Is the apocrine milk secretion process observed in the goat species rooted in the perturbation of the intracellular transport mechanism induced by defective alleles at the alpha(s1)-Cn locus?

The structural and quantitative variability of caprine alpha(s1)-casein induced by the extensive polymorphism recorded at the corresponding locus strongly influences the composition (proteins as well as lipids) and the technological behaviour of milk. Immuno-histo-chemistry studies coupled with electron microscopy analysis have shown that a dysfunction exists in the intracellular transport of caseins when alpha(s1)-casein is lacking. Casein accumulation in the endoplasmic reticulum leads to a dilation of the cisternae that could disturb the whole secretion process (including lipids). Despite a long controversy, goat milk secretion is still considered to occur through an apocrine process contrary to the merocrine process described for cow's milk. We suggest that the apocrine pathway of secretion described in the goat could be the consequence of the dysfunction observed in the intracellular transport of caseins when alpha(s1)-casein is lacking. To obtain further clues in the favour of such a hypothesis, we compared the protein and lipid fractions of milks from goats homozygous for different alpha(s1)-casein alleles.