The Integrity of Mammary Alveolar Cells in Two Consecutive Lactations

Abstract

Reece and Warbritton ( l ) , using the colchicine technique, reported that mammary glands of rats pregnant during the first lactation exhibited very little mitotic activity between the fifth day of lactation and the second parturition. This indirect evidence indicated that some of the secretory cells present in the first lactation were probably present in the second lactation in rats pregnant during the first lactation. Hence, it was desirable to obtain direct evidence on this problem.The use of tritiated thymidine to label alveolar cells has the advantage that labeled cells could be traced through successive lactations through autoradiography. Although cell numbers can be estimated by DNA content, the DNA procedure can not be used to determine whether or not cells are involuting in instances where hyperplasia is also occurring. However, by using tritiated thymidine to label cells and the DNA procedure to estimate cell numbers, the percentage of "carryover" of alveolar cells from the first lactation to the second lactation can be determined in animals pregnant during the first lactation.Materials and Methods. Virgin female rats of the hooded Norway strain, weighing 160 to 170 g, were mated. They were maintained at a temperature of 72 t 2°F and under 10 hr of artificial light daily. The animals received Purina Laboratory Chow and water ad libitum. Prior to parturition, female rats were either isolated or cohabitated with a sexually mature male rat in a lactating cage. In cohabitated female rats, vaginal smears were examined each morning following parturition for the presence of sperm (day 1 of pregnan-lPaper of the Journal Series, N.J. Agr. Exp. Sta., Rutgers University, The State University, New Brunswick, N J. cy) until day 3 of lactation at which time the male rat was removed and the litter was standardized at 6 pups. T o aid implantation, litters were removed from mother rats at 4:OO p.m. on day 5 of pregnancy and returned at 8: 00 a.m. on day 6 (2).Rats that received tritiated thymidine were injected subcutaneously on the third day of lactation with 0.3 pCi/g of body weight (sp act = 6.7 Ci/mmole; New England Nuclear, Boston, MA), since La Ganga (3) found that mammary proliferation was greatest at this time in lactating hooded Norway rats. The effect of tritiated thymidine on mammary function (litter growth index and nucleic acid content) was determined in rats sacrificed on day 30 of the first lactation and in rats pregnant during the first lactation and sacrificed on day 4 of the second lactation. The percentage of alveolar cells present in the first lactation that was present in the second lactation (carry-over percentage) of rats pregnant during the first lactation was based on two criteria: the mean labeled daughter cell index and the mean total mammary gland DNA content found after injecting tritiated thymidine on day 3 of the first lactation. Rats were sacrificed on day 3 of the first lactation (1 hr after injection), on day 4 of the second lactation (with and without a nonlactating period prior to the...

Effects of Dry Period Length on Milk Yield and Mammary Epithelial Cells

“…As mentioned previously, the importance of the extracellular matrix on mammogenesis also has been suggested in cattle (Capuco et al, 1997). Pitkow et al (1972) also demonstrated reduced milk yield in CM rats, but provided evidence that omitting the dry period resulted in an increase in mammary cell carryover from one lactation to the next. In humans, women who nursed through the last trimester of pregnancy had infants with a 15% reduction in weight gain during the first month postpartum (Marquis et al, 2002).…”

Omitting the dry period between lactations does not reduce subsequent milk production in goats

“…There is evidence in rats for increased carry-over of mammary secretory cells from one lactation to the next when the dry period is omitted (Pitkow et al 1972). We have no direct evidence for or against this in goats.…”

Continuous Lactation in Dairy Cows: Effect on Milk Production and Mammary Nutrient Supply and Extraction

“…As a result, a higher number of old alveolar cells are being carried over from the previous lactation to the next in the continuously milked mammary gland. This has also been observed in rats with no dry period in between 2 lactations (Pitkow et al, 1972). The cows in the study by Capuco and Akers (1999) were killed before parturition, and the effect on their milk production in the next lactation therefore not known.…”