Which of the following surface protein is most often expressed in human hematopoietic stem cells?
**Core Concept:** Hematopoietic stem cells (HSCs) are pluripotent cells that give rise to all blood cells and play a crucial role in maintaining blood cell homeostasis. They express specific surface proteins for identification and isolation purposes.
**Why the Correct Answer is Right:** CD34 is a transmembrane glycoprotein that belongs to the immunoglobulin superfamily. It is widely expressed on hematopoietic stem and progenitor cells, including human hematopoietic stem cells. The expression of CD34 allows for the identification and isolation of HSCs from bone marrow or peripheral blood samples.
**Why Each Wrong Option is Incorrect:**
A. CD13 is a surface protein expressed on immature myeloid cells, not HSCs.
B. CD19 is a surface protein expressed on mature B-lymphocytes, not HSCs.
C. CD235a (glycophorin A) is a band 3 protein present on erythroid cells, not HSCs.
D. CD38 is a surface protein expressed on hematopoietic stem cells, but not as broadly as CD34.
**Clinical Pearl / High-Yield Fact:** CD34 is the most common surface marker used for isolating human hematopoietic stem cells due to its broad expression on these cells. However, CD34 alone is not sufficient for the definitive identification of HSCs, as it is also expressed on other hematopoietic cells. Combining CD34 with other markers (e.g., CD133, CD117, CD90) can provide a more comprehensive and selective identification of HSCs.
**Correct Answer:** CD34