Book - An experimental analysis of the origin of blood and vascular endothelium (1915) 1: Difference between revisions
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==Introduction== | |||
The origin of blood presents an almost unique problem in embryologj'. First, on account of the fact that the initial blood anlage in many animals is contributed to by wandering cells. Second, owing to the establishment of an early flow or circulation of embryonic fluids before the blood corpuscles have arisen. | |||
Soon after the cells and corpuscles are formed they are swept into this circulating current and carried to all 'parts of the body. In this way the blood cells become associated and mixed with numerous other types of cells, and it is difficult, if not impossible, to estabhsh their true relationship mth their surroundings. For the above reasons one is often ready to believe that many of the even careful and long thought out contributions to the development of blood are, after all, largely a matter of the author's own interpretation rather than a record of the actual processes. | |||
The general current of opinion at the present time would seem to indicate that all blood cells arise from a mesenchymal tj'pe of cell. A number of very competent workers have described the change of tliis mesenchymal cell into a stem cell or mother cell. On one side from this mother cell are developed various leucocytes, which it is important to note always occur in an interstitial position, while on the other hand, this same general type of mother cell gives rise to other cells which later differentiate into tjT^ical erythroblasts, and finally eiythrocytes which are always found to be located within the vessels. These socalled indifferent mesenchymal cells probably, from the evidence contained in the Uterature, do form blood cells, but to the discriminating reader the evidence is not at all convincing that both white blood cells and red blood cells really arise from one common mother cell or common embryonic anlage. | |||
The possibihty, and even probability, is certainly present that these so-called stem mother cells may in reahty not all belong to one type, but are different and may already be destined to form either red cells or white cells. Yet on account of their wandering capacities as well as on account of the fact that the earliest blood cells are swept around in the circulating current they have become so mixed and confused that it is almost impossible to separate the cell groups or differentiate between them. One must in this connection, remember the fact that almost all authors have concluded that only red blood cells are formed in the blood islands of the yolk-sac in most vertebrates. All authors who have studied the development of the blood in Teleosts have invariably described only red blood cells as arising in the intermediate cell mass. No one has ever mentioned the presence of white blood cells in the true blood forming anlage of the Teleosts. | |||
The monophyletic school really goes still further and not only claims that all tjrpes of blood cells arise from a common mother mesenchymal cell, but also that the vascular endothehal cell is hkewise capable of giving rise to the various types of blood cells and is originally a cell of the same type as the stem mother cell. There are numerous descriptions and illustrations of the origin of blood cells from the vessel hnings in the literature of the past twenty-five years, since Schmidt in 1892 described the transformation of individual endothelial cells into white and red blood corpuscles. Yet again, I beheve that the really skeptical reader will not be at all convinced that such a thing ever takes place from the evidence presented in the literature, certainly not from any of the illustrations that have been made of this process. | |||
No real vascular endothehal cell has been actually observed to metamorphose into a blood cell or to divide off another cell which forms a blood cell, and until such a dhect observation is forthcoming one can only question the accm'acj^ of the interpretation of the various observations up to now recorded. | |||
a. | The mesenchyme is a very generalized embryonic tissue and from it arise the various kinds of blood cells, endothehal cells, connective tissue cells, etc. There can be no doubt of the great genetic difference between blood cells and connective tissue cells, yet their parent cells are with our present methods indistinguishable. We may wdth equal justification go further and hold Hkewise that the cells from which the vascular endothelium, red blood cells and white blood cells arise are mesenchymal cells really differing in nature according to whether they will give rise to one or the other of the three cell types. Yet they may not differ from one another in smy way by which we can at present distinguish them. If this proposition be true, or even if the weight of evidence lean in this direction, it is scarcely more justifiable to derive these completely different cells from a common mother cell than it would be to derive connective tissue and blood cells from a common mother cell. | ||
Of course, we are only considering the mesenchymal cell just before its differentiation is to begin. Carried back further, no doubt, all the cells become more and more ahke and possess more and more complex potentiahties as is so thoroughly demonstrated by the numerous studies of cell lineage. In the beginning, of coiuse, all cells arose from one single egg cell capable of giving rise to every tissue of the body, but after tendencies in differentiation have proceeded sufficiently far in the various cells some then form real mesenchymal cells. Later individual mesenchymal cells incline in certain directions and finally become incapable of giving rise to any other than the definite type of tissue or cells towards which their particular tendencies have directed just as certain endodermal cells become specialized to form the liver while others near by and at first indistinguishable from these give rise to the ducts and acini of the pancreas. | |||
All of the vertebrate classes present these many questions of blood origin, etc., but the forms upon which this investigation has been conducted, the Teleosts, possess in addition many extremely interesting special problems. In all other meroblastic embryos the majority of the earliest blood cells arise in yolk-sac blood islands. Yet in many of the Teleosts there are apparently no early blood islands on the yolk, but all of the blood forming cells are contained within the embryonic body. | |||
This intra-embryonal blood anlage has been frequently described by many authors as the "intermediate cell mass." The intermediate cell mass as has been suggested by Marcus ('05), Mollier ('06), and others, is really the homologue of the blood forming yolk-sac mesoderm in the other meroblastic types. | |||
The bony fish is important as an object of study on account of the fact that so many of its organs and tissues arise in a way pecuhar to the group and differing from the other vertebrate classes. The sohd gastrular invagination described by Sumner ('00), the original solid condition of the central nervous system, the solid optic knob which changes into the optic vesicle, and in the present connection, the very particularly interesting solid cord of cells, the intermediate cell mass, which is to give rise to the red blood corpuscles of the individual make the Teleosts a group of great embryological interest. | |||
The complexity of the problem concerning the origin of the various types of blood cells is then largely due to the migration and mixture of the cells involved. It is strange that up to now no investigator has attempted in an expermiental way to analyze the situation. It would seem to be one of the most favorable problems for an experimental analysis, and in the end it is certainly an analytical problem. | |||
If it were possible by any means to separate the anlage of the red blood cells from that of the white blood cells and prevent the flow of fluid in the embryonic body so that these cells would not frequently become intermixed, then it would seem possible to determine clearly the entire genesis of the various type ceUs. If all the types of blood corpuscles did arise from a common mother mesenchymal cell they should then be found in intimate association throughout all blood forming regions. Further, if the vascular endothelium really has blood forming power, it should be found that blood cells arise in any region of the embryo which possesses vessels lined by such endothehmn. | |||
There have been various experiments performed which have interfered more or less with the circulation of the body fluids of the embryo, but none of these experiments where aimed at a solution of the genesis of blood cells or have been used for such a purpose. Knower ('07) removed the heart anlage from early frog embryos and the}^ continued to develop in some cases with almost no circulation. In other specimens there was a very feeble sluggish circulation due to the pulsation of the lymph hearts or of remnants of the heart which remained after the operation. The embryos were not particularly adapted for the study of the blood questions since some circulation always took place, and this no doubt was sufficient to contaminate the original sources of blood cells and so confuse the situation. Loeb ('12) has reported experiments on bony fish hybrids and embr^^os treated with certain chemicals in which there was a heart beat but no circulation. These embryos were, however, not studied for either blood or vascular genesis. | |||
The first demonstration of the fact that the embryo could develop without the circulation of the blood was given by Loeb in 1893. He showed that Fundulus eggs developing in solutions of KCl had no heart beat and no circulation of the blood, yet some vessels formed. In 1906 the writer repeated this experiment and confirmed Loeb's results entirely, but found that the vascular system and general development of the embryo was extremely abnormal and was hardly reliable for conclusive studies on the origin of special tissues. | |||
With these experiments in mind, and appreciating the problems indicated above regarding the origin of blood as weU as vascular endothelium, I have undertaken an extensive experimental analysis of this subject in conjunction wdth a careful systematic study of the histogenesis of the blood and vessels in normal embryos. The results of the experimental study which has been carefully followed during the past three years are presented in the following pages of this paper. | |||
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Stockard CR. An experimental analysis of the origin of blood and vascular endothelium. (1915) Memoirs of the Wistar Institute of Anatomy and Biology No. 7.
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I. The Origin of Blood and Vascular Endothelium in Embryos Without a Circulation of the Blood and in the Normal Embryo
Contents
I. Introduction
II. Methods of experiment and material
III. The study of living embryos with and without the circulation of the blood
1. Normal development up to the establishment of a circulation
2. History of experimental embryos to the time when circulation should begin
3. Early formation of blood cells in living embryos
a. Intra-embryonic blood cells
b. Yolk-sac blood islands
4. The five-day embryos
5. The eight- and ten-day embryos
6. Condition of the heart in old embryos without a circulation
7. Development of the yolk-sac blood islands in life
IV. The origin and histogenesis of vascular endothelium and blood corpuscles as determined by study of microscopic sections
1. The structure of the heart in embryos without a circulation
2. The intermediate cell mass; its origin, position and significance as an intra-embryonic blood anlage
3. Blood islands of the yolk-sac, their origin and development
4. Fate of ihe blood corpuscles in embryos without a circulation
5. Has vascular endothelium haematopeotic power?
6. The origin of lymphocytes and leucocytes or so-called white blood corpuscles
7. Environmental conditions necessary for blood cell multiplication and differentiation
8. Question of haematopoetic organs?
V. A consideration of the experimental study on the origin of blood in Teleosts in relation to the more recent studies on the origin and development of vessels and blood cells
1. Introduction
2. The specific problems of blood and vessel formation in the bony fish
3. Vascular endothelium, and vascular growth and development
4. Haematopoesis, the monophyletic and polyphyletic views, etc
VI. Summary and conclusions
Literature cited
Introduction
The origin of blood presents an almost unique problem in embryologj'. First, on account of the fact that the initial blood anlage in many animals is contributed to by wandering cells. Second, owing to the establishment of an early flow or circulation of embryonic fluids before the blood corpuscles have arisen.
Soon after the cells and corpuscles are formed they are swept into this circulating current and carried to all 'parts of the body. In this way the blood cells become associated and mixed with numerous other types of cells, and it is difficult, if not impossible, to estabhsh their true relationship mth their surroundings. For the above reasons one is often ready to believe that many of the even careful and long thought out contributions to the development of blood are, after all, largely a matter of the author's own interpretation rather than a record of the actual processes.
The general current of opinion at the present time would seem to indicate that all blood cells arise from a mesenchymal tj'pe of cell. A number of very competent workers have described the change of tliis mesenchymal cell into a stem cell or mother cell. On one side from this mother cell are developed various leucocytes, which it is important to note always occur in an interstitial position, while on the other hand, this same general type of mother cell gives rise to other cells which later differentiate into tjT^ical erythroblasts, and finally eiythrocytes which are always found to be located within the vessels. These socalled indifferent mesenchymal cells probably, from the evidence contained in the Uterature, do form blood cells, but to the discriminating reader the evidence is not at all convincing that both white blood cells and red blood cells really arise from one common mother cell or common embryonic anlage.
The possibihty, and even probability, is certainly present that these so-called stem mother cells may in reahty not all belong to one type, but are different and may already be destined to form either red cells or white cells. Yet on account of their wandering capacities as well as on account of the fact that the earliest blood cells are swept around in the circulating current they have become so mixed and confused that it is almost impossible to separate the cell groups or differentiate between them. One must in this connection, remember the fact that almost all authors have concluded that only red blood cells are formed in the blood islands of the yolk-sac in most vertebrates. All authors who have studied the development of the blood in Teleosts have invariably described only red blood cells as arising in the intermediate cell mass. No one has ever mentioned the presence of white blood cells in the true blood forming anlage of the Teleosts.
The monophyletic school really goes still further and not only claims that all tjrpes of blood cells arise from a common mother mesenchymal cell, but also that the vascular endothehal cell is hkewise capable of giving rise to the various types of blood cells and is originally a cell of the same type as the stem mother cell. There are numerous descriptions and illustrations of the origin of blood cells from the vessel hnings in the literature of the past twenty-five years, since Schmidt in 1892 described the transformation of individual endothelial cells into white and red blood corpuscles. Yet again, I beheve that the really skeptical reader will not be at all convinced that such a thing ever takes place from the evidence presented in the literature, certainly not from any of the illustrations that have been made of this process.
No real vascular endothehal cell has been actually observed to metamorphose into a blood cell or to divide off another cell which forms a blood cell, and until such a dhect observation is forthcoming one can only question the accm'acj^ of the interpretation of the various observations up to now recorded.
The mesenchyme is a very generalized embryonic tissue and from it arise the various kinds of blood cells, endothehal cells, connective tissue cells, etc. There can be no doubt of the great genetic difference between blood cells and connective tissue cells, yet their parent cells are with our present methods indistinguishable. We may wdth equal justification go further and hold Hkewise that the cells from which the vascular endothelium, red blood cells and white blood cells arise are mesenchymal cells really differing in nature according to whether they will give rise to one or the other of the three cell types. Yet they may not differ from one another in smy way by which we can at present distinguish them. If this proposition be true, or even if the weight of evidence lean in this direction, it is scarcely more justifiable to derive these completely different cells from a common mother cell than it would be to derive connective tissue and blood cells from a common mother cell.
Of course, we are only considering the mesenchymal cell just before its differentiation is to begin. Carried back further, no doubt, all the cells become more and more ahke and possess more and more complex potentiahties as is so thoroughly demonstrated by the numerous studies of cell lineage. In the beginning, of coiuse, all cells arose from one single egg cell capable of giving rise to every tissue of the body, but after tendencies in differentiation have proceeded sufficiently far in the various cells some then form real mesenchymal cells. Later individual mesenchymal cells incline in certain directions and finally become incapable of giving rise to any other than the definite type of tissue or cells towards which their particular tendencies have directed just as certain endodermal cells become specialized to form the liver while others near by and at first indistinguishable from these give rise to the ducts and acini of the pancreas.
All of the vertebrate classes present these many questions of blood origin, etc., but the forms upon which this investigation has been conducted, the Teleosts, possess in addition many extremely interesting special problems. In all other meroblastic embryos the majority of the earliest blood cells arise in yolk-sac blood islands. Yet in many of the Teleosts there are apparently no early blood islands on the yolk, but all of the blood forming cells are contained within the embryonic body.
This intra-embryonal blood anlage has been frequently described by many authors as the "intermediate cell mass." The intermediate cell mass as has been suggested by Marcus ('05), Mollier ('06), and others, is really the homologue of the blood forming yolk-sac mesoderm in the other meroblastic types.
The bony fish is important as an object of study on account of the fact that so many of its organs and tissues arise in a way pecuhar to the group and differing from the other vertebrate classes. The sohd gastrular invagination described by Sumner ('00), the original solid condition of the central nervous system, the solid optic knob which changes into the optic vesicle, and in the present connection, the very particularly interesting solid cord of cells, the intermediate cell mass, which is to give rise to the red blood corpuscles of the individual make the Teleosts a group of great embryological interest.
The complexity of the problem concerning the origin of the various types of blood cells is then largely due to the migration and mixture of the cells involved. It is strange that up to now no investigator has attempted in an expermiental way to analyze the situation. It would seem to be one of the most favorable problems for an experimental analysis, and in the end it is certainly an analytical problem.
If it were possible by any means to separate the anlage of the red blood cells from that of the white blood cells and prevent the flow of fluid in the embryonic body so that these cells would not frequently become intermixed, then it would seem possible to determine clearly the entire genesis of the various type ceUs. If all the types of blood corpuscles did arise from a common mother mesenchymal cell they should then be found in intimate association throughout all blood forming regions. Further, if the vascular endothelium really has blood forming power, it should be found that blood cells arise in any region of the embryo which possesses vessels lined by such endothehmn.
There have been various experiments performed which have interfered more or less with the circulation of the body fluids of the embryo, but none of these experiments where aimed at a solution of the genesis of blood cells or have been used for such a purpose. Knower ('07) removed the heart anlage from early frog embryos and the}^ continued to develop in some cases with almost no circulation. In other specimens there was a very feeble sluggish circulation due to the pulsation of the lymph hearts or of remnants of the heart which remained after the operation. The embryos were not particularly adapted for the study of the blood questions since some circulation always took place, and this no doubt was sufficient to contaminate the original sources of blood cells and so confuse the situation. Loeb ('12) has reported experiments on bony fish hybrids and embr^^os treated with certain chemicals in which there was a heart beat but no circulation. These embryos were, however, not studied for either blood or vascular genesis.
The first demonstration of the fact that the embryo could develop without the circulation of the blood was given by Loeb in 1893. He showed that Fundulus eggs developing in solutions of KCl had no heart beat and no circulation of the blood, yet some vessels formed. In 1906 the writer repeated this experiment and confirmed Loeb's results entirely, but found that the vascular system and general development of the embryo was extremely abnormal and was hardly reliable for conclusive studies on the origin of special tissues.
With these experiments in mind, and appreciating the problems indicated above regarding the origin of blood as weU as vascular endothelium, I have undertaken an extensive experimental analysis of this subject in conjunction wdth a careful systematic study of the histogenesis of the blood and vessels in normal embryos. The results of the experimental study which has been carefully followed during the past three years are presented in the following pages of this paper.
