Age-associated changes in the shell gland transcriptomics and eggshell quality of broiler breeder hens
The egg production cycle of broiler breeder hens is comparatively shorter than laying hens, and as they age, their egg production and eggshell quality decline. The eggshell formation occurs in the shell glands, which are influenced by several factors, including aging. The objectives of the study were to 1) identify differentially expressed genes (DEGs) and biological pathways in the shell glands (young vs aged) and 2) determine the age-associated changes in eggshell quality. The shell glands tissues were collected from broiler breeder hens at peak-lay (35 weeks of age; termed as “young”) and late-lay phases (50 weeks of age; termed as “aged”) (n=30/group) at 10-15 hours post-ovulation (unclassified egg present in the shell glands). To delineate the genes and biological pathways associated with eggshell biomineralization, total RNAs extracted from the shell glands of young and aged hens (n=6/group) were analyzed using RNA sequencing and validated using real-time PCR. The ultrastructure quality of eggshells (n=10 eggs/group) was analyzed using a Scanning Electron Microscope (SEM), and the elemental composition of eggshells was measured using SEM-Energy Dispersive Spectrometry, and their variability was confirmed by t-test in RStudio. Eggshell strength, thickness, palisade layer, and mammillary width were significantly higher in the young, while mammillary knobs were wider in aged hens (p<0.05). The sulfur and potassium levels in eggshells were higher in young hens than aged ones. Although the young group had a higher calcium concentration in the eggshells, the difference was statistically insignificant (p>0.05). RNA-Seq data identified highly upregulated genes specific to eggshell biomineralization, such as SPP1 (binds to hydroxyapatite), OTOP2 (maintains high conc. of cytosolic Ca2+), PKD2 (helps in releasing Ca2+), SLC22A15 (transports organic ions), and STAB2 (binds to gram-positive and gram-negative bacteria). The DEGs showed significant enrichment for biological pathways (SLC6A6, KCNK7, UCP3, SCNN1A, PKD2, OTOP2) associated with the transport of monoatomic and inorganic cations across the cell membrane, molecular functions related to the transport of potassium ions and the activity of monoatomic cation channels (KCNK7, PKD2, OTOP2),  and the cellular components involved in the luminal side of the endoplasmic reticulum membrane (CALR, PKD2).  These findings suggest that the aging process downregulates the transcriptomes of the shell glands, negatively impacting the transportation of ions required for eggshell formation, resulting in poor eggshell quality. Overall design: RNA sequencing of the broiler breeder shell gland to compare between peak (35 weeks) and declining (50 weeks) production to see the traanscriptomic variation of the shell gland with the shell qualit changes.