Recent experiments, in which barriers were inserted between anterior and posterior tissues of the chick wing bud, resulted in deletion of structures anterior to the barrier (Summerbell, 1979). From these data it was concluded that blockage of morphogen from the polarizing zone by the barrier resulted in the observed failure of specification of anterior structures. We suggest an alternative interpretation, viz. the interruption of the apical ridge by the barrier caused the deletions. This hypothesis was tested by removal of increasing lengths of ridge. This was done beginning at either the anterior or posterior junction of the wing bud with the body wall and proceeding posteriorly or anteriorly, respectively, to each half-somite level between 16/17 and 19/20. With removal of progressively greater lengths of anterior ridge, more anterior limb elements failed to develop. These data were used to construct a map of the ridge responsible for each digit. To test our hypothesis we removed posterior sections of apical ridge, as described above. Removal of posterior ridge to a level which was expected to allow outgrowth of digits anterior to the level of removal resulted in wings without digits in the majority of cases. An exception occurred when ridge posterior to the mid-19 somite level was removed. In almost half of these cases digits 2 and 3 did develop. In most cases the retention of only a half-somite piece of ridge with all other ridge removed, also resulted in deletion of all digits. Again the exception occurred when ridge posterior to somite level mid-19 and anterior to level 18/19 was removed, leaving only that ridge between somite level 18/19 and mid-19. In many of these cases digit 3 did develop. We conclude from these data that, in the wing bud, ridge anterior to the mid-19 somite level must be connected to more posterior ridge to function.

The leg ridge does not exhibit the asymmetrical, low anterior, high posterior configuration, which appears in the wing. Because the leg ridge is symmetrically high anteriorly and posteriorly, we questioned whether or not leg would also require a continuity between anterior and posterior ridge for anterior ridge to function. It did not. When posterior ridge was removed, structures developed under remaining anterior ridge and the elements which developed were complementary to those which developed after anterior ridge removal to the same somite level. Those leg elements, which failed to develop, were truncated at the appropriate proximodistal levels as indicated by the fate map we have constructed for the leg.

The data reported here do not rule out a role for the polarizing zone in specification of anterior structures. It is apparent that posterior ridge removal in the wing results in loss of structures anterior to the removal. However, this is not true for the leg.

The zone of polarizing activity, or polarizing zone, has been operationally defined as the mesoderm of the limb bud which when grafted to an anterior or mid-distal position of a host limb bud, induces formation of supernumerary limb parts from host anterior tissues (Saunders & Gasseling, 1968). The supernumerary limb elements are polarized so that the most posterior structures form next to the graft. The polarizing zone is limited to the posterior border of the presumptive and developing limb bud during stages 15–28 in chicks (MacCabe, Gasseling & Saunders, 1973; Hornbruch, unpublished) and all amniote limb buds tested to date (Tickle, Shellswell, Crawley & Wolpert, 1976; MacCabe & Parker, 1976a; Fallon & Crosby, 1977).

Clearly, under experimental conditions the polarizing zone can be shown to have morphogenetic activity. However, the role of this region in normal limb development has not been established. The argument that it plays no role in normal development has arisen (Saunders, 1977; Iten & Murphy, 1980a, b). We have suggested an early role in the stabilization and polarization of the limb field before limb-bud outgrowth (Fallon & Crosby, 1975, 1977; Thoms & Fallon, 1980). Yet another hypothesis has proposed that the polarizing zone is acting to polarize the limb elements at limb-bud stages when those elements are determined. The strongest evidence in support of this position has come from experiments by Summerbell (1979). In those experiments permeable and impermeable barriers were inserted through the dorsoventral extent of the limb bud, perpendicular to the body wall and at varying anteroposterior levels, using the somites as a reference. In this way the distal anterior and posterior limb-bud tissues were separated. When impermeable barriers were used at somite level 17/18,19 of 23 limbs developed without a radius and 14 of 23 developed without digit 2. After insertion of a permeable barrier at the same level, a radius failed to develop in 14 of 20 cases. If the impermeable barrier was inserted only through the mesoderm, leaving the apical ectoderm intact, a normal limb developed. It was concluded from these data that the action of the polarizing zone upon limb structures can be blocked by a barrier, thus resulting in deletions of limb elements anterior to the barrier. Specifically, it was proposed that a diffusable morphogen from the polarizing zone was prevented from reaching anterior mesoderm and that resulted in the failure of anterior structures to develop. We offer another interpretation: that the interruption of the apical ectoderm by the implanted barrier caused the deletions of limb elements. To test this hypothesis, we have removed pieces of ridge from the anterior and posterior borders of the wing and leg buds and examined whether or not structures, expected to grow out under the remaining ridge, were deleted. With this approach we determined that structures under remaining anterior ridge fail to develop when posterior ridge is removed from the wing. This is not true for the leg. In the leg, structures which are expected to develop under remaining anterior ridge do develop when posterior ridge is removed.

Fertilized White Leghorn eggs were incubated for 3 days at 38°C, candled and fenestrated according to the technique of Zwilling (1959). Embryos of Hamburger & Hamilton (1951) stages 18–20 were used. A portion of the apical ectodermal ridge was removed from the right wing or leg bud with a glass needle. The eggs of operated embryos were sealed with tape and returned to the incubator for 7–9 days. The 10- to 12-day chicks were then fixed in 10% formalin, stained with Victoria blue and cleared.

Two groups of operations were performed on both wing and leg buds. In the first group, apical ridge was removed beginning at the junction of the limb-bud anterior border (somite level 14/15 for the wing and somite level 27/28 for the leg) with the body wall and proceeding back to a specific level of the bud determined by its relation to the somites. Removals beginning at the 14/15 level were performed to each half-somite level between somites 16/17 and 19/20 for the wing (Fig. 1A) and beginning at the 27/28 level to each half somite level between 28/29 and 31/32 for the leg. The second group of removals was complementary to the first group. In this group, apical ectodermal ridge was removed beginning at the junction of the limb-bud posterior border with the body wall and proceeding anteriorly to each half-somite level between somites 19/20 and

Fig. 1

(A) Diagram of the anterior apical ridge removals in the stage-19 wing bud. The removals begin at a level anterior to somite 15 and proceed posteriorly to one of seven positions as indicated by the arrows. (B) Diagram of the experiments complementary to those in (A). Removal of ridge begins posterior to the somite-20 level and extends anteriorly to one of the seven positions indicated by the arrows.

Fig. 1

(A) Diagram of the anterior apical ridge removals in the stage-19 wing bud. The removals begin at a level anterior to somite 15 and proceed posteriorly to one of seven positions as indicated by the arrows. (B) Diagram of the experiments complementary to those in (A). Removal of ridge begins posterior to the somite-20 level and extends anteriorly to one of the seven positions indicated by the arrows.

16/17 in the wing (Fig. 1B) and between 31/32 and 28/29 in the leg. Two additional groups of experiments were performed only on the wing bud. These were also complementary types of operations. In one group a small piece of apical ridge, one-half somite in length, was removed. The operations in the other group consisted of removing the entire apical ectoderm with the exception of a piece one-half somite in length.

The numbers which survived postoperatively in each group are given in the results.

Removal of anterior apical ectodermal ridge from the wing bud

Anterior pieces of apical ectodermal ridge were removed to determine which level of ridge is responsible for each digit. The results of these removals at stages 18–19 are given in Table 1 and typical examples of the resulting limbs appear in Fig. 2. At these stages, the amounts of radius and ulna which develop following complete ridge removal vary (Summerbell, 1974). Because of this variability, we will concentrate on the appearance or deletion of the digits, making only minor reference to the radius and ulna. As illustrated in Fig. 2, more posterior digits were deleted after removal of greater lengths of anterior apical ridge.

Table 1
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Fig. 2

Photographs of the typical results of anterior apical ridge removal described in Fig. 1A. (A) Somite level mid-17 removal -normal. (B) Somite level 17/18 removal -digits 2, 3, 4 and ulna. (C) Somite level mid-18 removal -digits 3, 4. (D) Somite level 18/19 removal -digit 4. (E) Somite level mid-19-proximal digit 4. (F) Somite level 19/20 removal -no digits.

Fig. 2

Photographs of the typical results of anterior apical ridge removal described in Fig. 1A. (A) Somite level mid-17 removal -normal. (B) Somite level 17/18 removal -digits 2, 3, 4 and ulna. (C) Somite level mid-18 removal -digits 3, 4. (D) Somite level 18/19 removal -digit 4. (E) Somite level mid-19-proximal digit 4. (F) Somite level 19/20 removal -no digits.

Removal of anterior ridge back to the level of somite 17/18 and to intervening levels (16/17 and mid 17, Fig. 2 A) resulted in development of a normal limb with respect to the digits. When done at stage 18 and early 19, about 50% of the removals to the 17/18 level resulted in deletion of the radius (Fig. 2B), 100% of removals to the mid-18 somite level resulted in wings missing digit 2, (Fig. 2C). All 12 cases of removals to the 18/19 somite level resulted in deletion of digit 2 and 92% in deletion of the proximal part (42%) or all (58%) of digit 3, (Fig. 2D). After removal of ridge to the mid-19 somite level, limbs developed without digits 2 and 3 in all 10 embryos and without the distal portion of digit 4 in 5 of these (Fig. 2E). Removal to the 19/20 level always resulted in deletion of all digits (Fig. 2F). The proximal portion of digit 4 was not deleted until the anterior removal of ridge extended to the 19/20 level. However, as noted, the distal portion of digit 4 could be deleted by anterior ridge removal to only the mid-19 level. At stage 18 and early 19, removals to the 19/20 level resulted in the development of limbs missing some or all of the ulna in 67% of cases.

We conclude the following from these data on deletions after anterior ridge removal: that the ridge at the level between somite 17/18 and mid-18 is responsible for the outgrowth of mesoderm which will form digit 2; the ridge between mid-18 and mid-19 is responsible for digit 3; the ridge at the level of mid-19 and immediately posterior to that is responsible for digit 4. These data are summarized in a map of the apical ectodermal ridge in Fig. 3.

Fig. 3

Diagram of the stage-19 wing bud with a map of the ridge responsible for outgrowth of each digit: digit 2 – between somite levels 17/18 and mid-18, digit 3 – between somite levels mid-18 and mid-19, digit 4-between somite levels mid-19 and 19/20.

Fig. 3

Diagram of the stage-19 wing bud with a map of the ridge responsible for outgrowth of each digit: digit 2 – between somite levels 17/18 and mid-18, digit 3 – between somite levels mid-18 and mid-19, digit 4-between somite levels mid-19 and 19/20.

Removal of posterior apical ectodermal ridge from the wing bud

Table 2 presents the results of removal of posterior apical ridge from the wing bud. As expected from data in which anterior ridge removal to 16/17, mid-17, and 17/18 somite levels resulted in normal development of digits, removal of the complementary posterior ridge to those levels resulted in deletion of all digits in all cases. However, in most cases, removal of reciprocal pieces of anterior and posterior apical ridge at other levels did not result in formation of complementary digital elements. Recall that removal of anterior ridge to the mid-18 somite level resulted in deletion of digit 2 (Fig. 4A, B). Therefore, formation of digit 2 was expected after removal of the complementary posterior ridge to that level. However, all 11 cases showed deletion of this digit following that operation (Fig. 4C, D). After removal of posterior apical ridge to the level of 18/19 or mid-19, we expected development of structures missing after anterior removal, i.e. digits 2 and 3 (Fig. 4E, F), and digits 2, 3 (Fig. 41, J), and perhaps distal 4, respectively. However, in all 10 cases of ridge removal to the 18/19 level, wings developed without digits (Fig. 4G, H). Removal to the mid-19 level resulted in limbs without digits in 56% of cases (Fig. 4K, L) and limbs missing only digit 4 in 44% of cases. Only in these 44% of cases did we find that, after posterior ridge removal, the wing elements developed which were missing after anterior removal. Posterior ridge removal resulted in deletion of structures expected to grow out under remaining anterior apical ridge in a majority of cases.

Table 2
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Fig. 4

Diagrams of the level of anterior and posterior ridge removals and photographs of typical resulting wings arranged so they can be compared directly. (A) Drawing of anterior ridge removal to somite level mid-18. (B) Resulting wing with only digits 3 and 4. (C) Drawing of posterior ridge removal to somite level mid-18. (D) Resulting wing with no digits. (E) Drawing of anterior ridge removal to somite level 18/19. (F) Resulting wing with only digit 4. (G) Drawing of posterior ridge removal to somite level 18/19. (H) Resulting wing with no digits. (I) Drawing of anterior ridge removal to somite level mid-19. (J) Resulting wing with only proximal part of digit 4. (K) Drawing of posterior ridge removal to somite level mid-19. (L) Resulting wing with no digits.

Fig. 4

Diagrams of the level of anterior and posterior ridge removals and photographs of typical resulting wings arranged so they can be compared directly. (A) Drawing of anterior ridge removal to somite level mid-18. (B) Resulting wing with only digits 3 and 4. (C) Drawing of posterior ridge removal to somite level mid-18. (D) Resulting wing with no digits. (E) Drawing of anterior ridge removal to somite level 18/19. (F) Resulting wing with only digit 4. (G) Drawing of posterior ridge removal to somite level 18/19. (H) Resulting wing with no digits. (I) Drawing of anterior ridge removal to somite level mid-19. (J) Resulting wing with only proximal part of digit 4. (K) Drawing of posterior ridge removal to somite level mid-19. (L) Resulting wing with no digits.

Removal of half-somite length pieces of apical ectodermal ridge from the wing bud

In order to examine further the precision of the correlation between the level of apical ridge removal and deleted elements, we removed pieces of apical ridge one-half somite in length from the wing bud (Table 3). After removal of ridge between somite levels 17/18 and mid-18, digit 2 did not develop in 67% of operated wings. However, digit 2 was also deleted after mid-18 to 18/19 (67%) and 18/19 to mid-19 level removals (50%). Digit 3 was absent after mid-18 to 18/19 level removals in 89% of the cases and also in 90% of the cases after removal of apical ectoderm between the 18/19 and mid-19 somite levels. Finally, with removal at the level of mid-19 to 19/20, digit 4 was deleted in 71% of cases.

Table 3
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These data are consistent with Fig. 3, in that outgrowth of digit 4 can be attributed to apical ridge between somite levels mid-19 and 19/20 and outgrowth of digit 3 to that between somite levels mid-18 and mid-19. Outgrowth for digit 2 can be attributed to ridge between levels 17/18 and mid-18. However, that digit also was deleted after removal of ridge posterior to the 17/18 to mid-18 level.

Retention of half-somite length pieces of apical ectodermal ridge in the wing bud

Next, a complementary set of experiments was performed in which only a piece of apical ridge one-half somite in length was left on the limb bud. The results of these experiments are summarized in Table 4. With ridge remaining between the 17/18 and mid-18 level, digits 3 and 4 should not grow out, according to Fig. 3, but digit 2 should develop. In 92% of cases, digit 2 was deleted. According to the map in Fig. 3, removal of all ridge except that between mid-18 and 18/19 or 18/19 and mid-19 should result in outgrowth of part of digit 3 and deletion of digits 2 and 4. But with the mid-18 to 18/19 piece remaining, digit 3 was absent in all seven embryos. However, the expected outgrowth of digit 3 did occur after operations in which ridge between 18/19 and mid-19 was left intact. In these operations, ridge anterior to 18/19 (responsible for digit 2 and part of digit 3 outgrowth) and that posterior to mid-19 (responsible for digit 4 outgrowth) was removed and limbs developed complementary to those from which only ridge between 18/19 and mid-19 was removed. Thus, only in cases where ridge at the level of 18/19 to mid-19 was removed or left intact were complementary wings obtained. In these cases, when ridge posterior to the mid-19 level was removed, digit 3 developed in 71% of embryos. Thus, at levels other than that between 18/19 and mid-19, the digit deleted by removal of a half-somite piece of apical ridge could not be obtained by leaving only that piece of ridge.

Table 4
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Anterior and posterior apical ridge removals from the leg

The ridge of the avian wing bud is asymmetrically distributed on the limb-bud tip. The anterior ridge is low pseudostratified columnar epithelium, while posterior ridge has a high pseudostratified columnar epithelial configuration. The apical ridge of the leg bud, unlike that of the wing, but similar to that of other amniote limb buds, is symmetrically distributed at the limb-bud tip. Therefore, it seemed reasonable to question whether or not the results of posterior ridge removal in the leg would mimic those we found for the wing. In order to determine which structures in the leg are susceptible to deletion with stage-19 to-20 ridge removal, we constructed a proximodistal fate map for the leg, employing the technique used by Saunders (1948) and Summerbell (1974) for the wing. The results of these operations are summarized in Fig. 5.

Fig. 5

Drawing of the right chick leg from a right lateral perspective. The stages at which the total ridge removals were done are listed on the right. The lines intersecting the limb represent the levels of truncation of the leg after ridge removal at the indicated stages.

Fig. 5

Drawing of the right chick leg from a right lateral perspective. The stages at which the total ridge removals were done are listed on the right. The lines intersecting the limb represent the levels of truncation of the leg after ridge removal at the indicated stages.

Data from experiments in which anterior or posterior apical ridge were removed from the leg are summarized in Table 5 and typical results shown in Fig. 6, 7. As can be seen in the figures, the removals of posterior ridge from the leg did not result in deletions of structures under remaining anterior ridge, as was the case for the wing. For example, when anterior ridge was removed to the level of 30/31 (Fig. 7 A), in all cases digits 1, 2 and 3 were deleted. With reciprocal removal of posterior ridge to the level of 30/31, legs developed which were missing digit 4 (Fig. 7B). Thus, in contrast to the wing, removal of reciprocal pieces of apical ridge from the leg does result in development of complementary limb elements.

Table 5
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Fig. 6

Photographs of the chick legs resulting from anterior (left) and posterior (right) ridge removals from stage-20 leg buds. (A) Anterior ridge removal to somite level mid-29 – normal leg. (B) Posterior ridge removal to somite level mid-29 – no digits or metatarsals. (C) Anterior ridge removal to somite level 29/30 -digits 2, 3 and 4 and metatarsals 2, 3 and 4. (D) Posterior ridge removal to somite level 29/30-digit 1. (E) Anterior ridge removal to somite level mid-30 – digits 3 and 4 and metatarsals 3 and 4. (F) Posterior ridge removal to somite level mid-30 – digits 1, 2 and proximal 3 and metatarsals 2 and 3.

Fig. 6

Photographs of the chick legs resulting from anterior (left) and posterior (right) ridge removals from stage-20 leg buds. (A) Anterior ridge removal to somite level mid-29 – normal leg. (B) Posterior ridge removal to somite level mid-29 – no digits or metatarsals. (C) Anterior ridge removal to somite level 29/30 -digits 2, 3 and 4 and metatarsals 2, 3 and 4. (D) Posterior ridge removal to somite level 29/30-digit 1. (E) Anterior ridge removal to somite level mid-30 – digits 3 and 4 and metatarsals 3 and 4. (F) Posterior ridge removal to somite level mid-30 – digits 1, 2 and proximal 3 and metatarsals 2 and 3.

Fig. 7

Photographs of chick legs resulting from anterior (left) and posterior (right) apical ridge removals from stage 20 leg buds. (A) Anterior ridge removal to somite level 30/31– digit 4 and metatarsals 3 and 4. (B) Posterior ridge removal to somite level 30/31 – digits 1,2 and proximal 3 and metatarsals 2 and 3. (C) Anterior ridge removal to somite level mid-31 -digit 4 and metatarsal 4. (D) Posterior ridge removal to somite level mid-31 – digits 1, 2, 3 and metatarsals 2, 3 and 4. (E) Anterior ridge removal to somite level 31 /32 – no digits or metatarsals. (F) Posterior ridge removal to somite level 31/32 – normal leg.

Fig. 7

Photographs of chick legs resulting from anterior (left) and posterior (right) apical ridge removals from stage 20 leg buds. (A) Anterior ridge removal to somite level 30/31– digit 4 and metatarsals 3 and 4. (B) Posterior ridge removal to somite level 30/31 – digits 1,2 and proximal 3 and metatarsals 2 and 3. (C) Anterior ridge removal to somite level mid-31 -digit 4 and metatarsal 4. (D) Posterior ridge removal to somite level mid-31 – digits 1, 2, 3 and metatarsals 2, 3 and 4. (E) Anterior ridge removal to somite level 31 /32 – no digits or metatarsals. (F) Posterior ridge removal to somite level 31/32 – normal leg.

The results of experiments reported in this paper show that the apical ectodermal ridge of the chick wing bud anterior to the mid-19 somite level must be connected to more posterior ridge to allow outgrowth of anterior mesoderm. If posterior ridge was removed extending anteriorly to at least the mid-19 somite level, limb elements that should develop beneath remaining ridge, failed to do so. The results of half-somite-piece apical ridge removal or retention in the wing bud suggest that the ridge specifically required for anterior ridge function may be that at the level of mid-19, the highe.st ridge epithelium in the wing at the stages used in these experiments. We infer that anterior ridge must be connected to posterior ridge for normal wing development.

The removal of progressively greater amounts of anterior ridge in a posterior direction allows determination of the anteroposterior section of ridge responsible for outgrowth of each limb element. Summerbell (1979) has estimated a fate map for the anteroposterior axis which correlates with that of Stark & Searls (1973). In both of these papers, determinations of which anteroposterior level of mesenchyme would become a particular limb element were made. In contrast, our experiments determine which anteroposterior level of the ridge at stages 18–19 allows outgrowth of digital elements. It is noteworthy that we find that all of the phalangeal elements of a single digit do not seem to develop from mesenchyme in the same horizontal plane with respect to the somites, unless that mesenchyme changes position with respect to overlying ridge. For example, ridge responsible for the outgrowth of the distal part of digit 4 seems to be anterior to that required for outgrowth of the more proximal part of that digit.

Unlike the wing, the leg does not seem to require the presence of the posterior ridge for the normal development of anterior structures. Anterior apical ectodermal ridge functions without the presence of the posterior ridge in the leg. Our original reason for examining whether or not the leg is the same in its requirement for posterior ridge stemmed from the fact that the leg apical ridge is more symmetrical than that of the wing bud. The ridge of the wing bud is higher posteriorly, while that of the leg is high both anteriorly and posteriorly. Whether or not the difference in ridge symmetry can be correlated with the difference in requirement for posterior ridge between wing and leg buds, cannot be determined from the present experiments. Although more anterior ridge is in the high configuration in the leg, as compared to the wing, it seems to contribute little to the formation of the anterior cartilage elements of the leg. At stage 20, ridge can be removed from the anterior junction with the body wall to the midpoint of the leg bud, before digit 1 is deleted. Determination of the factors responsible for the difference in posterior ridge requirement between wing and leg buds will require further examination.

Upon removal of pieces of apical ridge from the wing bud, we obtained results similar to those of Summerbell (1979) after barrier insertion. According to the map in Fig. 3, digits which were expected to develop beneath ridge remaining anterior to the level of removal failed to develop, as they did when a barrier was inserted at the same anteroposterior level. Our data do not permit a conclusion as to whether or not the polarizing zone or its morphogen are involved. Experiments in which dorsal ectoderm immediately proximal to the ridge was removed (unpublished observations) and experiments in which the wing bud was denuded of all ectoderm except the ridge (Zwilling, 1956; Gasseling & Saunders, 1961) give evidence that the deletions occurring after posterior ridge removal are not the result of leakage of a diffusible morphogen through the wound.

The simplest explanation for the results of the experiments reported in this paper is that in order for anterior apical ridge to function, it must be connected to posterior ridge. In support of this, we point out that numerous, very large gap junctions are characteristic of the apical ridge (Fallon & Kelley, 1977). Further, it has been demonstrated that ridge cells transport Lucifer yellow, which is evidence for coupling among ridge cells (Fallon & Sheridan, in preparation). Thus, it may be that ionic or metabolic coupling of posterior to anterior ridge cells is required for anterior ridge function. Aside from this possibility of intercellular coupling, it is also possible that something essential to anterior ridge function emanates from posterior ridge. This reasoning is complicated by the possibility that polarizing zone or its diffusible morphogen may act through the apical ridge (Tickle, 1980). However, there is no direct evidence for this proposal and we do not favour it. Rather, the experiments reported in this paper may demonstrate a property of the ridge of the wing bud which is not related to the polarizing zone directly.

It is pertinent that after removal of the entire posterior half of the wing bud, investigators (Warren, 1934; Amprino & Camosso, 1955) have shown that the anterior half of the wing bud fails to develop. This result is comparable to that obtained with barrier insertion and posterior ridge removal. Further, Hinchliffe & Gumpel-Pinot (1981) have shown that removal of the entire posterior half of the wing bud results in extensive cell death of the remaining anterior mesoderm. This is also true for the leg bud (Fallon & Rowe, unpublished). In the light of chick wing-bud fate maps, analysis of Warren’s data leads to the conclusion that anterior structures already determined at the time of removal of the posterior half of the wing bud are absent (e.g., Warren’s fig. 22). This is consistent with the observation of massive cell death of anterior mesoderm after posterior wing-bud removal. In contrast, we do not find mesodermal cell death under remaining anterior ridge following removal of posterior ridge (Rowe, Cairns & Fallon, 1980 and in preparation). It is likely than that anterior deletions seen after partial ridge removal and after posterior-half wing-bud removal demonstrate at least two different phenomena. In the former experiment, anterior ridge does not appear to function without continuity with posterior ridge. This is also likely to be true in the latter case, but cell death is an added effect of the removal of posterior mesoderm.

This would seem to implicate the polarizing zone. Simple removal of the polarizing zone can result in normal limb development (MacCabe et al. 1973; Fallon & Crosby, 1975). It is probable, however, that the polarizing zone morphogen remains in more anterior tissue for a period after removal of its source, the posterior border mesoderm. In fact, MaCabe & Parker (1976b) have shown a gradient of morphogenetic activity with an area of activity at the ‘anteroposterior center’ of the wing bud, intermediate to the high posterior and null anterior activities. This middle area would remain after removal of polarizing zone alone, but all activity (that from source and middle morphogen containing area) are removed from influencing the anterior mesoderm when the entire posterior half of the bud is excized. This removal from posterior mesodern influence would also occur when a piece of anterior tissue is isolated from the rest of the wing bud, as in MacCabe & Parker’s (1975),in vitro system. Cell death does occur when anterior mesoderm with its apical ectoderm is grown in culture. However, if polarizing zone is put in the same culture with isolated anterior wing-bud mesoderm, cell death does not occur. Thus, in experimental systems where anterior mesoderm is removed from posterior mesoderm, extensive cell death is observed in anterior wing-bud tissues. It would seem that the posteriorhalf limb-bud removal and the posterior ridge removal experiments are different. Posterior ridge removal does not result in cell death in anterior wingbud mesoderm. In contrast, removal of the entire posterior half of the wing bud results in extensive cell death in anterior mesoderm. It is not known whether the barrier experiments done by Summerbell (1979) were accompanied by anterior mesodermal cell death.

We still feel that there is no compelling evidence for a role for polarizing zone morphogen in the specification of the anteroposterior polarity of limb elements during the limb-bud stages of normal development. Consistent with this, we have proposed that the production of morphogen is residual during limb-bud stages (Fallon & Crosby, 1975, 1977). However, considering work in this laboratory and data from the laboratories of MacCabe and Hinchliffe, it is reasonable to assume that factors from the polarizing zone are required for the survival of anterior mesoderm cells during the limb-bud stages of normal development.

This investigation was supported by NSF Grant No. PCM7903980 and NIH Grant No. T32HD7118. We are grateful to Drs Allen W. Clark and David B. Slautterback, and Ms Eugenie Boutin for their constructive criticism of this manuscript. Speicial thanks are due to Ms B. Kay Simandl for technical assistance. We thank Ms Lucy Taylor for making the drawings and Ms Julie Michels and Ms Sue Leonard for typing the manuscript.

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