Download Important metabolic pathways in poultry embryos prior to hatch

World's Poultry Science Journal
http://journals.cambridge.org/WPS
Additional services for World's Poultry Science Journal:
Email alerts: Click here
Subscriptions: Click here
Commercial reprints: Click here
Terms of use : Click here
Important metabolic pathways in poultry embryos prior to hatch
J.E. DE OLIVEIRA, Z. UNI and P.R. FERKET
World's Poultry Science Journal / Volume 64 / Issue 04 / December 2008, pp 488 ­ 499
DOI: 10.1017/S0043933908000160, Published online: 12 November 2008
Link to this article: http://journals.cambridge.org/abstract_S0043933908000160
How to cite this article:
J.E. DE OLIVEIRA, Z. UNI and P.R. FERKET (2008). Important metabolic pathways in poultry embryos prior to hatch. World's Poultry Science Journal, 64, pp 488­499 doi:10.1017/
S0043933908000160
Request Permissions : Click here
Downloaded from http://journals.cambridge.org/WPS, IP address: 128.139.15.19 on 29 Oct 2012
doi:10.1017/S0043933908000160
Important metabolic pathways in poultry
embryos prior to hatch
J.E. DE OLIVEIRA1, Z. UNI2 and P.R. FERKET1*
1
North Carolina State University, Department of Poultry Science, Raleigh, NC, USA
27695; 2Hebrew University, Faculty of Agriculture, Department of Animal Science,
Rehovot, Israel 76100
*Corresponding author: [email protected]
Growth performance and meat yield of commercial broilers and turkeys has
improved linearly each year during the past four decades (Havenstein et al.,
2003b; Havenstein et al., 2003a; Havenstein et al., 2007), and this trend is likely
to continue in the future as new technologies in genetics, biotechnology and
developmental biology are adopted by the poultry industry. As the time it takes
meat birds to achieve market size decreases, the period of embryonic development
becomes a greater proportion of a bird's productive life. Therefore, incubation and
embryonic development towards hatch is of greater relative importance to the
successful rearing of meat poultry than ever before (Hulet 2007; Foye et al.,
2007b). Consequently, anything that supports or limits growth and development
during the incubation period will have a marked effect on overall growth
performance and health of modern strains of meat poultry. Many poultry
researchers now realize that future gains in genetic and production potential of
poultry will come from advancements made during the incubation period and
embryogenesis (Elibol et al., 2002; Peebles et al., 2005; Christensen et al., 2007;
Collin et al., 2007; Leksrisompong et al., 2007). The urgent need to explore and
understand the biology of incubation has been emphasised by several symposia: two
held at the annual conference of the U.S. Poultry Science Society (July 2006Edmonton, Alberta, Canada “Managing the embryo for performance”, and July
2007-San Antonio, TX Informal Nutrition Meeting “The impact of imprinting on
biological and economical performance in animals”), and one held by the European
Federation of World Poultry Science Society (October 2007-Berlin, Germany
“Fundamental physiology and perinatal development in poultry), which were
specifically devoted to demonstrating the importance of the embryonic period on
poultry performance. This review will summarise the metabolic events and
pathways in four of the most active tissues of embryos during the period just
prior to hatch, and the hormonal control that coordinates the marked changes as
the embryo prepares for its post-hatch life.
Keywords: poultry embryo; metabolic pathways; incubation; liver; breast muscle;
hatching muscle; intestine
© World's Poultry Science Association 2008
World's Poultry Science Journal, Vol. 64, December 2008
Received for publication May 22, 2008
Accepted for publication August 20, 2008
488
Embryonic metabolic pathways: J.E. de Oliveira et al.
Periods in poultry embryo development
According to Moran (2007), embryonic development can be divided into three major
phases as follows:
1) Establishment of germ. This phase includes the first week of incubation, which is
characterized by the formation of egg compartments (amnion, chorion, allantois, and yolk
sac) that support the survival of the growing embryo. Oxygen supply may be limited due
to immature blood cells and an under developed chorionic vascular system, so metabolic
energy is provided by anaerobic glycolysis of the limited amounts of glucose present in
the egg (Kucera et al., 1984; Moran, 2007).
2) Embryo completion. This phase of embryonic development is characterized by a
fully developed chorioallantois, which is able to assure adequate O2-CO2 exchange and
support rapid embryonic growth (Moran, 2007). Establishment of chorioallantoic
respiration occurs around day 8 of embryonic development in chicks (Baumann and
Meuer, 1992), while extra-embryonic blood vessels completely surround the entire inner
shell surface of turkey eggs around day 9 of incubation (Phillips and Williams, 1943).
Chick embryos are structurally completed around 14 days of incubation, and the same
happens around 22 days of incubation in turkeys. As the embryo approaches full body
mass, maximum chorioallantoic respiration capacity is reached in what is called the
plateau stage of oxygen consumption (Christensen, et al., 1996).
3) Emergence. This last phase is characterized by oral consumption of the amnion by
the embryo, accumulation of glycogen reserves in muscle and liver tissues and
glycogenolysis, initiation of pulmonary respiration, abdominal internalization of
remaining yolk, shell pipping, and emergence (Christensen and Biellier, 1982;
Christensen et al., 1982; Donaldson and Christensen, 1991; Donaldson et al., 1991;
Christensen et al., 1999; Christensen et al., 2001; Moran, 2007). During this period,
dramatic physiological and metabolic changes occur, and any disturbances during this
period may markedly affect embryonic survival and later performance (Christensen et al.,
1999; Collin et al., 2007; Leksrisompong et al., 2007). This review is focused on the
metabolic and physiological aspects of the ‘emergence’ phase of embryonic
development.
Importance of critical tissues during the last period of incubation
Current information on poultry embryo development towards hatch emphasises several
tissues that are most affected by changes during this period, including the liver, pectoral
muscle, hatching muscle, and intestine.
METABOLIC IMPORTANCE OF THE LIVER IN PERINATAL POULTRY
As in all animals, the liver is the most metabolically active tissue of the embryo. It is
the only organ in which all metabolic pathways and metabolic enzymes are active; some
of them are present exclusively in the liver (Krebs, 1972). Blood coming from vitelline
circulation, digestive tract or peripheral tissues pass though the liver, where nutrients can
be absorbed, stored, modified or exported to other parts of the body, and waste
components from other tissues can be metabolized for excretion, or recycled back into
useful metabolites (Romanoff, 1960).
During periods of limited oxygen supply just before hatch, the embryonic hepatic tissue
is essential to produce glucose through gluconeogenesis (Scott et al., 1981; Muramatsu et
al., 1990; Moran, 2007). Hepatic gluconeogenesis is crucial because it is the only source
of glucose for the embryo from the time internal pipping begins until the hatchling is fed
World's Poultry Science Journal, Vol. 64, December 2008
489
Embryonic metabolic pathways: J.E. de Oliveira et al.
(Donaldson, 1995). This is in contrast to what happens in mammals, where a constant
supply of glucose is provided by the gestating mother (Pearce and Brown, 1971). Another
reason for the increased importance of the avian liver compared to mammals is that their
hearts lack the enzymes of the Cori cycle, so in birds, lactate produced by other tissues
during oxygen limitation can only be recycled by hepatic tissue, after oxygen supply is
restored (Pearce and Brown, 1971; Christensen et al., 2003b).
In late-term avian embryos, protein is the major gluconeogenic substrate coming either
from consumed amniotic fluid or from tissue protein reserves (Keirs et al., 2002; Moran,
2007). In addition, the liver will metabolize nitrogen from amino acid deamination for
gluconeogenesis. This allows it to be transported in less toxic forms to the kidney
(Matthews and Holde, 1990b) and deposited with other waste compounds in the
allantois until hatch (Romanoff, 1960).
According to Romanoff (1967), the liver shows extensive growth during the last phase
of incubation, growing proportionally faster than the rest of the embryo to accommodate
its increased metabolic demand by the late-term embryo. Major energy metabolic
pathways, such as glycolysis, gluconeogenesis, tricarboxylic acid cycle (TCA),
pentose phosphate, glycogenesis, and glycogenolysis, are most active in the liver as
they maintain energy homeostasis. Extensive lipid metabolism also occurs in the liver,
like exporting stored and de novo-synthesized cholesterol, triacylglycerol and
phospholipids packaged or repackaged as low-density lipoproteins (LDL), very low
density lipoproteins (VLDL) and high-density lipoproteins (HDL). To keep energy
homeostasis, hepatic cells are under fine control of circulating hormones, including
insulin, glucagon, corticosteroids and thyroid hormones.
METABOLIC IMPORTANCE OF PECTORAL MUSCLE IN PERINATAL POULTRY
The pectoral or breast muscle (Pectoralis major) of the avian embryo is metabolically
important mainly because of its relatively large size and glycogen storage capacity. Even
though the pectoral muscle contains less glycogen per unit of mass than the liver, it
accounts for the greatest quantity of total glycogen stored in the body (John et al., 1988;
Christensen et al., 2001; Uni et al., 2005; Foye et al., 2006). The pectoral muscle is also
the predominant source of protein mobilized to supply amino acids for gluconeogenesis if
energy reserves are depleted after hatch (Donaldson 1995; Keirs et al., 2002; Warner et
al., 2006). Therefore, it is important to study the pectoral muscle when tracking the
destiny of circulating glucose during late incubation.
Muscle cells lack the enzyme glucose-6-phosphatase, so they cannot export glucose to
other tissues. Therefore, glucose stored as glycogen in pectoral muscle cells is ‘trapped’
inside and can only be utilized by the muscle itself (Krebs, 1972). Genetic selection for
increased pectoral muscle size has been emphasised in commercial meat poultry
(Velleman, 2007), which may contribute to some of the metabolic challenges
observed. These high yield meat birds may favour more caloric resources towards
muscle growth than fuelling the hatching process, but more research is necessary to
confirm this hypothesis.
METABOLIC IMPORTANCE OF HATCHING MUSCLE IN PERINATAL POULTRY
The Complexus muscle, or hatching muscle, is located in the back of the head, right
under the skull (George and Berger, 1966; Gross, 1985). Its primary function is to help
the embryo pierce the membranes and the shell during hatching (Bakhuis, 1974; Feher,
1988; Moran, 2007). The hatching muscle increases in size towards the time of pipping,
and then regresses some days after hatch (John et al., 1987). Enlargement of the hatching
muscle by the large accumulation of water and glycogen granules provides the resources
for vigorous contractions that force the egg-tooth to puncture the shell membranes,
490
World's Poultry Science Journal, Vol. 64, December 2008
Embryonic metabolic pathways: J.E. de Oliveira et al.
allowing the hatchling to emerge (Smail, 1964; John et al., 1987). During the course of
the hatching process, the hatching muscle hydrolyzes much of its glycogen reserves and
is reduced to half its size after hatch (John et al., 1987).
There are not many studies that relate the structure and metabolism of the hatching
muscle to its function. It is known, however, that the hatching muscle is comprised of
two types of muscle fibres: the majority are glycolytic only (able to use glucose
exclusively), and a few are oxidative-glycolytic (able to use glucose and fatty acids)
(John et al., 1987). Glycolytic fibres produce lactate and are more active during pipping
and hatching, while oxidative-glycolytic fibres are active after hatch, coinciding with
rising amounts of free fatty acids in this tissue (John et al., 1987).
The Complexus muscle has its own independent nerve control to coordinate its
vigorous movements (Gross, 1985; Moran, 2007). Because it is one of the most
active muscles during shell emergence, it would have a high metabolic demand,
taking precedence over other tissues during distressed incubation conditions.
METABOLIC IMPORTANCE OF INTESTINE IN PERINATAL POULTRY
During the incubation period, the avian embryo does not invest much metabolic
resources into gut development until the end of the incubation period when rapid
visceral growth and maturation occurs (Uni et al., 2003; Gilbert et al., 2007). The
sooner the intestinal tract achieves its functional capacity for digestion and nutrient
assimilation, the sooner the young bird can utilize dietary nutrients and grow to its
genetic potential (Uni et al., 2003; Tako et al., 2004; Uni and Ferket, 2004). Chicken
embryos have limited capacity to digest and absorb nutrients prior to hatch, as
demonstrated by relatively low mRNA levels of sucrase-isomaltase (SI) and laminopeptidase and the ATPase and sodium glucose transporter (SGT-1) in the small
intestinal mucosa (Uni et al., 2003; Tako et al., 2004). The activity of brush border
enzymes, leucine amino peptidase (LAP) and sucrase-isomaltase (SI) have been reported
in turkey embryos at 25E, and glucose transporter (SGLT-1) and alanine transporter (B+)
activity have been measured as early as 23E (Foye et al., 2007a). The absorption capacity
increases close to hatch and continues to increase during the first few days post-hatch
(Uni et al., 1999; Geyra et al., 2001; Uni et al., 2003; Tako et al., 2004). The enteric
villus height of chick embryos increases by 200-300% from 17 days of incubation until
hatch, and the small intestine weight increases faster than body mass (Katanbaf et al.,
1988; Sell et al., 1991; Uni et al., 1999). This rapid intestinal growth is due to a great
increase in cell numbers and size, due to accelerated enterocyte proliferation and
differentiation, and intestinal crypt formation (Uni et al., 2000; Geyra et al., 2001).
Therefore, intestinal tissue growth, maturation and metabolism become of great
importance during the last phase of embryonic development.
To further emphasise the importance of a mature gut, it is worth mentioning the study
done by Applegate et al. (2005) who compared turkey poults and pekin ducklings. Both
species have the same egg size and incubation length, but ducklings grow faster, the main
difference being their higher intestinal growth and maturation compared to poults of the
same age. Hypothetically, any improvement in early gut maturation and digestive
capacity could improve the hatching's post-hatch growth and survival.
Changes in metabolic pathways during the last phase of embryonic
development
Most of the challenges poultry embryos encounter during last phase of incubation are
associated with energy metabolism (Christensen et al., 1999; Uni and Ferket, 2004; Uni
World's Poultry Science Journal, Vol. 64, December 2008
491
Embryonic metabolic pathways: J.E. de Oliveira et al.
et al., 2005; Foye et al., 2006), but little is known about the energy-related metabolic
pathways and how they change before hatch. The metabolic pathways of interest include
glycolysis, gluconeogenesis, Cori cycle, glycogenesis, glycogenolysis, TCA cycle,
pentose phosphate pathway, fatty acid synthesis, and β-oxidation. The following is a
summary of the role of these metabolic pathways during the last period of embryonic
development in poultry.
IMPORTANCE OF FATTY ACID SYNTHESIS AND Β-OXYDATION IN
PERINATAL POULTRY
Metabolic pathways related to lipids are most active in embryos from mid incubation
until 2 or 3 days before internal pipping. During this phase, adequate oxygen is supplied
by the corioallantois and the embryo utilizes yolk fatty acids as their main energy source
(Romanoff, 1960, Sato, et al., 2006, Moran, 2007). Thus, fatty acid synthesis and βoxidation should be very active during this phase of embryonic development.
Fatty acid synthesis occurs when circulating glucose is abundant (Matthews and Holde,
1990a). This condition does not occur in avian embryos because there are little
carbohydrates present in the egg (Romanoff, 1967), so this paper will focus on βoxidation. Fatty acids, stored in egg yolk as triacylglycerol and phospholipids, can be
used for energy and membrane synthesis throughout incubation. During mid-incubation,
glucose availability is low and oxygen supply is not limited. Triacylglycerol is mobilized
from yolk fat to yield free fatty acids. The remaining glycerol backbone can generate
glucose through gluconeogenesis. Fatty acids liberate acetyl-CoA, which is oxidized to
produce ATP. This metabolic pathway is called β-oxidation. Acetyl-CoA is produced by
fatty acid oxidation in a process similar to the reverse of fatty acid elongation (Moran,
1994). Acetyl-CoA units can enter the TCA cycle to produce ATP by forming citrate, a
reaction catalyzed by the enzyme citrate synthase (Krebs, 1972).
IMPORTANCE OF GLYCOGENESIS AND GLYCOGENOLYSIS
As the embryo reaches the plateau stage of oxygen consumption, oxygen becomes a
limiting factor and energetic metabolism switches from using yolk fat to the predominant
substrate of carbohydrates (Donaldson and Christensen, 1991). Since the incubating egg
contains little carbohydrate (Romanoff, 1967), glucose arises from protein utilisation
(amino acids) by glocogenogenesis, and it is stored in liver and muscle tissues as
polymers called glycogen (Matthews and Holde, 1990a; Christensen et al., 2001).
Synthesis of glycogen (glycogenesis) and glycogen degradation (glycogenolysis) are
vital for embryonic survival during the last phase of incubation.
The enzymes of glycogenesis and glycogenolysis are homeostatically controlled by
hormones and other enzymes to ensure a constant supply of glucose to tissues. The
embryo depends on stored glycogen for muscle activity, heat production, and body
maintenance during and after hatch, until feed is consumed. If the embryo depletes its
glycogen reserves, hatchling vigour and survival are compromised (Christensen et al.,
1999; Christensen et al., 2003b).
IMPORTANCE OF GLYCOLYSIS AND GLUCONEOGENESIS
Glycolysis and gluconeogenesis are pathways that share many of the same enzymes
that are bidirectional throughout embryonic development, depending on substrate/product
supply. During the last phase of incubation, these glycolytic and glucogenic enzymes are
expected to be very active during the low oxygen availability that occurs during the
plateau stage of oxygen consumption until external pipping.
Through glycolysis, glucose can be reduced to 2 pyruvate units plus 6 moles of ATP.
Usually, pyruvate enters the TCA cycle for further oxidation to produce more ATP, a
492
World's Poultry Science Journal, Vol. 64, December 2008
Embryonic metabolic pathways: J.E. de Oliveira et al.
process that requires oxygen (Matthews and Holde, 1990a). During late incubation,
pyruvate is converted to lactate because oxygen supply is limited. As previously
discussed, birds can recycle lactate back to glucose in the liver through the Cori cycle
once oxygen availability is restored.
Gluconeogenesis is the major metabolic means to produce glucose from other carbonrich components, such as amino acids, glycerol and other carbohydrates. Glucose is
produced by gluconeogenesis only in the liver and kidney, so it can be exported to
other tissues or polymerized and stored as glycogen in liver and muscle cells for later use.
In the days that precede pipping, amniotic fluid and possibly muscle proteins are utilized
to accumulate glycogen reserves in the liver and muscle (Romanoff, 1967; Donaldson
and Christensen, 1991; Uni et al., 2005). These metabolic pathways are of great
importance during late incubation and early post-hatch life, because the late-term
embryo and hatchling relies on gluconeogenesis as the main energy source until feed
is consumed (Donaldson, 1995).
IMPORTANCE OF TCA CYCLE
During most of the incubation period, the TCA cycle generates most of the embryo's
ATP, as it receives acetyl-CoA from yolk fatty acid β-oxidation. Because its activity
depends on oxygen, TCA cycle activity is drastically reduced during the plateau stage of
oxygen consumption and internal pipping when chorioallantoic gas exchange capacity is
reached. Once external pipping occurs, oxygen is supplied by pulmonary respiration,
providing oxygen for TCA cycle activity. The TCA cycle can be summarised as
conversion of acetyl-CoA into several intermediates, plus energy-rich molecules of
NADH, GTP and FADH2. NADH and FADH2 are reducing equivalents that will later
result in 24 ATP produced from each glucose unit through the electron transport chain.
Other than producing energy, the TCA cycle may receive or donate intermediaries to
other pathways. Carbon backbones from many origins enter the TCA cycle in different
points as one of the intermediates. This feature is important during protein degradation
for gluconeogenesis, when deaminated carbon backbones enter the TCA cycle to produce
energy. The enzymes of the TCA cycle are also kept under strict control as the flux
through this pathway may change dramatically during the last period of incubation.
IMPORTANCE OF PENTOSE PHOSPHATE PATHWAY
The pentose-phosphate pathway is important in embryonic tissues because pentose
sugars are necessary for DNA and RNA synthesis. Since there are minimal amounts
of nucleic acids in the freshly laid egg, they must be synthesized by the embryo during
incubation (Reddy et al., 1952), especially during periods of rapid growth. This is the
case during the last phase of incubation when many organs are increasing in size and
maturing.
Hormonal control of metabolic pathways
Metabolic pathways are dependently connected with one another and share intermediate
metabolite substrates, thus they require precise homeostatic control. The amount and kind
of substrates available to the embryo will trigger the production of hormones that in turn
will control gene expression of the enzymes that regulate the flux within these pathways.
Genome-wide analysis has identified genes associated with insulin, glucagon, T3, T4,
IGF-I, IGFII as being most involved in metabolism that affect phenotypic traits in chickens
(Zhou et al., 2007). Therefore, a review of embryonic metabolism is not complete without
addressing the contribution of these hormones to embryo metabolism control.
World's Poultry Science Journal, Vol. 64, December 2008
493
Embryonic metabolic pathways: J.E. de Oliveira et al.
CONTROL BY INSULIN AND GLUCAGON
Insulin and glucagon are closely related to energy metabolism in all tissues of the bird
(Lu et al., 2007). These hormones are produced by the pancreas, and their release into
blood circulation depends on the animal's energy status.
Lu et al. (2007) found that insulin levels increased significantly as embryo
development progresses, reaching a plateau during pipping and hatching, and rising
again 24 hours after the chick starts feeding. Concurrently, glucagon levels are low
throughout incubation and then rise three days before hatch, peaking during pipping
and hatching. Glucagon levels drop by 40% about 24 hours after the chick starts feeding.
Plasma glucose levels follow the insulin pattern, increasing throughout incubation, and
plateau during pipping and at hatching. A significant reduction in plasma glucose levels
occurs 4 days prior to hatch. Concurrently, there is a peak in insulin levels as the embryo
consumes the amnion and glycogen accumulates in liver and muscle tissues.
Insulin is released after a meal, signalling abundance of circulating glucose. It permits
cells throughout the body to uptake glucose and cause the synthetic processes to occur
(Matthews and Holde, 1990c). Insulin stimulates glycolysis, the TCA cycle,
glycogenesis, and fatty acid synthesis. In chicken embryos, insulin regulates the
concentration of amino acids and related compounds in plasma, amniotic fluid, and
allantoic fluid. It also accelerates embryonic morphological development (Lu et al.,
2007). Because embryo growth hormone levels are low during incubation, Lu, et al.
(2007) hypothesized insulin's role as a possible growth factor for chicken embryos. They
observed insulin to act as an important promoter of protein deposition during the period
of rapid embryonic growth. Moreover, insulin levels are much more sensitive to amino
acids than to carbohydrate levels in chick embryos as compared to mammalian embryos.
This observation may be due to the low glucose content in the egg as compared to the
high levels of amino acids in the amnion and albumen.
Glucagon, the hormone of fasting, indicates low glucose availability, so only cells that
rely on glucose as their sole energy substrate (i.e. some cells of brain, blood, heart and
muscle) will uptake it (Krebs, 1972). Glucagon also triggers glycogen and fat mobilization
throughout the body. In the liver, gluconeogenesis is stimulated by glucagon to produce
glucose from other substrates so it can be made available to other tissues through
circulation. Glucagon appears to be the dominant pancreatic hormone in birds,
maintaining them in constant hyperglycemia. Glucagon levels in avian plasma are 2 to
3 times greater than in mammals. Glucagon rises when the embryo needs more glucose,
precisely during the first 10 days of incubation and during the last week before hatch (Lu et
al., 2007). Lu et al., (2007) confirmed previous research by Picardo and Dickson (1982),
who claimed that the mobilization of glycogen from hepatocytes is not affected by insulin
levels 3 days before hatch. Thus, liver cells have the capacity to synthesize and utilize
glycogen simultaneously. Glucagon also plays a role during the embryonic transition from
lipid to carbohydrate metabolism. As Langslow et al. (1979) proposed, adipocytes become
sensitive to glucagon levels only after hatch, which is important so that subcutaneous
adipose stores are not used until the first day post-hatch. In contrast to other species or
among chickens post-hatch, the avian embryo's glucagon levels may be low or high during
rapid growth stages of incubation. Glucagon apparently functions as a complex embryonic
growth-stimulating factor that interacts with other hormones (Lu et al., 2007). From a
general hormonal perspective, chick embryos may be in a constant state of fasting, while
hatched chicks may be in a state of feasting Langslow et al. (1979).
CONTROL BY INSULIN-LIKE GROWTH FACTORS I AND II
As in mammals, insulin-like growth factors I and II (IGF-I and IGF-II) in poultry
stimulate growth, development, and intermediary metabolism (Taylor et al., 2001;
494
World's Poultry Science Journal, Vol. 64, December 2008
Embryonic metabolic pathways: J.E. de Oliveira et al.
Langley et al., 2002; Joulia et al. 2003; Foye, 2005). Specifically important, IGF-I and II
stimulate hepatic glycogen, RNA and protein synthesis (McMurtry et al., 1998; Lu, et al.,
2007). IGFs were also found in amniotic fluid where they may play a role in regulating
amino acid utilization (Karcher et al., 2005; Lu, et al., 2007).
IGF-I levels increase during incubation, but are reduced during hatch. After hatch, IGFI levels increase rapidly and continue until 21 days post-hatch (Lu et al., 2007). IGFs
levels have been identified to stimulate intestinal mucosa growth and cell maturation
(Jehle et al., 1999; Foye, 2005). IGF-II levels also increases during incubation and
remain much higher than IGF-I until hatch when it quickly drops to very low levels.
Thus, IGF-II may play a predominant role in growth and development during incubation,
whereas IGF-I becomes predominant after hatch (Lu et al., 2007).
CONTROL BY THYROID HORMONES T3 AND T4
Thyroid hormones, thiiodothyronine (T3) and thyroxine (T4), are involved in numerous
physiological processes, including regulation of heat production, mobilisation of
glycogen reserves, embryo muscle growth (Nobikuni et al., 1989; Christensen, et al.,
1996), and stimulus for hatching (Christensen and Biellier, 1982, Christensen et al.,
1982, Christensen et al., 2003a; and Christensen et al., 2003b). Lu et al. (2007)
observed constant levels of T3 in chicken embryos during mid incubation, but it
peaked the day before hatch. T4 levels reached high levels and stayed high during
amnion consumption (between 17 and 20 days of incubation in chickens and 22 and
24 days in turkeys), and decreased after hatch. Elevated thyroxine levels are considered
important for stimulating a variety of developmental and metabolic processes necessary
for hatching. A sharp rise in T3 was associated to embryonic switching to lung
respiration. Both T3 and T4 are positively correlated with embryonic body weight
during late incubation (Lu et al., 2007). The same pattern of plasma thyroid hormone
levels was observed in turkey embryos (Christensen et al., 1996; Christensen et al.,
2003b).
In summary, the most important hormones contributing to embryonic growth are
insulin and IGF-II. IGF-I and T4 are important from hatch to 3 weeks of age.
Glucagon modifies growth and function, while thyroid hormones are critical for
hatching (Lu et al., 2007).
OUTSTANDING ISSUES REGARDING EMBRYONIC METABOLISM
Although hormonal control seems to be very important in controlling metabolism
towards hatch, the signals that trigger their release are still unknown. Changes in
metabolism could be triggered by a variety of factors. It could be environmental
conditions during incubation, such CO2 or O2 levels or temperature and humidity. It
could also be physical, as space for growth becomes limited within the shell. Another
factor that affects the metabolism of the embryo could be related to nutritional
biochemistry, such as lack of energetic substrates or nutrient cofactors, or osmotic
challenges due to water loss and waste accumulation. There is still no information on
how egg nutrient resources are utilized by the growing embryo, or the specific nutrient
requirements for best embryonic development. It is clear that there is still a lot we do not
know about this important period of development, so more studies are needed on this
subject.
Conclusions
The phases of embryonic development and the most important physiological and
World's Poultry Science Journal, Vol. 64, December 2008
495
Embryonic metabolic pathways: J.E. de Oliveira et al.
metabolic events in perinatal poultry are summarized in Figure 1 (chicken) and Figure 2
(turkeys). The conclusions of this review are summarized as follows:
1) The incubation period and embryogenesis are of critical importance to the
production of modern strains of meat-type poultry. More research is needed
to better understand the various phases of embryonic development.
2) Many significant changes in metabolism occur during the last days of incubation
as the embryo prepares for and succeeds to hatch. Before manipulating
embryonic development to benefit production efficiency and animal welfare, it
is necessary to understand the predominant pathways of carbohydrate, lipid, and
amino acid metabolism, and how these pathways interact and are homeostatically
controlled.
3) The signals that trigger shifts in metabolism and hormone release during the
critical events leading to hatch are still unknown. More information about the
mechanisms of perinatal metabolism could yield creative ways to manipulate
physiological or nutritional conditions that improve hatchability, and survival
and vigor of hatchlings.
embryo structurally
completed
plateau stage
incubation starts
establishment of chorioallantois
1
7
8
embryonic rapid growth
17
hatch
pipping
18
19
20
21
amnion consumption
glycogen storage
T4 and Insulin peak
limitedȱoxygenȱ
supplyȱ
limitedȱoxygenȱ
supply
plentyȱof oxygen
2) Embryo completion
Fuel: fatty acids
Source: yolk
Figure 1 Chicken embryo development periods and their main physiological and metabolic changes.
embryo structurally
completed
plateau stage
incubation starts
establishment of chorioallantois
pipping
embryonic rapid growth
1
8
22
9
hatch
24
25
26
28
amnion consumption
glycogen storage
T4 and Insulin peak
limitedȱoxygenȱ
supplyȱ
plenty ȱoof ȱooxygen ȱ
limitedȱoxygenȱ
supplyȱ
2) Embryo completion
Fuel: fatty acids
Source: yolk
Figure 2 Turkey embryo development periods and their main physiological and metabolic changes.
496
World's Poultry Science Journal, Vol. 64, December 2008
Embryonic metabolic pathways: J.E. de Oliveira et al.
References
APPLEGATE, T.J., KARCHER, D.M. and LILBURN, M.S. (2005) Comparative development of the small
intestine in the turkey poult and Pekin duckling. Poultry Science 84: 426-431.
BAKHUIS, W.L. (1974) Observations on hatching movements in the chick (Gallus domesticus). Journal of
Comparative Physiology and Psychology 87: 997-1003.
BAUMANN, R. and MEUER, H.J. (1992) Blood oxygen transport in the early avian embryo. Physiological
Reviews 72: 941-965.
CHRISTENSEN, V.L. and BIELLIER, H.V. (1982) Physiology of turkey embryos during pipping and
hatching. IV. Thyroid function in embryos from selected hens. Poultry Science 61: 2482-2488.
CHRISTENSEN, V.L., BIELLIER, H.V. and FORWARD, J.F. (1982) Physiology of turkey embryos during
pipping and hatching III. Thyroid function. Poultry Science 61: 367-374.
CHRISTENSEN, V.L., DONALDSON, W.E. and MCMURTRY, J.P. (1996) Physiological differences in
late embryos from turkey breeders at different ages. Poultry Science 75: 172-178.
CHRISTENSEN, V.L., DONALDSON, W.E. and NESTOR, K.E. (1999) Length of the plateau and pipping
stages of incubation affects the physiology and survival of turkeys. British Poultry Science 40: 297-303.
CHRISTENSEN, V.L., WINELAND, M.J., FASENKO, G.M. and DONALDSON, W.E. (2001) Egg
storage effects on plasma glucose and supply and demand tissue glycogen concentrations of broiler
embryos. Poultry Science 80: 1729-1735.
CHRISTENSEN, V.L., GRIMES, J.L., WINELAND, M.J. and DAVIS, G.S. (2003a) Accelerating
embryonic growth during incubation following prolonged egg storage. 2. Embryonic growth and
metabolism. Poultry Science 82: 1869-1878.
CHRISTENSEN, V.L., ORT, D.T. and GRIMES, J.L. (2003b) Physiological factors associated with weak
neonatal poults (Meleagris gallopavo). International Journal of Poultry Science 2: 7-14.
CHRISTENSEN, V.L., ORT, D.T., NESTOR, K.E., VELLEMAN, S.G. and HAVENSTEIN, G.B. (2007)
Genetic control of neonatal growth and intestinal maturation in turkeys. Poultry Science 86: 476-487.
COLLIN, A., BERRI, C., TESSERAUD, S., RODON, F.E., SKIBA-CASSY, S., CROCHET, S.,
DUCLOS, M.J., RIDEAU, N., TONA, K., BUYSE, J., BRUGGEMAN, V., DECUYPERE, E.,
PICARD, M. and YAHAV, S. (2007) Effects of thermal manipulation during early and late
embryogenesis on thermotolerance and breast muscle characteristics in broiler chickens. Poultry Science
86: 795-800.
DONALDSON, W.E. and CHRISTENSEN, V.L. (1991) Dietary carbohydrate level and glucose metabolism
in turkey poults. Comparative Biochemistry and Physiology A 98: 347-350.
DONALDSON, W.E., CHRISTENSEN, V.L. and KRUEGER. K.K. (1991) Effects of stressors on blood
glucose and hepatic glycogen concentrations in turkey poults. Comparative Biochemistry and Physiology A
100: 945-947.
DONALDSON, W.E. (1995) Carbohydrate, hatchery stressors affect poult survival. Feedstuffs 67(14): 16-17.
ELIBOL, O., PEAK, S.D. and BRAKE, J. (2002) Effect of flock age, length of egg storage, and frequency of
turning during storage on hatchability of broiler hatching eggs. Poultry Science 81: 945-950.
FEHER, G. (1988) The process of hatching in geese and ducks. Anatomia, Histologia, Embryologia 17: 107120.
FOYE, O.T. (2005) The biochemical and molecular effects of amnionic nutrient administration, “in ovo
feeding” on intestinal development and function and carbohydrate metabolism in turkey embryos and
poults. Ph.D. Dissertation. North Carolina State University. Raleigh, NC.
FOYE, O.T., UNI, Z. and FERKET, P.R. (2006) Effect of in ovo feeding egg white protein, beta-hydroxybeta-methylbutyrate, and carbohydrates on glycogen status and neonatal growth of turkeys. Poultry Science
85: 1185-1192.
FOYE, O.T., FERKET, P.R. and UNI, Z. (2007a) The effects of in ovo feeding arginine, beta-hydroxy-betamethyl-butyrate, and protein on jejunal digestive and absorptive activity in embryonic and neonatal turkey
poults. Poultry Science 86: 2343-2349.
FOYE, O.T., FERKET, P.R. and UNI, Z. (2007b) Ontogeny of energy and carbohydrate utilisation of the
precocial avian embryo and hatchling. Avian and Poultry Biology Reviews 18 (3): 93 -101.
GEORGE, J.C. and BERGER, A.J. (1966) Avian myology Academic Press, New York.
GEYRA, A., UNI, Z., and SKLAN, D. (2001) Enterocyte dynamics and mucosal development in the posthatch
chick. Poultry Science 80: 776-782.
GILBERT, E.R., LI, H., EMMERSON, D.A., WEBB JR., K.E. and WONG, E.A. (2007) Developmental
regulation of nutrient transporter and enzyme mRNA abundance in the small intestine of broilers. Poultry
Science 86: 1739-1753.
GROSS, G.H. (1985) Innervation of the complexus ("hatching") muscle of the chick. Journal of Comparative
Neurology 232: 180-189.
World's Poultry Science Journal, Vol. 64, December 2008
497
Embryonic metabolic pathways: J.E. de Oliveira et al.
HAVENSTEIN, G.B., FERKET, P.R. and QURESHI, M.A. (2003a) Growth, livability, and feed conversion
of 1957 versus 2001 broilers when fed representative 1957 and 2001 broiler diets. Poultry Science 82: 15001508.
HAVENSTEIN, G.B., FERKET, P.R. and QURESHI, M.A. (2003b) Carcass composition and yield of 1957
versus 2001 broilers when fed representative 1957 and 2001 broiler diets. Poultry Science 82: 1509-1518.
HAVENSTEIN, G.B., FERKET, P.R., GRIMES, J.L., QURESHI, M.A. and NESTOR, K.E. (2007)
Comparison of the performance of 1966- versus 2003-type turkeys when fed representative 1966 and
2003 turkey diets: growth rate, livability, and feed conversion. Poultry Science 86: 232-240.
HULET, R.M. (2007) Managing incubation: where are we and why? Poultry Science 86: 1017-1019.
JEHLE, P.M., FUSSGAENGER, R.D., BLUM, W.F., ANGELUS, N.K., HOEFLICH, A., WOLF, E. and
JUNGWIRTH, R.J. (1999) Differential autocrine regulation of intestine epithelial cell proliferation and
differentiation by insulin-like growth factor (IGF) system components. Hormone and Metabolic Research 31:
97-102.
JOHN, T.M., GEORGE, J.C. and MORAN JR., E.T. (1987) Pre- and post-hatch ultrastructural and
metabolic changes in the hatching muscle of turkey embryos from antibiotic and glucose treated eggs.
Cytobios 49: 197-210.
JOHN, T.M., GEORGE, J.C. and MORAN JR., E.T. (1988) Metabolic changes in pectoral muscle and liver
of turkey embryos in relation to hatching: influence of glucose and antibiotic-treatment of eggs. Poultry
Science 67: 463-469.
JOULIA, D., BERNARDI, H., GARANDEL, V., RABENOELINA, F., VERNUS, B. and CABELLO, G.
(2003) Mechanisms involved in the inhibition of myoblast proliferation and differentiation by myostatin.
Experimental Cell Research 286(2): 263-275
KARCHER, D.M., MCMURTRY, J.P. and APPLEGATE, T.J. (2005) Developmental changes in amniotic
and allantoic fluid insulin-like growth factor (IGF)-I and -II concentrations of avian embryos. Comparative
biochemistry and physiology. Part A, Molecular & integrative physiology 142: 404-409.
KATANBAF, M.N., DUNNINGTON, E.A. and SIEGEL, P.B. (1988) Allomorphic relationships from
hatching to 56 days in parental lines and F1 crosses of chickens selected 27 generations for high or low
body weight. Growth, Development, and Aging 52: 11-21.
KEIRS, R.W., PEEBLES, E.D., HUBBARD, S.A. and WHITMARSH, S.K. (2002) Effects of supportative
gluconeogenic substrates on early performance of broilers under adequate brooding conditions. Journal of
Applied Poultry Research 11: 367-372.
KREBS, H.A. (1972) Some aspects of the regulation of fuel supply in omnivorous animals. Advances in
Enzyme Regulation 10: 397-420.
KUCERA, P., RADDATZ, E. and BAROFFIO, A. (1984) Oxygen and glucose uptake in the early chick
embryo. In: R.S. SEYMOUR and JUNK, W. (Eds) Respiration and Metabolism of Embryonic Vertebrates.
pp. 299-309 (Dordrecht, The Netherlands).
LANGLEY, B., THOMAS, M., BISHOP, A., SHARMA, M., GILMOUR, S. and KAMBADUR, R. (2002)
Myostatin inhibits myoblast differentiation by down-regulating MyoD expression. The Journal of Biological
Chemistry 277: 49831-49840
LANGSLOW, D.R., CRAMB, G. and SIDDLE, K. (1979) Possible mechanisms for the increased sensitivity
to glucagon and catecholamines of chicken adipose tissue during hatching. General and Comparative
Endocrinology 39: 527-533.
LEKSRISOMPONG, N., ROMERO-SANCHEZ, H., PLUMSTEAD, P.W., BRANNAN, K.E. and
BRAKE, J. (2007) Broiler incubation. 1. Effect of elevated temperature during late incubation on body
weight and organs of chicks. Poultry Science 86: 2685-2691.
LU, J.W., MCMURTRY, J.P. and COON, C.N. (2007) Developmental changes of plasma insulin, glucagon,
insulin-like growth factors, thyroid hormones, and glucose concentrations in chick embryos and hatched
chicks. Poultry Science 86: 673-683.
MATTHEWS, C.K. and HOLDE, K.E. (1990a) Carbohydrate metabolism I: anaerobic processes in
generating metabolic energy. In: Biochemistry. pp. 670-703 (The Benjamin/Cummnings Publishing
Company, Inc., Redwood City, CA).
MATTHEWS, C.K. and HOLDE, K.E. (1990b) Metabolism of nitrogenous compounds: principles of
biosynthesis, utilization, turnover, and excretion. In: Biochemistry. pp. 670-703 (The Benjamin/Cummings
Publishing Company, Inc., Redwood City, CA).
MATTHEWS, C.K. and HOLDE, K.E. (1990c) Integration and control of Metabolic processes. In:
Biochemistry. pp. 779-814 (The Benjamin/Cummnings Publishing Company, Inc., Redwood City, CA).
MCMURTRY, J.P., ROSEBROUGH, R.W., BROCHT, D.M, FRANCIS, G.L., UPTON, Z. and PHELPS,
P. (1998) Assessment of developmental changes in chicken and turkey insulin-like growth factor-II by
homologous radioimmunoassay. The Journal of Endocrinology 157: 463-473.
MORAN JR., E.T. (2007) Nutrition of the developing embryo and hatchling. Poultry Science 86: 1043-1049.
MORAN, L.A. (1994) Lipid metabolism. In: L.A. MORAN (Ed) Biochemistry. pp. 20.21-20.14 (Neil Patterson/
Prentice Hall, Englewood Cliffs, NJ).
498
World's Poultry Science Journal, Vol. 64, December 2008
Embryonic metabolic pathways: J.E. de Oliveira et al.
MURAMATSU, T., HIRAMOTO, K., KOSHI, N., OKUMURA, J., MIYOSHI, S. and MITSUMOTO, T.
(1990) Importance of albumen content in whole-body protein synthesis of the chicken embryo during
incubation. British Poultry Science 31: 101-106.
NOBIKUNI, K., KOGA, K.O. and NISHIYAMA, H. (1989) The effects of thyroid hormones on liver
glycogen, muscle glycogen and liver lipids in chicks. Japanese Journal of Zootechnical Science 60: 346-348.
PEARCE, J. and BROWN, W.O. (1971) Carbohydrate metabolism (Academic Press, London).
PEEBLES, E.D., KEIRS, R.W., BENNETT, L.W., CUMMINGS, T.S., WHITMARSH, S.K. and
GERARD, P.D. (2005) Relationships among prehatch and posthatch physiological parameters in early
nutrient restricted broilers hatched from eggs laid by young breeder hens. Poultry Science 84: 454-461.
PHILLIPS, R.E. and WILLIAMS, C.S. (1943) External morphology of the turkey during the incubation
period. Poultry Science 22: 270-277.
PICARDO, M. and DICKSON, A.J. (1982) Hormonal regulation of glycogen metabolism in hepatocyte
suspensions isolated from chicken embryos. Comparative biochemistry and physiology. B, Comparative
biochemistry 71: 689-693.
REDDY, D.V., LOMBARDO, M.E. and CERECEDO, L.R. (1952) Nucleic acid changes during the
development of the chick embryo. The Journal of Biological Chemistry 198: 267-270.
ROMANOFF, A.L. (1960) The avian embryo; structural and functional development (Macmillian, New York,
NY).
ROMANOFF, A.L. (1967) Biochemistry of the avian embryo; a quantitative analysis of prenatal development
(Interscience Publishers, New York).
SATO, M., TACHIBANA, T. and FURUSE, M. (2006) Heat production and lipid metabolism in broiler and
layer chickens during embryonic development. Comparative biochemistry and physiology. Part A, Molecular
& integrative physiology 143: 382-388.
SCOTT, T.R., JOHNSON, W.A., SATTERLEE, D.G. and GILDERSLEEVE, R.P. (1981) Circulating
levels of corticosterone in the serum of developing chick embryos and newly hatched chicks. Poultry
Science 60: 1314-1320.
SELL, J.L., ANGEL, C.R., PIQUER, F.J., MALLARINO, E.G. and AL-BATSHAN, H.A. (1991)
Developmental patterns of selected characteristics of the gastrointestinal tract of young turkeys. Poultry
Science 70: 1200-1205.
SMAIL, J.R. (1964) A possible role of the Musculus complexus in pipping the chicken egg. The American
midland naturalist 72: 499-506.
TAKO, E., FERKET, P.R. and UNI, Z. (2004) Effects of in ovo feeding of carbohydrates and beta-hydroxybeta-methylbutyrate on the development of chicken intestine. Poultry Science 83: 2023-2028.
TAYLOR, W.E., BHASIN, S., ARTAZA, J., BYHOWER, F., AZAM, M. and WILLARD, D.H. (2001)
Myostatin inhibits cell proliferation and protein synthesis in c2C12 muscle cells. American journal of
physiology. Endocrinology and metabolism 280: E221-E228.
UNI, Z., NOY, Y. and SKLAN, D. (1999) Posthatch development of small intestinal function in the poult.
Poultry Science 78: 215-222.
UNI, Z., GEYRA, A., BEN-HUR, H. and SKLAN, D. (2000) Small intestinal development in the young
chick: crypt formation and enterocyte proliferation and migration. British Poultry Science 41: 544-551.
UNI, Z., TAKO, E., GAL-GARBER, O. and SKLAN, D. (2003) Morphological, molecular, and functional
changes in the chicken small intestine of the late-term embryo. Poultry Science 82: 1747-1754.
UNI, Z. and FERKET, P.R. (2004) Methods for early nutrition and their potential. World´s Poultry Science
Journal 60: 101-111.
UNI, Z., FERKET, P.R., TAKO, E. and KEDAR, O. (2005) In ovo feeding improves energy status of lateterm chicken embryos. Poultry Science 84: 764-770.
VELLEMAN, S.G. (2007) Muscle development in the embryo and hatchling. Poultry Science 86: 1050-1054.
WARNER, J.D., FERKET, P.R., CHRISTENSEN, V.L. and FELTS, J.V. (2006) Effect of season, hatch
time, and post-hatch holding on glycogen status of turkey poults. Poultry Science 85(Suppl. 1): 117 (Abstr.).
ZHOU, H., EVOCK-CLOVER, C.M., MCMURTRY, J.P., ASHWELL, C.M. and LAMONT, S.J. (2007)
Genome-wide linkage analysis to identify chromosomal regions affecting phenotypic traits in the chicken. IV.
Metabolic traits. Poultry Science 86: 267-276.
World's Poultry Science Journal, Vol. 64, December 2008
499