Avian Hematopoiesis: How Blood Cells Travel from Bone Marrow to Circulation

In birds, as in other vertebrates, blood cell production (hematopoiesis) occurs primarily in the bone marrow. The process involves five key stages:

1. Hematopoietic Stem Cells (HSCs) – Reside in bone marrow; self‑renew and differentiate into all blood lineages.
2. Differentiation – HSCs become committed progenitor cells (erythroid, myeloid, lymphoid).
3. Maturation – Progenitors mature into functional erythrocytes, leukocytes, and thrombocytes, acquiring specialized morphology and function.
4. Release into Circulation – Mature cells enter the bloodstream, regulated by systemic demand (e.g., erythropoietin for red cells, G‑CSF for neutrophils).
5. Circulation and Function – Cells distribute throughout the body: red cells transport O₂/CO₂, white cells provide immunity, platelets enable clotting.

Developmental Note: In avian embryos, hematopoiesis begins in the yolk sac, shifts to the liver, and finally establishes in the bone marrow.