The International Journal of Biochemistry & Cell Biology 34 (2020) 1513–1518

Cells in focus
Red blood cells
∗
S. Peter Klinken
Department of Biochemistry, Laboratory for Cancer Medicine, Western Australian Institute for Medical Research, Royal Perth Hospital,
The University of Western Australia, Level 6, Medical Research Foundation Building, Rear, 50 Murray Street, Perth 6000, Australia
Received 12 December 2020; received in revised form 20 May 2020; accepted 29 May 2020

Abstract
Red blood cells are derived from haemopoietic stem cells in bone marrow. Following a series of maturation steps, directed
largely by the hormone erythropoietin (Epo), red cells enucleate and enter the circulatory system. In circulation these small,
flexible biconcave cells containing haemoglobin transport O2 from the lungs to the periphery, and CO2 back from the
periphery to the lungs. The most common disorders associated with red blood cells are anaemias. While there are numerous
causes of anaemia, the reduced capacity for gaseous exchange is the underlying theme. Over the past 15 years, recombinant
Epo has been used extremely successfully in the treatment of several forms of anaemia. The single gene disorders collectively
known as haemoglobinopathies represent one of the best opportunities for gene therapy.
© 2020 Elsevier Science Ltd. All rights reserved.
Keywords: Red blood cells; Maturation; Erythropoiesis; Anemias; Haemoglobin

Cell facts

• Red blood cells transport O2 for respiration from the lungs to all parts of the body, and remove metabolically-generated CO2.
• In adults there are approximately 5 × 1012 red blood cells per litre of blood. The average life span of red blood cells is 110 days.
• Red blood cells travel approximately 250 km during their life span.
• Erythropoietin (Epo), the major hormone responsible for red blood cell production, is produced by the kidneys.
• Since red blood cells have no nuclei, the heart is relieved of pumping an extra 1000 t of inert nuclei daily.

1. Introduction

Red blood cells, or erythrocytes, are biconcave

Abbreviations: B, basophilic erythroblast; BFU-E, burst form-
discs which are critical for gaseous exchange [1].
ing unit-erythroid; CFU-E, colony forming unit-erythroid; Epo, Their ma- jor product is haemoglobin, the carrier for
erythropoietin; E, eythrocyte; IL-3, interleukin 3; GM-CSF, gran- O2 and CO2 throughout the body. Haemoglobin
ulocyte/macrophage colony stimulating factor; O, orthochromatic comprises of four globin subunits, normally 2α and
erythroblast; Po, polychromatic erythroblast; Pr, proerythroblasts;
R, reticulocyte; SCF, stem cell factor
2β chain in adults, each surrounding a core haem-
∗
Tel.: +61-8-9224-0334; fax: 61-8-9224-0322. moiety. At the center of the haem is iron which is
E-mail address: pklinken@cyllene.uwa.edu.au essential for gaseous trans- port. It is haemoglobin
(S. Peter Klinken). which produces the character- istic redness
associated with erythrocytes (Fig. 1).

1357-2725/02/$ – see front matter © 2020 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 7 - 2 7 2 5 ( 0 2 ) 0 0 0 8 7 - 0
151 S. Peter Klinken / The International Journal of Biochemistry & Cell Biology 34 (2002) 1513–
1518
The average diameter of erythrocytes is 8 µm, with
a mean volume of 90 fl [1]. The biconcave shape of
red blood cells, together with cell membrane flexibi-
lity, enables the cells to expand up to 150 fl, or to
en- ter capillaries with a diameter considerably less
than 8 µm. As red cells age, their membranes become
rigid and inflexible—they are then removed from
circula- tion by macrophages. Nearly all of the
constituents in 6–7 g of haemoglobin catabolized daily
are reused, except for bilirubin.

2. Cell origin

Red blood cells are produced from pluripotent

haemopoietic cells [2]. These stem cells reside in the
bone marrow and upon appropriate stimulation from
a variety of hormones or cytokines, undergo a series
of maturation events which eventually produce fully
functional erythrocytes (Fig. 2). Furthermore, there
is a massive increase in cell numbers which enables
thousands of erythrocytes to be generated from the
activation of individual stem cells.
The primary hormone which regulates erythro-
poiesis (i.e. the process of red blood cell production)
is Epo [3–6]. Kidneys are the main site of Epo syn-
thesis in adults, as they are capable of sensing the
O2 tension in blood [7]. Release of Epo into the
circulatory system by the kidneys, results in acceler-
ated erythropoiesis in the bone marrow. In utero, the
foetal liver produces Epo and is also the main organ
for red cell production. Under stressful condition, the
adult liver is capable of reactivating Epo synthesis to
promote the manufacture of erythrocytes.
The development of mature haemoglobin-synthesi-
zing red blood cells from haemopoietic stem cells is
orchestrated by the expression of crucial transcription
factors including GATA-1, EKLF, SCL and LMO2 [8].
The earliest committed progenitor in the erythroid
lin- eage is the burst forming unit-erythroid (BFU-E).
This immature red blood cell cannot be identified
Fig. 1. Morphology of cells in the erythroid lineage. (A) A mature morpho- logically, and is defined as a cell which
red blood cell, displaying the chracteristic biconcave discoid produces large, burst colonies of erythroid cells in
appearance. (B) Morphologically identifiable erythroid progenitor semi-solid culture (Figs. 1 and 2). BFU-E express
cells.
receptors for stem cell factor (SCF) and low levels of
Epo receptors.
The next cell in the erythroid maturation pathway
is the colony forming unit-erythroid (CFU-E) which
also cannot be defined morphologically, but
generates
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Fig. 2. Maturation along the erythroid lineage. Haemopoietic stem cells proliferate and differentiate through a series of stages, culminating
in the generation of mature red blood cells. Morphological changes are accompanied by alterations to cell surface receptors.

small compact colonies of mature red blood cells. ly associated with changes in appearance (Figs. 1
CFU-E express fewer receptors for SCF and more
and 2). Proerythroblasts (Pr) are large cells (20–25 µm)
Epo receptors, as the lineage becomes increasingly
with a high nucleus to cytoplasm ratio (∼ 80%).
depen- dent on Epo for maturation.
Haemoglobin synthesis begins in these cells which
Beyond the CFU-E stage, erythroid cells become
have a basophilic cytoplasm due to a high concentra-
morphologically recognizable and maturation is
tion of ribosomes.
clear-
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1518
Basophilic erythroblasts (Bs) are the next stage gaseous exchange across the placenta. Intriguingly,
along the pathway—these cells are smaller (16–18 haemoglobin may also be involved in NO transport
µm), extremely basophilic and develop the [12].
characteristic “clock face” nucleus due to chromatin Iron plays a central role in O2 transport by
changes. Next, the 12–15 µm polychromatic haemoglobin and is actively taken up by immature
erythroblasts (Pos) de- velop a pinkish tinge as red blood cells via the transferrin cycle [13]. Plasma
haemoglobin levels rise and dilute the ribosome- transferrin binds iron with higher affinity, and enters
induced basophilia. Then or- thochromatic cells by associating with membrane-bound transferrin
erythroblasts (O) of 10–15 µm appear with highly receptors. These receptors form endosomes within
∼
condensed nuclei occupying 25% of the the cell to release the iron, which is then transported
cell and an increasingly pink cytoplasm. The eccentric to the mitochondria for chelation in the final step in
nuclei of these cells are then extruded, forming retic- haem biosynthesis. Transferrin and the receptor are
ulocytes (Rs) which enter the circulation and continue then recycled in a process which takes only 4–5 min.
to mature for another 24–48 h as the final haemoglobin
content is synthesized. After the loss of mitochon-
dria and ribosomes, mature erythrocytes then engage
4. Associated pathologies
in gaseous exchange. By enucleating, erythrocytes off-
load 40 pg per cell of nuclear material. Not only does
The most common disorders associated with red
this facilitate flexibility of the cell by converting a
blood cells are anaemias, i.e. a decrease in
rigid, spheroid cell into a supple biconcave disc, but it
erythrocyte levels, or haemoglobin, which result in
also reduces the cardiac effort involved in pumping
poor gaseous exchange. Anaemias are diagnosed on
the cells devoid nuclei around the body. To
the basis of reduced red cell numbers, haemoglobin
accommodate these morphological changes, the
content and haematocrit (percentage of red cells in
cytoskeleton of ery- throblasts alters as they mature
blood). Normal levels are:
into erythrocytes [9].
Male Female
3. Functions Red cells (×1012 l−1) 5.1 4.6
Haemoglobin (g/dl) 15.3 13.9
The specialized function of red blood cells is the Haematocrit (%) 45.9 41.9
transport of O2 from pulmonary capillaries to tissue
capillaries, where it is exchanged for CO2 [10]. A Anaemias can be induced by numerous means
resting individual consumes 250 ml of O2 and ex- [10]. One cause is renal failure where patients have
hales 200 ml of CO2 every minute. The importance of restric- ted Epo production. Another is impaired
haemoglobin to O2 transfer is amazing—if O2 is dis- erythropoie- sis and release of red cells from the
solved in solution only 5 ml/min can be delivered in marrow—this is the reason for aplastic anaemia and
adults, whereas, haemoglobin can transport 50 times myelodysplastic syndrome. Anaemias of chronic
that amount, i.e. 250 ml/min. An essential feature of disease involve cyto- kines, such as TNFα,
O2 transport is that haemoglobin must bind O2 firmly interleukin 1 (IL-1) and inter- ferons, suppressing
enough to remove it from pulmonary capillaries at erythropoiesis [14]. Hypochromic anaemias result
high O2 tension, then unload the gas as the O2 tension from defective haemoglobin synthe- sis—even
falls in the periphery. To achieve this, binding of O2 though sufficient erythrocytes may be present,
by the haemoglobin tetramer is co-operative, i.e. the inadequate haemoglobin levels produce symptoms of
first O2 binds weakly to deoxyhaemoglobin, but the anaemia. Iron deficiency anaemias are caused by
initial association enhances binding of more O2 by failure of iron to be absorbed, transported, taken up
the other subunits. It is significant that the composi- by cells or incorporated into the haem-moiety of
tion of the globin chains varies during life. Different haemoglobin.
combinations of globin genes are used by embryos, Severe destruction of red cells in hemolytic anae-
foetuses and adults [11]. The subunits used in utero mias can be caused by infections, glycolytic defects
have a greater affinity for O2, ensuring efficient
 S. Peter Klinken / The International Journal of Biochemistry & Cell Biology 34 (2002) 1513– 151
1518
or chemical insults [15]. A group of diseases known The haemoglobinopathies may represent one of
as haemoglobinopathies involve mutations to globin the best opportunities for gene therapy [20]. Once
genes; these include thalassaemias which can result the defective gene in these unigenic diseases has
from large deletions of the adult α and β globin been identified, haemopoietic stem cells generated by
genes, to nonsense and splicing mutations, as well as, recombinant granulocyte/macrophage colony stimu-
subtle mutations which prevent transcription factors lating factor (GM-CSF) treatment can be collected
binding to the promoter region of these genes. The and the normal gene re-introduced. Cells bearing the
consequence of globin chain imbalance is intracel- normal gene may be returned to the individual, or
lular precipitation of these proteins which alters cell expanded along the erythroid lineage in vitro before
morphology and terminates in red cell destruction. re-infusion.
A classic example is sickle cell anaemia, where a
point mutation produces an abnormal globin protein
which precipitates, producing red cells with a char- Acknowledgements
acteristic sickle shape—these abnormal cells are then
eliminated. This work was supported by grants from the
In contrast, overproduction of red blood cells can NHMRC (139008), Cancer Foundation of WA and
cause polycythaemias or erythrocytosis [16]. This Medical Research Foundation of Royal Perth Hospi-
can be caused by increased Epo production, or hyper- tal. The author is indebted to Dr. Peta A. Tilbrook for
sensitivity of erythroid progenitors to the hormone. her outstanding assistance with this paper.
Mutations to the Epo receptor, removing the negative
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