Developement of the adrenergic innervation and chromaffin cells in the human fetal gut

Fetal superior mesenteric artery: longitudinal reference ranges and evidence of regulatory link to portal liver circulation

OBJECTIVE

To establish longitudinal reference ranges for the fetal superior mesenteric artery (SMA) flow velocity and pulsatility index (PI(SMA)). Also to examine the hemodynamic relationship to venous liver perfusion and umbilical flow distribution in the liver, to other splanchnic arteries, and more generally to the middle cerebral and umbilical artery.

METHODS

Prospective longitudinal study of 161 low-risk pregnancies using Doppler recordings including the SMA, repeated on 3-5 occasions at 3-5 weekly intervals. Umbilical venous flow was estimated, blood velocity in the shunt ductus venosus represented umbilico-caval (i.e. porto-caval) pressure gradient, and left portal vein blood velocity represented umbilical distribution within the liver. The correlation between PI(SMA) and the splenic and hepatic artery PI were analysed (PI(SA) and PI(HA)), and the association to middle cerebral and umbilical artery PI (PI(MCA) and PI(UA)) assessed.

RESULTS

Reference ranges for the SMA for gestational weeks 21-39 were based on 589 observations. Low impedance in the SMA (i.e. low PI(SMA)) was associated with low umbilical flow and porto-caval pressure gradient (i.e. <10th centile), and high distribution of umbilical flow to the right lobe (i.e. left portal vein blood velocity >90th centile). PI(SMA) correlated weakly with PI(SA) and PI(HA) (r=0.30, 95%CI 0.22-0.37, and r=0.39, 95%CI 0.27-0.51, respectively). PI(SMA) was positively associated with PI(MCA) and PI(UA).

CONCLUSION

We have provided longitudinal reference ranges for fetal SMA flow velocity and PI, and shown that the SMA, which perfuses the fetal gut, is also involved in the regulation of the liver perfusion.

Acute systemic complications in the preterm fetus after asphyxia: role of cardiovascular and blood flow responses

1. Poor perfusion of the kidneys and gut, and associated functional impairment, are major problems in the first days of life in very preterm infants. These complications can be associated with a substantial mortality and further problems such as reduced kidney growth and chronic renal problems in later childhood. 2. There is very little information, and consequently considerable debate, about how or even whether to improve perfusion of the vital organs of this most vulnerable group of babies. Current treatments simply do not consistently improve babies' perfusion generally or kidney and gut perfusion and function in particular. 3. In this review we critically examine clinical and experimental evidence that suggests that exposure to low oxygen levels before and during birth may be a significant contributor to impaired systemic perfusion, and highlight areas requiring further research. 4. This knowledge is essential to develop and refine ways of improving perfusion of the kidneys and other vital organs in premature babies.

The role of the sympathetic nervous system in postasphyxial intestinal hypoperfusion in the pre-term sheep fetus

Asphyxia in utero in pre-term fetuses is associated with evolving hypoperfusion of the gut after the insult. We examined the role of the sympathetic nervous system (SNS) in mediating this secondary hypoperfusion. Gut blood flow changes were also assessed during postasphyxial seizures. Preterm fetal sheep at 70% of gestation (103-104 days, term is 147 days) underwent sham asphyxia or asphyxia induced by 25 min of complete cord occlusion and fetuses were studied for 3 days afterwards. Phentolamine (10 mg bolus plus 10 mg h(-1)i.v.) or saline was infused for 8 h starting 15 min after the end of asphyxia or sham asphyxia. Phentolamine blocked the fall in superior mesenteric artery blood flow (SMABF) after asphyxia and there was a significant decrease in MAP for the first 3 h of infusion (33 +/- 1.6 mmHg versus vehicle 36.7 +/- 0.8 mmHg, P < 0.005). During seizures SMABF fell significantly (8.3 +/- 2.3 versus 11.4 +/- 2.7 ml min(-1), P < 0.005), and SMABF was more than 10% below baseline for 13.0 +/- 1.7 min per seizure (versus seizure duration of 78.1 +/- 7.2 s). Phentolamine was associated with earlier onset of seizures (5.0 +/- 0.4 versus 7.1 +/- 0.7 h, P < 0.05), but no change in amplitude or duration, and prevented the fall in SMABF. In conclusion, the present study confirms the hypothesis that postasphyxial hypoperfusion of the gut is strongly mediated by the SNS. The data highlight the importance of sympathetic activity in the initial elevation of blood pressure after asphyxia and are consistent with a role for the mesenteric system as a key resistance bed that helps to maintain perfusion in other, more vulnerable systems.

An electrophysiological study of developmental changes in the innervation of the guinea-pig taenia coli

A sucrose-gap technique was used to study the development of neuromuscular transmission in the taenia coli of fetal, 1- to 2-day-old, 3- to 4-week-old and 3-month-old guinea-pigs. In addition, the effects of exogenous, alpha,beta-methylene adenosine 5'-triphosphate (ATP), noradrenaline, vasoactive intestinal polypeptide (VIP) and carbachol were examined. Taking the response to a single pulse of electrical field stimulation as the index of a developed neuromuscular junction, it was apparent that the non-adrenergic inhibitory system arose before, and matured more quickly than, the cholinergic system. The inhibitory system was present by 8 weeks of gestation in some fetuses, but, in contrast, excitatory cholinergic transmission was not seen until birth. As evidenced by responses to carbachol, alpha,beta-methylene ATP and VIP, cholinergic, purinergic and VIP receptors were present on the smooth muscle at the earliest ages studied. No changes in sensitivity to these agents were noted throughout development, although in adults the level of the maximum responses increased.

The development of adrenergic innervation in some human foetal blood vessels

The development of adrenergic innervation was examined in saphenous and brachial veins, femoral, brachial and superficial temporal arteries from human foetuses of 7-19 weeks gestation. Fluorescence histochemical and electron microscopic techniques were used on transverse sections of the vessels. No specific adrenergic fluorescence was detected within any of the vessels before 11 weeks gestation, by which time the arteries showed specific adrenergic fluorescence. However, there was no evidence of adrenergic innervation of veins even at 19 weeks gestation. Incubation of the vessels with alpha-methylnoradrenaline did not lead to earlier detection of fluorescence. Electron microscopy showed that the adrenergic nerves found in the arteries were arranged as bundles of axons partially enclosed within Schwann cell processes, often lying close to capillaries in the outer regions of the tunica adventitia.

Small intensely fluorescent (SIF) cells and sympathetic nerves in the adult rabbit portal vein and during perinatal development

The ontogenesis of small intensely fluorescent (SIF) cells and the adrenergic nerve plexus is described in stretch preparations of the rabbit portal vein. On the 25 to 26th days of gestation there was a predominance of SIF cells (8 to 30 microns in diameter), but a few nerve fibres in bundles were also present. Each portal vein preparation contained 6 to 9 groups of cells. The distribution and number of SIF cells and nerve bundles remained constant until the 31st day of gestation at which stage the number of SIF cells had decreased, while the density of the nerve plexus had increased approximately 4-fold. The adult portal vein exhibited a dense adrenergic plexus, but SIF cells were absent from nine out of ten preparations.

Development and ageing of perivascular adrenergic nerves in the rabbit. A quantitative fluorescence histochemical study using image analysis

Age-related changes in vascular adrenergic nerves of 5 contrasting arteries in the rabbit were studied from before birth through to old age. Adrenergic nerves were demonstrated on stretch preparations using glyoxylic acid fluorescence histochemistry. Quantitative estimates of the densities of nerve fibres and varicosities were obtained by automated light microscopic image analysis. The early stages of development of vascular innervation were similar in all the vessels studied. They consisted of a period of outgrowth of axons; a period of rapid increase in density and formation of varicosities; and a later period of more gradual nerve growth. The timing of these stages varied greatly between the different vessels. The larger vessels, i.e. the carotid, renal and femoral arteries, had a well-developed innervation at birth, whilst the innervation of the smaller mesenteric and basilar arteries was sparse. Nerve loss occurred between 6 weeks and 6 months in the femoral artery and in old age (3 years or over) in the renal and carotid arteries. The large elastic arteries were, in general, more densely innervated than the smaller muscular arteries throughout life. The innervation of the different vessels became increasingly diverse between birth and adulthood, indicating a relationship between the pattern of vascular innervation and local physiological requirements. Factors which could influence age-related changes in nerve pattern and density are discussed.

Perinatal development of quinacrine-positive neurons in the rabbit gastrointestinal tract

Nerve showing positive reaction to quinacrine are described in rabbit ileum and stomach during perinatal development and compared to the distribution of nerves revealed by catecholamine fluorescence and by staining for acetylcholinesterase Acetylcholinesterase- and quinacrine-positive neurons were observed on the twenty-third day of gestation in both the ileum and stomach, but catecholamine fluorescence was not detected in nerves until the twenty-fifth and twenty-sixth days of gestation in the stomach and ileum respectively. The possibility that quinacrine is binding to high accumulations of ATP is discussed.

The cardiovascular effects of circulating catecholamines in fetal sheep

1. Adrenaline and noradrenaline have been infused into the fetal sheep to produce plasma concentrations comparable to those seen during hypoxia and the cardiovascular changes compared with those seen in response to hypoxia. The effect of isoprenaline, methoxamine, and beta- and alpha-adrenergic antagonists were also investigated. 2. There were no significant changes in the blood gas values during any of the infusions except for a mean fall in pH of 0.04 during adrenaline infusion. 3. Adrenaline caused a fall in the fetal heart rate followed by a rise above the control value after 15-20 min. The fall in heart rate was abolished when the rise in blood pressure was blocked with phentolamine. The rise in heart rate was blocked by propranolol. The exact cause of the biphasic changes in heart rate during adrenaline infusion is not clear. 4. A fall in heart rate was not seen with noradrenaline; a small rise was. Propranolol changed this into a fall in heart rate while phentolamine increased the size of the heart rate rise. 5. Phentolamine alone increased the fetal heart rate by 25% and reduced blood pressure by 12%; propranolol alone reduced heart rate by 14% and had no effect on blood pressure. Isoprenaline increased fetal heart rate and reduced blood pressure. 6. The incidence of fetal breathing movements was highly variable. Despite this a significant increase was observed during adrenaline infusion. None of the other infusions had consistent effects. 7. The role of the circulating catecholamines in mediating or modifying the cardiovascular responses to hypoxia in the fetal sheep is discussed.

Light and electron microscopic histochemistry of the monoamines in the human foetal sympathetic ganglion in culture

Sympathetic ganglia of 13 to 19-week-old human foetuses were cultured in small pieces with and without nerve growth factor for up to 5 weeks in vitro. The cultures were studied using phase-contrast, fluorescence and electron microscopy. Monoamines were demonstrated with the formaldehyde-induced fluorescence method, with and without pretreatment of the cultures with catecholamines or monoamine oxidase inhibitor. In the long-term cultures, primitive sympathetic cells, sympathicoblasts of types I and II, and young sympathetic neurons showed a fine structure identical to that described earlier in vivo. There were virtually no satellite or Schwann cells in the cultures. The neurons showed a considerable capacity to grow new nerve fibres in culture, even without nerve growth factor. Nerve terminals with accumulations of synaptic vesicles were regularly observed, occasionally in synapse-like contact with other nervous structures. Large granular vesicles were regularly found in the sympathicoblasts after glutaraldehyde-osmium tetroxide fixation. After permanganate fixation, dense-cored vesicles typical of adrenergic neurons were not seen, either in the perikarya, or in the processes, although it was possible to demonstrate specific fluorescence. No small intensity fluorescent (SIF) cells were observed. Variable formaldehyde-induced fluorescence was observed in the nerve cell perikarya and nerve fibres. The intensity of the fluorescence increased after treatment of the cultures with monoamine oxidase inhibitor and after incubation with catecholamines.

Changes in the physiology and fine structure of the taenia of the guinea-pig caecum following transplantation into the anterior eye chamber

1. The taenia of the guinea-pig caecum has been used as a model to study the re-establishment of autonomic innervation following transplantation into the anterior eye chamber. The ultrastructure, the histochemical localization of noradrenaline and acetylcholinesterase and the pharmacology of transmission to the taenia have been examined 1 day to 16 weeks following transplantation. Both ganglion-free strips of the taenia and caecal wall segments including the underlying Auerbach's plexus were used.2. Caecal wall preparations: nerve fibres from intramural ganglion cells retracted during the first 2 days following transplantation, but reappeared in the muscle soon afterwards. Adrenergic nerves from the iris formed terminals about ganglion cells at about 2-4 weeks. Both cholinergic excitatory and non-adrenergic (;purinergic') inhibitory transmission to the muscle was re-established by 2-4 days following transplantation.3. Taenia strip preparations: both adrenergic and cholinergic nerve fibres were demonstrated histochemically in muscle bundles by 2-4 weeks. Non-adrenergic inhibitory and cholinergic transmission was not re-established until 2-4 weeks following transplantation. There was an abnormally dense re-innervation of the muscle by adrenergic nerve fibres by about 8 weeks which is compared with the innervation of aganglionic bowel in Hirschsprung's disease.4. The origin of non-adrenergic inhibitory responses in the transplanted taenia is discussed and the results considered in relation to the re-innervation of transplants of the vas deferens and of intestine during early development.

Adrenoceptors in the human foetal small intestine

Human foetal small intestine was shown to contain both alpha- and beta-adrenoceptors with a predominance of beta-adrenoceptors. The tissue examined was obtained from foetuses of gestational age between 8 and 26 weeks.