The Haematopoietic System

Abstract

Circadian variations are present in all aspects of haematopoiesis, including the different cell lineages and stages of development, from stem and progenitor cells to mature leukocytes and erythrocytes. In humans, the circadian variations of cell proliferation in different cell types show covariation, whereby haematopoiesis occurs in synchronized waves. The highest activity is seen in the middle of the day, while the minimum is observed during the night. In nocturnal animals, such as mice and rats, haematopoiesis undergoes similar variations, with a maximum during the active phase. There is also evidence for seasonal variations, although they are not as well-characterized as the circadian variations. The clock genes are present and are actively transcribed in all hematopoietic cells. However, there are great variations in terms of their functions and their importance in individual cell types. Altogether, they create enormous complexity through cellular interactions and regulatory mechanisms, influenced both by the circulation of the nervous system, including signals from the central master clock, as well as by local cellular functions. The clock genes and their proteins modify other regulatory signals and cell functions, mainly mediated by chemokines.