The composition of colostrum is quite different from that of mature milk. In particular, colostrum contains relatively high concentrations of proteins responsible for the immunological defence of newborns, such as lactoferrin, lactoperoxidase, lysozyme and immunoglobulins (Farkye, 1992; Cals et al. 1994; Telemo & Hanson, 1996; Wang et al. 1997). Whereas lactoferrin and lactoperoxidase are synthesized in mammary epithelial cells (Cals et al. 1994; Molenaar et al. 1996), the immunoglobulins are derived from blood serum (Larson, 1992). Immunoglobulin A (IgA), predominantly found in milk, participates in the development of the gastrointestinal system and the immune system in newborn infants. It is transported by a system that involves formation of a complex with the polymeric IgA receptor (pIgR) exposed on the basolateral aspect of mammary gland epithelial cells, followed by internalization and release into milk as secretory IgA (Rosato et al. 1995; De Groot et al. 1999). Consequently, expression of pIgR in mammary epithelial cells also contributes to the development of the immune system. These proteins specifically expressed in the early stage of lactation are of interest. However, few studies have been carried out compared with those focusing on the major proteins in mature milk, in part owing to problems of mammary gland availability.

Imamura et al. (1996) reported the isolation of mRNA from bovine mammary epithelial cells derived from colostrum and showed that it is possible to detect αs1-casein mRNA, predominantly expressed in the bovine mammary gland, through the application of reverse transcriptase polymerase chain reaction (RT-PCR). We have applied this procedure to porcine colostrum, and successfully analysed the coding sequence of pIgR. In this paper we describe the possibility of detecting gene expression in mammary epithelial cells present in colostrum and the porcine pIgR sequence obtained is compared with those of other animal species.