Avascular and vascular phases of tumour growth in the chick embryo

Abstract

The chick embryo was used to study the relationship between the onset of tumor neovascularization and tumor growth. Walker 256 [rat] carcinosarcoma was implanted on the chorioallantoic membrane (CAM) of about 600 embryos aged 5-16 days. Tumor diameter and changes in the CAM vasculature in response to the implants were recorded daily. Representative tumors were examined by light microscopy of Epon-embedded tissue and autoradiography after injection of [3H]-thymidine. Tumors remained avascular for 72 h, after which they were penetrated by new blood vessels and began a phase of rapid growth. The rate of growth during this vascular phase was greatest for implants in 5 and 6 day old embryos and decreased the later the day of implantation. The time of onset of tumor angiogenesis appears to be independent of the immunological state of the chick embryo, although the rate of growth after vascularization may be modified by the onset of immunity. This study suggests that the avascular and vascular phases of tumor growth are separable, and the avascular tumor population lives under the growth constraints which limit the size of a tumor spheroid growing in soft agar or aqueous humor.