HISTOGENESIS OF HEMATOPOIETIC AND IMMUNE ORGANS DURING PRENATAL AND EARLY POSTNATAL DEVELOPMENT

This article reviews how hematopoietic and immune organs are built during prenatal life and how they rapidly adjust after birth. Developmental blood formation does not stay in one place. It shifts through a sequence of anatomical niches that provide distinct cellular signals and structural support. Early embryonic hematopoiesis begins outside the embryo, then definitive hematopoietic stem cells arise within embryonic vessels and seed fetal organs where expansion and lineage specification occur.