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Thomason, Ryan

Animal Science

2004

M.S.

Development of the mammary gland is crucial in determining the milk producing ability of the dairy cow. A wide variety of factors have been found to influence mammary development including photoperiod and hormonal regulation. Optimization of these factors could lead to improved milk production efficiency. Therefore, it is important to understand the mechanisms by which these factors affect mammary development. The objectives of this research were to investigate the effects of photoperiod, hormonal induction of lactation or ovariectomy on the expression of candidate proliferation-associated genes during mammary development. An animal trial was conducted to investigate the effects of photoperiod and hormone-induced lactation on the regulation of mammary development. Exposure to short-day (SD; 8 h light: 16 h dark) photoperiod during hormone-stimulated mammary development in pubertal heifers resulted in significantly higher (~ 20%) milk yields compared to heifers exposed to long-day (LD; 16h light: 8 h dark). Mammary cell proliferation did not differ between treatments at the times sampled during mammary development, however an overall effect of time occurred. No differences in mammary differentiation were detected between photoperiod treatments as histological analysis and alpha-lactalbumin mRNA were not different at the times sampled. Therefore, it is still unclear how this effect occurred. All functional indicators of hormone-induced lactation confirm the utility of induced lactation as a model for mammary development and demonstrate its potential for practical application.
The second aim of this research was to determine the expression profiles of candidate proliferation-associated genes and to determine their involvement in mammary growth. I first took a genomics approach using techniques from cDNA microarray experiments to develop and partially validate a low-density cDNA array. The assay was reproducible and linear for quantitation of high abundance genes. However, the assays sensitivity limited the detection of low abundance mRNAs such as proliferation associated genes. mRNA expression of candidate genes CYR61, BTG-1, PEDF, PLCE, and PEDF along with the proliferation marker PCNA were determined by quantitative RT-PCR and temporal expression profiles were examined in two experimental models that manipulate mammary growth through hormonal regulation. The expression profile of the proliferation marker, PCNA, agreed with previous proliferation data from both models thereby implying that the expression profiles of candidate genes associated with proliferation could be accurately determined. Candidate genes CYR61, BTG-1, and PEDF were differentially expressed in hormonally regulated models, thereby suggesting a role for each gene in regulation of mammary growth. However, PLCE and PSMA2 were not differentially expressed, therefore it was concluded that they are not obligately involved in regulation of mammary growth. It appears that CYR61, BTG-1, and PEDF are involved in regulation of mammary growth and development, however, further studies need to be conducted to describe their precise role in the mammary gland.