CHANGES IN THE MICROSCOPIC ANATOMY OF THE BOVINE TEAT CANAL DURING MAMMARY INVOLUTION

Changes in the microscopic anatomy of the bovine teat canal were examined during mammary involution. Morphometric analyses revealed a significant (P < 0.05), temporary dilatation of the teat canal lumen on day 7 of the nonlactating period. The teat canal epithelium physiologically atrophied as evidenced by decreased cross-sectional area and thickness during the first 30 days of the nonlactating period, significantly so (P < 0.05) between days 0 and 7. This physiologic atrophy was due mainly to a reduction in area and thickness of the stratum granulosum and may have resulted from continuing keratinization, a process that led to increased thickness of the keratin layer and formation of a functional plug during later stages of involution. Changes in cells of the stratum granulosum indicated a decrease in the rate of epithelial cell maturation during involution. The mitotic index (percentage of basal cells in mitosis) of the teat canal epithelium decreased significantly (P < 0.05) between days 0 and 7 of the nonlactating period. Bacteria, observed in histologic sections, appeared to colonize only certain regions of the keratin layer. Seemingly, changes in the teat canal during mammary involution may be important factors in changing susceptibility to new intrammmary infection during the early and mid-nonlactating periods.