Expression of genes associated with apoptosis in the residual yolk sac during the peri-hatch period of broiler chicks

The yolk sac (YS) consists of the yolk and the surrounding YS tissue, which provides essential nutrients and physiological functions for the developing embryo. After the YS is internalized into the abdominal cavity of the embryonic chick, the YS starts to degrade. Apoptosis, or programmed cell-death, is speculated to be the mechanism behind degradation of the YS. The objective of this study was to determine if degradation of the YS tissue was mediated by apoptosis during the perihatch period. The YS tissue was collected from broiler chicks from embryonic d 17 to d 7 posthatch. The mRNA abundance of genes that are involved in the regulation, initiation, and execution of apoptosis were analyzed by qPCR. The mRNA for Bcl2, Bcl2L11, cytochrome C and caspases 3, 6, 7, 8, 9, and 18 all showed a linear response from embryonic d 17 to d 7 posthatch. To confirm the role of apoptosis in the degradation of the YS tissue, a DNA fragmentation assay was performed. Degradation of genomic DNA in the YS tissue started on day of hatch. The characteristic ladder of oligonucleosomal-sized DNA fragments was observed on d 3, 5, and 7 posthatch. The combined gene expression and DNA fragmentation results demonstrate that degradation of the YS posthatch is mediated by apoptosis.