Na 1- 14 C stearate and Na 2- 14 C acetate metabolism and morphological analysis of late prepartum bovine mammary tissue

Short communication: Ability of cultured mammary epithelial cells in a bicameral system to secrete milk fat

Mammary epithelial cells from lactating cows were cultured onto inserts coated with type I collagen. Every second day, the rates of fatty acid synthesis and secretion were determined by measuring the amount of [14C]-labeled sodium acetate incorporated into lipids over a 4-h period. The [14C]-containing lipids were identified by thin layer chromatography fractionation. In parallel, the integrity of the cell layer was evaluated by measurement of transepithelial electrical resistance. The integrity increased progressively to reach a maximum after 8 d of culture. Cells incorporated acetate into lipids; 1.34% of acetate was incorporated into lipids produced by freshly isolated cells. This percentage decreased to 0.5% after 2 d of culture. Moreover, this capacity decreased with the duration in culture; on d 8, the rate of incorporation dropped to about 3% of that on d 2. In the cell extracts, the [14C]-labeled lipids were mainly triglycerides, although the proportion of diglycerides and phospholipids progressively increased as a part of total newly synthesized lipids. The proportion of triglycerides decreased 0.66 times between d 2 and 8 when the proportion of diglycerides and phospholipids increased 1.33 and 2.18 times, respectively. About 28% of the newly synthesized lipids were secreted within 4 h of incubation. Around 65 to 85% of these labeled lipids were found in the apical compartment, suggesting a partially vectorial secretion. But 58 to 80% of labeled lipids found in the apical and basolateral medium were free fatty acids. Functional tight junctions and incorporation of labeled fatty acids into triglycerides are not compatible with an inferred status of complete dedifferentiation of the cell layer. Moreover, triglyceride secretion seems compromised, probably due to the lack of an appropriate cell environment and cell shape.

Changes in the lipid composition of the secretions of the bovine mammary gland during the dry period

To study initiation of milk fat synthesis, lipid composition of mammary secretions at -60, -40, and -10 d prepartum was studied in lactating and nonlactating Holstein cows. Eleven cows were dried off, and 13 cows were milked twice per day throughout the normal dry period. Total neutral lipid was similar in late lactation milk (-60 d) from lactating cows, 2.1 g/dl, and in milk from the dry group, 2.2 g/dl. Neutral lipids decreased to 1.3 and .9 g/dl in quarters from dry cows at -40 and -10 d prepartum. In secretions from dry quarters, triglycerides were 97% of total lipids at -60 d and decreased to 85 and 91% at -40 and -10 d, respectively. Conversely, FFA and monoglycerides increased during the dry period. Lipids associated with fat globule membrane components increased during the dry period. These increases were 10 times for cholesterol, 20 times for cholesteryl esters, and twice for phospholipids. In general, the content of fat globule core lipids (triglycerides) exhibited a pattern opposite that of membrane lipids (cholesterol, cholesteryl esters, phospholipids) during the prepartum period. Proportions of core lipids tended to decrease, whereas proportions of membrane lipids increased in prepartum mammary secretions. Lipid composition of prepartum secretions may be influenced by blood lipids, somatic cells, and alterations in mammary lipid synthesis.

Mammary secretion of triglycerides in the cow pre-partum

1. Regular removal of fluid from the mammary gland, before parturition, stimulated its secretion of triglycerides, and particularly C18 fatty acids in the triglycerides, in some individual cows. 2. Cows that did not respond to pre-partum emptying were stimulated to secrete triglycerides by parturition. 3. The onset of secretion of milk triglycerides correlated with the onset of secretion of mammary fluid. 4. No increase in the ratio of C18:1 to C18:0 in the triglycerides, reflecting an onset of mammary desaturase activity, was observed.

Effect of prepartum milk removal on quantitative morphology of bovine lactogenesis

The effect of prepartum milking on bovine mammary histology and milk composition was studied in five Holstein cows. One udder half was milked twice daily starting 10 days before the estimated calving date. On udder half was not milked until 2 days postpartum when all quarters were milked and two tissue samples per quarter were taken for morphological analyses. Lactose, casein, and fat concentrations differed from prepartum-milked to postpartum-milked quarters (.82, -1.44, and 3.29%). Quarters milked prepartum were more developed: mature epithelium 24.8%, immature epithelium - 8.9%, total epithelium 15.6%, stroma -24.8%, and alveolar lumen 8.5%. Most alveolar cells in prepartum-milked halves had prominent rounded basal nuclei, hypertrophied secretory vesicles, and a mixed array of lipid droplets. Cells within alveoli were morphologically similar. Secretory epithelial cells from postpartum-milked glands had irregular randomly located nuclei, indistinct Golgi, variable lipid content, and less cytoplasm. These differences demonstrate the importance of prepartum-product removal on secretory cell development and subsequent milk production.

Hormonal control of mammogenesis and onset of lactation in cows--a review

Estrogen stimulates development of mammary ducts, and progesterone and estrogen stimulate proliferation of secretory tissues. In vivo, sequential addition of insulin (step 1), glucocorticoid (step 2), and prolcatin (step 3) leads to biosynthesis of casein and lactose. In cows, mammogenesis continues until termination of pregnancy and overlaps onset of lactation. Progesterone probably inhibits differentiation of secretory cells at step 2 or step 3. Sensitivity of individual cells to progestational inhibition may decrease variably which may be interdependent upon relative increases in estrogen, prolactin, corticoids, and growth hormone to cause asynchronies among them at calving. Since prolactin in plasma is not correlated with progesterone or the estrogens, factors other than feed-back effects of ovarian steroids may be responsible for its sustained increase periparturiently. Also, elevated prolactin periparturiently may be unrelated to subsequent rates of lactation because its "basal" concentrations may meet requirements when inhibiting effects of progesterone are removed. This concept is attractive because mammary cells neither are synchronized highly for biosynthesis nor secrete normal milk for several days after calving. At the latter time, concentrations in plasma are low for progesterone and estrogen, similar to 3 days before calving for glucocoiticoids and prolactin, and increasing for insulin. Evidence of lactation under unusual circumstances was discussed.

Histology of induced bovine lactogenesis

Nine cycling nonlactating dairy cows were treated hormonally to induce lactation with progesterone, estradiol-17beta, and dexamethasone. Mammary tissue biopsies were taken on days 18, 21, and 23 after initiation of treatment (different three cows for each period). First milking was initiated on day 21 in the latter two groups. Mean differences in tissue development (percent area) between day 18 and 23 were, epithelium + 11.1%, stroma -25.9%, alveolar lumen + 13.9%, adipose + 1.58%, duct - 974%, and + 15 nuclei/alveolar section. On day 18 epithelia containing small lipid droplets predominated but were replaced with mixed lipid droplets by day 23. Epithelia day 18 had elongated dark staining nuclei. Epithelia on day 23 had spherically shaped nuclei often containing several nucleoli and an increased cytoplasmic/nuclear ratio. Marked histological development occurred from 18 to 23 days after initiation of treatment. Mammary tissue development in nongravid hormonally treated animals was equal to or greater than 2-wk-prepartum tissue but less than 2-day-prepartum tissue for natural lactations.

Metabolic adaptations during lactogenesis. Fatty acid and lactose synthesis in cow mammary tissue

1. Mammary-tissue biopsies were obtained from multiparous cows at 30 and 7 days pre partum and 7 and 40 days post partum. Investigations of the effect of lactogenesis on fatty acid and lactose synthesis involved measurements of biosynthetic capacity (tissue-slice incubations in vitro) and activities of relevant enzymes. 2. Fatty acid synthesis from acetate increased over 20-fold from 30 days pre partum to 40 days post partum. Changes in the lipogenic capacity of mammary-tissue slices more closely paralleled increases in the activities of acetyl-CoA carboxylase (EC 6.4.1.2) and acetyl-CoA synthetase (EC 6.2.1.1) than of other enzymes involved in acetate incorporation into fatty acids or in NADPH generation. 3. Lactose biosynthesis by mammary-tissue slices, lactose synthetase activity (EC 2.4.1.22) and alpha-lactalbumin concentration were all negligible at 30 days pre partum but increased 2.5-4-fold between 7 days pre partum and 40 days post partum. Phosphoglucomutase (EC 2.7.5.1), UDP-glucose pyrophosphorylase (EC 2.7.7.9) and UDP-glucose 4-epimerase (EC 5.1.3.2) had substantial activities at 30 days pre partum and increased less dramatically during lactogenesis. 4. Results are consistent with acetyl-CoA carboxylase and perhaps acetyl-CoA synthetase representing the regulatory enzyme(s) in fatty acid synthesis, with lactose synthetase (alpha-lactalbumin) serving a similar function in lactose biosynthesis.