Embryo and Yolk Compositional Relationships in Broiler Hatching Eggs
During Incubation1,2
E. DAVID PEEBLES,*,3 LUMU LI,*,4 SHERMAN MILLER,* TOMAS PANSKY,* SHARON WHITMARSH,*
MICKEY A. LATOUR,*,5 and PATRICK D. GERARD†
*Department of Poultry Science and †Experimental Statistics Unit, Mississippi State University,
Mississippi State, Mississippi 39762
ABSTRACT Developmental relationships between
yolk, embryo body, and embryo liver compositions during incubation were determined in two trials. In Trial 1,
embryo body moisture, fat, and CP contents and embryo
liver moisture and fat contents were determined. In Trial
2, relative yolk weights, moisture, fat, and fatty acid contents, relative wet and dry embryo weights and moisture
contents, and relative wet and dry liver weights and moisture contents were determined. In Trial 1, embryo moisture decreased sigmoidally between Days 6 and 21,
whereas embryo fat increased between Days 12 and 21 of
incubation; embryo CP displayed sequential fluctuations
throughout incubation. However, an overall significant
decrease in embryo CP occurred between Days 6 and
21. Liver fat content increased between Days 12 and 21,
whereas liver moisture decreased through Day 18, with
a subsequent increase by Day 21. In Trial 2, relative yolk
weight and moisture content decreased, whereas percentage yolk lipid content increased between Days 6 and 15.
Relative wet and dry embryo weights changed in a similar
manner, with rapid increases between Days 12 and 18 of
incubation. Embryo moisture and CP were negatively
correlated to embryo fat content. Furthermore, relative
embryo and liver DM were related to yolk palmitic acid
concentration, whereas yolk oleic acid was correlated
only with liver DM. In conclusion, embryos and their
livers displayed differential accumulations of moisture
and DM during incubation, and these differences exhibited distinctive associations with various yolk fatty acids.
(Key words: carcass fat, carcass protein, embryo, liver, yolk fatty acids)
1999 Poultry Science 78:1435–1442
INTRODUCTION
The chemical nature of whole yolk in hen’s eggs (Burley
and Vadehra, 1989) and the transfer, utilization, and deposition of yolk lipids, including fatty acids, during incubation in avian eggs (Noble and Cocchi, 1990; Whitehead,
1991) have been described. Carbohydrate and protein metabolism predominates during the first 2 wk of incubation,
with the last 7 d of incubation being noted as an intense
period of lipid metabolism and rapid embryo growth in
which some 80% of the entire lipid content is mobilized
and absorbed into the embryonic tissues (Romanoff, 1960;
Noble and Cocchi, 1990). The transfer of fat from the
Received for publication December 21, 1998.
Accepted for publication July 1, 1999.
1
This is Journal Article Number J-9430 from the Mississippi Agricultural and Forestry Experiment Station supported by MIS-2990.
2
Use of trade names in this publication does not imply endorsement
by Mississippi Agricultural and Forestry Experiment Station of the products, nor similar ones not mentioned.
3
To whom correspondence should be addressed: [email protected]
4
Current address: Institute of Animal and Veterinary Sciences, Anhui
Academy of Agricultural Sciences, Hefei, Anhui 230031, P.R. China.
5
Current address: Department of Animal Sciences, 1151 Lilly Hall,
Purdue University, West Lafayette, IN 47907-1151.
yolk to the embryo is also associated with notable fat
alterations prior to deposition (Noble and Cocchi, 1990).
Noble and Connor (1984) have discussed lipid compositional changes that occur in embryos during the last week
of incubation. Furthermore, the fatty acid compositions
of various fats in the yolk, liver, and extrahepatic tissues of avian embryos (Noble and Moore, 1964) and the
interconversions of fatty acids in the yolk sac membrane
during embryogenesis (Noble and Shand, 1985) have
been investigated.
Ar (1991) reported in detail the compartmentalization
and movement of water within the egg during incubation.
During the fast growth phase, the influx of water to the
embryo from other compartments increases with an increase in the osmotic pressure of the embryo in comparison with that of the rest of the egg. This influx of water
is largely a result of an increase in electrolyte concentration in embryonic tissues through active transport (Ar,
1991). The uptake and utilization of lipids in embryos
Abbreviation Key: ECP = percentage embryo CP content; EL = percentage embryo lipid content; EM = percentage embryo moisture content; LL = percentage liver lipid content; LM = percentage liver moisture
content; YL = percentage yolk lipid content; YM = percentage yolk
moisture content.
1435
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PEEBLES ET AL.
may also have an inverse relationship with tissue water
content. The water content of hatching chicks and residual
yolk sacs are affected by incubational egg water loss; the
moisture content is more tightly maintained in the yolk
than in the embryo (Tullett and Burton, 1982). However,
near the time of hatch, associated increases in eggshell
porosity and dry embryo weight follow hyperbolic functions (Burton and Tullett, 1985).
There is, nevertheless, a paucity of information describing associated changes in the moisture and organic compositions of embryonic and extra-embryonic tissues
throughout incubation. These temporal relationships are
important to consider to predict more accurately subsequent effects that changes in the incubational environment may have on embryonic composition and metabolism. This study was, therefore, conducted to examine
specifically relationships among moisture, fat, and CP
contents in the yolk, embryo, and embryonic liver during incubation.
MATERIALS AND METHODS
General
In two trials, commercial broiler eggs (Avian female ×
Peterson male) were obtained from a local company and
set for incubation at the Mississippi State University Poultry Research Center within 24 h of being layed. The broiler
eggs were from hens that were 38 or 39 and 30 or 40 wk
of age in Trials 1 and 2, respectively; hens of different
ages belonged to different flocks. Approximately 1,100
eggs in Trial 1 and 1,020 eggs in Trial 2 were incubated
in a Model 5 incubator6. In both trials, eggs were randomly set with regard to hen age and incubated for 18
d at a dry bulb temperature of 37.5 C and a wet bulb
temperature of 28.9 C. On Day 18 of incubation, the eggs
were transferred to hatcher trays in a Jamesway Model
252 B incubator7 for hatching. The dry and wet bulb temperatures of the hatcher were maintained at 37.5 and 30.0
C, respectively. All temperatures were recorded twice
daily.
Trial 1
Samples were collected at Days 6, 9, 12, 15, 18, and 21
of incubation. Between Days 6 and 18, embryos in eggs
from each of six incubator trays were collected, and, at
Day 21, chicks from each of six hatcher trays were collected. There were approximately 180 eggs that were set
in each tray, and trays with eggs occupied only the middle
portion of the incubator. Because this procedure might
have caused the air flow rate to be different between trays
(French, 1997), equal numbers of embryos or chicks were
sampled from each tray to prevent an incubational stratification effect on the parameters examined. The number
6
Petersime Incubator Co., Gettysburg, OH 45318.
Butler Manufacturing Co., Fort Atkinson, WI 53538.
7
of embryos or chicks sampled from each tray at Days 6,
9, 12, 15, 18, and 21 equalled 49, 20, 5, 2, 1, and 1, respectively. The differences in sampling size were accounted
for in modeling of results. Multiple samples from each
tray were pooled prior to analysis. Wet and dry embryo
weights and total body moisture, CP, and fat contents
were determined. For total embryo fat and CP, two replicate samples per pooled dry embryo sample were analyzed separately and were then averaged prior to data
analysis. At Days 12, 15, 18, and 21 of incubation, another
group of embryos or chicks from each of the six trays
were collected to determine wet and dry liver weights
and liver moisture and fat contents. The number of livers
sampled from each tray at Days 12, 15, 18, and 21 equalled
48, 20, 10, and 6, respectively. Samples from each tray
were, similarly, pooled prior to analysis; variation in sample size was accounted for in modeling as previously
mentioned.
Eggs were opened, and embryos were separated from
all surrounding egg contents (Peebles et al., 1998). The
embryos and chicks were killed by cervical dislocation.
Embryos and organs were blotted on an absorbent paper
towel to remove excess moisture before obtaining a wet
weight. Embryos and livers were dried at 85 C for 96 h
or until moisture loss ceased. The dry embryos and livers
were then allowed to reach room temperature for 2 h
before obtaining a dry weight (Peebles et al., 1998). Embryo and liver moisture contents were calculated as the
difference between their wet and dry weights and were
expressed as a percentage of wet embryo weight or liver
weight, respectively. Embryo body CP and embryo body
and liver lipid contents were determined as described by
Latour et al. (1994). Crude protein and lipid concentrations were expressed as percentages of total DM.
Trial 2
Eggs were incubated and sampled from trays as in Trial
1. There were approximately 170 eggs that were set per
tray. However, at Day 0, eggs in each tray, randomly
designated for sampling, were weighed and labeled.
Three eggs were collected from each of six incubator trays
at 6, 9, 12, 15, and 18 d for wet and dry embryo weights
and embryo moisture content and at 12, 15, and 18 d for
wet and dry liver weights and liver moisture content.
Livers were removed from embryos after determination
of total wet embryo weight and were then combined
with embryos after drying for determination of total dry
embryo weight. Embryo and organ preparation prior to
weighing was as described in Trial 1. Of the three eggs
sampled per tray at each time period, two at Day 6 and
one at Days 9, 12, and 15 were also used for determination
of yolk lipid content. One egg of those sampled for yolk
lipid content was further analyzed for yolk fatty acid
content. Total yolk palmitic, stearic, oleic, palmitoleic,
linoleic, and arachidonic acid concentrations were determined. Total lipid extractions and determinations of fatty
acid concentrations in the yolk were as described by Latour et al. (1998).
EMBRYO AND YOLK COMPOSITION IN BROILER HATCHING EGGS
A separate egg from each tray was also sampled at
Days 6, 9, 12, and 15 for determination of wet and dry yolk
weights and yolk moisture content. Total yolk weight
included the weight of the yolk sac membrane; separation
of the yolk sac and embryo was as described by Peebles
et al. (1998). As in Trial 1, multiple samples from each
tray during incubation were pooled prior to analysis.
Tissue moisture levels were determined as in Trial 1 and
were expressed as percentages of wet weight. Wet and
dry embryo weights were expressed as percentages of
total preset egg weight, because variation in embryo and
chick weights may, in part, be accounted for by the weight
of the egg prior to incubation and by the weight loss of
the egg during the incubation period (Tullett and Burton,
1982). Yolk weight was, similarly, expressed as a percentage of Day 0 egg weight. Total yolk lipid was expressed
as a percentage of total yolk weight, and fatty acid concentrations were expressed as percentages of total yolk fatty
acids. Wet and dry liver weights were expressed as percentages of wet and dry embryo weights, respectively.
1437
(P ≤ 0.001) between Days 9 and 12 then constantly increased throughout the rest of incubation with increases
occurring between Days 12 and 15 (P ≤ 0.0003), Days 15
Statistical Analysis
In both trials, a completely randomized design was
utilized. Within each trial, correlation analysis was performed only on those parameters that were determined
on common embryos. Correlations between parameters
were determined at each day by Pearson Product Moment
Correlation (Steel and Torrie, 1980). Functional relationships between each variable and day of incubation were
examined graphically, utilizing spline smoothing. Differences caused by day of incubation were analyzed using
the Mixed procedure (Littell et al., 1996). In Trial 1, differences in variability across days as a result of sample size
variation were accounted for by modeling variances separately for each day. Angular transformations (arc sine of
the square root of the proportion affected) were performed on all percentage data prior to analysis (Steel and
Torrie, 1980). A three-parameter logistic growth model
(Ratowsky, 1983) was fit for absolute wet embryo weight
separately for each trial using the NLIN procedure. Statistical procedures were those of the SAS Institute (1994).
Statements of significance were based on P ≤ 0.05 unless
otherwise indicated.
RESULTS
Trial 1
Percentage embryo moisture content (EM) decreased
sigmoidally throughout incubation (Figure 1a). Significant changes occurred between each consecutive time
period examined, with the smallest and greatest changes
being displayed between Days 6 and 9 and Days 12 and
15, respectively. Levels of significance among all time
periods examined were P ≤ 0.0001, except between Days
15 and 18, which was P ≤ 0.001. Percentage body lipid
content of embryos (EL) followed a quadratic curve response during incubation (Figure 1b). Body fat decreased
FIGURE 1. Spline curves of percentage embryo or chick a) moisture
content (EM; r2 = 0.9764), b) lipid content (EL; r2 = 0.7068), and c) CP
content (ECP; r2 = 0.2794) at Days 6, 9, 12, 15, 18, and 21 of incubation
in Trial 1 (n = 6 at each day).
1438
PEEBLES ET AL.
moisture and fat content at 21 d were negatively correlated (r = −0.94; P ≤ 0.005).
Trial 2
FIGURE 2. Spline curves of percentage embryo or chick liver a)
moisture content (LM; r2 = 0.9787) and b) lipid content (LL; r2 = 0.9896)
at Days 12, 15, 18, and 21 of incubation in Trial 1 (n = 6 at each day).
and 18 (P ≤ 0.0003), and Days 18 and 21 (P ≤ 0.04). The
percentage CP content of embryos (ECP) displayed a sinusoidal pattern of change with repetitive fluctuations
(Figure 1c). However, an overall decrease (P ≤ 0.0005)
occurred between 6 and 21 d of incubation. A decrease
(P ≤ 0.005) and increase (P ≤ 0.01) also occurred between
Days 6 and 9 and Days 9 and 12, respectively. Percentage
liver moisture (LM) displayed a sigmoidal curve, with
large decreases occurring between Days 12 and 15 (P ≤
0.0001) and Days 15 and 18 (P ≤ 0.0001) (Figure 2a). No
change (P = 0.75) was noted between Days 18 and 21.
Percentage liver lipid content (LL) also displayed a sigmoidal curve (Figure 2b), with increases (P ≤ 0.0001) between all consecutive time periods examined.
The ECP was positively correlated with embryo moisture at Days 15 (r = 0.84; P ≤ 0.03) and 18 (r = 0.85; P ≤ 0.03)
and was negatively correlated with embryo fat content at
Day 21 (r = −0.92; P ≤ 0.01). Furthermore, chick body
Relative yolk weight (Figure 3a) and moisture content
(Figure 3b) decreased in a similar manner (P ≤ 0.0001)
between 6 and 15 d of incubation. Levels of significance
for respective decreases in relative yolk weight and yolk
moisture content were P ≤ 0.004 and P ≤ 0.04 between
Days 6 and 9, P ≤ 0.0001 and P ≤ 0.0001 between Days 9
and 12, and P ≤ 0.003 and P ≤ 0.02 between Days 12 and
15. Conversely, percentage yolk lipid content (YL) (Figure
3c) increased (P ≤ 0.0006) between 6 and 15 d of incubation. There were no significant increases observed in any
of the intervening consecutive time periods. Relative wet
(Figure 4a) and dry (Figure 4b) embryo weights (percentages of Day-0 egg weight) increased similarly (P ≤ 0.0001)
between 6 and 18 d. There were increases (P ≤ 0.0001)
between all consecutive time periods examined after Day
9 for both parameters. Between 6 and 9 d, only relative
wet embryo weight increased (P ≤ 0.004).
Results of correlation analyses included relationships
between relative wet and dry weights of embryos and
their livers, relative weights and moisture contents of
embryos and livers, yolk lipid concentrations and relative
wet and dry weights of embryos and livers, and yolk
concentrations of various fatty acids. Percentage liver DM
(percentage of embryo DM) at Day 15 was negatively
correlated with relative wet (r = −0.86; P ≤ 0.03) and dry
(r = −0.86; P ≤ 0.03) embryo weights (percentages of Day0 egg weight) but, at Day 18, was positively correlated
(r = 0.90; P ≤ 0.02) with relative wet embryo weight.
Embryo moisture content was positively correlated with
relative wet embryo weight at 6 (r = 0.89; P ≤ 0.02) and
18 d (r = 0.92; P ≤ 0.01), but had a negative relationship
with relative wet embryo weight at Day 15 (r = −0.85; P
≤ 0.03). Relative liver weight and moisture content were
positively related at Days 12 (r = 0.98; P ≤ 0.001), 15 (r =
0.89; P ≤ 0.02), and 18 (r = 0.84; P ≤ 0.04).
At Day 15 of incubation, relative dry liver weight was
positively correlated with yolk palmitic acid (r = 0.91; P
≤ 0.03) but negatively correlated with yolk oleic acid (r
= −0.97; P ≤ 0.006) concentration. Furthermore, yolk palmitic acid concentration was negatively correlated with
relative embryo DM at 9 (r = −0.90; P ≤ 0.04) and 12 d (r
= −0.96; P ≤ 0.01) and was negatively correlated with
relative wet embryo weight at 12 d (r = −0.95; P ≤ 0.01).
Yolk stearic acid concentration was negatively correlated with linoleic acid at Day 6 (r = −0.96; P ≤ 0.04) and
oleic acid at Day 12 (r = −0.88; P ≤ 0.05), but was positively
correlated with arachidonic acid at Days 6 (r = 0.96; P ≤
0.04) and 15 (r = 0.92; P ≤ 0.02). Linoleic acid was negatively correlated with oleic acid at Day 9 (r = −0.95; P ≤
0.01) and arachidonic acid at Day 6 (r = −0.95; P ≤ 0.05).
Oleic acid was negatively correlated with palmitic acid
at Day 15 (r = −0.95; P ≤ 0.01) and arachidonic acid at
EMBRYO AND YOLK COMPOSITION IN BROILER HATCHING EGGS
1439
FIGURE 4. Spline curves of a) wet embryo weight as a percentage
of Day-0 egg weight (WEW; r2 = 0.9941) and b) dry embryo weight as
a percentage of Day-0 egg weight (DEW; r2 = 0.9970) at Days 6, 9, 12,
15, and 18 of incubation in Trial 2 (n = 6 at each day).
Because eggs from Trials 1 and 2 were incubated at
different times, the repeatability of data between trials
was examined by fitting absolute wet embryo weight
during incubation in each trial to a three-parameter (α,
β, and γ) logistic growth model [absolute wet embryo
weight = α/(1 + exp (β − γ × day))]. Analysis showed
that there were no differences between parameters fit for
each trial (P = 0.34, 0.20, and 0.19). This lack of differences
would indicate that the embryos in each trial grew similarly and that the relationships among yolk, embryo, and
liver at each day of sample collection should be comparable.
FIGURE 3. Spline curves of a) wet yolk weight as a percentage of
Day-0 egg weight (WYW; r2 = 0.9341), b) percentage yolk moisture
content (YM; r2 = 0.8879), and c) percentage yolk lipid content (YL; r2
= 0.5434) at Days 6, 9, 12, and 15 of incubation in Trial 2 (n = 6 at each day).
Day 12 (r = −0.89; P ≤ 0.04). In addition, yolk palmitoleic
acid was negatively correlated with arachidonic acid at
Day 9 (r = −0.92; P ≤ 0.03).
DISCUSSION
Somatic Composition Patterns
The initial water concentration in the young embryo is
about 95% and declines with development and maturation (Romanoff, 1967). Similarly, EM in this study de-
1440
PEEBLES ET AL.
creased as the embryo grew during incubation. This
change occurred as relative wet embryo weight increased
and relative yolk weight and yolk moisture content decreased. Davis and Ackerman (1987) concluded that embryo growth per se, as determined by increased dry body
mass, is not altered in response to a change in the amount
of water available in the egg. In the present investigation,
embryo moisture content changed little prior to Day 12
of incubation, but, after that time, it appeared that the
relative contribution of DM to total embryo mass began
to increase at a rapid rate. A negative correlation between
relative embryo moisture and wet embryo weight (percentage of Day-0 egg weight) was observed at Day 15.
Nevertheless, positive correlations between relative wet
embryo weight and embryo moisture content at Days 6
and 18 would implicate a significant contribution of water
toward embryo mass throughout incubation, although its
relative contribution to total mass decreased with embryonic growth.
Water transport from the albumen to the yolk occurs
primarily during the first 20% of incubation (Ar, 1991).
Observed significant decreases in percentage yolk moisture (YM) after 6 d, and particularly after 9 d, would,
therefore, be expected to result from decreased water
transport from the albumen to the yolk, with concurrent
uptake of yolk-derived water into the embryo and its
ultimate loss through pores in the eggshell. The continual
loss of water through the eggshell, along with the compartmentalization of water throughout the egg, serve in
embryonic osmoregulation despite the maintenance of a
consistent total egg water content with the oxidation of
yolk and the production of metabolic water (Ar, 1991;
Davis et al., 1988).
Embryonic uptake of water from other compartments
in the egg during the last half of incubation is osmotically
driven (Ar, 1991). The allantoic compartment stores and
recycles fluid to help prevent large changes in embryo
water content (Hoyt, 1979) and to provide the embryo
with water necessary for its growth (Ar, 1991); however,
water accumulation in the embryo is regulated so as to
not create an osmotic stress to the blood or amniotic fluid
(Ar, 1991). Furthermore, moisture is maintained in the
yolk over that in the embryo when fluid reserves in the
egg decrease just prior to hatch (Tullett and Burton, 1982).
Davis and Ackerman (1987) have noted that wet masses
of yolk and albumen were not affected during the last
week of incubation by increased water loss in chicken
eggs; however, Davis et al. (1988) have reported the accumulation of excess albumen and a slower decline in allantoic fluid volume in eggs with low rates of incubational
water loss.
Decreased relative yolk weight during the last 1 to 2
wk of incubation is apparently largely due to an increase
in YM loss. Moreover, total body water volume of the
embryo increases during embryogenesis (Whitehead,
1991). In the present experiments, however, as relative
moisture declined, percentage body fat content of the
embryos increased and, integral to a period of rapid
growth between Days 15 and 21, the body fat content of
the embryo underwent its greatest increases. In addition,
relative embryo DM and CP were negatively correlated
at Days 15 and 18, and, by 21 d, ECP was lowest, and fat
content was greatest. These relationships were further
accentuated because embryo fat content had significant
negative correlations with CP and moisture and a positive
correlation with embryo DM at Day 21. Therefore, as the
embryo matured past Day 12, an accelerated contribution
of DM to total embryo mass occurred, with an increased
percentage of DM as fat by Day 15 and after. Thus, with
the rapid absorption of yolk lipid by the embryo during
the last week of incubation, associated accumulations of
lipid occur in embryonic tissues. Noble and Cocchi (1990)
have reported that, by Day 21, lipid may account for as
much as 25 to 30% of embryo DM. Stored fat during
growth may serve as an energy source, especially during
the period prior to the time a chick receives feed.
Liver Composition Patterns
During rapid yolk uptake, there is a large accumulation
of lipid in the liver, so that, by Day 19, liver lipid accounts
for approximately 5% of that absorbed from the yolk
(Noble and Cocchi, 1990). Similarly, results from the present study show that liver lipid accumulation is most rapid
between 15 and 18 d, with an increase of more than 15%
of liver DM, which was coincidental with a decrease in
relative moisture content between 15 and 18 d. Nevertheless, relative liver weight is positively influenced by moisture content as exemplified by positive correlations between the two parameters at Days 12, 15, and 18. The
greatest increase in absolute liver weight occurred between 18 and 21 d. This accelerated increase may be associated with the greater metabolic role that the embryonic
liver must assume, particularly in increased linoleic acid
desaturation activity coincident with its decrease in the
yolk sac membrane (Noble and Shand, 1985). Ding and
Lilburn (1996) found in turkey poults that all yolk sac
fatty acids declined greatly during the second half of incubation.
Yolk Fatty Acid, Liver, and
Somatic Contents
In the present study, percentage embryo DM accumulation was related to fatty acid concentration in the yolk.
Relative embryo DM (percentage of preset egg weight)
was negatively correlated, and EM was positively correlated, with yolk palmitic acid concentration at Days 9 and
12. Additionally, yolk palmitic acid concentration was
negatively correlated with relative wet embryo weight at
Day 12. These relationships suggest that as the embryo
absorbs yolk fat during those times, a concomitant decrease in yolk palmitic acid may occur. Furthermore, the
utilization of yolk palmitic acid with embryonic growth
during the second week of incubation may be necessary
for increased embryo DM synthesis. On the other hand,
yolk palmitic acid was positively correlated, and oleic
acid was negatively correlated, with relative liver DM at
EMBRYO AND YOLK COMPOSITION IN BROILER HATCHING EGGS
Day 15, suggesting that liver DM accumulation at this
later time may involve the metabolism of yolk oleic acid
and that the utilization of yolk oleic acid may be important to increased liver DM synthesis. Noble et al. (1988)
observed that oleic acid concentration in the yolk sacs of
chick embryos decreased with age. Ding and Lilburn
(1996) also found that oleic acid comprised the largest
proportion of total yolk fatty acids and that it largely
increased in the liver of the embryo during incubation.
Noble (1987) has further described a particularly high
content of oleic acid (70 to 75% of total long-chain fatty
acids) present in chick embryo liver cholesterol esters.
Donaldson (1981) found that among de novo synthesized fatty acids, unsaturated fatty acids can be converted
to saturated fatty acids, but the saturated fatty acids cannot be changed to unsaturated forms in embryonic tissues, such as the liver. This limitation might have promoted the selective uptake of oleic acid, an unsaturated
form, from the yolk at Day 15 in the present experiments.
Embryo and liver DM increases were not associated with
yolk concentrations of all of the same types of fatty acids.
Furthermore, the relationships of DM in the whole embryo and its liver to fatty acid absorption changed during incubation.
A negative correlation between yolk linoleic and arachidonic acids on Day 6 supports earlier findings that arachidonic acid is derived from linoleic acid through appreciable ∆6-desaturation activity of the yolk sac membrane
(Noble and Cocchi, 1990; Noble and Shand, 1985). As
more arachidonic acid is produced by the yolk sac membrane, the level of linoleic acid in the yolk would be
expected to decrease. A negative correlation between yolk
stearic and oleic acids at Day 12 would also support earlier
evidence of ∆9-desaturase activity in the yolk sac membrane, converting stearic to oleic acid (Noble and Cocchi,
1990; Noble and Shand, 1985). Furthermore, yolk stearic
acid content was positively correlated with arachidonic
acid at Days 6 and 15 and was negatively correlated with
linoleic acid at Day 6. Oleic and arachidonic acids were
negatively correlated at Day 12, and oleic and linoleic
acids were negatively correlated at Day 9. These relationships may, in part, be due to lower rates of absorption
and conversion of stearic acid relative to that for linoleic
acid, as arachidonic acid is among other long-chain polyunsaturated fatty acids preferred by the embryo (Cherian
and Sim, 1993). Noble and Shand (1985) have noted high
levels of both stearic and linoleic acid desaturation in the
yolk sac membrane at Day 15 of incubation. However,
the absence of these relationships after Day 15 may also
be due to the decrease in activity of both desaturation
enzyme systems in the yolk sac membrane with the approach of hatch (Noble and Cocchi, 1990; Noble and
Shand, 1985).
There were negative correlations between palmitic and
oleic acids at Day 15 and palmitoleic and arachidonic
acids at Day 9. However, no negative correlation was
noted between palmitic and palmitoleic acids. These results may be due to the lack or minimal conversion of
palmitic to palmitoleic acid through ∆9-desaturation by
1441
the yolk sac membrane, the differential utilization of palmitic and oleic acids by the liver at Day 15, and the
differential absorptions of palmitoleic and arachidonic
acids by the embryo at Day 9.
Conclusion
Our data demonstrate that changes in embryo mass
during incubation were related to differential accumulations of moisture and DM and that the relative proportions of total DM and DM fat content increased toward
hatch. Not only is embryo mass influenced by the uptake
of moisture and fat from the yolk, but, in this study,
embryo and liver DM percentages were associated with
different fatty acid fractions in yolk lipid. It is suggested
that this differential influence is significant to embryonic
composition and metabolism because of the liver’s role in
fatty acid interconversions and oxidation during growth.
The yolk and embryo compositional relationships described in this study may also serve to predict more accurately potential environment-embryo interactions
throughout the incubational period.
ACKNOWLEDGMENTS
This work was funded in part by a grant from the
U.S. Poultry and Egg Association and by the Mississippi
Agricultural and Forestry Experiment Station. The authors also express appreciation to Janice Orr for her expert
secretarial assistance in the preparation of this manuscript.
REFERENCES
Ar, A., 1991. Egg water movements during incubation. Pages
157–173 in: Avian Incubation. S. G. Tullett, ed. ButterworthHeinemann Ltd., London, UK.
Burley, R. W., and D. V. Vadehra, 1989. The macroscopic structure, physical properties, and chemical composition of avian
eggs. Pages 1–17 in: The Avian Egg Chemistry and Biology.
A Wiley-Interscience Publication, New York, NY.
Burton, F. G., and S. G. Tullett, 1985. The effects of egg weight
and shell porosity on the growth and water balance of the
chicken embryo. Comp. Biochem. Physiol. 81A(2):377–385.
Cherian, G., and J. S. Sim, 1993. Net transfer and incorporation
of yolk n-3 fatty acids into developing chick embryos. Poultry
Sci. 72:98–105.
Davis, T. A., and R. A. Ackerman, 1987. Effects of increased
water loss on growth and water content of the chick embryo.
J. Exp. Zool. Suppl. 1:357–364.
Davis, T. A., S. S. Shen, and R. A. Ackerman, 1988. Embryonic
osmoregulation: Consequences of high and low water loss
during incubation of the chicken egg. J. Exp. Zool.
245:144–156.
Ding, S. H., and M. S. Lilburn, 1996. Characterization of changes
in yolk sac and liver lipids during embryonic and early posthatch development of turkey poults. Poultry Sci. 75:478–483.
Donaldson, W. E., 1981. Lipid metabolism in chick embryos.
Poultry Sci. 60:1964–1970.
French, N. A., 1997. Modeling incubation temperature: The effects of incubator design, embryonic development, and egg
size. Poultry Sci. 76:124–133.
Hoyt, D. F., 1979. Osmoregulation by avian embyos: The allantois functions like a toad’s bladder. Physiol. Zool. 52:354–362.
1442
PEEBLES ET AL.
Latour, M. A., E. D. Peebles, C. R. Boyle, and J. D. Brake, 1994.
The effects of dietary fat on growth performance, carcass
composition, and feed efficiency in the broiler chick. Poultry
Sci. 73:1362–1369.
Latour, M. A., E. D. Peebles, S. M. Doyle, T. Pansky, T. W. Smith,
and C. R. Boyle, 1998. Broiler breeder age and dietary fat
influence the yolk fatty acid profiles of fresh eggs and newly
hatched chicks. Poultry Sci. 77:47–53.
Littell, R. C., G. A. Milliken, W. W. Stroup, and R. D. Wolfinger,
1996. SAS威 System for Mixed Models. SAS Institute Inc.,
Cary, NC.
Noble, R. C., 1987. Lipid metabolism in the chick embryo: Some
recent ideas. J. Exp. Zool. Suppl. 1:65–73.
Noble, R. C., and M. Cocchi, 1990. Lipid metabolism and the
neonatal chicken. Prod. Lipid Res. 29:107–140.
Noble, R. C., and K. Connor, 1984. Lipid metabolism in the
chick embryo of the domestic fowl (Gallus domesticus).
World’s Poult. Sci. 40:114–120.
Noble, R. C., and J. H. Moore, 1964. Studies on the lipid metabolism of the chick embryo. Can. J. Biochem. 42:1729–1741.
Noble, R. C., D. Ogunyemi, and S. G. Tullett, 1988. Understanding the chick embryo. V. A close view of the uptake of yolk
fat. Poult. Misset Int. 4(5):32–33.
Noble, R. C., and J. H. Shand, 1985. Unsaturated fatty acid
compositional changes and desaturation during the embryonic development of the chicken. Lipids 20:278–282.
Peebles, E. D., T. Pansky, S. M. Doyle, C. R. Boyle, T. W. Smith,
M. A. Latour, and P. D. Gerard, 1998. Effects of dietary fat
and eggshell cuticle removal on egg water loss and embryo
growth in broiler hatching eggs. Poultry Sci. 77:1522–1530.
Ratowsky, D. A., 1983. Sigmoidal growth models. Pages 61–
91 in: Nonlinear Regression Modeling. Marcel-Dekker, Inc.,
New York, NY.
Romanoff, A. L., 1960. The Avian Embryo. Structural and Functional Development. MacMillan Co., New York, NY.
Romanoff, A. L., 1967. Biochemistry of the Avian Embryo. Macmillan, New York, NY.
SAS Institute, 1994. SAS威 User’s Guide: Basic (Release 6.08 Edition). SAS Institute, Inc., Cary, NC.
Steel, R.G.D., and J. H. Torrie, 1980. Principles and Procedures
of Statistics. A Biometric Approach. 2nd ed. McGraw-Hill
Book Co., New York, NY.
Tullett, S. G., and F. G. Burton, 1982. Factors affecting the weight
and water status of the chick at hatch. Br. Poult. Sci.
23:361–369.
Whitehead, C. C., 1991. Nutrition of the breeding bird and developing embryo. Pages 227–238 in: Avian Incubation. S. G.
Tullett, ed. Butterworth-Heinemann Ltd., London, UK.