Formed Elements of the Blood
The cells found in the circulating blood arise from two sources in the body. Myeloid tissue or red marrow is the source of erythrocytes, the granulocytes and the platelets. These cell types arise from a common ancestral cell, the hemocytoblast. Two other populations of cells making up the formed elements are the lymphocytes and monocytes. These cells develop from a precursor of the hemocytoblast, the reticular cell. Lymphocytes and monocytes are formed in the bone marrow and in the lymphoid tissues.
Occurrence red marrow is the soft vascular tissue found in the marrow cavities of the diaphyses of long bones and in the spaces of the spongy bone located in the epiphyses of long bones and the middle portion of flat bones. These cavities are lined with an endosteum.
Components of Marrow Tissue
Marrow tissue consists of:
1. Blood cells in various stages of maturation from primitive stem cells to mature blood cells.
2. Adipose cells are common. During puberty and adolescence, adipose tissue replaces the blood-forming red marrow converting it to yellow marrow. This fatty tissue is primarily a filler tissue which can be compressed when the inactivated marrow proliferates during periods of accelerated hemopoiesis.
3. Blood vessels can be seen. Nutrient marrow arterioles branch into capillaries which empy into large, abundant sinusoids lines by flattened, phagocytic reticular cells. Sinusoids then converge into venules. Lymphatic vessels have not been demonstrated in bone marrow.
4. The structural framework of marrow tissue consists of a syncitium of reticular fibers associated with reticular cells. This network is connected to the endosteum and closely associated with blood vessels. These reticular cells are primitive connective tissue cells. They are the progenitors of macrophage (either fixed or wandering).
5. Nerves are also found in marrow. They are associated with blood vessels and follow their pathways. These nerves may have a vasomotor function.
Proliferative Function of Bone Marrow
There is about 0.56 grams of marrow for each gram of blood, about 3.5 to 6% of body weight. This corresponds to 1600 to 3700 grams, the weight of the liver. About one half of this material is fat.
The development of the formed elements begins with the division and differentiation of stem cells in the marrow. These cells are distinguished from one another using polychrome Romanowsky stains (eosin, azure and methylene blue dissolved in absolute alcohol). The three most commonly used Romanowsky stains are Wrights, Giemsa and May Grunwald stains.
Formed Elements Produced by the Red Marrow Exclusively
I. Granulocytes General Characteristics
B. Lobes of the nucleus are connected by thin strands of chromatin.
C. Cytoplasm is acidophilic and contains granules. These granules have a size and stain affinity which allows the granulocytes to be placed into three categories.
b. Granules are pale staining (neutral).
c. 12 to 14 um in diameter.
d. They make up about 60% of the circulating leukocytes.
e. A phagocyte, it plays a primary role in inflammation..
b. The granules are richly eosinophilic (acidophilic).
c. About 12 to 17um in diameter.
d. They make up about 3% of the circulating leukocytes.
e. Their functions are associated with allergic reactions and parasitic infections.
b. The large, deeply basophilic granules may obscure the nucleus.
c. About 12 to 15 um in diameter.
d. They make up less than 1% of the circulating leukocytes.
e. They have a purely secretory function. The contents of their granules function
in inflammatory, immune and hypersensitivity reactions.
4. Platelets form from giant myeloid cells called megakaryocytes
5. Erythrocytes are formed by a complex process which occurs through several stages beginning with the hemocytoblast.
a. Pronormoblast arises by division and differentiation of the hemocytoblasts.
i. 20 to 25 um in diameter.
ii. Large nucleus (80% of cell volume).
iii. Cytoplasm is deeply basophilic.
b. Basophilic Normoblast arises by division and
differentiation of the
i. 16 to 18 um in diameter.
ii. Nuclear size is reduced.
iii. Chromatin forms a "spoke wheel" configuration.
iv. Cytoplasm is still basophilic.
c. Polychromatic Normoblast arises by division and
differentiation of the
d. Acidophilic Normoblast arises by differentiation
from the polychromatic
Formed Elements Produced in non-Marrow Tissues
f. They are voracious phagocytes and become highly activated after making contact with foreign antigen.
g. They make up about 5% of the circulating leukocytes
h. The monocytes and macrophages plays
a crucial role in inflammatory and