restriction as utilized in
this study (Han et al., 2004; Dong et al., 2005). Further,
when the nutrient-restricted UW ewes were realimented to 100% NRC from d 79 to term and allowed
to lamb, male offspring developed hypertension by 9
mo of age (Gilbert et al., 2005). These data suggest that
the growth restriction experienced by fetuses gestated
by nutrient-restricted UW ewes led to altered cardiovascular function in later life, which could affect production efficiency and health.
IMPLICATIONS
We have observed different placental responses to
a nutritional challenge during pregnancy in ewes of
similar breeding but chronically adapted to both different environments and nutrition. The central difference
is that ewes adapted to both harsh environments and
limited nutrition may initiate the early conversion of
type A placentomes to more efficient placentomal types
when subjected to an early to mid-gestational nutrient
restriction, thereby facilitating nutrient delivery to
their developing fetus. These data are consistent with
a possible alleviating effect of early placentomal conversion on fetal growth and development in nutrient-restricted Baggs ewes. The exact process that allows
Baggs ewes, but not University of Wyoming ewes, to
initiate the earlier conversion of placentomes to more
advanced types when subjected to early maternal nutrient restriction is unknown at the present time but warrants further investigation.
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