Development of different mast cell types in the opossum Didelphis albiventris

Previous studies have disclosed three types of mast cell in opossums: connective tissue (CTMC), mucosal (MMC), and lymphatic sinus (LSMC). In contrast to most opossum lymph nodes, the mesenteric lymph node is virtually devoid of LSMC, displaying medullary cord CTMC. The present study aimed to describe the development of these mast cell populations. Toluidine blue staining and a histochemical method for demonstrating heparin allowed the identification of immature and mature mast cells. Immature CTMC devoid of detectable heparin were rare until postnatal day 10. Mature CTMC filled with heparin-containing granules became numerous by day 30 to day 40. In the ileum, despite the presence of mature CTMC in the submucosa and mucosa (villus base), immature mast cells first appeared in the villus core by day 65 and adult features were apparent by day 100. In LSMC-containing lymph nodes, immature mast cells were found in lymphatic sinuses by day 10. Clear signs of LSMC differentiation were observed from day 20. Compared with the 10-day value, the mean diameter of cytoplasmic granules at day 40 had doubled and that at day 110 had tripled. In the mesenteric lymph nodes, immature mast cells differentiated into lymphatic sinus CTMC-like cells. After day 80, most of them were located in medullary cords. Weaning and complete maturation of mucosa preceded the differentiation of MMC. In lymph nodes, LSMC differentiation occurred in parallel with the development of the medullary region and deep cortex units.