Related Experiment Videos
Macrophages are the first differentiated blood cells formed in human embryonic liver
Abstract:
Up to 70% of free, intravascular hemopoietic cells belonged to the macrophage series in half-thin sections prepared from the liver of a 4.5 week old human embryo (fertilization age) and 50 to 10% were found in the liver sinusoids up to the end of the 7th week. The number of apparent macrophage precursors could reach 10% of cells of the series. Intravascular macrophage clusters were discovered; the majority of contributing cells were phagocytic. The percentage of circulating macrophages in smears was only 0.5% or less during the investigated time period. Peak values for yolk sac macrophages did not exceed 10% of intravascular cells, and apparently followed peak values found in the liver. In half-thin sections large, pale, endodermally located hemocytoblasts of the definitive series also appeared during the 5th gestational week, but the amount of these cells was less than 5%. Their relation to the intrasinusoidal macrophage has not been proved till now. So, the first differentiated definitive blood cells apparently formed in, or at least preferentially attracted to the liver are not definitive erythroblasts, but macrophages.
Insights
Early human embryos show a high proportion of macrophages in the liver, suggesting these cells, not erythroblasts, are the first definitive blood cells to develop. This finding impacts our understanding of embryonic hematopoiesis.
Area of Science:
- Embryology
- Hematology
- Cell Biology
Background:
- The liver is a primary site of hematopoiesis during embryonic development.
- The origin and differentiation of early blood cells, particularly macrophages, are not fully understood.
Purpose of the Study:
- To investigate the prevalence and characteristics of intravascular hemopoietic cells in the early human embryonic liver.
- To determine the identity of the first differentiated definitive blood cells in the embryonic liver.
Main Methods:
- Analysis of half-thin sections of human embryonic liver tissue (fertilization age 4.5 to 7 weeks).
- Quantification of intravascular hemopoietic cells, focusing on the macrophage series and their precursors.
- Comparison of intravascular macrophage populations with yolk sac macrophages and definitive hemocytoblasts.
Main Results:
- Macrophages constituted up to 70% of intravascular hemopoietic cells in the embryonic liver by 4.5 weeks, decreasing to 10-50% by 7 weeks.
- Intravascular macrophage clusters, predominantly phagocytic, were observed.
- Circulating macrophages in smears were rare (<0.5%), and yolk sac macrophages peaked after liver macrophage values.
- Definitive hemocytoblasts appeared but in lower numbers (<5%) and their relationship to macrophages was unproven.
Conclusions:
- Macrophages, not erythroblasts, appear to be the first differentiated definitive blood cells to form or be preferentially attracted to the embryonic liver.
- The liver plays a crucial role in the early development and proliferation of the macrophage lineage during human embryogenesis.