Quantitative Histological Changes in Murine Tail Skin Following Photodynamic Therapy

Mice were treated by an intravenous injection of 2 mg of the photosensitising drug meso-tetra (sulphonatophenyl) porphine (TPPS) and 24 h later a 2.5 cm length of their tails was exposed to visible light (photodynamic therapy, PDT). Using cross-sections from the centre of the treatment field, the absolute areas occupied by epidermis, dermis, hypodermis, tendon and bone, and also the total number and area of the blood vessels in the dermis and hypodermis, were compared between control and PDT-treated animals. There was a significant increase in the mean cross-sectional area of the epidermis, dermis and hypodermis following both 90J cm-2 (a dose expected to produce a low incidence of tail necrosis) and 180J cm-2 (expected to produce a 100% tail necrosis rate), on day 1 and day 5 following light exposure. The cross-sectional area of the vascular compartment was also significantly increased by day 5 at both dose levels. Differences were observed between the two doses when the total number of blood vessels were compared. There was a significant increase in the number of blood vessels by day 5 following 90 J cm-2 in both the dermis and hypodermis, but not following 180J cm-2. This appeared to be due to a significant increase in blood vessels with a cross-sectional area of less than 100 microns2 by day 5 at the lower dose. It is concluded that angiogenesis plays an important role in vascular recovery following PDT.