CD34 derives its name from the cluster of differentiation protocol that identifies cell surface antigens. CD34 was first described on hematopoietic stem cells independently by Civin et al. and Tindle et al. as a cell surface glycoprotein and functions as a cell-cell adhesion factor. It may also mediate the attachment of hematopoietic stem cells to bone marrow extracellular matrix or directly to stromal cells. Clinically, it is associated with the selection and enrichment of hematopoietic stem cells for bone marrow transplants. Due to these historical and clinical associations, CD34 expression is almost ubiquitously related to hematopoietic cells however it is actually found on many other cell types as well.
The CD34 protein is a member of a family of single-pass transmembrane sialomucin proteins that show expression on early hematopoietic and vascular-associated tissue. However, little is known about its exact function.
CD34 is also an important adhesion molecule and is required for T cells to enter lymph nodes. It is expressed on lymph node endothelia, whereas the L-selectin to which it binds is on the T cell. Conversely, under other circumstances CD34 has been shown to act as molecular "Teflon" and block mast cell, eosinophil and dendritic cell precursor adhesion, and to facilitate opening of vascular lumina. Finally, recent data suggest CD34 may also play a more selective role in chemokine-dependent migration of eosinophils and dendritic cell precursors. Regardless of its mode of action, under all circumstances CD34, and its relatives podocalyxin and endoglycan, facilitates cell migration.
It is important to mention that Long-Term Hematopoietic Stem Cells (LT-HSCs) in mice and humans are the hematopoietic cells with the greatest self-renewal capacity. Human HSCs express the CD34 marker.
CD34 is expressed in roughly 20% of murine hematopoietic stem cells, and can be stimulated and reversed.
Antibodies are used to quantify and purify hematopoieticprogenitor stem cells for research and for clinical bone marrow transplantation. However, counting CD34+ mononuclear cells may overestimate myeloid blasts in bone marrow smears due to hematogones (B lymphocyte precursors) and CD34+ megakaryocytes.
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