Page 27 - Essential Haematology

Chapter 1 Haemopoiesis / 13

cells swell, the plasma membrane loses integrity. 1 Immunoglobulin superfamily Th is includes

There is usually an infl ammatory infi ltrate in receptors that react with antigens (the T - cell
response to spillage of cell contents. Autophagy is receptors and the immunoglobulins) and antigen -
the digestion of cell organelles by lysosomes. It may independent surface adhesion molecules.

be involved in cell death but in some situations also 2 Selectins These are mainly involved in leucocyte
in maintaining cell survival by recycling nutrients. and platelet adhesion to endothelium during

inflammation and coagulation.
3 Integrins These are involved in cell adhesion

Transcription f actors to extracellular matrix (e.g. to collagen in
wound healing and in leucocyte and platelet
Transcription factors regulate gene expression by
adhesion).

controlling the transcription of specific genes or

gene families. Typically, they contain at least two The adhesion molecules are thus important in the
domains: a DNA - binding domain such as a leucine development and maintenance of infl ammatory
zipper or helix - loop - helix motif which binds to a and immune responses, and in platelet – vessel wall
specific DNA sequence, and an activation domain and leucocyte – vessel wall interactions. Expression

which contributes to assembly of the transcription of adhesion molecules can be modifed by extracel-
complex at a gene promoter (Fig. 1.9 ). Mutation, lular and intracellular factors and this alteration of
deletion or translocation of transcription factors expression may be quantitative or functional. IL - 1,
underlie many cases of haematological neoplasms. TNF, IFN - γ , T - cell activation, adhesion to extracel-
lular proteins and viral infection may all up - regulate
expression of these molecules.
Adhesion m olecules

The pattern of expression of adhesion molecules
A large family of glycoprotein molecules termed on tumour cells may determine their mode of
adhesion molecules mediate the attachment of spread and tissue localization (e.g. the pattern of
marrow precursors, leucocytes and platelets to metastasis of carcinoma cells or non - Hodgkin lym-

various components of the extracellular matrix, to phoma cells into a follicular or diffuse pattern). Th e
endothelium, to other surfaces and to each other. adhesion molecules may also determine whether or

The adhesion molecules on the surface of leucocytes not cells circulate in the bloodstream or remain

are termed receptors and these interact with mole- fixed in tissues. They may also partly determine
cules (termed ligands) on the surface of potential whether or not tumour cells are susceptible to the

target cells. Three main families exist: body ’ s immune defences.

■ Haemopoiesis (blood cell formation) arises infants and young children haemopoietic

from pluripotent stem cells in the bone tissue extends down the long bones of the
marrow. Stem cells give rise to progenitor arms and legs.

cells which, after cell divisions and ■ Stem cells reside in the bone marrow in SUMMARY

differentiation, form red cells, granulocytes niches formed by stromal cells and
(neutrophils, eosinophils and basophils), circulate in the blood.
monocytes, platelets and B and T ■ Growth factors attach to specifi c cell

lymphocytes. receptors and produce a cascade of

■ Haemopoetic tissue occupies about 50% phosphorylation events to the cell
of the marrow space in normal adult nucleus. Transcription factors carry the
marrow. Haemopoiesis in adults is message to those genes that are to be
confi ned to the central skeleton but in ‘ switched on ’ , to stimulate cell division,
(Continued)