THE AVIAN EMBRYO

Chick and quail (Coturnix coturnix japonica) embryos have advantages for studying embryological events because they can be accessed through the eggshell or in whole embryo culture for microsurgery, microinjection, or electroporation (1). The monoclonal antibody QH-1 that labels quail angioblasts and endothelial cells (ECs) has been used extensively to study the origin of ECs and their assembly into the initial vascular pattern (2–5). This also allowed observation of quail angioblast migration in quail–chick chimeras (4). Although no equivalent monoclonal antibody exists for the chick embryo, fluorescent lectins have been used that bind to the endothelium when injected into the vasculature (6). Mouse tissue also has been successfully grafted into developing quail embryos. The mammalian angioblasts migrate extensively in the avian embryo and respond to host signals that pattern the vasculature (7). The accessibility in whole embryo culture, even into the later developmental stages, has made the quail embryo the ideal organism to follow nerve and blood vessel development in the limb, where antibodies and growth factors have been delivered by bead implantation to study the molecular basis of neurovascular congruence (8). In summary, the ability to manipulate the avian embryo has led to important discoveries of the relative roles of vasculogenesis (vessel formation from angioblast assembly) and angiogenesis (sprouting of new vessels from preexisting vessels) in embryonic vascular development.

MOLECULAR MARKERS AND BLOOD FLOW

Arterial (ephrinB2, neuropilin 1) and venous (EphB4, neuropilin 2) markers are present on the endothelium of chick, mouse, and zebrafish embryos before blood flow begins (9). The chick embryo has been used to demonstrate that changes in these markers can be experimentally induced by changes in the pattern of blood flow (10).