Colostrogenesis and Colostral Components
Colostrogenesis begins 5 weeks before calving under the influence of prolactin and other lactogenic hormones, with peak production occurring 1 to 3 days prior to parturition.2 Colostrum is nutrient dense and has a higher level of energy and protein than is found in milk.
Bovine colostrum contains approximately 45 mg/mL3 of immunoglobulin and 1 ? 106 leukocytes/mL.4 Immunoglobulin G1 (IgG1) is transported from blood in endosomes through the polarized mammary epithelial cell by a process known as transcytosisl IgG1 is bound by the neonatal Fc receptor (FcRn) at the basal side and transferred to the apical side under the control of intracellular proteins termed small GTPases.6 The FcRn consists of two subunits: an integral membrane α-chain (Fc fragment of IgG, receptor, transporter, alpha; FcGRT) and a β light chain (β2 microglobulin; β2M). In vitro the expression of FcGRT and several of the small GTPases controlling the transfer of IgG are upregulated by estrogen and progesterone, and it is thought that this mechanism results in the marked increase in transport of IgG1 observed in the 4 weeks prior to calving.6,7 Immunoglobulins account for more than 95% of the whey protein at calving, and IgG1 accounts for approximately 80% of this amount.8 IgG2, IgA, and IgM account for 10%, 7%, and 5%, respectively. The amount of IgA remains constant throughout all lactations, whereas the colostrum of cows in their first lactation contains less IgG1, IgG2, and IgM. Immunoglobulin levels in mature milk are much lower, comprising only 7% of the total whey protein, with an increase in the relative contribution of IgA. Older cows tend to have a higher ratio of IgG1 to IgG2.9 This is consistent with a lower level of IgG1 in the serum of heifers. The transport of IgG into bovine colostrum has been associated with a transient decrease in serum total protein, globulin, and IgG concentrations during the preparturient period, particularly in dairy cows. Up to 500 g of IgG1 a week are removed from blood during the most active period of colostrum formation.10Colostrum also contains hormones (growth hormone, relaxin, prolactin, insulin, glucagon), essential fatty and amino acids, minerals, trace elements and vitamins (especially betacarotene; vitamins A, B, D, and E) (Table 19.1), as well as cytokines, growth factors (members of the insulin-like growth factor [IGF] system, including IGF binding proteins [IGFBPs]), and nonspecific antimicrobial factors such as lactoferrin. Colostrum is a significant source of copper to the newborn calf but not 11
a source of selenium or zinc.11