Page 202 - Atlas of Histology with Functional Correlations

erythrocytes, eosinophils, neutrophils, basophils, monocytes, and

megakaryocytes. Lymphoid stem cells also develop in the red bone marrow.
Some lymphoid cells remain in the bone marrow, proliferate, mature, and
become B lymphocytes. Other lymphocytes leave the bone marrow and migrate
via the bloodstream to lymph nodes and the spleen, where they proliferate and
differentiate into B lymphocytes, after which they colonize peripheral lymphoid

tissues (connective tissues, lymphoid tissues, and lymphoid organs).

Other undifferentiated lymphoid cells migrate to the thymus gland, where
they proliferate and differentiate into immunocompetent T lymphocytes.
Afterward, T lymphocytes enter the bloodstream and migrate to reside in the

connective tissues and specific regions of peripheral lymphoid organs of the
body. Both B and T lymphocytes reside in numerous peripheral lymphoid
tissues, lymph nodes, and spleen. Here, they initiate immune responses when
exposed to antigens. Although both the B and T lymphocytes are

morphologically indistinguishable under a light microscope, their cell lines have
separate pathways for growth, development, and function. Only the different
protein markers on their cell surfaces allow these cells to be distinguished by
immunohistochemical means.

Because all blood cells have a limited life span, the pluripotential
hematopoietic stem cells continually divide and differentiate to produce new
progeny of cells. When the blood cells become worn out and die, they are
destroyed by macrophages in different lymphoid organs such as the spleen.

FORMED ELEMENTS: MAJOR BLOOD

CELL TYPES

Microscopic examination of a stained blood smear reveals the major blood cell
types. Mature erythrocytes, or RBCs, are nonnucleated cells and are the most

numerous blood cells. During their final maturation process, the erythrocytes
extrude their nuclei and assume a biconcave shape, and the mature blood cells
enter the bloodstream. Erythrocytes remain in the blood and perform their major
functions within the blood vessels.

In contrast, leukocytes, or WBCs, are nucleated and subdivided into

granulocytes and agranulocytes. Granulocytes are the neutrophils, eosinophils,
and basophils. Both eosinophils and basophils contain distinct stained granules

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