Haemopoietic Cells

Haemopoiesis

During foetal development, the formation of blood cells (haemopoiesis) commences in wall of the yolk sac. After the second month of foetal development, the liver, and, slightly later, the spleen, become the dominant sites of haemopoiesis. From the 6th month, and dominating from the 7th month onwards, the formation of blood cells occurs in bone marrow, which is the major site of formation of blood cells in normal adult humans.

Yellow bone marrow, which harbours mainly adipocytes, dominates in the hollow of the diaphysis of adult long bones. Haemopoiesis occurs in red bone marrow, which is typically found between the trabeculae of spongy bone in the epiphysis of adult long bones. Both age and demands on haemopoiesis may effect the relative amounts of red and yellow bone marrow. Haemopoietic cells surround the vascular sinusoids and are supported by reticular connective tissue. In addition to the endothelial cells of the sinusoids and the reticulocytes of the connective tissue, macrophages are frequent in red bone marrow.

Haemopoietic Cells

The basis of haemopoiesis is a small population of self-replicating stem cells, which ultimately can generate all types of blood cells. Their progeny may develop into either lymphocytic stem cells or pluripotent haemal stem cells (colony-forming unit - stem cell - CFU-S). The latter type gives rise to stem cells which can form the major groups of blood cells other than lymphocytes. Depending on their progeny it is possible to differentiate

• burst-forming unit of the erythroid line (BFU-E),
• colony-forming unit - granulocytes and macrophages (CFU-G/M), and
• colony-forming unit - megakaryocytes (CFU-Mk).

Erythrocytes The first identifiable stage of erythropoiesis is the proerythroblast - a large, slightly basophilic cell, which contains a large, lightly stained nulceus. Proerythroblasts proliferate to generate a sequence of cells which show a gradual decrease in size and condensation of their chromatin. They are named after changes in the staining characteristic of their cytoplasm (basophilic erythroblast, polychromatophilic and orthochromic normoblasts). The nucleus is finally extruded from the normoblast. The cell enters circulation as a reticulocyte, which still contains some organelles. Reticulocytes remain for a few days in either the bone marrow or the spleen to mature to erythrocytes. In some blood smears reticulocytes may be recognisable because of a very slight basophilic staining - either homogeneous or in the form of a basophilic stippling.

Granulocytes

Myeloblast appear light-microscopically similar to proerythroblast. They proliferate to generate promyelocytes. Promyelocytes begin to accumulate nonspecific granules, but they are still able to divide. The maturation of their progeny, the myelocytes, is characterised by the accumulation of specific granules and changes in nuclear morphology. Metamyelocytes have a C-shaped nucleus.
 

Blood Platelets (Thrombocytes)

are, as mentioned above, fragments of the cytoplasm of megakaryocytes. Megakaryocytes are very large cells (up to 160 µm in diameter), which contain very large, highly lobulated, polyploid nuclei. Megakaryocytes are in turn the product of the differentiation of basophilic megakaryoblasts.

Precursors of blood cells which are usually only found in the bone marrow can be found in peripheral blood in a variety of pathological conditions.
If a Rh-negative mother has been immunised by erythrocytes of a Rh-positive foetus, a condition called Erythroblastosis fetalis may develop during subsequent pregnancies. It would show itself in the foetus or newborn by the presence of erythrocyte precursors in peripheral blood - although other, more severe symptoms should be obvious. Chronic myeloid leukemia is another condition - in this case showing itself by the presence of all types of granulocyte precursors in peripheral blood.

The nomenclature employed for haemopoietic cells (but not the number of stages recognized) is somewhat variable across texts. Note also that these cell types refer to stages of development along a morphologically more or less continuous spectrum.

Suitable Slides

sections of red bone marrow - H&E or a bone marrow smear - Leishman, Wright's, Giemsa or May-Grünwald-Giemsa stains

Red Bone Marrow, rabbit - H&E
Most of the haemopoietic cells visible will be of the erythroblastic line. The only cell type of this line which is easy to distinguish in H&E stained sections are normoblasts.
A very condensed nucleus is seen in late (orthochromic)normoblast. Granulocyte and erythrocyte precursors will mostly intermingle, but may be distinguished by nuclear morphology and/or size. A bent nucleus is found in metamyelocytes - this shape is very pronounced in the last, immature form of neutrophils, which are also called stab or band cells. If the cell (1) is large, with a distinct "clearing" in the otherwise pink cytoplasm and (2) has an ovoid or slightly indented nucleus, it is likely to be a myelocyte. Cells with large light nuclei and almost unstained cytoplasm are either reticulocytes or macrophages.

Macrophages are frequently associated with normoblasts, and together these cells form erythroblastic islands. The name for macrophages in these islands, nurse cells, may tell you a bit about their function in addition to the scavenging of the expelled nuclei.
Identify normoblasts, myelocytes and metamyelocytes and include them in your drawing of the megakaryocyte/blast.