/ . Embryol. exp. Morph. Vol. 15, 1, pp. 67-76, February 1966
With 3 plates
Printed in Great Britain
67
The reopening process of the esophagus in the
normal chick and the crooked neck dwarf mutant
By ALLAN L. ALLENSPACH 1
Department of Biology, Albright College, Reading, Pennsylvania
INTRODUCTION
In an earlier publication (Allenspach & Hamilton, 1962) an account was
given of chick esophageal development and histogenesis up to 10 days of incubation. The esophagus becomes completely occluded at 5 days (stage 26, Hamburger & Hamilton, 1951) and reopens rapidly at 7-J-8 days (stages 33-34).
Detailed observations were made of the histological changes in the organs as
correlated with the localization of ribonucleic acid and alkaline phosphatase.
The occlusion of the developing esophagus was apparently due to collapse
and adhesion of roof and floor epithelia to form a solid epithelial bar and not
to cell proliferation, as stated in Lillie's Development of the Chick (Hamilton,
1952, pp. 384-5). This process was the object of further investigation in which
the role of proliferation in occlusion was determined by the use of colchicine.
Results indicated that cell proliferation is not involved in the closing process
(Allenspach, 1964).
The mechanisms which control the reopening process have also been partially
elucidated. Reopening is initiated by the appearance of bilateral primary
vesicles. Subsequent fusion of primary and secondary vesicles with concomitant
desquamation of degenerating epithelial cells restores the lumen (Allenspach
& Hamilton, 1962; Allenspach, 1964). During this period the esophageal epithelium changes from non-oriented cells in the occluded region to a stratified
epithelium similar to that of the definitive organ. Reorganization is initiated
around the most lateral vesicles. Another subtle mechanism apparently operates
in the reopening process. Orientation of dorsal, epithelial cells suggested they
were the object of vertical tension. By treating the esophagus with versene, a
compound which disaggregates tissues, a cleft was induced between roof and
floor. This suggests that (1) adhesions play a significant role during occlusion,
and (2) forces exerted on the dorsal epithelium assist in separating the roof from
the floor (Allenspach, 1964).
The present investigation was prompted by the report of Pun (1954) that the
esophagus of the crooked neck dwarf mutant embryo is still largely occluded
1
Author's address: Department of Biology, Albright College, Reading, Pennsylvania, U.S.A.
5-2
68
A. L. ALLENSPACH
beyond 12 days. He suggested that affected embryos surfer from chronic starvation since they cannot swallow nutritious amniotic fluid that is essential for normal
development (Pun, 1954). The object of this study is to approach an understanding of the controlling mechanisms of esophageal histogenesis by a
comparison of mutant and normal embryos at critical stages of occlusion and
reopening. The results obtained support the thesis that, even though vesiculation
occurs, epithelial degeneration plays a major role in the reopening process which
may itself be influenced by tensions not yet understood.
MATERIALS AND METHODS
The material used in this study was collected at the University of Connecticut
during the summers of 1962 and 1963. (The author is deeply indebted to Dr
Louis J. Pierro and the Department of Animal Genetics, University of Connecticut, Storrs, Connecticut, for their co-operation in collecting and shipping
the material used in this study.) Eggs from a cross of carriers heterozygous for
the crooked neck dwarf mutant were incubated for 5^—12 days. Embryos were
recovered, fixed and shipped in Bouin's fixative and upon receipt were staged
(Hamburger & Hamilton, 1951) and stored in 70% ethanol.
The nature of the study required recognition of mutants as early as possible.
Embryos of 9-10 days and older were recognized by edema, spindly legs, and
abnormally short necks (Herrmann, Clark & Landauer, 1963) and were collected along with a few normal embryos. Since the gene involved is an autosomal recessive, earlier embryos were necessarily studied in greater numbers to
ensure an accurate histological picture of esophageal development.
For this investigation the neck region of the embryo was removed, dehydrated, cleared in benzene, infiltrated in 56-58 °C paraffin-bayberry wax mixture, and sectioned at 7 JLL. With the exception of one series stained with toluidine
blue, all were stained by a modified haematoxylin method.
RESULTS
The crooked neck dwarf mutation apparently had no effect on early esophageal organogenesis in embryos of stage 29 and younger. The occluded,
vesiculated, and posterior open regions appear unchanged in all embryos
studied.
Stages 30-33
Mutant embryos could be recognized from stage 30 onward by the appearance
of deeply chromatophilic, degenerating mesenchymal cells outside the basement
membrane as shown in Plate 1, fig. A. This degeneration results in deterioration
of the submucosal region to a greater or lesser extent and occasionally the esophageal epithelium is partially destroyed (Plate 1, fig. A). Degradation always
progresses from the submucosal layer through the basement membrane into
Esophagus in crooked neck dwarf embryo
69
the epithelial layer. More extensive deterioration is observed in the virtual
destruction of the epithelial wall of the esophagus at stage 32 (Plate 1, fig. B).
Lysed epithelial cells can be seen floating in the lumen.
With the exception of the 'degenerate cell' marker in the mutant mesenchyme,
histological examination of all embryos within stages 30-33 demonstrated
normal esophageal organogenesis. Briefly stated, the closing process is brought
about by fusion of roof and floor epithelia. The vesiculated region contains
many mitotic figures adjacent to the numerous vesicles, an indistinct basement
membrane limiting the dorsal epithelium, and prominent nucleoli. Rearrangement of the epithelium from an unorganized to a pseudostratified type occurs
around the primary vesicles. The posterior open region, with pseudostratified
columnar epithelium, is limited by a distinct basement membrane. As the
lumen increases in diameter the epithelium is reduced to a low stratified type
characterized by few mitoses and small nucleoli.
It is noteworthy that there is little secretion exuding from the roof of the
posterior open esophagus in the mutant embryos (Plate 1, fig. C). Small
amounts of basophilic secretion appear at the free surface of the organ, but the
lumen is never full of mucus, as it is in normal embryos.
Stage 34
Up to stage 34, development of the mutant esophagus is apparently normal;
closing by fusion and vesiculation occur without change. However, significant
differences in the reopening process are observed.
Since major histological changes occur at this stage, a detailed histological
study along the cephalo-caudal axis of the mutant esophagus is of interest.
A representative section of the broad, occluded epithelial bar just behind the
pharynx is shown in Plate 1, fig. D. Vesicles appear posteriorly, but the intervesicular epithelium shows no degeneration (Plate 1, fig. E). As the broad
esophagus narrows, the middle segment becomes vesiculated (Plate 2, fig. A).
The two primary, bilateral vesicles exist with several secondary vesicles in the
medial epithelial bridge which unites the roof and floor, but no degeneration
occurs within the organ (see Plate 1, fig. F, or Plate 2, fig. A). However, the
mutant esophagus does contain, with some variability, degenerating cells,
always in the posterior part of the vesiculated region (Plate 2, fig. I). The dearth
of degenerative cells contrasts sharply with the massive desquamation observed
in the normal embryo where comparable regions are already reopened (Plate 2,
figs. E, F), and where full-scale degeneration has proceeded anteriorly into the
broad epithelial bar just posterior to the tracheal bifurcation (Plate 2, figs. C, D).
The most extensive degeneration observed through stage 38 is observed in an
embryo of stage 34 (Plate 1, fig. G).
The vesiculated region extends posteriorly by the persistence of a thin epithelial bridge connecting the roof and floor in mutant embryos (Plate 1, fig. H).
There is no evidence of pycnosis, and the large nucleoli reflect a period of syn-
70
A. L. ALLENSPACH
thesis rather than degradation (Plate 2, fig. H). Also, the epithelial bridges are
coated with a generous supply of extracellular matrix. Projecting from the floor
in the reopened region is a pinnacle of living epithelial cells, a remnant of the
median cellular connexion (Plate 1, fig. I). This phenomenon prevails in the
open region even at later stages.
Perhaps the most curious phenomenon in the vesiculated region is the cleft
observed infrequently in some cytoplasmic bridges (Plate 1, fig. F). The cleft
partially severs, for a short distance, the connexion between roof and floor.
Cells adjacent to the break are living, with intact nuclei and prominent nucleoli,
even though neighbouring cell membranes may be ruptured as a result of the
separation. Epithelial cells dorsal to the cleft are vertically oriented along an
indistinct, basement membrane. The indistinct, dorsal basement membrane,
best observed in Plate 3, fig. I, and also directly above the cleft in fig. J, same
plate, is characteristic of the region and the stage of development.
EXPLANATION OF PLATES
Most of thefiguresare photographs of cross-sections of the esophagus in the crooked neck
dwarf embryo. Several other photographs are included for comparative purposes. Unless
otherwise stated, sections were stained with iron haematoxylin. The magnification is indicated
by a scale-line on each photograph.
PLATE 1
Fig. A. Through the vesiculated region in the crooked neck dwarf (stage 30). Note the
degradation of the dorsal epithelial wall and the presence of numerous degenerate, deeply
basophilic cells in the mesenchyme below the basement membrane.
Fig. B. Through the vesiculated region in the crooked neck dwarf at stage 32 to illustrate
the severity of effect of the mutant gene. Note the complete destruction of the epithelial wall
and muscularis.
Fig. C. Through the open lumen in the crooked neck dwarf (stage 33). Note the empty lumen
and the reorganized, pseudostratified epithelium thrown into longitudinal folds.
Fig. D. Typical example of the anterior occluded region in the crooked neck dwarf (stage 34).
The broad epithelial bar lacks vesicles.
Fig. E. Through the vesiculated region of the crooked neck dwarf (stage 34). Intervesicular
degeneration is lacking. Compare with Plate 2,fig.C.
Fig. F. Through the vesiculated region in the crooked neck dwarf (stage 34) posterior to
fig. E. Note the lack of degenerating epithelial cells in the median bridge, very small amounts
of mucus in the lumina, and the cleft in the left side of the epithelial bridge.
Fig. G. Through the most posterior segment of the vesiculated region in the crooked neck
dwarf (stage 34). Of all mutants studied at this stage, this one most closely resembles the
normal embryo (compare with Plate 2,fig.F). Note the massive sloughing of deeply basophilic degenerate cells in the lumen. See also Plate 3,fig.I.
Fig. H. Through a posterior segment of the vesiculated region in the crooked neck dwarf
(stage 34). The vital epithelial bridge connecting roof and door is characteristic of many
mutant embryos. See Plate 2,fig.H for higher magnification.
Fig. I. Through the open region in the crooked neck dwarf (stage 34). Note the pinnacle
of vital epithelial cells which persists even at later stages. Normally these cells would have
degenerated and sloughed into the lumen.
J. Embryol. exp. Morph., Vol. 15, Part 1
A. L. ALLENSPACH
PLATE 1
facing p. 70
/ . Embryol. exp. Morph., Vol. 15, Part 1
PLATE 2
H
A. L. ALLENSPACH
facing p. 71
Esophagus in crooked neck dwarf embryo
71
Stage 35
While a broad epithelial bar exists, it is not lengthy, for it is rapidly penetrated
by vesicles. In the mutant it becomes multi-vesiculated and resembles the
normal esophagus at stage 34 (see Plate 1, fig. E).
Frequent mitoses are observed in the unorganized intervesicular epithelium,
and large, basophilic nucleoli are common. There was no evidence of pycnosis.
The extent to which epithelial cells degenerate is shown more posteriorly in
the vesiculated region (Plate 2, fig. B). Only a small population of cells has
degenerated (Plate 2, fig. I). Occasionally free-floating masses as well as projecting clusters of living cells are observed in the vesicles. That this embryo is a
mutant is confirmed by the presence of deeply basophilic, degenerating mesenchyme cells below the basement membrane.
The open region is partially bisected by a mid-ventral projection of living
epithelial cells (cf. Plate 1, fig. I). This is a remnant of the median, cellular bridge
observed in the vesiculated region. More posteriorly in the open region the
PLATE 2
Fig. A. Typical picture of the vesiculated region in the crooked neck dwarf (stage 34). Note
the small amounts of basophilic mucus in the lumina but no degeneration of epithelial cells.
Compare withfig.E, below.
Fig. B. Through the vesiculated region in the crooked neck dwarf at stage 35. Note the
paucity of degenerating cells, always located at the median side of primary, bilateral vesicles.
Fig. C. Through the vesiculated region in the normal chick embryo showing the process of
reopening (stage 34). Note the loose aggregates of cells lining the vesicles. Compare with
Plate 1,figs.D, E. Toluidine blue.
Fig. D. Through the vesiculated region in a normal embryo (stage 35). Note that the vesicles
fuse with a concomitant sloughing of epithelial cells to restore the lumen. Compare with
fig. B, above. Toluidine blue.
Fig. E. Through the completely restored lumen in the normal chick embryo (stage 34). The
single lumen has been formed by fusion of large vesicles with aggregates of necrotic, deeply
basophilic masses desquamated into the cavity. Compare withfig.A, above. Toluidine blue.
Fig. F. Through the open esophagus in the normal chick embryo (stage 34), showing massive
desquamation of necrotic cells into the lumen. Necrotic cells are concentrated medially in
the organ. Compare with fig. A, above. Gomori's technique for alkaline phosphatase.
Fig. G. Through the vesiculated region in the normal chick embryo (stage 32), showing the
induced cleft in the epithelial bridge following treatment with versene. Note the indistinct
dorsal basement membrane. The induced cleft resembles that observed occasionally in
mutants (see Plate 3,fig.J).
Fig. H. High magnification of the vital epithelial bridge shown in Plate 1,fig.H. The epithelial cells show no signs of degeneration.
Fig. I. High magnification of a section adjacent to that shown infig.B, above. Note the
small number of degenerating cells on the median side of the large lateral vesicle. Vital epithelial cells show prominent nucleoli. The deeply basophilic mesenchymal nuclei adjacent to
the basement membrane in the upper right side of the photograph indicate that this embryo
is a mutant.
72
A. L. ALLENSPACH
principal changes include further development of the longitudinal folds and the
rearrangement of epithelium into a low stratified type. In addition, numerous
vesicles are observed within the thin wall of stratified epithelium.
Stage 36
Occasionally a short occluded zone is observed in the esophagus of mutant
embryos; however, the anterior region is typically represented in Plate 3, fig. A.
The organ is compartmentalized by extremely long, thin epithelial bridges
which are coated with considerable extracellular matrix. More posteriorly the
organ is greatly restricted (Plate 3, fig. B) and characterized by nuclei with pro-
PLATE 3
Fig. A. Cross-section of the crooked neck dwarf (stage 36) near the pharyngeal region,
showing multiple vesiculation. Epithelial bridges persist, causing the lumen to remain
restricted. Esophagus in upper third of section is virtually occluded.
Fig. B. Typical picture of the vesiculated region of the crooked neck dwarf (stage 36) posterior
to that shown infig.A, above. Note the many vesicles along the broad epithelial bar. Epithelial bridges persist between vesicles even at this late stage.
Fig. C. Through the anterior region in a normal embryo (stage 38). Note the much larger
lumen as compared with that in the mutant (fig. G, below).
Fig. D. Through the esophagus of the crooked neck dwarf embryo more posterior tofig.B,
above, at a later stage (stage 37). Note the very thin cytoplasmic strands uniting roof and
floor (see alsofig.K, below). The normal embryonic esophagus is fully reopened by this time.
Fig. E. Through the posterior region in the normal embryo (stage 38). Note the much larger
lumen as compared with that in the mutant.
Fig. F. Through the posterior region in the crooked neck dwarf embryo (stage 38), showing
the organ thrown into longitudinal folds. In contrast tofig.E, the epithelium in the mutant
esophagus contains numerous vesicles.
Fig. G. Through the esophagus just posterior to the tracheal bifurcation in the crooked neck
dwarf (stage 38). Note the broad epithelial bridge uniting roof and floor which greatly
occludes the opening. The inset is a higher magnification of the central epithelial cells, showing
some signs of vacuolation, but no extensive degeneration is seen in the organ at this late stage.
Fig. H. Through the middle portion of the esophagus in the crooked neck dwarf embryo
(stage 38). Note the broad zone of occlusion between roof and floor. Even the lateral longitudinal folds have typically persisting cytoplasmic bridges causing vesiculation in these
regions.
Fig. I. A higher magnification of Plate 1,fig.G, demonstrating degenerate, deeply basophilic
epithelial cells. Of all the mutants studied, this represents the most extensive degeneration
during the period when reopening should occur.
Fig. J. A higher magnification of the epithelial bridge in the mutant embryo represented in
Plate 1,fig.F. The cleft in the epithelium resembles the induced, premature cleft shown in
Plate 2, fig. G. The epithelial cells have prominent nucleoli and show no indications of
degeneration.
Fig. K. A higher magnification of the epithelial bridges infig.D, above, (a) Note the epithelial cells serving to connect the roof and floor. The nuclei have prominent nucleoli and the
cytoplasmic bridges appear to be coated with a mucoid-like substance, (b) This representative
section shows the few deeply basophilic, degenerating cells which occur along the epithelial
bridges. Extensive degeneration normally occurs at earlier stages.
/. Embryol. exp. Morph., Vol. 15, Part 1
A. L. ALLENSPACH
PLATE 3
facing p. 72
Esophagus in crooked neck dwarf embryo
73
minent nucleoli and mitotic figures adjacent to the vesicles. Degenerate, desquamated masses of epithelial cells are lacking.
Of questionable significance is the presence of an epithelial cleft in one stage 36
mutant embryo, similar to that described earlier (cf. Plate 1, fig. F).
The open region and the small vesicles within the stratified epithelium of the
stellate-shaped esophagus have been described earlier.
Stages 37-38
Mutant embryos of stages 37-38 do not exhibit a distinct region of occlusion,
but the broad anterior esophagus has a reduced diameter by virtue of a median
connexion between roof and floor (Plate 3, fig. G). The median epithelial bridge
contains small vesicles. Some intervesicular epithelial cells located farthest
from the basement membrane are highly vacuolated, but their nuclei never show
indications of pycnosis (Plate 3, fig. G, inset).
The vesiculated region more posteriorly takes on the character shown in
Plate 3, fig. D. The large, bilateral vesicles never fuse with medial vesicles, and
the latter enlarge to fashion thin, cellular bridges between roof and floor. The
majority of the cellular strands are living (Plate 3, fig. K(a)); occasionally
degenerate cells are observed in lateral epithelial connexions while medial cellular strands persist (Plate 3, fig. K(6)). Degeneration is never extensive enough
to effect reopening since the organ is still occluded in 18-19-day embryos
(Pun, 1954). A representative section showing well-developed longitudinal
folds in the posterior esophagus at stage 38 is seen in Plate 3, fig. H. Note that
the median part of the organ, including one longitudinal fold, is occluded.
Small vesicles occur in the medial, occluded epithelium and are particularly
abundant within the epithelium of the longitudinal folds. Typically the esophagus is reopened at this time (cf. Plate 3, fig. C).
The open region remains unchanged, characterized by remnants of cellular
connexions protruding from the floor, groups of free-floating cells in the lumen,
and numerous vesicles within the stratified epithelium (Plate 3, fig. F).
DISCUSSION
Our observations support those reported on older material by Pun (1954) but
give details on earlier stages. Studies on 12- and 18-day mutant embryos
revealed that the size of the esophageal lumen was slightly smaller and, in some
cases, completely obliterated while the normal esophagus was open along its
length. Cellular strands 'suggesting closure or anastomosis between all folds
of the esophagus' were observed connecting walls of longitudinal grooves
(Pun, 1954, p. 107).
Esophageal histogenesis in the mutant embryo resembles the normal pattern
up to stage 33. Beyond this stage, however, significant differences are noted;
perhaps the most important is the failure of the esophagus in the mutant embryo
to reopen at the normal time.
74
A. L. ALLENSPACH
Vesiculation and degeneration are important in reopening the normal,
occluded organ. However, that reopening is not necessarily caused by vesiculation alone is shown by observations in the mutant embryo where vesiculation
occurs extensively in the occluded epithelium through stage 38 to produce a
broad, much restricted, multi-vesicular organ (Plate 3, figs. G, H), yet the roof
and floor fail to separate and reopen the organ. This evidence precludes reopening of the esophagus by vesiculation alone and points to epithelial degeneration
as an essential mechanism for removing connexions between roof and floor.
One curious phenomenon observed is the appearance of apparently natural
clefts in the epithelial bridge between roof and floor in mutant embryos of
stages 34 and older (Plate 2, fig. F ; Plate 3, fig. J). In an earlier work, treatment
of the occluded, embryonic esophagus with versene induced separation of roof
and floor into clefts similar to those observed in mutants (Plate 2, fig. G). Versene-induced clefts were noted at stages before epithelial degeneration commences, namely stages 33-34 (Allenspach, 1964). The interpretation was offered
that a force exerted on the esophageal epithelium must exceed the cellular
adhesiveness to effect reopening. The clefts noted in mutant material occur at
stage 34 and later and always in regions where epithelial cells appear to be
subject to tensions, as noted by their specific orientation near a subjacent dorsal,
indistinct basement membrane (Plate 3, fig. J). These clefts may indicate that an
intrinsic force exists in the mesenchyme and plays a role, along with degeneration,
in the separation of roof and floor. The possibility of these clefts being artifacts
is not justified since fixation of other tissues was excellent.
The insight gained from studies on earlier developmental processes helps in
the interpretation of observations reported on 18-19-day mutant embryos
(Pun, 1954). Pun discovered that the entrance to the esophagus was occluded
in 18-19-day mutant embryos. Some of the longitudinal grooves were completely closed while others were united by anastomosing, epithelial strands
which greatly reduced the lumen (Pun, 1954). The persisting occlusion of longitudinal grooves (Plate 3, fig. H) and cellular strands (Plate 3, fig. F) is due to
failure of epithelial cells to degenerate at 8 days. The lack of occluded longitudinal grooves and cellular strands is evident in the normal esophagus (Plate 3,
fig. E). The net result is the same: chronic starvation due to failure of nutrients
to pass from amnion to stomach.
Very little secretion was observed in the mutant esophagus, resembling the
situation in the crooked neck dwarf turkey embryo where the epithelium is
apparently non-secretory (Asmundson & Pun, 1956). Considerable mucus is
secreted by normal epithelial cells, but it is believed that the secretion, acidic
mucopolysaccharides by nature, is not responsible for the reopening process
(Allenspach & Hamilton, 1962). On the other hand, degenerate epithelial cells
are sloughed into the lumen, and their by-products may be responsible for
autolysis of other epithelial cells. The role, if any, of secretory products in
esophageal reopening remains unclear.
Esophagus in crooked neck dwarf embryo
75
Since the crooked neck dwarf gene is lethal, the question arises as to which
abnormality of the pleiotropic pattern is the primary cause of death. Several
organ systems are known to be involved. That mutant embryos suffer from
chronic starvation because of their inability to swallow amniotic fluid has been
advanced by Pun (1954). However, a similar mutant in the turkey results in
harmful effects before esophageal atresia prevents starvation (Asmundson &
Pun, 1956). Recent results of experimental work on normal and abnormal
muscle have led Herrmann et al. (1963) to declare the 'starvation hypothesis'
an unlikely cause of abnormal development. Observations reported here and
others (Hadorn, 1961; Allenspach, unpublished) support this contention.
Abnormal developmental processes of the lethal embryo can be recognized
before any effects of starvation occur.
Esophageal organogenesis and more specifically the reopening process may
be controlled by a minimal number of mechanisms: cell degeneration, vesiculation and intercellular tensions. To understand these controlling mechanisms
further in highly organized tissues requires, first of all, an understanding of cell
contacts and intracellular aspects of cell degradation. The fact that some epithelial cells in the esophagus degenerate, while adjacent cells do not, causes
speculation as to what happens at the molecular level within these cells. These
problems are currently under investigation.
SUMMARY
1. The development of the esophagus in the crooked neck dwarf was studied
between 5£ and 12 days of incubation (stages 26-38).
2. Esophageal histogenesis in the crooked neck dwarf resembles normal
development up to stage 33.
3. Vesiculation alone does not cause the reopening of the esophagus, which
depends on the participation of epithelial degeneration to complete the process.
4. Studies of epithelial degeneration in the mutant esophagus emphasize its
caudo-cephalic gradient in the reopening process.
5. The appearance of clefts in the intervesicular epithelium suggests that
tension plays a role in the reopening process.
6. The occluded segments and persisting epithelial strands of the 18-day
mutant embryonic esophagus result from critical developmental failures at
8 days of incubation (stage 34).
7. The crooked neck dwarf mutant has a histogenetic marker in the form of
' degenerate mesenchyme cells' as early as stage 30.
76
A. L. ALLENSPACH
RESUME
Le processus de reouverture de Vcesophage chez le poulet en cours de
developpement, etudie sur le mutant nain a cou tordu
1. On a etudie le developpement de l'oesophage chez le mutant nain a cou
tordu, entre 5^ jours et 12 jours d'incubation (stades 26-38).
2. L'histogenese cesophageienne du mutant ressemble a celle du developpement normal jusqu'au stade 33.
3. Le vesiculisation seule ne provoque pas la reouverture de l'oesophage qui
depend de la participation de la degenerescence de l'epithelium pour achever
ce processus.
4. Les recherches sur la degenerescence epitheliale du mutant font ressortir
son gradient caudo-cephalique dans le processus de reouverture.
5. L'apparition de fissures dans l'epithelium intervesiculaire suggere que la
tension joue un role dans le processus de reouverture.
6. Les segments occlus et les cordons epitheliaux persistants del cesophage
embryonnaire du mutant du 18e jour, resultent de defectuosites critiques du
developpement au 8e jour de l'incubation (stade 34).
7. Le mutant nain a cou tordu presente un marqueur histogenetique sous la
forme de 'cellules mesenchymateuses degenerees', des le stade 30.
This investigation was supported by a special grant from Albright College. The technical
assistance of Linda J. Doerr and Russell Reidinger is hereby acknowledged.
Special thanks are due to Mr Forest Moyer, Chief Microphotographer, Reading Hospital,
Reading, Pennsylvania, whose generous time and valuable criticisms are deeply appreciated.
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HADORN, E. (1961). Developmental Genetics and Lethal Factors. London: Methuen and Co.,
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(Manuscript received 2 September 1965)