Page 16 - Essential Haematology

2 / Chapter 1 Haemopoiesis

Th is first chapter is concerned with the general marrow consists of fat (Fig. 1.1 ). Th e remaining
aspects of blood cell formation (haemopoiesis). Th e fatty marrow is capable of reversion to haemopoiesis
processes that regulate haemopoiesis and the early and in many diseases there is also expansion of
stages of formation of red cells (erythropoiesis), haemopoiesis down the long bones. Moreover, the
granulocytes and monocytes (myelopoiesis) and liver and spleen can resume their fetal haemopoietic
platelets (thrombopoiesis) are also discussed. role ( ‘ extramedullary haemopoiesis ’ ).

Site of h aemopoiesis Haemopoietic s tem and
p rogenitor c ells
In the first few weeks of gestation the yolk sac is the

main site of haemopoiesis. However, defi nitive hae- Haemopoiesis starts with a pluripotential stem cell
mopoiesis derives from a population of stem cells that can self - renew but also give rise to the separate

first observed on the dorsal aorta termed the AGM cell lineages. These cells are able to repopulate a

(aorta - gonads - mesonephros) region. Th ese common bone marrow from which all stem cells have been
precursors of endothelial and haemopoietic cells eliminated by lethal irradiation or chemotherapy.
(haemangioblasts) are believed to seed the liver, Th is haemopoietic stem cell is rare, perhaps 1 in
spleen and bone marrow and from 6 weeks until every 20 million nucleated cells in bone marrow.
6 – 7 months of fetal life the liver and spleen are the Although its exact phenotype is unknown, on
major haemopoietic organs and continue to produce immunological testing it is CD34 CD38 and
+
−
blood cells until about 2 weeks after birth (Table negative for lineage markers (Lin ) and has the
−
1.1 ; see Fig. 7.1 b ). The bone marrow is the most appearance of a small or medium - sized lymphocyte

important site from 6 to 7 months of fetal life. (see Fig. 23.3 ). The cells reside in specialized niches ’ .
‘

During normal childhood and adult life the marrow Cell differentiation occurs from the stem cell via

is the only source of new blood cells. Th e develop- committed haemopoietic progenitors which are
ing cells are situated outside the bone marrow restricted in their developmental potential (Fig.
sinuses; mature cells are released into the sinus 1.2 ). The existence of the separate progenitor cells

spaces, the marrow microcirculation and so into the can be demonstrated by in vitro culture techniques.
general circulation. Very early progenitors are assayed by culture on
In infancy all the bone marrow is haemopoietic bone marrow stroma as long - term culture initiating
but during childhood there is progressive fatty cells whereas late progenitors are generally assayed
replacement of marrow throughout the long bones
so that in adult life haemopoietic marrow is con-

fined to the central skeleton and proximal ends of
the femurs and humeri (Table 1.1 ). Even in these
haemopoietic areas, approximately 50% of the

Table 1.1 Sites of haemopoiesis.
Fetus 0 – 2 months (yolk sac)
2 – 7 months (liver, spleen)
5 – 9 months (bone marrow)
Infants Bone marrow (practically all bones)

Figure 1.1 A normal bone marrow trephine biopsy
Adults Vertebrae, ribs, sternum, skull,
(posterior iliac crest). Haematoxylin and eosin stain;
sacrum and pelvis, proximal ends
approximately 50% of the intertrabecular tissue is
of femur
haemopoietic tissue and 50% is fat.