Engine and radiator: fetal and placental interactions for heat dissipation

Abstract

SUMMARYThe 'engine' of fetal metabolism generates heat (3-4 W kg-' in fetal sheep) which has to be dissipated to the maternal organism. Fetal heat may move through the amniotic/allantoic fluids to the uterine wall (conductive pathway; total conductance, 1-1 W 0C-' kg-') and with the umbilical arterial blood flow (convective pathway) to the placenta. Because resistance to heat flow is larger than zero fetal temperature exceeds maternal temperature by about 0 5°C (0.3-1°C). Probably 85 % of fetal heat is lost to the maternal organism through the placenta, which thus serves as the main 'radiator'. Placental heat conductivity appears to be extremely high and this may lead to impaired heat exchange (guinea-pig placenta). A computer simulation demonstrates that fetal temperature is essentially clamped to maternal temperature, and that fetal thermoregulatory efforts to gain thermal independence would be futile. Indeed, when the late gestational fetus in uter^o is challenged by cold stress, direct and indirect indicators of (non-shivering) thermogenesis (oxygen consumption, increase of plasma glycerol and free fatty acid levels) change only moderately. In prematurely delivered lambs, however, cold stress provokes summit metabolism and maximum heat production. Only when birth is imitated in utero (by cord clamping, external artificial lung ventilation and cooling) do thermogenic efforts approach levels typical of extra-uterine life. This suggests the presence of inhibitors of thermogenesis of placental origin, e.g. prostaglandins and adenosine. When the synthesis of prostaglandins is blocked by pretreatment with indomethacin, sheep fetuses react to intra-uterine cooling with vigorous thermogenic responses, which can be subdued by infusion of prostaglandin E2 (PGE2). Since the sheep placenta is known to produce sufficient amounts of PGE2, it seems that the placenta controls fetal thermogenic responses to some extent. This transforms the fetus into an ectothermic organism, and yet allows the newborn the full exploitation of thermoregulatory responses typical of endothermic animals.

Effects of age and pregnancy on the circulatory activin response of sheep to acute inflammatory challenge by lipopolysaccharide

“…The experiments reported here showed that, despite robust temperature, activin A, follistatin and cytokine responses in the mother, with the exception of temperature concomitant increases in the fetal compartment were not seen. Fetal temperature did rise simultaneously with that of the mother, consistent with the idea that most heat is lost from the fetal compartment across the placenta into maternal blood (Schröder & Power 1997). However, in terms of TNF-and IL-6 concentrations in the fetus, there were no significant changes in the 24-h period following LPS treatment.…”

Life-History Correlates of Placental Structure in Eutherian Evolution

“…The efficiency of exchange can be predicted by interdigitation and the placental shape. In labyrinthine placentae, counter-current flow with cross-current components is the most common arrangement Miglino et al 2004), with counter-current flow considered the most effective for maternofetal exchange (Faber et al 1992); whereas the predominant cross-current flow in villous placentae are buttressed by the ratio of maternofetal blood flow (Schroder and Power 1997; also see Moll 1972;Benirschke and Kaufmann 2000, pp. 35-38).…”

Fluorescent sensors reveal subcellular thermal changes

“…Because the heat from thermo-generating organelles diffuses rapidly throughout the cell and beyond [28], it is ideal if the sensor can be targeted directly to the organelles where the heat is generated. In these points of view, each category of technique has its own unique properties, and the characteristics and advantages/disadvantages of these methods are summarized in Table 2.…”