Endocrine changes during the peripartal period related to colostrogenesis in mammalian species.

This review discusses endocrine and functional changes during the transition from late gestation to lactation that are related to the production of colostrum in different mammalian species. Species covered in this article include ungulate species (cattle, sheep, goats, pigs, horses), rodents (rat, mouse), rabbits, and carnivores (cats, dogs), as well as humans. An immediate availability of high quality colostrum for the newborn after birth is crucial in species where a transfer of immunoglobulins (Ig) does not or only partially occur via the placenta during pregnancy. Declining activity of gestagens, in most species progesterone (P4), is crucial at the end of pregnancy to allow for the characteristic endocrine changes to initiate parturition and lactation, but the endocrine regulation of colostrogenesis is negligible. Both, the functional pathways and the timing of gestagen withdrawal differ considerably among mammalian species. In species with a sustaining corpus luteum throughout the entire pregnancy (cattle, goat, pig, cat, dog, rabbit, mouse, and rat), a prostaglandin F2α (PGF2α)-induced luteolysis shortly before parturition is assumed to be the key event to initiate parturition as well as lactogenesis. In species where the gestagen production is taken over by the placenta during the course of gestation (e.g., sheep, horse, and human), the reduction of gestagen activity is more complex, as PGF2α does not affect placental gestagen production. In sheep the steroid hormone synthesis is directed away from P4 towards estradiol-17ß (E2) to achieve a low gestagen activity at high E2 concentrations. In humans the uterus becomes insensitive to P4, as parturition occurs despite still high P4 concentrations. However, lactogenesis is not completed as long as P4 concentration is high. Early colostrum and thus Ig intake for immune protection is not needed for the human newborn which allows a delayed onset of copious milk secretion for days until the placenta expulsion causes the P4 drop. Like humans, horses do not need low gestagen concentrations for successful parturition. However, newborn foals need immediate immune protection through Ig intake with colostrum. This requires the start of lactogenesis before parturition which is not fully clarified. The knowledge of the endocrine changes and related pathways to control the key events integrating the processes of colostrogenesis, parturition, and start of lactation are incomplete in many species.

This manuscript reviews and compares hormonal and functional changes occurring in the conceptus (embryo and its extra-embryonic membranes) and their effects on the mammary gland during development from pregnancy to colostrum formation and milk production in multiple mammalian species. Declining activity of gestagens at the end of pregnancy is crucial to allow for both parturition and onset of milk production in most mammals. Strategies to achieve this state of low gestagen activity are different among species. In species where the corpus luteum is sustained throughout the entire pregnancy, luteolysis is the key event to initiate parturition and onset of milk secretion (cattle, goat, pig, cat, dog, rat, mouse, rabbit). However, in species where the placenta takes over gestagen production during the course of pregnancy, the achievement of a state of low gestagen activity is more complex. It ranges from redirection of the hormone production pathway away from gestagens in sheep, to decreasing sensitivity of the uterus towards gestagens in humans. In the horse, there is evidence pointing towards redirection of the hormone production as well as a decrease in sensitivity towards gestagens, but the exact mechanisms are still not clarified.

Implications of placentation type on species-specific colostrum properties in mammals.

Maternal care is essential to optimally support survival of the offspring. During evolution of mammalian species, different phenotypes have evolved in relation to gestation length, number, size, and maturation stage of the offspring at parturition, as well as colostrum and milk composition. The aim of the present review is to describe relationships between placental function and colostrum and milk composition in different mammalian species. Species covered in this article include humans, rabbits, rodents (rat and mouse), carnivores (cats and dogs), and a variety of ungulate species (cattle, sheep, goats, pigs, and horses). Species-specific aspects are elucidated with a special focus on the transfer of passive immunity. In this regard, the structure and thus the capability of the placenta to transport immunoglobulins from maternal to fetal circulation in utero dictates the necessity of the passive transfer of immunity via colostrum. Consequently, species with exclusive postpartal transfer of immunity such as in all ungulate species have greater immunoglobulin G concentrations in colostrum than species with a prepartal transfer in utero, where especially immunoglobulin A with its local immune function in the gastrointestinal tract is present in colostrum (e.g., rabbit and human). In terms of the nutritional purpose, suckling frequency is an important factor determining the gross composition of colostrum as well as in the mature milk of these species. Milk of nidicolous animals with long intervals in-between suckling events contains more fat than milk of nidifugous animals with constant access to their mother. However, the importance of colostrum and milk consumption for newborn animals and human babies goes beyond nutrition and the transfer of immunity. Numerous bioactive components such as growth factors, hormones, and oligosaccharides are enriched in colostrum and transition milk, which support the development of the intestinal tract and local immune system.

During evolution of mammalians, intrinsic strategies and components of maternal care during the transition from pregnancy to lactation have evolved into a broad variety in gestation length, number, size, and maturation stage of the offspring at parturition, and colostrum and milk composition. The original purpose of immuno-protective glandular secretions is still conserved in many mammalian species, where colostrum, that is, the very first milk obtained after parturition, contains the greatest amounts of immunoglobulins, leukocytes, lysozyme, lactoferrin, oligosaccharides, etc. Apart from its nutritive function, bioactive components in colostrum and milk support the development of gastrointestinal structures and intestinal microflora. Depending on the placentation type and intrauterine transfer of immunoglobulins, the survival of the neonate depends more or less on the passive transfer of immunoglobulins via colostrum. The aim of the present review is to describe relationships between placenta function and colostrum (and milk, respectively) composition in different mammalian species. Special attention is paid to the transfer of passive immunity from the dam to the offspring.