Chapter summary
Introduction
Lactation is critical in multiple aspects of animal production involving mammals. This is most apparent in dairy production but rearing of young to reach market weight or to produce new breeding stock also depends on successful lactations and mothering.
Within the mammary gland the multicellular alveolus is the key structure that is responsible for synthesis and storage of milk between milking or suckling episodes. Composed of an inner spherical layer of cuboidal epithelial cells with an internal lumen, it is surrounded by myoepithelial cells that contract and reduce the volume and elicit removal of milk from storage. The space that surrounds alveoli is filled with blood vessels, connective tissue proteins and cells (fibroblasts, adipocytes, white blood cells), and interstitial fluid. Study of the mammary gland and lactation involves scientists with multiple discipline interests, including molecular biologists, cancer biologists, dairyspecialists, endocrinologists, and others. Milk production and milk composition (other than genetic selection in dairy breeds) have evolved as part of reproductive strategies to ensure successful rearing of offspring.
Mammogenesis, lactogenesis, and galactopoiesis
Changes in the growth (mammogenesis) and development of the mammary gland are most apparent when considered in the context of physiological stages. The mammary gland is also unique compared with most organs because there is relatively little development until the postpartum period. Development during the early neonatal period has also traditionally been viewed as relatively unimportant. However, it is becoming increasingly clear that physiological, nutritional, and management conditions can have both positive and negative impacts on future development and function. Regardless, expressed on a mass or DNAbasis, growth of the mammary parenchymal begins in earnest with the onset of puberty.
But by far the largest proportion of mammary growth occurs during pregnancy. For the first two-thirds of gestation, growth consists of expansion of the mammary ductular tree. Estrogen, progesterone, and growth hormone (likely in the guise of IGF-I) are the primary drivers of mammary ductular growth (Purup et al., 2000). But a variety of growth factors are likely involved in multiple aspects of ductal cell proliferation and morphogenesis of the developing mammary gland. In the last third of gestation the mammary alveoli appear, but the alveolar epithelial cells are poorly differentiated. As parturition approaches, decreases in progesterone along with increasing serum estradiol, glucocorticoids, and prolactin induce the progressive differentiation of the mammary secretory cells so that synthesis and secretion of specific milk components (lactogenesis) can begin. Once lactation is established, it is essential that milk removal continues and that secretion of supportive hormones continues (growth hormone, glucocorticoids, thyroid hormones, etc.) so that milk secretion is maintained (galactopoiesis).Milk composition
When compared across species, milk composition is highly variable. But the composition has been tailored through evolutionary pressures to supply milk that is best suited for the survival of offspring. For example, milk of artic mammals or sea mammals is typically very high in fat (50% or more) but very low in lactose and water. The high fat supplies the energy necessary for survival and/or metabolic water for animals (seals for example) that are born on floating sea ice. For most mammals, the first secretions produced are usually high in protein due to the presence of immunoglobulins that accumulate in the alveolar spaces during late gestation. For mammals in which there is minimal or no transfer of antibodies to the fetus in utero, passive immunity supplied via the consumption of colostrum and gut transfer is vitally important. Drivers for accumulation of antibodies include the presence of "leaky" tight junctions between alveolar cells prior to parturition as well as expression of IgG receptors on the undifferentiated the alveolar cells.
As parturition approaches, increased prolactin and glucocorticoid secretion induces alveolar cell differentiation and inhibition of IgG receptors. With the induction of lactose synthetase and induction of specific milk proteins (caseins, OC-Iactalbumin, β-lactoglobulin), the composition of mammary secretions becomes progressively similar to the mature milk for the particular mammal.Review questions and answers are available ¾ online.
References
Akers, R.M. 1985. Lactogenic hormones: binding sites, mammary growth, secretory cell differentiation, and milk biosynthesis in ruminants. J. Dairy Sci. 68: 501-519.
Akers, R.M. 2002. Lactation and the Mammary Gland. Iowa State Press, Ames, Iowa, 278 pages.
Akers, R.M. 2006. Major advances associated with hormone and growth factor regulation of mammary growth and lactation in dairy cows. J. Dairy Sci. 89: 1222-1234.
Akers, R.M., D.E. Bauman, A.V. Capuco, G.T. Goodman, and H.A. Tucker. 1981a. Prolactin regulation of milk secretion and biochemical differentiation of mammary epithelial cells in periparturient cows. Endocrinology 109: 23-30.
Akers, R.M., D.E. Bauman, G.T. Goodman, A.V. Capuco, and H.A. Tucker. 1981b. Prolactin regulation of cytological differentiation of mammary epithelial cells in periparturient cows. Endocrinology 109: 31-40.
Akers, R.M., W.E. Beal, T.B. McFadden, and A.V. Capuco. 1990. Morphometric analysis of involuting bovine mammary tissue after 21 or 42 days of non-suckling. J. Anim. Sci. 68: 3604-3613.
Akers, R.M., A.V. Capuco, and J.E. Keys. 2006. Mammary histology and alveolar cell differentiation during late gestation and early lactation in mammary tissue of beef and dairy heifers. Livest. Sci. 105: 44-49.
Asimov, GJ. and N.K. Krouze. 1937. The lactogenic preparations from the anterior pituitary and the increase of milk yield in cows. J. Dairy Sci. 20: 289-306.
Bar-Peledz U.z E. Maltzz I. Bruckentalz Y. Folmanz Y Kali, H.
Gacituaz A.R. Lehrer, C.H. ICnightz B. Robinzonz H. Voetz and H. Tagari. 1995. Relationship between frequent milking or sucking in early lactation and milk production of high producing dairy cows. J. Dairy Sei. 78: 2726-2736.Bauman, D.E. 1999. Bovine somatotropin and lactation: from basic science to commercial application. Domest. Anim. Endocrinol. 17: 101-116.
Bauman, D.E.z R.W. Everett, W.L. Weiland, and RJ. Collier. 1999. Production responses to bovine somatotropin in northeast dairy herds. J. Dairy Sei. 82: 2564-2573.
Beal, W.E.z D.R. Notterz and R.M. Akers. 1990. Techniques for estimation of milk yield in beef cows and relationships to calf weight gain and postpartum reproduction. J. Anim. Sci. 68: 937-943.
Brennan, A.J.z J.A. Sharp, C. Lefevre, D. Topcic, A. Auguste, M. Digby, and K. Nicholas. 2007. The tammar wallaby and fur seal: models to examine local control of lactation. J. Dairy Sci. 90: (E. Suppl.): E66-E75.
Capuco, A.V. and R.M. Akers. 1999. Mammary involution in dairy animals. J. Mammary Gland Biol. Neoplasia 4: 137-144.
Capuco, A.V. and R.M. Akers. 2010. Management and environmental influences on mammary gland development and milk production. In Managing Prenatal Development to Enhance Livestock Productivity, pp. 259-292. Edited by P.L. Greenwood, A.W. Bell, RE. Vercoe, and GJ. Viljoen. Springer, Amsterdam, The Netherlands, IAEA.
Capuco, A.V., PA. Feldhoff, R.M. Akers, J.L. Wittliff, and H.A. Tucker. 1982. Progestin binding in mammary tissue of prepartum, nonlactating and postpartum, lactating cows. Steroids 40: 503-517.
Capuco, A. V, S. Ellis, D.L. Wood, R.M. Akers, and W. Garrett. 2002. Postnatal mammary ductal growth: three-dimensional imaging of cell proliferation, effects of estrogen treatment and expression of steroid receptors in prepubertal calves. Tissue Cell 34: 143-154.
Capuco, A.V., R.K. Choudhary K.M. Daniels, R.W. Li, and C.M.
Evock-Clover. 2012. Bovine mammary stem cells: cell biology meets production agriculture. Amimal 6: 382-393.Collier, RJ., J.C. Byatt, S.C. Denham, PJ. Eppard, A.C. Fabel- lar, R.L. Hintz, M.F. McGrath, C.L. McLaughlin, J.K. Shearer, JJ. Veenhuizen, and J.L. Vicini. 2001. Effects of sustained release bovine somatotropin (Sometribove) on animal health in commercial dairy herds. J. Dairy Sei. 84: 1098-1108.
Cowie, A.T. 1969. Lactogenesis. In The Initiation of Milk Secretion at Parturition, pp. 157-169. Edited by M. Reynolds and SJ. Folley University of Pennsylvania Press, Philadelphia.
Cowie, A.T., LA. Forsyth, and I.C. Hart. 1980. Hormonal Control of Lactation. Springer-Verlag Berlin, Heidelberg.
Cowie, A.T., J.S. Tindal, and A. Yokoyama. 1966. The induction of mammary growth in the hypophysectomized goat. J. Endocrinology 34: 185-195.
Ellis, S., R.M. Akers, A.V. Capuco, and S. Safayi. 2012. Bovine mammary epithelial cell lineages and parenchymal development. J. Anim. Sci. 90: 1666-1673.
Filep, R. and R.M. Akers. 2000. Casein secretion and cytological differentiation in mammary tissue from bulls of high or low genetic merit. J. Dairy Sci. 83: 2261-2268.
Francis, C.M., E.L.P. Anthony, J.A. Brunton, and T.H. Ktmz. 1994. Lactation in male fruit bats. Nature 367: 691-692.
Garst, A.S., S.F. Ball, B.L. Williams, C.M. Wood, JW. Knight, H.D. Moll, C.H. Aardema, and EC. Gwazdauskas. 1999a. Technical note: machine milking of sows—lactational milk yield and litter weights. J. Animal Sci. 77: 1620-1623.
Garst, A.S., S.F. Ball, B.L. Williams, C.M. Wood, JW. Knight, H.D. Moll, C.H. Aardema, and EC. Gwazdauskas. 1999b. Influence of pig substitution on milk yield, litter weights, and milk composition of machine milked sows. J. Anim. Sci. 77: 1624-1630.
Goodman, G.T., R.M. Akers, K.H. Friderici, and H.A. Tucker. 1983. Hormonal regulation of Oi-Iactalbumin secretion from bovine mammary tissue cultured uι vitro. Endocrinology 112: 1324-1330.
Hanwell, A. and M. Peaker. 1977. Physiological effects of lactation on the mother. Symp. ZooL Soc. Lond. 41: 297-312.
Jenness, R. 1974. The composition of milk. In Lactation: A Comprehensive Treatise, Vol. Ill, pp. 3-96. Edited by B.L. Larson and VR. Smith. Academic Press, New York and London.
Keys, J.E., A.V. Capuco, R.M. Akers, and J. Djiane. 1989. Comparative study of mammary gland development and differentiation between beef and dairy heifers. Domest. Anim. Endocrinol. 6: 311-319.
Ktmz, T.H. and D.J.s Hosken. 2009. Male lactation: why, why not and is it care? Trends EcoL Evol. 24: 80-85.
Lamb, G.C., B.L. Miller, J.M. Lynch, D.M. Grieger, J.S. Stevenson, and M.C. Lucy. 1999. Suckling reinitiated milk secretion in beef cows after an early postpartum hiatus of milking or suckling. J. Dairy Sci. 82: 1489-1496.
Larson, B.L. 1985. Biosynthesis and cellular secretion of milk. In Lactation. Edited by B.L. Larson. Iowa State University Press, Ames, Iowa.
Lyons, W.R., C.H. Li, and R.E. Johnson. 1958. The hormonal control of mammary growth. Recent Prog. Horm. Res. 14: 219-254.
McFadden, T.B., T.E. Daniel, and R.M. Akers. 1990. Effects of plane of nutrition, growth hormone, and Imsaturated fat on mammary growth in prepubertal lambs. J. Anim. Sci. 68: 3171-3179.
Mepham, T.B., 1983. Physiological aspects of lactation. In Biochemistry of Lactation, pp. 4-28. Edited by TB. Mepham. Elsevier Science Publishers, B.V. Amsterdam.
Nandi, S. 1958. Endocrine control of mammary gland development and function in the C3H∕He Crgl mouse. J. Natl. Cancer Inst. 21: 1039-1360.
Nickerson, S.C. and R.M. Akers. 1984. Biochemical and Ultrastructural aspects of milk synthesis and secretion. Int. J. Biochem. 16: 855-865.
Nickerson, S.C. and J.W. Pankey. 1983. Cytological observations of the bovine teat end. Am. J. Vet. Res. 44:1433-1441.
Oftendal, O.T. 1984. Milk composition, milk yield and energy output at peak lactation: a comparative review. Symp. ZooL Soc. Lond. 51: 33-85.
Oftendalz O.T. 1997. Lactation in whales and dolphins: evidence of divergence between baleen- and toothed-species.
J. Mammary Gland Biol. Neoplasia 2: 205-230.
Peaker, M. 1994 Male suckling. Nature 371: 292.
Purupz S.z K. Sejrsenz J. Foldagerz and R.M. Akers. 1993. Effect of exogenous bovine growth hormone and ovariectomy on prepubertal mammary growth, serum hormones and acute in-vitro proliferative response of mammary explants from Holstein heifers. J. Endocrinol. 139: 19-26.
Purupz S.z K. Sejrsenz and R.M. Akers. 1995. Effect of bovine GH and ovariectomy on mammary sensitivity to IGF-I in prepubertal heifers. J. Endocrinol. 144: 153-158.
Purupz S.z M. Vestergaardz M.S. Weber, K. Plautz R.M. Akers, and K. Sejrsen. 2000. Local regulation of pubertal mammary growth in heifers. J. Anim. Sci. 78(Suppl. 3): 36-47.
Raceyz D.N.z M. Peaker, and PA. Racey 2009. Galactorrhoea is not lactation. Trends EcoL Evol. 24: 354.
Riddle, O.z R.W. Bates, and S.W. Dykshorn. 1933. The preparation, identification and assay of prolactin- a hormone of the anterior pituitary. Am. J. Physiol. 105: 191-216.
Rosen, J.M.z S.L. Wyszomierski, and D. HadselL 1999. Regulation of milk protein gene expression. Annu. Rev. Nutr. 19: 407-436.
Rowsonz A.R.z K.M. Daniels, S.E. Ellis, and R.C. Hovey. 2012. Growth and development of the mammary glands of livestock: a veritable barnyard of Opportrmities. Semin. Cell Dev. Biol. 23: 557-566.
Sejrsen, K. and S. Purup. 1997. Influence of prepubertal feeding level on milk yield potential of dairy heifers: a review. J. Anim. Sci. 75: 828-835.
Sejrsen, K., J.T. Huber, H.A. Tucker, and R.M. Akers. 1982. Influence of plane of nutrition on mammary development in per-and post-pubertal heifers. J. Dairy Sci. 65: 793-800.
Sejrsen, K., J. Foldager, M.T. Sorensen, R.M. Akers, and D.E. Bauman. 1986. Effect of exogenous bovine somatotropin on pubertal mammary development in heifers. J. Dairy Sci. 69: 1528-1535.
Sejrsen, K., S. Purup, M. Vestergard, and J. Foldager. 2000. High body weight gain and reduced bovine mammary growth: physiological basis and implications for milk yield potential. Domest. Anim. Endocrinol. 19: 93-104.
Sharp, J.A., K.N. Cane, C. Lefevre, J.P.Y. Arnould, and
K. R. Nicholas. 2006. Fur seal adaptations to lactation: insights into mammary gland function. Curr. Top. Dev. Biol. 72: 275-308.
Sheffield, L.G. 1988. Organization and growth of mammary epithelia in the mammary fat pad. J. Dairy Sci. 71: 2855-2874.
Sinha, Y.N. and H.A. Tucker. 1969. Mammary development and pituitary prolactin level of heifers from birth through puberty and during estrus cycle. J. Dairy Sci. 52: 507-512.
Smith, JJ., A.V. Capuco, and R.M. Akers. 1987. Quantification of progesterone binding in mammary tissue of pregnant ewes. J. Dairy Sei. 70: 1178-1185.
Smith, JJ., A.V. Capuco, W.E. Beal, and R.M. Akers. 1989. Association of prolactin and insulin receptors with mam- mogenesis and lobulo-alveolar formation in pregnant ewes. Int. J. Biochem. 21: 73-81.
Smith, K.L. and F.L. Schanbacher. 1973. Hormone induced lactation in the bovine. I. Lactational performance following injections of 17β-estradiol and progesterone. J. Dairy Sei. 56: 738-745.
Stelwagen, K. 2001. Effect of milking frequency on mammary functioning and shape of the lactation curve. J. Dairy Sei. 84:(E. Suppl.):E204-E211.
Stinnakre, M.G., J.L. Vilotte, S. Soulier, and J.C. Mercier. 1994. Creation and phenotypic analysis of Oc-Iactalbumin- deficient mice. Proc. Natl. Acad. Sci. U.S.A. 91: 6544-6548.
Stricker, P. and R. Grueter. 1928. Action du lobe anterieur de Fhypophyse sur la montee Iaiteuse. C.R. Soc. Biol. (Paris) 99: 1978-1980.
Swett, WW., PC. Underwood, C.A. Matthews, and R.R. Graves. 1942. Arrangement of the tissues by which the cow's udder is suspended. J. Agric. Res. 65: 19-42.
Topper, YJ. and C.S. Freeman. 1980. Multiple hormone interactions in the developmental biology of the mammary gland. Physiological Rev. 60: 1049-1106.
Tucker, H. A. 1981. Physiological control of mammary growth, lactogenesis, and lactation. J. Dairy Sci. 64: 1403-1421.
Tucker, H.A. 1987. Quantitative estimates of mammary growth during various physiological states: a review. J. Dairy Sci. 70: 1958-1966.
Tucker, H.A. 1994. Lactation and its hormonal control. In The Physiology of Reproduction, 2nd edition, pp. 1065- 1098. Edited by E. Knobil and J.D. Neill. Raven Press, New York.
Tucker, H.A. 2000. Hormones, mammary growth, and lactation: a 41-year perspective. J. Dairy Sci. 83: 874-884.
Wilde, CJ., C.H. Knight, and DJ. Flint. 1999. Control of milk secretion and apoptosis during mammary involution. J. Mammary Gland Biol. Neoplasia 4: 129-136.