Anatomy

The bone marrow is a specialised tissue of haemopoietic elements, and comprises bony and stromal tissue with definite spatial organisation. The supporting stromal tissue consists of a fine reti-culin meshwork, fat and blood vessels. The amount of fat varies with age and quantity of haemopoiesis. From 50 to 80% fat is seen in marrows of adults and elderly patients. Little or no fat is seen in marrows of children and neonates as haemopoiesis occurs throughout the marrow in all bones. In adults, haemopoiesis is limited to certain areas within bone marrow of skull, vertebrae, sternum, ribs, pelvic bones and proximal long bones. The row of fat cells separating haemopoietic cells from bone trabeculae is called the first fat space. This is lost in leukaemic infiltration. The lamellar bone trabeculae contain osteoblasts and multinuclear osteoclasts on the endosteal surface and osteocytes within lacunae. In elderly patients with osteoporosis the trabeculae are thinned. Thin-walled venous sinuses are seen throughout the marrow and mature haemopoietic cells have a perisinusoidal location. Small muscular arteries and capillaries are also present.

Haemopoietic elements include the myeloid, erythroid, megakaryocytic and lymphoid cells; all of these have a common precursor stem cell. The committed stem cells give rise to the distinct cell lines. The various stages of maturation are shown in Figure 44.4. The morphology of haemopoietic cells in general is better appreciated in Giemsa-stained thin/semi-thin sections. The different types of cells and some of their features are presented in Table 44.6.

Artefacts including non-haemopoietic cells such as epidermis, skin appendages, muscle and bone tissue may be introduced into the trephine biopsy inadvertently during the biopsy procedure. The presence of epithelial elements can result in confusion with metastatic malignancy.

Lymphoid stem cell

^^ Myeloid stem cell

Lymphoid stem cell

^^ Myeloid stem cell

Anatomy Location Lymph Nodes

BFUE

Pre B cell Medullary thymocyte

CFUG CFUM CFUMeg

BFUE

Pre B cell Medullary thymocyte

CFUG CFUM CFUMeg

Intermediate B cell

Intermediate B cell

Mature B cell

Promyelocyte

Promyelocyte

Myelocyte

Myelocyte

Metamyelocyte

Metamyelocyte

Myeloblast Monoblast Megakaryoblast Erythroblast

Anatomy Location Lymph Nodes

Monocyte Platelet

Figure 44.4. Development and maturation of haemopoietic cells.

Mature B cell

Mature Neutrophil T cell

Monocyte Platelet

CFU E

CFU E

Myeloblast Monoblast Megakaryoblast Erythroblast

Normoblast

Normoblast

Megakaryocyte Reticulocyte

Erthrocyte

Erthrocyte

Figure 44.4. Development and maturation of haemopoietic cells.

Table 44.6. Haemopoietic cells

Precursors

Usual location within marrow

Numbers

Cytology

Clues to pathological proliferation

Granulocytes

Myeloblasts

Immature-

2 to 4 times the

Immature cells - high

Excess of blasts, present

including

paratrabecular.

number of

N/C ratio and granular

in both paratrabecular

neutrophils &

Mature-central.

erythroid cells

cytoplasm. Mature -

and central areas.

eosinophils

lobated nuclei and

specialised granules

in cytoplasm.

Erythroid

Erythroblasts

Colonies in

1/3 tol/4 of the

Immature cells - dark

Paratrabecular

central

myeloid cells.

blue cytoplasm, round

proliferation.

intertrabecular

nuclei, coarse

areas.

chromatin, and

nucleoli attached to

nuclear membrane.

Mature - dark, densely

staining, perfectly

round nuclei.

Megakaryocytes

Megakaryoblast Perisinusoidal

Variable, usually

Largest cells. Occur

Clustering,

at least 1 per

singly. Multilobated

paratrabecular location

field

nuclei with abundant

and nuclear

cytoplasm.

hypolobation.

Lymphoid

Lymphoblast

Interstitial

Up to 50% in

Mixed population of

> 3 aggregates/ diffuse

infiltrates or

children,

small mature and larger

heavy interstitial

< 3 nodular

5-10% in adult

lymphoid cells ± germinal

proliferation particularly

aggregates

marrows

centres.

if monoclonal.

Plasma cells

Lymphoblast

Perivascular

Up to 2% in

Mature plasma cells -

Large clusters and

adult marrows.

eccentric nuclei with

nucleolated plasma

clock face chromatin,

cells.

no nucleoli.

Immature plasma cells -

nucleolated.

Mast cells

Myeloblast

Perivascular or

Few

Basophilic coarse granules

Increased numbers,

paratrabecular

in cytoplasm, round/oval

spindle cells, fibrosis.

nucleus.

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