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

ABSTRACT

The internal anal sphincter [IAS] plays an important role in the maintenance of anorectal continence and in the pathophysiology of constipation and incontinence. Failure of a full-term newborn to pass meconium within the first 24 h should raise a suspicion of intestinal obstruction. However, the causes of functional intestinal obstruction in infants are still unclear; however, one cause might be immaturity of the IAS. This study aimed at illustrating the developmental changes in IAS in guinea pigs of different ages. Forty-one guinea pigs were selected and divided into four groups. They were sacrificed and the rectoanal canal specimens were dissected at the appropriate time 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 aged 4 weeks; and group IV - from animals aged 16 weeks. The rectoanal canal specimens was prepared for histological and immunohistochemical studies. Morphometric analysis was also performed. Thickness of the IAS increased with advancing age, whereas that of the connective tissue component decreased. Moreover, alpha-smooth muscle actin filaments increased with age. There was also age-related development of the enteric nervous system, especially the myenteric plexus in the rectoanal junction region. Results of the current study revealed pronounced developmental changes in the histological structure of guinea pig IAS. Therefore, development of the IAS might have a possible role in the functional intestinal obstruction seen in preterm and full-term infants