C. Vanginneken et al., NITRIC-OXIDE SYNTHASE EXPRESSION IN ENTERIC NEURONS DURING DEVELOPMENT IN THE PIG DUODENUM, Anatomy and embryology, 198(5), 1998, pp. 399-408
The expression of the constitutive neural isoform of nitric oxide synt
hase (bNOS) is dynamic and thus forms an ideal parameter to evaluate w
hether development and region affect the enteric nervous system. By ap
plying NADPH-diaphorase histochemistry on wholemount preparations of t
he myenteric and submucosal plexuses and by using the 'unbiased counti
ng frame', a qualitative and quantitative description of bNOS-expressi
on in enteric neurons in the pig duodenum in various developmental sta
ge and region was obtained. Examinations were carried out on the oral
and aboral duodenum of fetal pigs from the second half of gestation, o
f 1-2-day-old pigs and of 6-8-week-old pigs. In the pig duodenum, thre
e enteric plexuses were readily distinguished: the inner submucous, th
e outer submucous and the my enteric plexuses. All three plexuses alre
ady harboured, to different degrees, bNOS-expressing neurons at midges
tation. Although the enteric nervous system was present at midgestatio
n, the enteric neurons had not yet reached their adult phenotype and m
orphology. During gestation, the number of inner submucous bNOS-expres
sing neurons increased approximately 50-fold, whereas after birth that
number fell to about 10% of the prenatal value. During further postna
tal development it returned to prenatal values. In addition, the numbe
r of bNOS-expressing myenteric neurons doubled postnatally. These chan
ges favour a role for NO in mediating the development of enteric neuro
ns and point to a greater necessity for inhibitory innervation in the
adult pig as compared with the fetal pig. Furthermore, the number of b
NOS-expressing outer submucosal and myenteric neurons was significantl
y higher in the oral duodenal segment compared with the aboral duodena
l segment. This regional difference suggests that the oral duodenal se
gment is more prominently involved in the regulation of NO-mediated ga
strointestinal processes than the aboral one. The developmentally and
regionally dependent bNOS-expression can be explained by shifts and di
fferences in the balanced system of hormones, presynaptic input and ta
rget-derived signals that affects neurotransmitter expression.