16. The only bone marrow cell that never appears in peripheral blood is
    (1) myeloblast (2) myelocyte.
    (3) lymphoblast.                                             (4) megaloblast.

Introduction

Blood cell formation or hematopoiesis primarily takes place in the bone marrow, producing various precursor cells that mature and eventually enter peripheral blood circulation. However, not all marrow precursor cells appear in the bloodstream under normal conditions. It is important to know which immature cells are confined to the marrow and which may occasionally circulate, as this knowledge is critical in clinical diagnostics especially when evaluating blood smears for disease states like leukemia.

Bone Marrow Cell Types and Their Circulatory Presence

The four cells in question are:

1. Myeloblast: The earliest precursor in the granulocyte lineage, a large immature cell residing under normal conditions exclusively in the bone marrow.

2. Myelocyte: A more differentiated granulocytic precursor that sometimes appears in peripheral blood during regenerative or reactive states (“left shift”) but mainly matures in marrow.

3. Lymphoblast: Immature lymphoid precursors generally found in lymphoid tissues and marrow; they can appear in peripheral blood in pathological conditions such as leukemia.

4. Megaloblast: Enlarged, immature red blood cell precursors appearing in marrow during vitamin B12 or folate deficiency causing ineffective erythropoiesis; they do not normally enter peripheral blood except in severe disease.

Why Myeloblasts Never Appear in Peripheral Blood Normally

• Myeloblasts are normally confined to the bone marrow due to their immature state, size, adhesive properties, and lack of homing receptors necessary to enter circulation.

• The bone marrow microenvironment or “bone marrow barrier” retains these immature cells to ensure orderly maturation.

• Presence of myeloblasts in peripheral blood indicates pathological processes, particularly acute myeloid leukemia (AML), which causes abnormal release of blasts into blood.

When Do Other Precursor Cells Appear in Blood?

• Myelocytes can appear transiently in increased numbers during infection or marrow stimulation but usually indicate a reactive process rather than malignancy.

• Lymphoblasts may be found in blood during lymphoblastic leukemias or severe immune responses.

• Megaloblasts are typically marrow-confined but may be released into blood in severe vitamin deficiencies or marrow dysplasia.

Clinical Importance of Detecting Blast Cells in Peripheral Blood

• The presence of myeloblasts in circulation is a key diagnostic marker for acute myeloid leukemia.

• Blast percentage in blood and marrow guides diagnosis, prognosis, and treatment planning.

• Careful differentiation between reactive (infection-induced, transient) and malignant blast cells is critical.

Summary

• The only bone marrow cell that never normally appears in peripheral blood is the myeloblast (Option 1).

• Other precursors like myelocytes and lymphoblasts may appear due to pathological or reactive conditions.

• Megaloblasts are primarily in marrow and rarely enter blood under severe conditions.