Endocytosis and vesicular traffic in fetal and adult colonic goblet cells

Structural and functional differences between adult and fetal colonic goblet cells have not been clearly defined. To compare the binding, uptake, and intracellular pathway of internalized apical membrane in fetal and adult goblet cells, cationic ferritin (CF) was used as a nonspecific probe. The initial distribution of membrane anionic sites was determined in segments of proximal and distal colon from fetal (18–22 days) and adult rats that were fixed prior to a 10‐minute exposure to CF at 4°C. Uniform binding along the apical membrane and microvilli was noted at all ages. To assess uptake and intracellular transport, segments of proximal and distal colon from fetal and adult rats were exposed to CF for 10 minutes at 4°C prior to a saline wash and incubation in saline or Liebovitz L‐15 medium for 3, 6, 15, 30, or 60 minutes at 37°C. In addition, fetal rats between 18 and 22 days gestation (birth) were exposed to CF continuously for 10–30 minutes via an intracaecal injection. The results showed extensive uptake of CF in the fetal goblet cells and a more variable intracellular pathway than in with the adult. Within 3 minutes, numerous CF positive vesicles and tubules were present within the apical cytoplasm as well as interspersed among the secretory granules of the fetal goblet cell. Most of these vesicles were smooth surfaced, although some were coated. By 15 minutes, CF was frequently seen in multivesicular bodies, and occasionally in vacuoles in the vicinity of the Golgi. No CF was detected in Golgi cisternae. In contrast to fetal cells, only limited uptake was noted in adult goblet cells and the vesicles were restricted to the periphery of the secretory granule mass. In addition, transport to lysosomes was the exclusive route of these vesicles in the adult. These findings indicate that vesicular uptake is extensive in fetal goblet cells and the intracellular pathway is more variable and less restricted than in the adult.