Science Reviews - Biology, 2023, 2(2), 21-29 Ananya Bhattacherjee
Avian Erythrocytes and Agranulocytes - a Review
Ananya Bhattacherjee, PhD
Independent Researcher, Bhubaneswar, India;
Received June 19, 2023. Revised June 23, 2023. Published online July 04, 2023.
Abstract: This study reports on the morphology of red blood cells and agranulocytes in various types of birds,
such as rheas, emus, ostriches, chickens, and geese. Though all of them have the typical avian similarity in blood
cells, yet there are some differences. Along with normal and mature erythrocytes, lymphocytes, and monocytes,
the present study also focuses on rubricytes, reticulocytes, poikilocytes, polychromatophilic erythrocytes,
reactive lymphocytes, pleomorphic monocyte nuclei, etc.
Keywords: Erythrocytes, lymphocytes, monocytes, blood cells, hematology, birds, Aves, morphology
Birds inhabit a vast area from sea to mountains and
deserts to rainforests. Their physiology differs
based on habit, habitat, age, sex, feed, and breeding
season. Blood cell morphology plays an important
role in diagnosing physiological and pathological
aspects of birds’ health. The study of the blood of
birds revealed various types of cells just like other
vertebrates. These cells are red blood cells or eryth-
rocytes, white blood cells or leukocytes, and throm-
bocytes which are equivalent to mammalian plate-
lets. There are two types of leukocytes, viz., agran-
ulocytes and granulocytes. Agranulocytes are of
two types, namely, lymphocytes and monocytes.
Granulocytes are of three types, heterophils same as
neutrophils in mammals, eosinophils, and baso-
phils. In the following paragraphs, facts about avian
erythrocytes and agranulocytes are discussed.
An erythrocyte carries oxygen and carbon dioxide
and the exchange of gases depends on the cell’s sur-
face area to size ratio. Avian erythrocytes are large,
ovoid, or elliptical with homogenous eosinophilic
cytoplasm and a similarly shaped nucleus at the
center containing condensed chromatin (Narkkong
et al., 2011 and Lazăr et al., 2012). Such red blood
cells (RBCs) are more efficient than spherical ones
of the same volume. Aves are the most active verte-
brates and have a high metabolic rate. Due to this,
the demand for gaseous exchange is higher than
other animals. Among Aves, aquatic birds have a
higher metabolic rate than terrestrial birds. As a re-
sult, their erythrocytes are larger so that sufficient
gaseous exchange can take place as reported by
Hartman and Lessler, 1963. Nucleated RBCs of
birds maintain better homeostasis thus helping in
better utilization of water. Due to this reason,
aquatic birds’ erythrocytes bear larger nuclei com-
pared to others which help them to survive during
osmotic stress.
A comparative study on erythrocytes of Vanaraja
chicken (terrestrial bird) and White Embden goose
(aquatic bird) performed by Tripathy and Acharya,
2019, reported that both red blood cells and their
nuclei in geese are larger compared to chickens. The
measurement of a bird’s RBC ranges from 11-16μm
in length and 6-10μm in breadth. Among birds,
Rheiformes bear the largest blood cells. Their RBCs
stain orange-pink with deep purple-colored nu-
cleus (Gallo et al., 2015). Samour et al., 2010 noticed
that the dimensions of peafowl RBCs are
13.74±0.77μm and 6.56±0.65μm, with nuclear di-
mensions of 5.28±0.45μm and 2.20±0.40μm. The cy-
toplasm was slightly eosinophilic, with a strongly
basophilic nucleus. The size of fully mature eryth-
rocytes of chicken and turkey found in peripheral
blood is 12×6µm. The nuclear periphery having
densely populated clumps of condensed chromatin
surrounds the central loosely arranged chromatin
(Khan et al., 2016). Dimensions of erythrocytes de-
crease with an increase in age as nuclear chromatin
Ananya Bhattacherjee Science Reviews - Biology, 2023, 2(2), 21-29
condenses as mentioned by Kramer, 2015 and Khan
et al., 2016. Macrocytes, microcytes, and anisocytes
reflect changes in the size of erythrocytes. These can
be observed in mono-layer blood film and the de-
gree of anisocytosis can be determined by the pres-
ence of different sizes of erythrocytes (Campbell,
Since, erythropoiesis is intravascular or intrasinus-
oidal, occasional rubricytes or basophilic erythro-
blasts are found in blood smears of healthy birds
(Haile and Chanie, 2014). These are round imma-
ture erythrocytes with circular nuclei. They have a
very thin ring-like basophilic cytoplasm around the
nucleus. Like this, other immature RBCs are me-
tarubricytes or polychromatic erythroblasts. These
cells are less circular with basophilic and more
abundant cytoplasm than rubricytes in the case of
Rhea. The nuclei of such cells are round with par-
tially condensed nuclear chromatin. Young poly-
chromatic erythrocytes were also observed in Rheas.
They are slightly oval with a larger non-basophilic
cytoplasm and elongated nucleus having very con-
densed chromatin and a 1/1 Nucleus to Cytoplas-
mic (N:C) ratio. The presence of immature erythro-
cytes in the blood smears of birds does not always
indicate any disease. Occasionally binucleated
erythrocytes or mitotic cells are seen in the blood
smear of birds, but if present in large numbers may
indicate any neoplastic process, viral infection, or
genetic disorder. (Gallo 2015).
Immature erythrocytes are also known as reticulo-
cytes which have a distinct ring of aggregated retic-
ular material (reticulofilamentous substances) cir-
cling the nucleus (Khan et al 2016). Chickens and
turkeys have a greater number of reticulocytes in
peripheral blood compared to mammals. These au-
thors also observed more polychromasia in young
birds. Artifacts on slides occur during blood smear
preparation and cause hindrances in the correct
identification and counting of blood cells. Some of
the common artefactual problems include cytoplas-
mic vacuoles, smudge cells, and stretching cells like
a spindle, bi-lobed nuclei, bare nuclei, and erythro-
plastids (Haile and Chanie, 2014 and Wakenell,
Round immature erythrocytes are also noticed in
emus but with an irregular nuclear periphery. The
mature RBCs which differ from typical oval shapes
are called poikilocytes, such as schistocytes and
dacryocytes. Schistocytes are erythrocyte fragments
with two pointed ends found in peripheral blood.
These fragments form when RBCs have to cross
through defective blood vessels forcibly. Dacryo-
cytes commonly known as teardrop RBCs have the
shape of a drop of tear (Dash, 2020). Sometimes,
tear-drop erythroplastids or enucleated cells are
also found.
According to Tadjalli et al., 2013, in adult male os-
triches, the centrally located reticulocyte nucleus
contained reticular chromatin without nucleolus.
The cytoplasm of erythrocytes was light blue to
gray. These were oval-shaped but smaller and with
wider nuclei compared to mature erythrocytes.
Like rubricytes, small number of polychromato-
philic erythrocytes are sometimes present in avian
blood smears. These cells are the last stage of ma-
ture erythrocyte development. The cytoplasm of
such cells has more ribosomal RNA and appears
bluish than mature RBCs. Their chromatin material
is denser compared to mature erythrocytes. More
than 1-5% polychromatophilic red blood cells, indi-
cates increased erythropoiesis to overcome anemia
(Clark et al., 2009).
Hypochromatic RBCs are also occasionally ob-
served. These cells are paler compared to mature
erythrocytes and have an area of cytoplasmic pallor
covering more than half of the cytoplasm. These
cells have vacuoles and their nuclei are pyknotic
(Campbell, 2012).