Hemopoesis

Hemopoiesis: process of red blood cell (RBC) synthesis.

Pluripotent stem cells differentiate  into ‘differentiated’ hemopoietic progenitors influenced by:

• Cytokines

• Transcription factors (TF)

• Epigenetic events

As differentiation occurs there is lineage commitment ( plasticity and self renewal capacity)  eventually the formation of mature RBC (or as dictated by the specific lineage).

Stem cells: are characterised by

• No lineage specific markers (CD34⁺, KIT⁺, CD38^-, Dr^-)

• Quiescence (low % in S phase)

Genetically marked hemopoietic stem cells can differentiate into many ‘lineage’ and eventually cell types:

• Skeletal

• Cardiac

• Hepatocytes

• Astrocytes

• Endothelial cells

• Epithelial cells

Bone marrow contains 3 populations of stem cells.

Cytokines: are proteins that modulate function, activity and number of various other cells, e.g.:

• Non-lineage specific: KIT ligand / stem cell factor and IL-3

• Lineage specific: G-CSF (used extensively in bone marrow transplant), GM-CSF, thrombopoietin and erythropoietin.

Transcription factors: are signalling molecules which control gene expression by binding to regulatory sequences (promoter & enhancer regions), e.g.:

• GATA-1: (erythroid development); if knocked out  no erythroid progenitors  severe anaemia

• PU1: (myeloid development); if knocked out  few early myeloid progenitors (numerous molecules, combination and dose all play a role)

• Ikaros (lymphoid development)

Alterations in levels of TF help determine lineage commitment of multipotent progenitor cells.

Epigenetic factors: genetic memory (for lineage) is not just a matter of transcription factors and cytokines: e.g.

• Chromatin accessibility: expressed regions of the genome have open chromatin (i.e. DNA is not double stranded allowing access by TF) – modulated by acetylated histones

• Methylation changes: methylation status of DNA can determine transcription of some genes

• Nuclear location: active and silent genes occupy different sub nuclear compartments (silent B globin genes located near heterochromatin as opposed to active B globin genes which is located away from heterochromatin)

• Replication timing: transcriptionally active regions are replicated in S phase

Others include CoA recruitment, transcriptional activation, TF recruitment, linker histones, histone remodelling and chromatin remodelling.

B globin gene fragments taken from cells at different stages of development  placed in host cells  show pattern of gene expression of donor (rather than host). Therefore the developmental stage expression is determined largely by the epigenetic template carried by the chromosome fragment (rather than that of the host cell).

Assessment of bone marrow function: many different measurements can be taken to assess the bone marrow:

• Peripheral blood counts (e.g. Hb, WCC & platelets) – crude

• Reticulocyte count (e.g. LDH & bilirubin) – useful for situations with  turnover (e.g. in haemolytic anaemia); gives an indication of erythropoiesis

• Haemoglobin electrophoresis (e.g. in thalassemia)

• Bone marrow biopsy

• CD34 enumeration; antigen for primate progenitors – common practical test for stem cell numbers.

• CFU assays

Category: Pathology Notes