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Fig. 5. ZCs in Mist1-/- mice abutting the neck zone (TZ2
cells) form normal serous granules but basal ZCs have fewer, smaller
granules. (A) TEM of lower neck zone and upper base of a
Mist1-/- gastric unit (neck zone to left). Notice how
large, normal zymogenic granules occur only in cells immediately adjacent to
the neck zone. A cell (TZ2, shown at higher magnification in the inset at
left) with large, chimeric granules that are similar to those of normal ZCs
but have a smaller electron-lucent mucus-like component as well (chimeric
vesicle labeled SV, left arrow; normal vesicle indicated with arrow to right)
occurs at the transition between the last parietal cell of the neck zone and
the base. Outlined by a dashed box in A and shown at greater magnification in
the right inset, adjacent to the TZ2 cell on the right is the highest ZC in
the base; it has no residual mucus in its granules, but unlike a wild-type ZC,
it has a mixture of large, normal zymogenic granules with occasional,
abnormally small granules, and several granules appear in the process of
fusing with the apical plasma membrane. In addition, in this region, there are
invaginations of the apical plasma membrane, evidence of recently released
granules. The remaining cells, farther away from the neck zone (i.e. to the
right) are all abnormal: rather than having a pyramidal acinar cell
morphology, they are cuboidal or columnar with irregular apical plasma
membranes, and have scant supranuclear cytoplasm with abnormally small
secretory granules. (B) Neck-to-base transition region in another
Mist1-/- gastric unit from a section from another mouse.
Again, note the abundant large granules in the immature ZC that is immediately
adjacent to the last parietal cell of the neck zone and thus has not yet
entirely entered the base. Residual mucus in the granules is not observed, but
may be detectable by immunofluorescence; thus, this cell may either be a TZ2
cell or an early ZC. (C) Neck-to-base transition in a wild-type gastric
unit for reference. Notice how all ZCs in this transition area (whether
adjacent to neck zone or more mature) have large, abundant secretory granules.
PC, last parietal cell of neck zone; ZC, immature ZC adjacent to neck
zone.
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mean neck cell GSII and GIF within 1 s.d. of mean ZC GIF. Fraction of
total cells showing this phenotype is in parentheses. Blue box outlines the
cluster of GIFHiGSII+ cells ('immature ZCs' or
transition stage 2,'TZ2' cells) with GIF significantly above (i.e. 
half of
transitional cells are Mist1-positive). A single apparently negative ZC is
marked by an arrow (these cells are rare). (B) Expression from the
Mist1 promoter in the Mist1-/- mice. Labeling and
layout as in A, except: purple, expression of nuclear lacZ `knocked
in' to Mist1 promoter (anti-ß-galactosidase antibody). Note how
Mist1 promoter activity is first detectable in a subset
(promoter-positive cells are labeled `1,3,4') of the four GIFHi
cells at the transition between the neck and base zones and is not detectable
in neck cells or the earlier transition cell with equal levels of mucous and
zymogenic granules (TZ1). (C) Brdu immunolabeling in representative
gastric units at 90 minute timepoint. Labeling as in A, except: purple,
polyclonal goat anti-Brdu. Note single positive cell in isthmus or pit region
of wild type and Mist1-/- (arrows) and early neck cell of
Mist1-/- mouse. Transitional cells with negative nuclei
are also indicated. (D) Brdu pulse-chase experiments. All animals were
injected with Brdu concomitantly and sacrificed on different days. Note that
the day 9 timepoint comprised only one genotype:
Mist1-/-.
