Bone Marrow and Hemopoiesis
Bone marrow seen with low power (to the left). This marrow cavity lies within a shaft of compact bone (middle, pink band). Attaching muscle is to the right.
Bone marrow surrounding a pink, Y-shaped piece of spongy bone. Notice the small osteocytes scattered within the bony matrix. In the marrow there are clumps of small blood-forming cells scattered among the large round fat cells. (The lipid content of the fat cells has been dissolved out in fixation of the tissue). The elements formed most abundantly in marrow are r.b.c.'s, granular leukocytes, and platelets. Lymphocytes and monocytes are also formed here, but go elsewhere to proliferate. Another name for blood-forming tissue such as this is hemopoietic or myeloid tissue.
Bone marrow showing the typical cellular masses of developing blood cells lying between the round, empty fat cells. There are two large megakaryocytes in the field, one just about in the center and the other to the extreme right. Notice orange-colored rbc's in thin-walled sinusoids.
Bone marrow - higher power - identifiable by the fat cells, clusters of developing blood cells, and the large megakaryocytes. Also, in the middle of the field is a cross section of a sinusoid filled with rbc's.
Sinusoid of bone marrow seen here in longitudinal section. There is a good nucleus of a lining endothelial cell near the lower center of the field. Junctions between lining cells are loose so that newly formed blood cells can enter the vessels.
Megakaryocyte as seen in an H & E stained section. Note its multilobed nucleus and its comparatively giant cell size. (Remember that the other giant cell of bone, the osteoclast, has multiple separate nuclei. The osteoclast lies next to bone, while the megakaryocyte lies out in the middle of the marrow.)
Another megakaryocyte, this time as seen in a marrow smear with the May-Grunwald-Giemsa blood stain. In a smear the whole cell is here, though somewhat flattened. The lobed nucleus seems drawn together into a compact mass. Fragments of cytoplasm will form platelets.
In a section like this, stained with H & E, the developing blood cells are hard to identify. However, about in the middle of the field one can recognize a nearly mature eosinophil with bright red granules.
EM of eosinophil. Notice the crystalloid bar in the granules.
Identification of cells is somewhat easier in marrow stained with phloxine - methylene blue - azur II. Here we see a megakaryocyte near the rim of the fat cell to the left. Immediately below is a brightly stained eosinophil. The pale oval nucleus just to the right of the eosinophil belongs either to a reticular cell or a hemocytoblast (stem cell); both are primitive cells and similar in appearance in a section like this.
A reminder that bone marrow is one place where you find a stroma of reticular tissue. Here the tissue has been silvered so that you can see the network of fine reticular fibers that support all the blood forming cells. Large spaces represent fat cells.
Diagrammatic summary of the events that take place during maturation of red blood cells (erythropoiesis). Staining is with special blood stains (Giemsa, etc.). Primitive status is on the left; mature status is on the right.
- Top line: there's a decrease in cell size (from left to right) and a decrease in basophilia (blueness) of cytoplasm. At the same time, hemoglobin increases, making the cytoplasm more and more acidophilic (pink). Basophilia is due to presence of abundant polyribosomes.
- Second line: there's a decrease in nuclear size and ultimately extrusion and loss of the nucleus.
- Third line: there's increased condensation of nuclear chromatin and eventual pyknosis of the nucleus (very dark, compact, dying). Also the nucleoli, evident at first, are soon lost
Maturational stages in development of granular leukocytes:
- Extreme left - myeloblast (the most primitive stage)
- Next - promyelocyte (these first two stages are undifferentiated precursors of all three granulocyte types)
- Next four: myelocyte, metamyelocyte, band cell, and mature cell
The top row represents the eosinophilic cell line, the middle row represents the neutrophilic line, and the bottom row represents the basophilic line. Note the decrease in cell size, the decrease in cytoplasmic basophilia (meaning decrease in polyribosomes), the increase in cytoplasmic granules (these first become specific and distinguishable as eosinophilic or basophilic at the myelocyte stage), and an increase in lobulation of the nucleus. The next few slides are of smears of bone marrow stained with modified Giemsa stains, so we can rely on the color of cells in identifying their developmental status
The cells shown here are all stages in the development of erythrocytes. Generally in the red blood cell line: (1) the cells become progressively smaller, (2) the cytoplasm changes from blue to pink, and (3) the nucleus becomes smaller and more condensed and ultimately is lost altogether. Cells shown here include (in developmental order):
- Top cell - proerythroblast
- Lower row
- left = basophilic normoblast or erythroblast. It is still blue, but is smaller; the nucleus is more condensed
- middle = polychromatophilic normoblast or erythroblast. Cytoplasm is grayer or muddier; nucleus is even more condensed.
- right = orthochromatic (or eosinophilic) normoblast. Cytoplasm is pinker and cell is smaller; nucleus is pyknotic
Reticulocytes with polyribosomal remnants (RNA) staining dark in their cytoplasm. They are slightly larger than the completely mature erythrocytes and are often found in the peripheral bloodstream at times when blood cells are being formed unusually rapidly (as during or after certain blood diseases). Remember not to confuse reticulocytes of the blood with reticular cells of connective tissue!
The large cell with blue cytoplasm is a "blast" cell although simple observation cannot tell us whether it's a myeloblast or an erythroblast. To the left is a group of neutrophilic band cells; the Iower two are probably more advanced, judging by their more segmented appearance. At bottom center is an orthochromatic normoblast with pyknotic nucleus. In the upper right corner (and probably lower left corner) is a lymphocyte.
There is a large "blast" cell in the upper left group and a large promyelocyte at upper center. The latter is recognizable by the non-specific azurophilic granules in its cytoplasm, foretelling that it is heading toward one of the granulocyte lines. A basophilic normoblast with blue cytoplasm is in lower center. To the right of it are two early orthochromatic normoblasts. At bottom center is a late polychromatophilic normoblast with muddy cytoplasm. A slightly younger polychromatophilic cell is in the extreme lower left corner, with a slightly larger and less condensed nucleus. A neutrophilic metamyelocyte with indented nucleus also lies near the lower edge of the field.
NOTE: Remember in our terminology that:
- erythroblast and normoblast are used interchangeably - as in basophilic erythroblast or basophilic normoblast.
- orthochromatic, orthochromic, acidophilic, eosinophilic are all used interchangeably as applied to the red cell line. Of course, eosinophilic as applied to the granulocytes refers strictly to those cells that are becoming eosinophils