Published online by Cambridge University Press: 10 December 2009
When the gut evaginates and the cloacal and oral membranes rupture, the interface between amniotic fluid and the fetus is established. This interface serves as a conduit for the transfer of nutrients that are external to the fetus and the neonate. The aboral movement of amniotic fluid occurs as early as 18 weeks' gestation, and up to 450 ml of amniotic fluid move aborally through the intestine by term. While the intrauterine environment is sterile, the introduction of feedings presents a major challenge to host defense. Thus, the neonatal intestine is a digestive organ as well as an important component of the immune system. Both aspects of intestinal function will be reviewed in this chapter.
Digestion and absorption
Mucosal differentiation
During the second and third trimesters of pregnancy, growth and maturation of the gastrointestinal tract occur in preparation for postnatal life. The timing of structural and functional maturation is summarized in Tables 6.1 and 6.2. The gut lengthens to 250–300 cm by term, and gastric capacity is about 30 mL. During the second trimester, the glycocalyx appears, and the brush border is structurally well defined. Superficial glands are present in the pharyngeal and esophageal mucosa by 20 weeks and squamous cells by 28 weeks. Mucous and lingual lipases are also secreted. Endocrine, chief, mucus and parietal cells appear in the stomach by 12 weeks; by 16 weeks, these cells actively secrete hydrochloric acid, intrinsic factor, pepsin, gastrin and mucus.
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