1. ¹Department of Toxicologic Pathology, Azabu University School of Veterinary Medicine, Kanagawa, Japan
2. ²Department of Pharmacology and Toxicology, Kyorin University School of Medicine, Tokyo, Japan
3. ³Department of Biochemistry, Azabu University School of Veterinary Medicine, Kanagawa, Japan
4. ⁴Department of Pathology, The Jikei University School of Medicine, Tokyo, Japan
5. ⁵Department of Pathology, Kyorin University School of Medicine, Tokyo, Japan

Correspondence: Dr S Wakui, DVM, PhD, Department of Toxicologic Pathology, Azabu University School of Veterinary Medicine, 1-17-71 Fuchinobe, Sagamihara, Kanagawa 229-8501, Japan. E-mail: wakui@azabu-u.ac.jp

Received 31 May 2006; Revised 14 August 2006; Accepted 15 August 2006; Published online 11 September 2006.

Abstract

Endothelial cells and pericytes play critical role in angiogenesis, which is controlled, in part, by the angiopoietin (Ang)/Tie-2 system and vascular endothelial growth factor (VEGF). Here, we investigated Ang, Tie-2, and VEGF expression within endothelial cells and pericyte interdigitations (EPI), which consist of cytoplasmic projections of pericytes and corresponding endothelial indentations. After subcutaneous implantation of a thermoreversible gelation polymer disc in rats, the capillary density was low on day 5, increased to a peak on day 7, and then decreased on days 10–20. A small number of EPI were observed on day 5, then increased sharply to a peak on day 10, but had decreased on day 20. Light and electron microscopy immunohistochemical and RNA in situ hybridization analyses revealed that Tie-2 localized at endothelial cells, and Ang-2 localized at endothelial cells and pericytes, while Ang-1 and VEGF localized at pericytes, and Ang-1 was most intensely observed at EPI of pericytes. Conventional quantitative RT-PCR and Western blot analyses revealed that the level of Ang-1 was low on days 5–7, then increased on days 10–20, while the level of VEGF was high on days 5–10, but had decreased on day 20. The level of Ang-2 remained high and Tie-2 remained at the level of the control on days 5–20. The present study showed that the angiogenic phase might be initiated by increases in Ang-2 and VEGF, while the microvessel maturation phase might be initiated by a relative increase in Ang-1 and a decrease in VEGF. Moreover, EPI might serve as a pathway for the Ang-1/Tie-2 system, with VEGF promoting pericyte recruitment for microvascular integrity.