Passage of a heterologous protein through ileal enterocytes of newborn piglets: immunolabelling of bovine serum albumin at the light- and electron microscopic level.

The passage of bovine serum albumin through ileum enterocytes of neonatal pigs was studied by light microscopy with indirect immunoperoxidase and by electron microscopy with post-embedding direct immunogold methods. Vacuoles filled with the heterologous protein were seen as early as 10 min after the administration of either bovine serum or solutions of bovine serum albumin by gavage. The sizes of vacuoles increased with time, their electron densities and immunoreactivities were at variance. The formation of albumin-containing vacuoles was independent of the concentration of the solutions fed, ranging from 1 to 7%. Bovine serum albumin becomes discernible in the capillaries at 4 h after feeding. By then, the intact albumin transported through enterocytes amounted to more than 10% of the circulating plasma proteins. Of several thousand enterocytes screened in the whole study only one--from the piglet 4 h after feeding--contained lysosomes.