200
LEO LEMEÈ
Parent cells of various lines of chick embryo blood corpuscles are all
related, according to Dantschakoff (1908a), to one stem cell, called the
large lymphocyte or haemocytoblast. Because during development
different types of these haemocytoblasts appear and because they have
been differently evaluated by various authors, it seems pertinent to
review them first: (1) Primitive blood cells, as found on the 2nd day of
incubation, are the haemocytoblasts of the primitive erythrocyte line
(Ε I) found in smears up to the 3rd or 4th day of incubation. The cells
are large and thick showing protrusions of the strongly basophilic
narrow cytoplasmic rim which contains at one place a sphere with the
centrosome, and possesses two large nucleoli in a nucleus with very
little chromatin. (2) Very soon after the 2nd day of incubation the parent
cells of the thrombocytic series also appear; these are much smaller
(dwarf lymphocytes), and have long narrow cytoplasmic protrusions
(Section II, Β). (3) On the 4th day of incubation the haemocytoblast of
the definitive erythrocytic series appears; it is intermediate in size
between the first two described, and is less irregular in outline or even
completely regular. These cells are the only haemocytoblasts found in
blood smears after the 5th day of incubation till hatching. Because
Dantschakoff (1908a) has conclusively shown that erythrocytes differentiate inside vessels, whereas white blood cells differentiate outside, one
must conclude that all the haemocytoblasts present after the 5th day in
the circulatory system necessarily belong to the definitive erythrocyte
series, being their precursors or even proerythroblasts. We do not
therefore include them among white blood cells as many authors have
done. (4) The fourth haemocytoblast may closely resemble the last
mentioned, but is localized extravascularly, giving rise first to neutrophil
leucocytes (Dantschakoff, 1908a), so that it is probably not to be found
in circulating blood under normal circumstances. (5) The circulating
large lymphocytes of hens, which first appear in circulation on the 15th
day of incubation (Lemez, 1955) can synthesize DNA (Szenberg and
Warner, 1961) and might be considered, therefore, to be pluripotent cells.
Numerically, the haemocytoblasts (definitive erythrocyte series)
present in the circulation represent 30% of the number of leucocytes on
the 6th day, and 15% on the 7th day, decreasing in relative number
from then until the 12th day when they form 1 % of the leucocyte count.
Up to the end of the incubation period they do not exceed 0-5% of the
leucocytes, which are, however, still increasing in number (Lemez, 1955)
(Section II, C). Sandreuter (1951) reported a much higher proportion of
haemocytoblasts from the 8th day of incubation onwards. The ratio of
haemocytoblasts to white blood cells seems to be higher also in the data
given by Fennell (1947).
The primitive erythrocytes (Ε I) (Fig. ΙΑ, B) are large elements,
LEO LEMEÈ
Parent cells of various lines of chick embryo blood corpuscles are all
related, according to Dantschakoff (1908a), to one stem cell, called the
large lymphocyte or haemocytoblast. Because during development
different types of these haemocytoblasts appear and because they have
been differently evaluated by various authors, it seems pertinent to
review them first: (1) Primitive blood cells, as found on the 2nd day of
incubation, are the haemocytoblasts of the primitive erythrocyte line
(Ε I) found in smears up to the 3rd or 4th day of incubation. The cells
are large and thick showing protrusions of the strongly basophilic
narrow cytoplasmic rim which contains at one place a sphere with the
centrosome, and possesses two large nucleoli in a nucleus with very
little chromatin. (2) Very soon after the 2nd day of incubation the parent
cells of the thrombocytic series also appear; these are much smaller
(dwarf lymphocytes), and have long narrow cytoplasmic protrusions
(Section II, Β). (3) On the 4th day of incubation the haemocytoblast of
the definitive erythrocytic series appears; it is intermediate in size
between the first two described, and is less irregular in outline or even
completely regular. These cells are the only haemocytoblasts found in
blood smears after the 5th day of incubation till hatching. Because
Dantschakoff (1908a) has conclusively shown that erythrocytes differentiate inside vessels, whereas white blood cells differentiate outside, one
must conclude that all the haemocytoblasts present after the 5th day in
the circulatory system necessarily belong to the definitive erythrocyte
series, being their precursors or even proerythroblasts. We do not
therefore include them among white blood cells as many authors have
done. (4) The fourth haemocytoblast may closely resemble the last
mentioned, but is localized extravascularly, giving rise first to neutrophil
leucocytes (Dantschakoff, 1908a), so that it is probably not to be found
in circulating blood under normal circumstances. (5) The circulating
large lymphocytes of hens, which first appear in circulation on the 15th
day of incubation (Lemez, 1955) can synthesize DNA (Szenberg and
Warner, 1961) and might be considered, therefore, to be pluripotent cells.
Numerically, the haemocytoblasts (definitive erythrocyte series)
present in the circulation represent 30% of the number of leucocytes on
the 6th day, and 15% on the 7th day, decreasing in relative number
from then until the 12th day when they form 1 % of the leucocyte count.
Up to the end of the incubation period they do not exceed 0-5% of the
leucocytes, which are, however, still increasing in number (Lemez, 1955)
(Section II, C). Sandreuter (1951) reported a much higher proportion of
haemocytoblasts from the 8th day of incubation onwards. The ratio of
haemocytoblasts to white blood cells seems to be higher also in the data
given by Fennell (1947).
The primitive erythrocytes (Ε I) (Fig. ΙΑ, B) are large elements,