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SummaryThe composition of human milk from mothers delivering prematurely (PT) and at term (T) was studied over the first weeks of lactation. Complete 24 h milk expressions were obtained by electric pump at weekly or biweekly intervals through 44 wk conceptual age (120 samples from 20 PT mothers and 28 samples from 7 T mothers). PT milk was found to contain significantly higher concentrations than T milk of the following nutrients: total nitrogen, protein nitrogen, sodium, chloride, magnesium and iron. No differences were found between T and PT milk for nonprotein nitrogen, volume, solids, total calories, lactose, fat, fatty acids, potassium or calcium. The nutrients supplied to a 33 wk preterm infant fed 200 ml/kg/day of "average" PT milk were in excess of theoretic intrauterine requirements for all substrates except calcium and phosphorus. PT human milk is theoretically more suitable for the premature infant than either mature or term human milk, but may be deficient in specific nutrients for the very low birth weight baby. SpeculationThe compositional differences in preterm milk (increased protein and mineral content) are generally characteristic of colostrum. These changes may therefore reflect a prolonged colostral phase in premature mothers who are establishing lactation by artificial means during periods of emotional stress. Thus, the changes denoted for preterm milk may reflect the circumstances surrounding the onset of lactation rather than the nutritional requirements of the preterm infant.Controversy persists regarding the propriety of human milk for the feeding of preterm infants. Although insufficient data are available to determine exact nutritional requirements of the premature baby (16), Fomon et al. (10) have appropriately questioned the adequacy of mature breast milk in relation to estimated needs. While providing theoretically beneficial nonnutritional factors, mature human milk may be quantitatively deficient in protein, calcium, sodium and possibly other nutrients for the premature (10). Clinical studies of premature infants fed mature breast milk have yielded conflicting results (8,14,19,25), although the data generally indicate that mature human milk does not provide optimal nutritional support to the low birth weight infant.Recent investigations offer intriguing data which indicate that the composition of milk from mothers delivering prematurely differs from that of term mothers during the early weeks of lactation. Atkinson et al. (2) and Gross et al. (15) have confirmed a higher concentration of protein nitrogen in preterm milk, while Gross et al. also found an increased level of sodium in the premature mothers' milk. These findings suggest that preterm milk is theoretically more suitable for the premature infant than is mature milk. The present investigation was performed to assess, in a comprehensive fashion, the composition of milk from mothers delivering prematurely and at term over the same period of lactation. Further, the study was designed to control for many variables known to af...

A combination of cytogenetic and molecular analyses has shown that several different transposable elements are involved in the restructuring of Drosophila chromosomes. Two kinds of elements, P and hobo, are especially prone to induce chromosome rearrangements. The mechanistic details of this process are unclear, but, at least some of the time, it seems to involve ectopic recombination between elements inserted at different chromosomal sites; the available data suggest that these ectopic recombination events are much more likely to occur between elements in the same chromosome than between elements in different chromosomes. Other Drosophila transposons also appear to mediate chromosome restructuring by ectopic recombination; these include the retrotransposons BEL, roo, Doc and I and the foldback element FB. In addition, two retrotransposons, HeT-A and TART, have been found to be associated specifically with the ends of Drosophila chromosomes. Very limited data indicate that transposon-mediated chromosome restructuring is occurring in natural populations of Drosophila. This suggests that transposable elements may help to shape the structure of the Drosophila genome and implies that they may have a similar role in other organisms.

In preterm infants, patency of the ductus arteriosus may represent a normal physiologic adaptation to allow shunting from either systemic-to-pulmonary circulation (eg, in the first day of life) or from pulmonary-to-systemic circulation (eg, in the presence of severe lung disease). Therapies designed to close the ductus arteriosus are contraindicated in some settings and should not be considered a standard of care at any time until these therapies are proven to decrease long-term clinical morbidities in randomized, placebo-controlled trials.

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The effect of variations in arterial O2 content (CaO2) on the cerebrovascular bed of seven unanesthetized newborn lambs was studied as the hematocrit and arterial PO2 (PaO2) were varied. Each subject was studied at a high hematocrit [44 +/- 3% (SD)] and a low hematocrit [24 +/- 3%]. At each hematocrit level the PaO2 was changed over a range of 30-150 mmHg. The relationship between cerebral blood flow and PO2 depended on hematocrit and vice versa. To the contrary, the relationship of blood flow to CaO2 was independent of hematocrit and/or PO/. As CaO2 fell, regardless of whether this was due to a fall in PO2 hematocrit or both, there was a reciprocal increase in cerebral blood flow such that cerebral O2 delivery (cerebral blood flow x CaO2) was constant. These data show that CaO2 is a variable of fundamental importance to the regulation of cerebral blood flow. Changes in CaO2 are accompanied by reciprocal changes in cerebral blood flow to maintain constant cerebral O2 delivery. Data among species with differing cerebral O2 consumption show that cerebral O2 delivery, in turn, is regulated according to cerebral O2 consumption.

Developmental effects on the response of cerebral blood flow (Qc) and cerebral O2 consumption (CMRO2) to changes in CO2 tension were assessed in unanesthetized fetal, newborn, and adult sheep. Blood flow was measured using the radioactive microsphere technique. CMRO2 was calculated as the product of Qc and the difference in O2 content between arterial and sagittal sinus blood (CaO2 -- CVO2). The response of Qc to changes in arterial CO2 tension increased from fetus [3.53 +/- 0.56 ml.100 g-1.min-1.mmHg PaCO2(-1) (SE)] to newborn (5.16 +/- 0.59) to adult (6.20 +/- 0.63). Only the fetal-adult difference was significant (P less than 0.05). It has been suggested that developmental differences in CO2 responsiveness of cerebral blood flow are the result of differences in CMRO2. We corrected for differences in CMRO2 by looking at the response to CO2 of the variable 1/(CaO2--CVO2). According to the Fick principle 1/(CaO2--CVO2) = Qc/CMRO2, i.e., blood flow per unit O2 consumption. The fetal response was not significantly different from the newborn, but the adult was significantly different from both (P less than 0.05). Thus the difference in CO2 response of cerebral blood flow between fetus and adult cannot be explained by differences in CMRO2.