All vertebrate embryos pass through a developmental period of remarkably low morphological variability. This period has been called phylotypic period. During the phylotypic period, organogenesis takes place, including blood vessel development. Before the phylotypic period, the embryos rely on diffusion for the internal oxygen transport. Diffusion, however, is an efficient way of transport only over small distances.
Analytical models were constructed to investigate whether physical constraints (i.e. diffusional limitations) demand the development of an internal oxygen transport system as the embryos grow bigger.
These models showed that teleost embryos are smaller than their theoretically maximum size during the phylotypic period, based on oxygen diffusion. Lack of oxygen does therefore not demand blood vessel development.
Subsequently, numerical models of oxygen diffusion in a zebrafish embryo (Danio rerio) were developed, thereby including the realistic shape of the embryo.
These models were tested and refined with oxygen micro-electrode measurements of the oxygen partial pressure profile in and around the zebrafish embryo.
This numerical-experimental procedure revealed a high oxygen permeability in the yolk of the zebrafish embryo. Furthermore, lowest oxygen partial pressures were found in the head region with a gradient of posteriorly increasing oxygen partial pressures along the midline of the embryo. The three-dimensional oxygen partial pressure profile was compared with the expression pattern of the angiogenic factor (vegf), which is known to be expressed under hypoxic conditions.
The apparent colocalization of low oxygen partial pressure and the expression of vegf suggests oxygen to play an important role in regulating blood vessel development rather than posing a direct request for its development.