Development of hematological respiratory variables in late chicken embryos: The relative importance of incubation time and embryo mass

Oxygen demand increases during embryonic development, requiring an increase in red blood cells (RBCs) containing hemoglobin (Hb) to transport O₂ between the respiratory organ and systemic tissues. A thorough ontogenetic understanding of the onset and maturation of the complex regulatory processes for RBC concentration ([RBC]), Hb concentration ([Hb]), hematocrit (Hct), mean corpuscular indices (mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration ([MCHb])) is currently lacking. We hypothesize that during the last half of incubation when the respiratory organ (the chorioallantoic membrane) envelops most of the egg contents, mean corpuscular indices will stabilize. Accordingly, Hct, [RBC] and [Hb] must also all change proportionally across development. Further, we hypothesize that the hematological respiratory variables develop and mature as a function of incubation duration, independently of embryonic growth. As predicted, a similar increase in Hct (from 18.7 ± 0.6% on day 10 (d10) to 34.1 ± 0.5% on d19 of incubation), [RBC] (1.13 ± 0.03 × 10⁶/μL to 2.50 ± 0.03 × 10⁶/μL) and [Hb] (6.1 ± 0.2 g% to 11.2 ± 0.1 g%) occurred during d10–19. Both [RBC] and [Hb] demonstrated high linear correlation with Hct, resulting in constant [MCHb] (~ 33 g% from d10 to d19). The decrease in MCV (from ~ 165 μ³ on d10 to ~ 140 μ³ on d13) and MCH (~ 55 pg to ~  45 pg) during d10–13, may be attributed to a changeover from larger primary to smaller secondary and adult-type erythrocytes with MCV and MCH remaining constant (~ 140 μ³ and ~ 45 pg respectively) for the rest of the incubation period (d13–19). Hematological respiratory values on a given incubation day were identical between embryos of different masses using either natural mass variation or experimental growth acceleration, indicating that the hematological variables develop as a function of incubation time, irrespective of embryo growth.