Rheology of embryonic avian blood

Abstract

Shear stress, a mechanical force created by blood flow, is known to affect the developing cardiovascular system. Shear stress is a function of both shear rate and viscosity. While established techniques for measuring shear rate in embryos have been developed, the viscosity of embryonic blood has never been known but always assumed to be like adult blood. Blood is a non-Newtonian fluid, where the relationship between shear rate and shear stress is nonlinear. In this work, we analyzed the nonNewtonian behavior of embryonic chicken blood using a microviscometer and present the apparent viscosity at different hematocrits, different shear rates, and at different stages during development from 4 days (Hamburger-Hamilton stage 22) to 8 days (about HamburgerHamilton stage 34) of incubation. We chose the chicken embryo since it has become a common animal model for studying hemodynamics in the developing cardiovascular system. We found that the hematocrit increases with the stage of development. The viscosity of embryonic avian blood in all developmental stages studied was shear rate dependent and behaved in a non-Newtonian manner similar to that of adult blood. The range of shear rates and hematocrits at which nonNewtonian behavior was observed is, however, outside the physiological range for the larger vessels of the embryo. Under low shear stress conditions, the spherical nucleated blood cells that make up embryonic blood formed into small aggregates of cells. We found that the apparent blood viscosity decreases at a given hematocrit during embryonic development, not due to changes in protein composition of the plasma but possibly due to the changes in cellular composition of embryonic blood. This decrease in apparent viscosity was only visible at high hematocrit. At physiological values of hematocrit, embryonic blood viscosity did not change significantly with the stage of development. microelectromechanical systems; hematocrit; shear rate; shear stress; hemodynamic; rouleaux; vascular development HEMODYNAMICS, or blood fluid dynamics, are important not only for cardiovascular function but also for the development of the cardiovascular system. Blood flow creates a force called shear stress. Chronic changes in shear stress levels lead to a remodeling of the vasculature that normalizes the level of shear stress in the adult (20). Shear stress has also been found to be important during cardiovascular development, affecting heart formation (17), vascular remodeling (25, 36), arterial-venous differentiation (21), and hematopoiesis by the vascular endothelium (1, 29). For these reasons, there has been a significant effort in recent years to measure the shear stress levels during early embryonic development and link specific flow patterns or levels of shear stress to events in vascular development.Shear stress is a function of the shear rate and the viscosity of the fluid. The development of flow visualization techniques with micrometer-scale resolution, such as Doppler optical coherence tomography (12) and microparticle ima...

Blood flow through the embryonic heart outflow tract during cardiac looping in HH13–HH18 chicken embryos

“…Embryonic chick blood has a fairly constant viscosity in the physiological shear rate range [43], so that WSR should be proportional to wall shear stress. Al-Roubaie et al reported small changes in viscosity during later development over large periods of developmental time (4 days) [44], indicating that the changes in viscosity between HH13 and HH18 (20 h) are likely minimal and can be assumed constant.…”

“…We used four-dimensional embryo-specific geometries of the OFT lumen (segmented from OCT images), which consisted of a sequence of meshes that depicted the dynamic motion of the OFT walls over the cardiac cycle. Blood was modelled as a Newtonian fluid having density r ¼ 1060 kg m 23 [43] and viscosity m ¼ 0.003 Pa s. Embryo-specific blood flow through the OFT was modelled using a recently developed inverse method based optimization procedure [45]. Briefly, normal tractions (pressures) at the OFT lumen inlet and outlet surfaces (or more practically their difference) were iteratively imposed, until computed and measured Doppler velocities at an optimization point in the interior of the lumen differed by less than 1%.…”

Simultaneous imaging of blood flow dynamics and vascular remodelling during development

“…We were obliged, however, to extrapolate our previous viscosity data with respect to embryonic stage. Our previous results had shown that there was no change in apparent viscosity of blood with respect to stage when the haematocrit is below 50% (Al-Roubaie et al, 2011). Therefore, extrapolation results in similar values for the viscosity as using the data directly for the older embryos.…”

“…The data was fit to a cubic equation as a means to easily calculate the haematocrit in our computational analysis. We previously published values for the viscosity of avian blood at different shear rates and haematocrits for embryos between 4 days and 6 days of incubation, or HH22 to HH34 (Al-Roubaie et al, 2011). Embryos used in this current work were much younger, starting at 30 h of incubation (13 somites or HH11).…”

“…A cubic fit was used as a method to interpolate between stages in the computational model. Blood viscosity was calculated based on embryonic stage and vessel diameter, using a Carreau relationship that we previously developed to describe the non-Newtonian behaviour of embryonic blood flow (Al-Roubaie et al, 2011). The constants were re-evaluated from the paper to include a larger number of haematocrit values.…”