Here we review the aging of the hematopoietic system from its earliest stages in the embryo through fetal and adult life. We focus on the waves of hematopoietic cell generation during embryonic life and how cells of varying lineages, functions and life spans contribute to the normal development of the adult blood system and its progressive gain/loss of function in the aged adult.

Aging is a set of complex processes that occur temporally and continuously. It is generally a unidirectional progression of cellular and molecular changes occurring during the life stages of cells, tissues and ultimately the whole organism. In vertebrate organisms, this begins at conception from the first steps in blastocyst formation, gastrulation, germ layer differentiation, and organogenesis to a continuum of embryonic, fetal, adolescent, adult, and geriatric stages. Tales of the “fountain of youth” and songs of being “forever young” are dominant ideas informing us that growing old is something science should strive to counteract. Here, we discuss the normal life stages of the blood system, particularly the historical recognition of its importance in the early growth stages of vertebrates, and what this means with respect to progressive gain and loss of hematopoietic function in the adult.