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through the common duct into the gut. By the 13 th
day the pancreas is "greatly branched," and by
the 15 th day it is still "diffusely spread" in the
mesentery 93 .
We studied 95 the growth pattern of the hamster
pancreas by examining fetuses and newborns
immediately after birth and for two to four weeks
afterward by histology, immunohistochemistry
and, in part, by scanning electron microscopy
(SEM). Our observations, which are in line with
those in other species, including humans,
indicated the development of endocrine cells from
the pancreatic tubules. Glucagon cells were found
to be the first endocrine cells identifiable in the
budding of the primitive pancreas from the
duodenum at around the 11th day of the
gestation. Even at this early stage, single or a few
glucagon cells could be demonstrated within the
undifferentiated cells 96-98 . At the later stages
(during the 12th and 13th day of gestation), single
or a small group of glucagon cells appear,
distributed along the length of the primitive
branches (Fig. 8). During 13th to 15th day of
gestation, a tubular structure with primitive cells
extended along the greater curvature of the
stomach attached by intserstitium to the primitive
spleen. As the spleen moves to the left side of the
abdomen, it pulls the pancreas with it. At this
stage, scattered α-cells but none or only a few βcells
(cells immunoreactive to anti-insulin
antibody) could be visualized. A small cell
conglomerate of β-cells appeared one day before
birth (day 15 or 16 of gestation). We could not
detect any δ-cells before birth. In a 1.3-g fetus,
the pancreas was already differentiated. The fetal
pancreas seems to release an insulin-like
substance, however, islets of Langerhans could
not be seen in the neonatal hamster’s pancreas
neither in our study nor in that of Sak et al. 94
According to these investigators, aldehyde fuchsin
positive granulation could first be observed in the
hamster’s pancreas one hour after birth, which
are found either singly or in scattered, irregular
nests of three to four cells. At day 13, a large
number of the glucagon cells contrasts with the
relatively small number of somatostatin cells and
14
a few, if any, insulin cells. At day 14 of gestation
(a day before birth) cells immunoreactive with
anti-somatostatin and anti-insulin appear and their
increased number correlated with the decreased
number of the glucagon cells (Fig. 8). This
reverse relationship between the number of the
glucagon and insulin cells would indicate a shift in
the synthetic pathway of hormones (from
glucagon to insulin) within the same cells rather
than the generation of these two cell types from
the stem cells. In fact, the presence of two
different hormones within the same cell has also
been found in the embryonic human pancreas 96,
99 . These findings strongly suggest that, even
during the embryonic development, the mature
cells have potential to shift their differentiation
from one mature cell type to another.
Discrete small islets were still relatively rare in
two-week-old hamsters. Thereafter, islets grew
rapidly for the next two weeks. In fact, islet size
almost doubled to 87.4 ± 28.44 mm and continued
to increase in size with age.
Unlike the islets, acini were recognizable one day
before birth in clusters, buds, and rows that
formed glandular structures with tiny lumens (Fig.
8f). They reached their final size (37.61 ± 12.42 X
26 / 66 ± 6.85 mm) between two and four weeks
after birth and did not show, in contrast to islets,
any size variations by age. Also, the structures of
centroacinar cells, ductules, and ducts were
recognizable in the fetus shortly before birth.
Epithelial cells were ill defined in the small ducts.
Otherwise, their structures did not differ from
those in adults. The scanning electron
microscopic findings of the fetal pancreas are
illustrated in Figures 9 and 10.
4b. Cellular and Sub-cellular Anatomy of the
Pancreas
The presence of a variety of exocrine and
endocrine cells in the pancreas has hampered our
understanding of the function of each individual
cell component and especially their interaction.
Recent investigations point to a complex dialogue
between the individual exocrine cells on the one
hand, between the endocrine cells a well as