Mucocele: a human model for lymphangiogenesis.

The mechanism of lymphangiogenesis is poorly understood, and controversy exists whether it is part of the inflammatory response to tissue injury. Utilizing markers specific to lymphatics, we aimed to study if lymphangiogenesis plays a role in the tissue response of mucoceles. Twenty-three extravasated mucoceles were selected. They were grouped by using widely accepted histologic criteria of wound healing into early-, intermediate-, and late-phase lesions. To identify lymphatic vessels we used lymphatic endothelium-specific antibodies (VEGFR3, Prospero-related homeobox gene-1 [Prox-1], and D2-40). To assess the proportion of lymphatic channels to all lesional vessels we used the panendothelial marker CD31. The presence, distribution, and proportion of lymphatic channels were assessed and compared among the groups. To investigate the involvement of lymphangiogenic signals, the expression of VEGFC was determined. To assess for proliferative activity of lymphatic endothelial cells we utilized Ki-67 antibody. Early-phase lesions (n = 6) were characterized by the presence of centrally located mucicarmine-positive material (mucin pools) with numerous inflammatory cells dominated by mucin-laden CD163-positive macrophages. Only scattered peripheral thin-walled large and small vessels were seen in the stroma surrounding the central mucin pool. Less than half of these vessels were of lymphatic nature as determined by Prox-1, VEGFR3, and D2-40 positivity. The histology of the intermediate-phase lesions (n = 6) was dominated by numerous lymphatics of varying size, not seen in the early phase. The histology of late-phase lesions (n = 11) resembled a "pseudo-cyst," with dense granulation tissue containing rare macrophages and rare lymphatic vessels. Although VEGFC was present in all phases, the highest expression was in the early phase. Low-grade proliferative lymphatic endothelium was noted in the intermediate lesions with a Ki-67 index of 4%. Early lymphangiogenesis and late lymphatic vessel regression were observed during mucocele evolution. The abundant newly formed ectatic lymphatic vessels seen in the intermediate phase may play a role in the clearance of extravasated material (mucin, edema, and lymph fluid) and in the initiation of the young fibroblast-rich granulation tissue. Mucocele appears to be an excellent human model for studying the factors that play a role in new lymphangiogenesis and regression.