Changes in yolk sac membrane absorptive area and fat digestion during chick embryonic development.

The capacity of yolk sac (YS) utilization by the chick embryo may be affected by structural changes in the YS membrane (YSM) and by the mechanisms within its cells for digestion, absorption, and transfer of nutrients. Two experiments were conducted to examine structural and digestive changes in the YS of the broiler chick embryo; weights of embryo, YS, and YSM, as well as the total area of the YSM and the absorptive area of the YSM, were measured between embryonic day (E) 5 and E21. In addition, fat content, lipase activity, and bile acid concentration in the YSM and YS contents (YSC) were measured between E11 and E21. Results showed that YSM weight increased from 0.19 g on E5 to 6.46 g on E15, and decreased by 3.74 g between E17 and E21. The absorptive YSM area increased from 536 mm² on E5 (51% of total YSM area) to 6,370 mm² (86% of total area) on E17, and decreased to 4,439 mm(2) on E21 (85% of total area). The smaller YSM area between E17 and E21 did not decrease the rate of YS fat utilization, which could suggest that YSM mechanisms for fat absorption, digestion, and secretion increased during that period. Total YSM lipase activity relative to fat content (units per g of YSM fat) increased from approximately 1,000 units on E15 to 1,500 units on E21. The detection of lipase in the YSM lends support to the hypothesis that YS lipids are hydrolyzed in the lipolysosomes of the YSM. The current study also confirmed for the first time that bile acids are present in the YS, with levels that ranged from 0.61 to 1.06 µmol/g in the YSM, and may suggest that bile is synthesized in the YSM of the chick embryo. Results of the current study contribute to our understanding of the developmental changes that affect YS functionality and could give insight into the coordination between the embryo's demands and YSM morphological, absorptive, digestive, and secretive changes.

Gene expression of nutrient transporters and digestive enzymes in the yolk sac membrane and small intestine of the developing embryonic chick.

Chick embryos derive nutrients from the yolk during incubation and transition to intestinal absorption of nutrients posthatch. The uptake of nutrients is mediated by a variety of membrane-bound transporter proteins. The objective of this study was to determine the expression profiles of nutrient transporters and digestive enzymes during incubation in the yolk sac membrane (YSM) and embryonic intestine of egg-laying (Leghorn) and meat-producing (Cobb) chickens derived from 22 to 30 wk (young) and 45 to 50 wk (old) breeder flocks. Transporters examined included the peptide transporter PepT1, the glutamate/aspartate (EAAT3), cationic (CAT-1) and neutral (B0AT) amino acid transporters, and the fructose (GLUT5) and glucose (SGLT1) transporters. Digestive enzymes included aminopeptidase N (APN) and sucrase-isomaltase (SI). Expression of these genes was assessed by real-time PCR using the absolute quantification method in YSM at embryonic day (E) 11, 13, 15, 17, 19, 20, and 21 and intestine at E15, 17, 19, 20, and 21. The PepT1 and APN gene expression in the YSM increased until E15 and then decreased until E21, whereas expression in the intestine increased from E15 to E21. The B0AT showed a similar pattern, with greatest expression in the YSM occurring at E17/E19. The CAT1 and GLUT5 genes showed decreased expression in the YSM and increased expression in the intestine until E17/E19 and then a decrease until E21. Expression of SGLT1 and EAAT3 showed increased gene expression over time in both the intestine and YSM. Expression of SI showed little to no gene expression in the YSM, whereas the intestine exhibited consistently high levels of gene expression. In YSM and intestine, SI expression was greater in Leghorn than Cobb, whereas CAT1 and GLUT5 expression was greater in Cobb than Leghorn. Expression of the APN, CAT1, and SI genes was greater in embryos from young flocks than old flocks in YSM and intestine. These results demonstrate that the YSM expresses many of the digestive enzymes and nutrient transporters typically associated with the intestine and that these genes show tissue- and development-specific patterns of expression.

Yolk sac carbohydrate levels and gene expression of key gluconeogenic and glycogenic enzymes during chick embryonic development.

Glycogen and glucose concentrations (mg/g of tissue) and amounts (mg) were determined in the yolks of fertile eggs on the day of set and in the yolk sac (YS) and liver of broiler chick embryos between 11 and 21 embryonic days of age (E). On the day of set, the yolk contained 50 mg of glucose (0.31% of yolk) but did not contain glycogen. During incubation, the amount of glucose in the YS increased from 20 mg on E11 to 60 mg on E19. A parallel increase in YS and liver glycogen concentrations (mg/g) during the last week of incubation implied a similar capacity for glycogen synthesis per gram of tissue. However, due to its larger size, the YS capacity for glycogen storage far exceeded that of the liver, which stored less than 12 mg of glycogen up to E19, as compared with more than 200 mg in the YS. Between E19 and 21, liver and YS glycogen amounts decreased by 10 mg and 100 mg, respectively. These results indicated that the YS is a glycogenic and perhaps gluconeogenic organ. We therefore evaluated the gene expression of glycogen synthase and glycogen phosphorylase as well as gluconeogenic enzymes (fructose 1,6-bisphosphatase, phosphoenolpyruvate carboxykinase, and glucose 6-phosphatase) in the YS membrane and liver by real-time reverse-transcription PCR. Although the YS membrane and liver displayed different patterns of mRNA abundance, the high abundance of fructose 1,6-bisphosphatase mRNA in the YS membrane between E11 and 15, and the expression of phosphoenolpyruvate carboxykinase and glucose 6-phosphatase, supported the postulated gluconeogenic abilities of the YS membrane and indicated its role in providing glucose to the embryo. Thus, glucose is probably synthesized in the YS, stored in the form of glycogen, and toward hatch, the YS may have the potential to transfer 10 times more glycogen-derived glucose to the embryo as compared with the liver. As such, the YS may play a major role in the synthesis and storage of glucose and its supply to the chick embryo toward hatch.

The chick embryo yolk sac membrane expresses nutrient transporter and digestive enzyme genes.

The yolk sac membrane plays a major role in the transport of nutrients from the yolk contents to the chick embryo. We examined whether the yolk sac membrane expresses genes for nutrient digestion, enzymes, and nutrient transporters. We evaluated relative mRNA abundance of the digestive enzymes aminopeptidase N (APN) and sucrase-isomaltase (SI); the nutrient transporters oligopeptide transporter Pept1, cationic amino acid transporter CAT1, and sodium glucose transporter SGLT1; and the micronutrient transporters type IIb sodium phosphate cotransporter NPT2b, calcium transporter TRPV6, and zinc transporter ZnT-1 from embryonic d 11 (11E) to 21E (day of hatch) by real-time reverse-transcription PCR. The yolk sac membrane expressed all the examined genes, which exhibited several patterns of expression. Relative abundance of APN mRNA increased in the yolk sac membrane from 11E to 17E and decreased from 17E to 20E. Expression of PepT1 increased from 11E to 15E and decreased from 15E to 20E. In contrast, CAT1 expression decreased from 11E to 13E and increased from 15E to 17E. Expression of SGLT1 increased between 15E and 20E and decreased substantially between 20E and 21E. Expression of NPT2b increased during incubation and exhibited the highest relative expression of all the examined genes, particularly on 20E to 21E. Expression of TRPV6 decreased from 11E to 13E and increased substantially from 15E to 19E. No significant difference was found between the sampled days for ZnT-1 or SI expression, with the latter exhibiting the lowest relative expression of all the genes studied. These results present the first documentation of nutrient transporter and digestive enzyme gene-expression patterns in the yolk sac membrane, and provide a basis for future research on the capacity of the yolk sac membrane for nutrient digestion and transport.

The viability and performance under hot conditions of featherless broilers versus fully feathered broilers.

Hot conditions decrease the difference between ambient temperature (AT) and the average temperature of the body surface. A smaller difference reduces the rate of sensible heat loss of excessive internal heat, elevates the body temperature (BT), and may lead to mortality during heat waves. Under conditions of chronic heat, broilers avoid lethal BT elevation by reducing their feed intake; consequently, growth rate and meat yield are lower. Practices to avoid hot conditions are costly, whereas breeding for heat tolerance offers a sustainable approach. Being featherless was shown to provide heat tolerance; this was reevaluated in experimental broilers with a growth rate similar to that of contemporary commercial broilers. In experiment 1, 26 featherless birds and 49 feathered siblings (sibs) were reared at warm AT and exposed to moderate and acute heat waves. The featherless birds maintained normal BT under a moderate heat wave, with a slight elevation under an acute heat wave, and only 1 bird died. In contrast, the heat waves led to a significant elevation in BT of the feathered sibs, and 34% of them died. In experiment 2, featherless broilers were compared with feathered sibs and commercial broilers at 2 AT treatments: a constant temperature of 25°C (control AT) or a constant temperature of 35°C (hot AT). The birds were reared to 46 or 53 d at the control and hot AT, respectively, and the measured traits included BT, growth, and weight of the whole body and carcass parts (breast meat, legs, wings, and skin). At the hot AT, only the featherless broilers maintained a normal BT; their mean d 46 BW (2,031g) was significantly higher than that of birds maintained at the control AT, and it increased to 2,400 g on d 53, much higher than the corresponding means of all feathered broilers (approximately 1,700 g only). Featherless broilers had significantly higher breast meat yield (approximately 20% in both AT), lower skin weight, and supposedly better wing quality. These results confirmed that being featherless improved the livability and performance of fast-growing broilers in hot conditions and suggests that introduction of the featherless phenotype into commercial broiler stocks would facilitate highly efficient yet low-cost production of broiler meat under hot conditions.

Yolk sac nutrient composition and fat uptake in late-term embryos in eggs from young and old broiler breeder hens.

In the present study, we examined the composition, amount, and uptake of yolk nutrients [fat, protein, water, and carbohydrates (COH)] during incubation of eggs from 30- and 50-wk-old broiler breeder hens. Eggs were sampled at embryonic d 0 (fresh eggs), 13, 15, 17, 19, and 21 (hatch). Egg, embryo, yolk content, and yolk sac membrane were weighed, and the yolk sac (YS; i.e., yolk content + yolk sac membrane) composition was analyzed. From 30 to 50 wk of age, the albumen weight increased by 13.3%, whereas the yolk increased by more than 40%. The proportion of fat in the fresh yolk of the 30-wk-old group was 23.8% compared with 27.4% in the 50-wk-old group, whereas the proportion of protein was 17.9% compared with 15.6%, respectively. During incubation, results indicated that water and protein infiltrated from other egg compartments to the YS. Accordingly, the calculated change in the content of water and protein between fresh yolk and sampled YS does not represent the true uptake of these components from the YS to the embryo, and only fat uptake from the YS can be accurately estimated. By embryonic d 15, fat uptake relative to embryo weight was lower in the 30-wk-old group than in the 50-wk-old group. However, by embryonic d 21, embryos of both groups reached similar relative fat uptake, suggesting that to hatch, embryos must attain a certain amount of fat as a source of energy for the hatching process. The amount of COH in the YS increased similarly during incubation in eggs from hens of both ages, reaching a peak at embryonic d 19, suggesting COH synthesis in the YS. At hatch, the amount of protein, water, and COH in the residual YS, relative to the weight of the yolk-free chick, was similar in eggs from young and old hens. However, chicks from the younger hens had less fat in the YS for their immediate posthatch nutrition compared with those from the older hens.