Heredity and Development Second Edition (1972) / Chapter Skim
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12 Differentiation
12 Differentiation
Pages 256-278
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From page 256... ...
Cell division divides the fertilized ovum into the many cell types of the adult: eye cells, heart cells, kidney cells, nerve cells, and so on -- all traceable to an identical beginning. How can the fact of differentiation be made to conform to the fact of mitotic cell division, a process that seems to give daughter cells identical with each other and with the parent cell?
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From page 257... ...
There is no problem of differentiation for the preformationists: the adult structures are already differentiated at the beginning of embryonic life; development consists only of growth. The theory of preformation had one inter esting corollary.
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From page 258... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 258 12–2 (legend on facing page)
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From page 259... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 259 12–2 Four stages in the development of the chick (from Malpighi, ‘De Formatione pulli in ovo,' in Opera Omnia, Scott and Wells, London, 1686)
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From page 260... ...
In spite of an almost universal belief in preformation, there were always men of renown who thought that development was epigenetic. They believed that adult structures were absent from the early embryo, and that they made their appearance during the course of embryonic life.
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From page 261... ...
The plane of first cleavage cuts through the center of the gray crescent. Still later the dorsal lip of the blastopore forms in the center of the area where the gray crescent had been.
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From page 262... ...
The uninjured cell cleaved as though it were still part of the entire embryo. 12–3 Roux's theory of the segregation of determinants during cleavage.
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From page 263... ...
The uninjured cell formed only that part of the embryo it would have formed in an uninjured embryo. Roux interpreted this to mean that each cell was capable of forming only the structures it would in normal development.
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From page 264... ...
Formation of the Neural Tube in the Amphibian Embryo. If we examine the cells of an amphibian embryo in the late blastula stage we find that they are essentially the same in appearance throughout the entire embryo.
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From page 265... ...
They alone develop into the neural plate, while the remaining ectoderm cells produce the epidermal covering of the body. It is possible to trace the positions of the presumptive neural plate and the presumptive epidermis cells back to the early gastrula, as was done by Vogt (Chapter 11)
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From page 266... ...
Two alternative hypotheses to explain this phenomenon could be suggested: Hypothesis 1. The presumptive neural tube cells of an early gastrula possess an inherent capacity to form neural tissue.
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From page 267... ...
We must not be too quick to accept this conclusion, however. Perhaps the manipulation of the embryo injured the presumptive neural tube cells in such a way as to prevent them from differentiating into neural tissue.
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From page 268... ...
. Explants of presumptive epidermis form only epidermis but explants of presumptive neural tube cells differentiate 12–6 Explantation of presumptive epidermis and presumptive neural tissue in early (above)
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From page 269... ...
This is our second hypothesis, which was stated thus: ‘The presumptive neural plate cells of an early gastrula do not possess an inherent capacity to form neural tissue. Influences from outside the presumptive neural plate area are necessary for differentiation.' One deduction that we might make from this second hypothesis is: The neural tube should form in the same position, relative to the non ectodermal parts of the embryo, no matter how the presumptive ectoderm is oriented.
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From page 270... ...
In normal development these cells form the roof of the archenteron, which is immediately beneath the cells that will form the neural tube. If we regard this position as significant, we might restate the hypothesis more precisely: ‘The presumptive neural plate cells of an early gastrula do not possess an inherent capacity to form neural tissue.
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From page 271... ...
The roof of the archenteron then induces the overlying ectoderm cells to form a neural tube. We now have the theoretical basis to interpret the results of the experiments on explanting pieces of presumptive neural tube cells.
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From page 272... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 272 12–8 Dorsal Lip Transplantation experiment of Spemann and Mangold. a is a diagram of the operation.
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From page 273... ...
The only inherent ability they possess is to form simple epidermal cells. The ability to form a neural tube results from the induction of the presumptive neural tube cells by the organizer in the archenteron roof.
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From page 274... ...
. The optic cup area is in the presumptive neural plate, while the lens area is in the presumptive epidermis.
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From page 275... ...
The period during which this response is possible is very short. At or about the stage when the neural folds close, the presumptive epidermis will no longer respond to the archenteron-roof organizer.
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From page 276... ...
If presumptive epidermis of a frog embryo is transplanted to the region behind the mouth in a salamander embryo, the transplant forms mucus glands. The embryo shown in Figure 12–9 is the result of an operation of this sort.
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From page 277... ...
This brief survey has revealed some of the factors responsible for differentiation but embryology has not reached the point where we can say that we know in detail why an embryonic cell differentiates in a specific way. We know some of the answers and when we know more we should be able to answer a question of vital interest: ‘Why do normal cells sometimes change into cancerous cells?
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