Mammals placental transfer

The precursors of both n-6 and n-3 polyunsaturated fatty acids (PUFAs), linoleic acid and (/.-linolenic acid, respectively, are essential for mammals as they are required for normal physiological function and cannot be synthesized de novo (Holman, 1968). They can only be accumulated by placental transfer or by dietary intake. Once accretion of these fatty acids has occurred, metabolic, conservation and recycling pathways sustain them (B azan et al., 1994). Unlike mammals, plants can synthesize these precursor PUFAs (linoleic and a-linolenic acids) so they are found in abundance in the chloroplast membranes of plants, in certain vegetable oils, and in the tissues of plant-eating animals (Nettleton, 1991). The best sources of a-linolenic acid are vegetable oils, such as perilla (Yoshida et al., 1993) rapeseed (canola), linseed, walnut, and soybean (Nettleton, 1991). They are also abundant in shellfish, fish, and fish products and can be found in low amounts in green, leafy vegetables and baked beans (Nettleton, 1991 Sinclair, 1993). 

Most rodents are herbivorous and, therefore, do not have access to preformed DHA in their diet. Consequently, in order to produce a brain with the same DHA content of other mammals, they have developed an efficient system for converting the n-3 precursors of DHA (e.g., linolenic acid LNA), which are commonly found in fruits, nuts, and other dietary vegetable sources, into DHA (Pawlosky, et al., 1996). In addition, they also have an efficient system for the transfer of preformed DHA from the mother to the growing fetus in utero (placental transfer) or postnatally to the infant through their mother s milk. Because there is very little brain growth in the rodent after weaning, the requirement for DHA in the diet of the adult animal is reduced. 

Increasing mercury concentrations in tissues of marine mammals were also associated with poor health due to leptospirosis, with proximity to urbanized areas, and with starvation. Accumulations were usually highest in adult females, than adult males placental transfer of mercury to developing pups is low. Methylmercury concentrations in seal... 

Physiological studies have shown that the placental transfer of vitamin A is limited in most mammals. Thus, it is normal for the liver and plasma vitamin A levels of newborns to be much lower than those in adults. In humans, premature infants often have lower plasma retinol levels than full-term infants. The period of breastfeeding is important for the accrual of vitamin A reserves as shown by... 

The protection afforded the fetus by the mother occurs by transfer of immunoglobulins from the maternal circulation to the fetal circulation across the placental barrier (the chorioallantoic placenta). This is not true of all species of mammals. In some the yolk sac appears to be the major route across which immunoglobulins are transferred (e.g., rabbit, guinea pig, and mouse). In others, such as goats, cows, and sheep, no immunoglobulins are transferred during intrauterine life but are obtained by the neonate from maternal colostrum by absorption across the gut. In some mammals (mouse, rat, dog) both pre- and postnatal transfer occurs (Schlamowitz, 1976). 

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