POULTRY PUNCH - FEBRUARY 2019

ARTICLE
Avian stem cell – A model for stem cell therapy
in mammals
G.Kalaiselvi, K.Manimaran
Assistant Professor, Central University Laboratory
Tamil Nadu Veterinary and Animal Sciences University, Chennai -51
Stem cells are biological cells undifferentiated cells that can differentiate into other types of cells and can divide to
produce more of the same type of stem cells. They are found in multicellular organisms. In mammals, there are two broad
types of stem cells one is embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and another one
is adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair
system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized
cells from ectoderm, endoderm and mesoderm ( induced pluripotent stem cells)but also maintain the normal turnover of
regenerative organs, such as blood, skin, or intestinal tissues.
Avian embryos are a powerful model to study developmental and stem cell biology. They offer several advantages as
a model for studying stem cell biology including their convenient size and ease of obtaining eggs, easy availability and easy
of access to the embryo for manipulations, which among other applications led it to be used as a favorite model for toxicity
testing since very early days . Avian species are the only non-mammalian group from which stable embryonic stem cell and
germ cell lines have been established. Both chick embryonic stem cells (Ces) and chick embryonic germ (cEG) cells are
considered to be pluripotent, but cES cells have been shown to be able to contribute only to somatic tissues and not to the
germ line, while chick embryonic germ cells can contribute to the germ line. But little attention has been given to the biology
of avian stem cells, especially regarding similarities and differences between chick embryonic stem (cES) cells, germ cells,
and stem cells obtained from other embryonic and adult tissues.
The avian embryo spends its first 20 h or so in utero and the shell is deposited as the egg descends down the maternal
oviduct. During this time, cell division occurs in a meroblastic pattern (open cleavage planes, from the centre out to the yolk)
to generate a disc. By the time the egg is laid, the blastodisc comprises 20,000–50,000 cells arranged mainly as a singlecell-thick
layer (epiblast) underlain by islands of more yolky cells (hypoblast — extra embryonic endoderm of the future
yolk sac stalk). The entire embryo will arise from the centre of the epiblast, but it retains a remarkable ability to regenerate.
Fragments of blastodisc can regenerate the entire embryo and re-establish polarity, suggesting plasticity of the embryo and
perhaps pluripotency of the component cells. It is from these early (pre-primitive streak) stages of development that cell lines
analogous to mammalian embryonic stem cells (ESCs) can be established from cells dissociated from the central epiblast
and these cells can be perpetuated in culture indefinitely.
The biology of germ cells in bird embryos is particularly interesting and unique. Primordial germ cells (PGCs) appear to
arise at pre-primitive streak stages by ingression from the epiblast, joining the hypoblast cells below. The hypoblast forms a
continuous layer of cells that then moves to the most anterior part of the embryo, under the pre-amnion, carrying the PGCs
to this region, known as the Germinal Crescent. One remarkable feature is that primordial germ cells use the embryonic
116 February, 2019