ABSTRACT
Immune transmission from mother to offspring first requires the transfer of immunoglobulin from the maternal serum across cellular barrier(s) to one of the secretions or to the yolk and, hence, the transfer of the same immunoglobulin across cellular barrier(s) to the serum of the offspring. The immunoglobulin passes from the thecal vessels between the granulosa cells to the oocyte and accumulates in the yolk. During embryogenesis the developing yolk sac surrounds the yolk, and the endodermal cells take it up. The yolk will be digested intracellularly and the nutrients will be released into the circulation, while the immunoglobulin somehow escapes digestion during transmission. Lacteal secretion is one of the most important common characteristics of mammalian species. This evolutionary “innovation” opened an alternative means for the postnatal provision of the young with nutrients and immunoglobulins, yet the yolk remained important in the most primitive mammals, the Monotremata.
