Developmental changes of internal anal sphincter in guineapigs: a histological and immunohistochemical study

IntroductionThe internal anal sphincter (IAS) plays an important role in the maintenance ofanorectal continence and in the pathophysiology of constipation and incontinence.Failure of a full-term newborn to pass meconium within the first 24 h should raisea suspicion of intestinal obstruction. However, the causes of functional intestinalobstruction in infants are still unclear; however, one cause might be immaturityof the IAS.Aim of the workThis study aimed at illustrating the developmental changes in IAS in guinea pigs ofdifferent ages.Materials and methodsForty-one guinea pigs were selected and divided into four groups. They weresacrificed and the rectoanal canal specimens were dissected at the appropriatetime from each group as follows: group I – from embryos in the third trimester;group II – from animals on their first postnatal day; group III – from animals aged4 weeks; and group IV – from animals aged 16 weeks. The rectoanal canalspecimens was prepared for histological and immunohistochemical studies.Morphometric analysis was also performed.ResultsThickness of the IAS increased with advancing age, whereas that of the connectivetissue component decreased. Moreover, α-smooth muscle actin filaments increasedwith age. There was also age-related development of the enteric nervous system,especially the myenteric plexus in the rectoanal junction region.ConclusionsResults of the current study revealed pronounced developmental changes in thehistological structure of guinea pig IAS. Therefore, development of the IAS mighthave a possible role in the functional intestinal obstruction seen in preterm andfull-term infants.