Hematopoiesis - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 19

Chapter 2
Hematopoiesis
Hematopoietic cells , or hemopoietic cells, represent bone marrow-derived cell
types that circulate in blood (including mature cell types and their precursors).
Hematopoietic cells are categorized into myeloid cells (basophils, eosinophils, neu-
trophils, erythrocytes, thrombocytes, monocytes and macrophages, and mastocytes)
and lymphoid cells (B lymphocytes, various types of T lymphocytes, which are
the only hematopoietic cells that can be generated elsewhere than in the bone
marrow, and natural killer [NK] cells, that are cytotoxic lymphocytes also called
large granular lymphocytes).
Stromal cells is another collective term for different cell types in a given tissue
at a given time that do not pertain to functional components of this tissue, i.e.,
parenchymal cells . In particular, the bone marrow stroma yields a hematopoietic
microenvironment that consists of various cell types, such as mesenchymal progen-
itors, fibroblasts, adipocytes, monocytes and macrophages, endothelial and smooth
muscle cells, T-lymphocytes, among others. These cells can support short- and long-
term hematopoiesis via membrane-bound and secreted hematopoietic factors that
operate on hematopoietic stem and progenitor cells.
Hematopoietic stem cells produce by mitosis either stem cells of the same type
(self-renewal) or progenitors leading to precursor, immature, and mature blood
cells. In steady state, most stem cells are dormant, whereas few are active [ 37 ].
Hematopoietic stem cell self-renewal is regulated by intrinsic and extrinsic signals.
Blood-cell progenitors and mature cells of the granulocytic, monocytic,
megakaryocytic, and erythroid lineages can be generated from differentiated
cells. For example, human dermal fibroblasts that bypass the pluripotent stem
cell state can give rise to multipotent hematopoietic progenitors. These fibroblasts
synthesize predominantly octamer-binding transcription factor Oct4, or POU
domain, class-5, homeodomain-containing transcription factor POU5F1, as well
as PTPRc phosphatase, or panhematopoietic marker CD45, during the multipotent
reprogramming [ 38 ].