The inverted teat defect is characterized by the failure of teats to protrude from the udder surface and has a negative effect on the economic efficiency of pig production. The inverted teat defect is influenced by genetic factors, but the number and identity of relevant genes are unknown. In this study, we compared the mRNA expression of teat tissues from unaffected pigs and affected pigs by using microarrays. Simultaneously, 24,123 probe sets were screened, of which some 15,000 had present calls and were analyzed for differential expression between mesenchymal and epithelial tissue of 3 categories of teats (i.e., normal teats of unaffected and affected animals, and inverted teats of the latter). Differential expression was more pronounced in epithelial than in mesenchymal tissue, and the comparisons among the 3 categories of teats showed that local processes at the side of the affected area as well as processes taking place at the level of the organ contribute to the development of inverted teats. Genes related to biofunctions of cell maintenance, proliferation, differentiation, and replacement; organismal, organ, and tissue development; genetic information and nucleic acid processing; and cell-to-cell signaling and interaction were differentially expressed, depending on the teat phenotype and the status of the animal as affected or unaffected. In particular, genes encoding members of canonical pathways of growth factor signaling were highlighted. Complementary to previous real-time quantitative reverse-transcription PCR experiments showing upregulation of growth factors (epidermal growth factor, fibroblast growth factor, hepatocyte growth factor, platelet-derived growth factor, vascular endothelial growth factor) and their receptors in the inverted teat, here it is shown that the abundance of transcripts encoding subordinated proteins (acid phosphatase 1, soluble; activating transcription factor 2; casein kinase 2, α 1 polypeptide; casein kinase 2, α prime polypeptide; actinin, α 2; and Homo sapiens growth factor receptor-bound protein 2) within the growth factor signaling pathways are also affected. Tuning of the expression of genes of these pathways balances the differentiation and proliferation of epithelial and mesenchymal teat tissue and finally affects the shape and structure of the teats.

BackgroundThe mammary gland is key to all mammal species; in particular in multiparous species like pigs the number and the shape of functional mammary gland complexes are major determinants of fitness. Accordingly, we aimed to catalog the genes relevant to mammogenesis in pigs. Moreover, we aimed to address the hypothesis that the extent and timing of proliferation, differentiation, and maturation proccesses during prenatal development contribute to postnatal numerical, morphological and functional properties of the mammary gland. Thus we focused on differentially expressed genes and networks relevant to mammary complex development in two breeds that are subject to different selection pressure on number, shape and function of teats and show largely different prevalence of non-functional inverted teats. The expression patterns of fetal mammary complexes obtained at 63 and 91 days post conception (dpc) from German Landrace (GL) and Pietrain (PI) were analyzed by Affymetrix GeneChip Porcine Genome Arrays.ResultsThe expression of 11,731 probe sets was analysed between the two stages within and among breeds. The analysis showed the largest distinction of samples of the breed GL at 63 dpc from all other samples. According to Ingenuity Pathways Analysis transcripts with abundance at the four comparisons made (GL63-GL91, PI63-PI93, GL63-PI63 and GL91-PI91) were predominantly assigned to biofunctions relevant to `cell maintenance, proliferation, differentiation and replacement´, `organismal, organ and tissue development´ and `genetic information and nucleic acid processing´. Moreover, these transcripts almost exclusively belong to canonical pathways related to signaling rather than metabolic pathways. The accumulation of transcripts that are up-regulated in GL compared to PI indicate a higher proliferating activity in GL, whereas processes related to differentiation, maturation and maintenance of cells are more prominent in PI. Differential expression was validated by quantitative RT-PCR of five genes (GAB1, MAPK9, PIK3C2B, PIK3C3 and PRKCH) that are involved in several relevant signaling pathways.ConclusionsThe results indicate that mammary complex development in PI precedes GL. The differential expression between the two breeds at fetal stages likely reflects the prenatal initiation of postnatal phenotypes concerning the number and shape as well as functionality of teats.

The inverted teat defect is the most common disorder of the mammary complex in pigs. It is characterized by the failure of teats to protrude from the udder surface, preventing normal milk flow and thus limiting the rearing capacity and increasing the risk of mastitis. The inverted teat defect is a liability trait with a complex mode of inheritance. We previously identified QTL for inverted teats. As a complementary approach that integrates map-based efforts to identify candidate genes for the inverted teat defect with function-driven expression analysis, application-specific microarrays were constructed that cover 1525 transcripts mapping in QTL regions on pig chromosomes 2, 3, 4, 6 and 11. About 950 transcripts were expressed in epithelial and mesenchymal teat tissue. The expression of three categories of teats was compared: normal teats of both non-affected and affected animals and inverted teats of affected animals. In epithelium and mesenchyme, 62 and 24 genes respectively were significantly differentially expressed (DE). The majority of biofunctions to which a significant number of DE genes were assigned are related to the following: (1) cell maintenance, proliferation, differentiation and replacement; (2) organismal, organ and tissue development; or (3) genetic information and nucleic acid processing. Moreover, the DE genes belong almost exclusively to canonical pathways related to signaling rather than metabolic pathways. This is in line with findings obtained by genome-wide catalogue microarrays. This study adds another piece to the puzzle of the etiology of inverted teats by indicating that causal genetic variation leading to the disorder is likely among the genes encoding for members of the signaling cascades of growth factors.