Page 154 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

136 Veterinary Histology of Domestic Mammals and Birds

lymphoid (lymphocyte) and myeloid (granulocyte/mono- (erythroblastus polychromatophilicus). Due to the high
VetBooks.ir cyte, erythrocyte/megakaryocyte) cell lines. haemoglobin content, the cytoplasm of polychromat-
Progenitor cells are morphologically similar to one ophilic erythroblasts gradually becomes acidophilic with
another, being relatively small and strongly basophilic with little remaining basophilia (Figure 7.2). With continued
a round nucleus. maturation and increasing haemoglobin concentration,
In vitro stimulation of progenitor cells using cytokines the proportion of basophilic organelles (ribosomes,
or other growth factors results in the formation of clus- rER) declines, giving rise to the orthochromatophilic
ters or colonies of cells. These clonogenic cells are thus (oxyphilic or acidophilic) erythroblast (normoblast,
referred to as a ‘colony-forming unit’. erythroblastus acidophilicus) (Figure 7.2).
Progenitor cells differentiate in the bone marrow into
mature blood cells that are found in the circulation. These Species variation
cells (and their differentiation) are termed: Mammals: Following an increase in the density and com-
paction of the nuclear chromatin (pyknosis), the nucleus
· erythrocytes (erythropoiesis), is ejected (enucleation). Loss of the nucleus during
· granulocytes (granulopoiesis), erythropoiesis is characteristic of mammals, includ-
· lymphocytes (lymphopoiesis), ing humans. The remaining organelles (Golgi vesicles,
· monocytes (monopoiesis) and polyribosomes, ER) become clumped together in the
· platelets (thrombocytes) (thrombopoiesis).
cytoplasm. The enucleated cell is referred to as a reticu-
locyte (proerythrocyte). Within 24 hours, reticulocytes
Red blood cells (erythrocytes) lose their internal organelles, becoming the haemoglo-
A major function of the blood is the transport of O and CO ,
2 2 bin-carrying cells known as erythrocytes (Figure 7.2).
bound to the pigment haemoglobin. This globular chro-
moprotein with a prosthetic haem group is concentrated Birds, reptiles, amphibians and fish (except cyclostomes):
in red blood cells, constituting over 30% of the cytoplasm Enucleation does not take place. In these species, erythro-
(95% of dry weight). The high, reversible binding capacity cytes are nucleated and larger than those of mammals.
of haemoglobin enables erythrocytes to take up oxygen in
the lungs and deliver it to the peripheral tissues, and to trans- The morphological changes occurring during erythro-
port carbon dioxide in the opposite direction. The bright poiesis can be summarised as follows:
red colour of arterial blood is derived from its content of
oxyhaemoglobin. Venous blood contains haemoglobin in its · reduction in cell volume,
reduced state and is thus dark red in colour. · decrease in basophilia,
· reduction and loss of nucleoli and cellular organelles,
Development of red blood cells (erythropoiesis) · alteration of nuclear structure,
In the initial stage of erythropoiesis, haemopoietic stem · increase in acidophilia and
cells differentiate into precursor cells referred to as pro- · loss of the nucleus (mammals).
erythroblasts (Figure 7.2). This process is driven by
hormonal factors, particularly by erythropoietin, a sialo- Erythrocytes
glycoprotein produced by the kidney in response to low In most mammals, the red blood cell, or erythrocyte, is a
blood oxygen levels (anaemia or hypoxia). Steroid hor- non-nucleated, biconcave, round disc (Figure 7.3). The size
mones, peptides and Vitamin B also act as regulators of of erythrocytes varies considerably with species, ranging in
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erythrocyte development. diameter from 4.1 μm in the goat to 7.3 μm in the dog (Table
Proerythroblasts are basophilic cells measuring 20–25 7.1). In camels and dromedaries, the erythrocyte is oval. The
μm in size. They undergo rapid replication, during which the number of circulating erythrocytes is related to their species-
size of the nucleus decreases. Accumulation of haemoglobin dependent variation in size. The smaller the cells, the greater
in the cytoplasm begins in this early stage of differentia- their unit volume, as seen particularly in the goat.
tion. Further mitotic division gives rise to the basophilic Circulating erythrocyte numbers are further influenced
erythroblast (erythroblastus basophilicus) (Figure 7.2). By by muscular effort, training, condition (horses), housing
this stage the nucleus has become noticeably smaller and (pigs), altitude (e.g. alpine cattle) and sex (greater in males
heterochromatic. The cytoplasm is basophilic. Through than females). Lacking a nucleus, erythrocytes exhibit a
endocytosis, basophilic erythroblasts acquire increasing high degree of plasticity and elasticity and undergo con-
amounts of ferritin from surrounding macrophages and tinuous, reversible modifications of their shape. This
reticular cells to support increased haemoglobin synthesis. facilitates their passage through even the smallest capil-
The basophilic erythroblast undergoes further differen- laries. In hypotonic solution, erythrocytes absorb fluid,
tiation to become the polychromatophilic erythroblast resulting in swelling and possible disintegration. Under

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