Waddington1956 11 - Revision history

Z8600021: /* 4. Mammals */

2020-01-23T02:38:08Z

4. Mammals

← Older revision		Revision as of 13:38, 23 Ocak 2020
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	In the lowest group, the prototherian mammals or monotremes, such as Echidna, the egg is still reptilian in gencral configuration, cleavage 1s partial, and a blastoderm is formed. Flynn and Hill (1939, 1942) find that endoderm formation takes place mainly by delamination, though there is also some movement of isolated cells out of the upper layer ito the deeper one.		In the lowest group, the prototherian mammals or monotremes, such as {{Echidna}}, the egg is still reptilian in gencral configuration, cleavage 1s partial, and a blastoderm is formed. Flynn and Hill (1939, 1942) find that {{endoderm}} formation takes place mainly by delamination, though there is also some movement of isolated cells out of the upper layer ito the deeper one.


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	The mesoderm is formed from the outer surface of the inner cell masss. In this region the outermost layer of all, the “enveloping layer’ which is continuous with the main sphere of the blastocyst, either lifts away to leave a cavity (thus forming an amnion), or breaks down and disappears. The face of the inner cell mass thus exposed has in general an oval form in plan view, and on it there appears an elongated thickening, which is the primitive streak. In some forms, such as the rabbit, it is extremely like that seen in birds; in others, such as the mole and in man, the streak is shorter and there is a greater development of a chorda-mesodermal canal such as that found in reptiles. The invagination of the mesoderm has never been followed in detail by vital staining, but a few cinematograph films of rabbit blastoderms growing in vitro have been taken by Waddington, and a convergence of lateral material towards the streak from both sides has been seen; there is little doubt that the process is essentially similar to that of the bird and reptile. There are, of course, certain differences in detail. Thus in the rabbit the presumptive neural plate probably occupies a greater proportion of the embryonic area which corresponds to the area pellucida of the chick; and Waddington (1937) points out that the results of cultivation in vitro of posterior halves of rabbit embryonic areas suggests that there is less longitudinal movement up and down the streak than in birds.		The {{mesoderm}} is formed from the outer surface of the inner cell masss. In this region the outermost layer of all, the “enveloping layer’ which is continuous with the main sphere of the blastocyst, either lifts away to leave a cavity (thus forming an amnion), or breaks down and disappears. The face of the inner cell mass thus exposed has in general an oval form in plan view, and on it there appears an elongated thickening, which is the primitive streak. In some forms, such as the {{rabbit}}, it is extremely like that seen in birds; in others, such as the mole and in man, the streak is shorter and there is a greater development of a chorda-mesodermal canal such as that found in reptiles. The invagination of the mesoderm has never been followed in detail by vital staining, but a few cinematograph films of rabbit blastoderms growing in vitro have been taken by Waddington, and a convergence of lateral material towards the streak from both sides has been seen; there is little doubt that the process is essentially similar to that of the bird and reptile. There are, of course, certain differences in detail. Thus in the {{rabbit}} the presumptive neural plate probably occupies a greater proportion of the embryonic area which corresponds to the area pellucida of the chick; and Waddington (1937) points out that the results of cultivation in vitro of posterior halves of rabbit embryonic areas suggests that there is less longitudinal movement up and down the streak than in birds.


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	A little work has been done on the early cleavage stages. Nicholas and Hall (1942) succeeded in getting some development (up to just before the time the embryo makes its appearance) of one of the first two blastomeres, the other having been destroyed or removed. Pincus (1936) got rather similar results with the rabbit. Recently Seidel (19525) had been much more successful, One of the first two blastomeres of a rabbit egg was killed by pricking with a glass needle, without removing the jelly layer. The egg was then injected into the Fallopian tube of another rabbit at a suitable stage in the reproductive cycle, and fully normal young were born. The evidence that one blastomere was really killed seems quite convincing, and the operated egg differed genetically (in colour) from the host, so that there is no doubt that the young animal was derived from it and not from an uninjured egg of the foster-mother. Only two such animals have so far been described; one was fully normal, but the other showed defects on one side, rather similar to those which are found in newts’ eggs from half blastomeres when the first cleavage furrow does not divide the grey crescent region into equal parts. One may conclude that the rabbit egg (and presumably that of other mammals) is certainly not of the mosaic type, but may contain a localised organisation centre comparable to the amphibian grey crescent; this would probably be related to the bilateral structures described by Dalcq. (For experiments involving the alteration of chromosome numbers, induction of polyploidy etc. in mammals, see Beatty 1955.)		A little work has been done on the early cleavage stages. Nicholas and Hall (1942) succeeded in getting some development (up to just before the time the embryo makes its appearance) of one of the first two blastomeres, the other having been destroyed or removed. Pincus (1936) got rather similar results with the rabbit. Recently Seidel (19525) had been much more successful, One of the first two blastomeres of a {{rabbit}} egg was killed by pricking with a glass needle, without removing the jelly layer. The egg was then injected into the Fallopian tube of another rabbit at a suitable stage in the reproductive cycle, and fully normal young were born. The evidence that one blastomere was really killed seems quite convincing, and the operated egg differed genetically (in colour) from the host, so that there is no doubt that the young animal was derived from it and not from an uninjured egg of the foster-mother. Only two such animals have so far been described; one was fully normal, but the other showed defects on one side, rather similar to those which are found in newts’ eggs from half blastomeres when the first cleavage furrow does not divide the grey crescent region into equal parts. One may conclude that the rabbit egg (and presumably that of other mammals) is certainly not of the mosaic type, but may contain a localised organisation centre comparable to the amphibian grey crescent; this would probably be related to the bilateral structures described by Dalcq. (For experiments involving the alteration of chromosome numbers, induction of polyploidy etc. in mammals, see Beatty 1955.)


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	The first genes from which important embryological consequences could be deduced had their most obvious effects on the tail. The Brachyury gene T is a dominant, the heterozygous mice having short tails. The T/T homozygotes die before birth and in the embryos the whole posterior region of the body is missing (Chesley 1935). Detailed investigation showed that no notochord (or almost none) is ever formed in these homozygous embryos, and Chesley suggested that this was the primary action, the effect on other structures such as the nerve-cord being the result of secondary reactions of an inductive nature. This was confirmed, and the evidence made more convincing, when Gluecksohn-Schoenheimer investigated embryos of animals which were heterozygous both for T and for one or other of the genes f° and t1. These embryos have normallooking tails till an age of about eleven days, after which the tail becomes constricted at its base and degenerates, the young being quite tailless when born. Histological examination showed that even in the apparently normal tails of early stages, no notochord is present, although the neural tube, somites and tailgut are formed as usual. It seems then that the later degeneration is a consequence of the lack of the notochord, and some ‘inductive’ relation is indicated. It should be noted, however, that the relation is not quite that of evocation; even in the absence of the notochord, the neural tube is formed, presumably induced by the remainder of the mesoderm, but it is unable to persist. There is no exact parallel to this in other vertebrates, since the operative removal of the notochord, e.g. in Amphibia, does not lead to the regression of the neural tube, but only to a failure of its normal elongation and an alteration of its usual cross-sectional shape.		The first genes from which important embryological consequences could be deduced had their most obvious effects on the tail. The Brachyury gene T is a dominant, the heterozygous mice having short tails. The T/T homozygotes die before birth and in the embryos the whole posterior region of the body is missing (Chesley 1935). Detailed investigation showed that no notochord (or almost none) is ever formed in these homozygous embryos, and Chesley suggested that this was the primary action, the effect on other structures such as the nerve-cord being the result of secondary reactions of an inductive nature. This was confirmed, and the evidence made more convincing, when Gluecksohn-Schoenheimer investigated embryos of animals which were heterozygous both for T and for one or other of the genes f° and t1. These embryos have normallooking tails till an age of about eleven days, after which the tail becomes constricted at its base and degenerates, the young being quite tailless when born. Histological examination showed that even in the apparently normal tails of early stages, no notochord is present, although the neural tube, somites and tailgut are formed as usual. It seems then that the later degeneration is a consequence of the lack of the notochord, and some ‘inductive’ relation is indicated. It should be noted, however, that the relation is not quite that of evocation; even in the absence of the notochord, the neural tube is formed, presumably induced by the remainder of the mesoderm, but it is unable to persist. There is no exact parallel to this in other vertebrates, since the operative removal of the notochord, e.g. in Amphibia, does not lead to the regression of the neural tube, but only to a failure of its normal elongation and an alteration of its usual cross-sectional shape.

	The homozygous f°/t® embryos show more profound abnormalities. The whole endoderm tends to lift away from the rest of the inner cell mass some time before the beginnings of embryo formation are visible. The cells of the mass remain alive for some time, and some growth takes place, but there is no organisation of a primitive streak and no mesoderm appears; a day or two after the onset of the condition, the embryos die and are resorbed. Gluecksohn-Schoenheimer suggests that this may indicate that in the mammals, as in the birds, the endoderm may play an essential role in inducing the formation of mesoderm.		The homozygous f°/t1 embryos show more profound abnormalities. The whole endoderm tends to lift away from the rest of the inner cell mass some time before the beginnings of embryo formation are visible. The cells of the mass remain alive for some time, and some growth takes place, but there is no organisation of a primitive streak and no mesoderm appears; a day or two after the onset of the condition, the embryos die and are resorbed. Gluecksohn-Schoenheimer suggests that this may indicate that in the mammals, as in the birds, the endoderm may play an essential role in inducing the formation of mesoderm.

Z8600021: /* 1. Cyclostomes and other primitive fish */

2020-01-23T02:35:12Z

1. Cyclostomes and other primitive fish

← Older revision		Revision as of 13:35, 23 Ocak 2020
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	==1. Cyclostomes and other primitive fish==		==1. Cyclostomes and other primitive fish==

	The eggs of a representative of the cyclostomes, the river lamprey (Lampetra or Petromyzon) have been studied in some detail in recent years. The egg is about 2 mm. in diameter. Its cleavage is total and very similar to that of the Amphibia. It gives rise to a blastula which is also very similar to the amphibian. At gastrulation a small knob begins to protrude in the spherical embryo and the blastopore appears just below it. The blastopore is small and remains so throughout the whole period of invagination and one sees nothing resembling the yolk plug of the frog.		The eggs of a representative of the cyclostomes, the river lamprey (''Lampetra'' or ''Petromyzon'') have been studied in some detail in recent years. The egg is about 2 mm. in diameter. Its cleavage is total and very similar to that of the Amphibia. It gives rise to a blastula which is also very similar to the amphibian. At gastrulation a small knob begins to protrude in the spherical embryo and the blastopore appears just below it. The blastopore is small and remains so throughout the whole period of invagination and one sees nothing resembling the yolk plug of the frog.

	Weissenberg (1934, 1936) has studied the gastrulation by means of vital stained marks, and has derived a map of presumptive areas (Fig. 11.1). It shows a considerable resemblance to that of the Amphibia except that the axial mesoderm is, from the beginning, more concentrated towards the dorsal plane. According to Weissenberg, indeed, the presumptive mesoderm does not extend right round the egg but on the ventral side the presumptive ectoderm comes into direct contact with the presumptive endoderm. This situation, which would constitute a considerable difference from that of the Amphibia, has been questioned by Pasteels (1940) who points out that in the early neural plate stage, when invagination is still proceeding around the blastopore, this structure is undoubtedly surrounded by a complete ring of mesoderm, some of which is being invaginated over the ventral lip at this time. Although Pasteels’ material was not sufficient to enable him to draw an alternative map of the presumptive areas in the early gastrula, he concludes that the mesoderm miust in fact extend right round the egg between the ectoderm and endoderm. He suggests that the main difference between the gastrulation of the lamprey and the frog is that, in the former, the invagination of mesoderm over the lateral and ventral lips is delayed so that it does not begin until the main mass of endodermal material has already passed into the interior. The two processes in fact differ only in this comparatively minor matter of timing. Some attention has also been paid to the causal embryology or epigenetics of these forms. It was shown as long ago as 1900 by Bataillon that if the first two blastomeres are separated cach may give a complete embryo. The experiment has been repeated more recently (Montalenti and Maccagno 1935; Bytinski-Salz 19372) and it has been shown that the situation is almost exactly the same as that revealed by constriction experiments in the Amphibia. Further, fragments from the dorsal lip of the blastopore can be grafted just as in newts (Bytinski-Salz 1937); Yamada 1938). When they come in contact with gastrula ectoderm they cause the induction of a secondary neural axis and, so far as experiment has gone, repeat in every way the behaviour we have seen in Urodele material. Unfortunately there has been little work by modern methods on other groups of primitive fish. Those types which have markedly telolecithal eggs and very unequal, but still total, cleavage would be particularly interesting as providing a transition to the higher forms, which are provided with very large masses of yolk and develop through the stage of a blastoderm. No vital stain experiments seem however to have been made on these transitional types. Ginsburg and Dettlaff (1944) have shortly reported a small number of experiments on the embryos of the sturgeon Acipenser. This has an egg, about 3 mm. across, which is rather amphibian-like in the cleavage and carly gastrulation stages. Grafts of the blastopore lip showed that it behaved as an organiser and could induce secondary neural folds in a host embryo. The donors from which the blastopore lip had been removed developed no nervous system or axial organs, and the authors suggest that the presumptive materials for these are confined to a smaller area than in amphibian eggs, although presumably located in a similar position.		Weissenberg (1934, 1936) has studied the {{gastrulation}} by means of vital stained marks, and has derived a map of presumptive areas (Fig. 11.1). It shows a considerable resemblance to that of the Amphibia except that the axial mesoderm is, from the beginning, more concentrated towards the dorsal plane. According to Weissenberg, indeed, the presumptive mesoderm does not extend right round the egg but on the ventral side the presumptive ectoderm comes into direct contact with the presumptive endoderm. This situation, which would constitute a considerable difference from that of the Amphibia, has been questioned by Pasteels (1940) who points out that in the early neural plate stage, when invagination is still proceeding around the blastopore, this structure is undoubtedly surrounded by a complete ring of mesoderm, some of which is being invaginated over the ventral lip at this time. Although Pasteels’ material was not sufficient to enable him to draw an alternative map of the presumptive areas in the early gastrula, he concludes that the mesoderm miust in fact extend right round the egg between the ectoderm and endoderm. He suggests that the main difference between the gastrulation of the lamprey and the frog is that, in the former, the invagination of mesoderm over the lateral and ventral lips is delayed so that it does not begin until the main mass of endodermal material has already passed into the interior. The two processes in fact differ only in this comparatively minor matter of timing. Some attention has also been paid to the causal embryology or epigenetics of these forms. It was shown as long ago as 1900 by Bataillon that if the first two blastomeres are separated cach may give a complete embryo. The experiment has been repeated more recently (Montalenti and Maccagno 1935; Bytinski-Salz 19372) and it has been shown that the situation is almost exactly the same as that revealed by constriction experiments in the Amphibia. Further, fragments from the dorsal lip of the blastopore can be grafted just as in newts (Bytinski-Salz 1937); Yamada 1938). When they come in contact with gastrula ectoderm they cause the induction of a secondary neural axis and, so far as experiment has gone, repeat in every way the behaviour we have seen in Urodele material. Unfortunately there has been little work by modern methods on other groups of primitive fish. Those types which have markedly telolecithal eggs and very unequal, but still total, cleavage would be particularly interesting as providing a transition to the higher forms, which are provided with very large masses of yolk and develop through the stage of a blastoderm. No vital stain experiments seem however to have been made on these transitional types. Ginsburg and Dettlaff (1944) have shortly reported a small number of experiments on the embryos of the sturgeon Acipenser. This has an egg, about 3 mm. across, which is rather amphibian-like in the cleavage and carly gastrulation stages. Grafts of the blastopore lip showed that it behaved as an organiser and could induce secondary neural folds in a host embryo. The donors from which the blastopore lip had been removed developed no nervous system or axial organs, and the authors suggest that the presumptive materials for these are confined to a smaller area than in amphibian eggs, although presumably located in a similar position.


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	FIGURE II.1		FIGURE II.1

	Map of presumptive areas in the young gastrula of the lamprey (~~cyclo stomes~~). Close dots, chorda; spaced dots, mesoderm (somitic mesoderm with horizontal lines); close vertical lines, neural plate; spaced vertical lines, epidermis; white endoderm. The position of the mesoderm on the ventral side is uncertain. (Modified from Weissenberg).		Map of presumptive areas in the young gastrula of the lamprey (cyclostomes). Close dots, chorda; spaced dots, mesoderm (somitic mesoderm with horizontal lines); close vertical lines, neural plate; spaced vertical lines, epidermis; white endoderm. The position of the mesoderm on the ventral side is uncertain. (Modified from Weissenberg).

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