Moss - What genes cant do - 2003 (522929), страница 31
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Taxawithin an embranchment varied according to the extent of differentiation of the organs. A more differentiated organ developmentally passedthrough all the prior stages of less differentiated organs. Von Baerdepicted the process of embryological development within an embranchment as a series of radiations from out of a central node of greatestpotential. Each path of differentiation leads through new nodes thatalso constitutes Stamm of Keime und Anlagen, but a smaller Stamm.Such a developmental node might be common to the embryology ofa whole taxonomic subgroup (e.g., a “class”) of organisms.
From outof this node then would be several paths of greater differentiation leadingto nodes which are common to smaller taxonomic categories (“orders,”then “families,” and so on (see a detailed discussion of this in Chapter1). The kinship of members of an embranchment can be empiricallyverified on the basis of this model. If two species are members ofa common order then their pattern of development should be seen tocoincide up to that point which represents the Kieme und Analgen ofthat order.
Subsequently, their developmental patterns should be seento diverge. The nature of the divergence—that is, why some organsfurther differentiate and others do not, should be explicable in terms offunctional requirements—the specific adaptative needs of their mode oflife.What determines the degree of differentiation is the functional contextof the whole organism such that the level of differentiation of one organwill be related to the developmental degree of differentiation of othersets of organ Anlagen.
Unlike the transcendentalists who would grouptaxa along the continuum of a single organ, von Baer would expectcertain organs of one taxa to be more differentiated and others less differentiated than that of a related taxa, depending on the nature of thefunctional adaptations of the respective organisms. Von Baer characterized the vertebrate type by five fundamental organs out of which all otherorgans were understood to be derived.Dialectics of Disorder: Normalization and Pathology as Process123Each and every organ is a modified part of a more general organ, and in thisrespect we might say that each organ is already contained in all its specificity inthe fundamental organ . .
. The respiratory apparatus is a further developmentof an originally small part of the mucous tube (Lenoir 1982, chap 2).The fundamental vertebrate organs, in turn, are derived from the germlayers. Building on Pander’s idea of germ layers, von Baer distinguishedtwo primary divisions of the germ: an upper which he called animal anda lower which he called the vegetative, or mucous, layer. From the upperlayer come the sensory organs, skeleton, muscle, and nerves. From thelower come the mesentary organs and digestive and vascular tissue.
Amiddle, or mesoderm, germ layer was first identified by Müller’s studentRemak in 1850 (Lenoir 1982).Johannes Müller met Heinrich Rathke and von Baer for the firsttime in 1828 at the Versammlung Deutscher Naturforscher und Årtzte,organized by Alexander von Humboldt in Berlin. Von Baer was askedto demonstrate the existence of the mammalian ovum for them(described in chapter 1). Müller reported that he was deeply impressedby this demonstration. This experience and the discussion with vonBaer and Rathke led, upon Müller’s return, to his own work on theWolfian body, which was published in 1830 and dedicated to Rathke.Müller’s seminal work on the human urogenitial system constituted oneof clearest examples of the power a “teleomechanistic” developmentalmorphology.Müller, after his own early brush with the Naturphilosophen, becamea lifelong opponent of both transcendentalism and mechanistic reductionism.
He was ever wary of any investigative methodology that disrupted the unity of the organism. Müller adopted from von Baer the viewthat the members of a common type hold the same fund of structuralelements, with differences in organ formation being a function of different grades of differentiation along a common pathway. Anotherclosely related assumption was that any claim of a causal relationbetween an earlier and later structure had to be established by a sequenceof observable structural transformations capable of being shown to bematerially interconnected with one another. This doctine was exemplified by his elucidation of the developmental pathway of the female urogenital system described in chapter 1.124Chapter 4Müller published two papers on tumor microscopy as early as 1836.Developmental morphology already provided the basis for a tumorhistology, but it took the achievements of cell theory to provide for thebeginnings of a truly modern cancer biology.
The formation of cell theoryby Müller’s students Schleiden and Schwann began in 1838, and its quickappropriation by Müller for cancer theory must also be understood inthe context of the teleomechanist outlook.Although Müller referred to “cells” in his 1836 papers, he intendedlittle more by this word than it had denoted since the time of classicalmedicine, that is, not a living entity but merely a potential space as, forexample, in reference to the cells of a honeycomb. Such was also thesense of Robert Hooke, who looked microscopically at both organicand inorganic specimens, finding cellular arrangements in both sets ofcases (Rather, Rather et al. 1986).
While the notion of a cell as a livingentity doesn’t come on the scene until early 19th century, the concept offibers as basic constituents of organic tissue dates back to Aristotle andGalen.Galen posited three basic fiber types. His categorization largely heldsway until Giorgio Baglivi (1668–1707) examined macerated animaltissue under the microscope and settled on two basic fiber types (Rather,Rather et al. 1986). Hermann Boerhaave (1668–1738) pushed fibertheory in the direction of a single hollow structure deemed to be filledwith a tenuous fluid or spirit.
Fibers construed as containers can be seenas a kind of middle ground between classical fibers and the idea of cells.It is worth noting that although both simple and compound microscopeshad been in use since the middle of the seventeenth century, they did nothave much influence on what was perceived. Unlike the case of astronomy where the telescope largely expanded the range of a familiar objectdomain, in biology microscopes opened up whole new worlds and thuscrises of interpretation.
The analysis of that which appeared de novounder the microscope awaited new criteria for which objects shouldbe counted as real and important and which should be ignored asephemeral or artifactual (Rather 1978). New findings were met withmuch skepticism. Some of the so-called globules reported by early microscopists indeed were likely to have been bona fide cells.The first clear move in the direction of modern cell theory was madeby Johann Christian Reil, whose 1795 essay on the Lebenskraft (life-Dialectics of Disorder: Normalization and Pathology as Process125force) postulated the crystallization of fibers into a living form based onthe presence of a nucleating germ (Rather, Rather et al.
1986). Reil, whohad also studied in Göttingen between 1779 and 1789, overlapping withBlumenbach, had conducted his own appropriation of Kantian ideas forbiology. Reil pursued a theory of a Lebenskraft based on the idea ofchemical affinities. The specificity of a Lebenskraft, which was for Reilthe basis of the specificity of a species, was constituted by the arrangement of chemical affinities in the Kern or Stamm, an arrangement whichwas passed from one generation to the next by way of the egg (Lenoir1982). The idea of an egg as the repository of a developmental potential embedded in the organization of germinal particles went on to playa crucial role in the conceptualization of cell theory.As recounted in chapter 1, modern cell theory proper began inMüller’s laboratory in the 1830s with Schlieden’s work on plant cells,quickly followed by Schwann’s extension of cell theory to animalcells.
Within months of Schwann’s generalized exposition of animal celltheory in 1838 and with the aid of the newly invented achromaticmicroscope, Müller reported finding cellular structure in a pathologicalgrowth in which he had failed to see it before. The cell theory providedMüller with a powerful basis for the classification of pathologies usingthe analysis of cell type and tissue development.
With cartilage as hisexemplar, Müller found that the hallmark of pathology is an arrested oraberrant path of differentiation, in both cases an expression of cellularpotential which has become separated from the principle of the wholeorganism.The differences between pathological and healthy cartilaginous developmentsconsists principally in the continuation of embryonic cell formation. In numerous other tumors the same observation can be easily made. It is not the form ofthe elementary parts that distinguishes diseased structures.
The problem lies inpart in the formation of normally primitive structures where they are not necessary and do not contribute to the purposiveness of the whole, and particularlyin the incomplete development of these tissues, which usually only reach a particular stage of development that is transient in healthy life. This is the mode ofoperation of diseased vegetative life. In the development of sound primitive cartilage, however, the monadic life of the cells is controlled by the Lebensprincipof the entire individual; it reaches its limit, the cells coagulate and the interstitial, unclear fibrous mass of the cartilage emerges between the cavities of thegerminal cells. In the Enchondroma on the other hand the regulated life of thepart no longer attains a particular limit, and it slowly continues to increase in126Chapter 4size. The cell walls in this case do not thicken normally; the cartilage remains inits embryonic condition and this embryonic structure is continually repeated.Über die krankhaften Geschwülste (in Lenoir 1982, pp.
144–145).With the conceptualization of the cell as the now more specified repository of the Keime und Anlagen, development and cancer emerge as ineffect complementary possibilities in a story about the relationship ofmonadic parts to that organismic whole that is constituted by them. Inthis view, far removed from classic notions of humoral-based inflammations, we can see a model of cancer recognizable in its antecedentrelationship to strains of current thought. It was through Müller’s teleomechanistically informed reflections upon the relationship of parts towhole, in which the parts possess the potential of the whole, yet a potential whose realization is mediated by the interaction of the parts underthe auspices of the whole, that such a model takes shape.
Against thepractices of more simple-minded empirical contemporaries, RudophVirchow, student of Müller and heir to cellular pathology, criticizedthe practice of ontologizing pathological structures. He suggested thatMuller’s greatest contribution was that of understanding the “law of theidentity of embryonal and pathological development” and its corollarythat histopathological lesions of “pathological products” should not beconsidered as “given, ontologically complete things but merely as tissuesin stages of development” (Rather 1978).It was not until the 1850s that Virchow and a fellow former Müllerstudent named Remak could definitively rule out the possibility that cellsarise not only from other cells but also from acellular structures—“cytoblastema.” Having established his famous dicta that Omnis cellula acellula (cells come only from cells), Virchow could then proceed to classify all tissues in cellular terms.
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