Page 199 - Atlas of Histology with Functional Correlations

SECTION 1 Blood

Blood is a unique form of connective tissue in which its cells are suspended in a

fluid component called plasma. The major cell types found in plasma are the
erythrocytes (red blood cells [RBCs]) and leukocytes (white blood cells
[WBCs]), which are divided into two categories agranulocytes and
granulocytes. These cell types constitute the formed elements of the blood.
Also, circulating in blood are cell fragments called platelets derived from large

bone marrow cells, the megakaryocytes. Blood cells transport gases, nutrients,
waste products, hormones, antibodies, various chemicals, ions, and other
substances in plasma to and from different cells, tissues, and organs in the body.

Blood cells have also a limited life span, and, as a result, they constantly wear
out and are continuously replaced. The production of the formed elements of the
blood is called hematopoiesis.

SITES OF HEMATOPOIESIS

Hematopoiesis occurs in different organs of the body, depending on the stage of
development of the organism. In a developing embryo, hematopoiesis initially

occurs in the yolk sac and later in the development in the liver, spleen, lymph
nodes, and bone marrow. After birth, hematopoiesis continues almost
exclusively in the red marrow of different bones. In the newborn individual, all

bone marrow is red and functions in hematopoiesis.

The red bone marrow is a highly cellular structure and consists of
hematopoietic stem cells and the precursors of different blood cells. Red marrow
also contains a loose arrangement of fine reticular fibers that form an intricate
connective tissue network. As the individual ages and reaches adulthood, the red

marrow is primarily confined to the flat bones of the skull, sternum and ribs,
vertebrae, and pelvic bones. The remaining long bones in the limbs of the body
gradually accumulate fat, and their red marrow is replaced by fatty yellow
marrow. Consequently, these sites lose the hematopoietic functions.

HEMATOPOIESIS

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