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背景知識補充...
THE MORPHOREGULATOR HYPOTHESIS
We can now sketch a hypothesis framed to answer these questions. As originally proposed, it was called the regulator hypothesis and was formulated mainly around CAMs. It will be extended here to cover all morphoregulatory molecules and in recognition of this will be called the morphoregulator hypothesis. I will first give an overview of its major tenets taking heed of the summary just presented, and then describe the salient details.
We may conclude from our summary that there are four main tasks in formulating an adequate hypothesis. The first is to account for the epigenetic component of morphogenesis in terms of cellular processes. The second is to identify those regulatory and structural genes that are chiefly responsible for the genetic component of pattern formation and morphogenesis. The third is to link the epigenetic and genetic components in order to provide a signal path both to these regulatory genes and to the regulatory genes governing histodifferentiation. These three tasks are the concern of this chapter. The fourth task is to explain how particular combinations and responses of two kinds of regulatory genes in a phenotype can give rise during evolution to a particular animal form in a species. This will be considered in the next chapter which discusses the evolutionary question.
According to the morphoregulator hypothesis the epigenetic component of morphogenesis comes from the three driving force processes that assure the presence of cells in a sufficient number--cells that are capable of movement in certain states and that are subject to selective removal by death. All of these processes-cell division, movement and death--depend upon the action or removal of mechanochemical factors and, in addition to their particular detailed consequences at a particular place and time, their exercise has mechanical consequences that alter form at the scale of both cells and tissues.
Regulation of these driving force processes comes from the processes of adhesion and induction. According to the hypothesis, the major genetic aspects of morphogenesis result from the interaction of these two regulatory processes, as mediated by gene products we have called morphoregulatory molecules. Under cell control morphoregulatory molecules are capable of promoting cell-cell adhesion, causing epithelial-mesenchymal transformation, and providing substrate pathways for cell movement.
The morphoregulator hypothesis (see table 8.1 for a brief summary of its assumptions and proposals in a slightly different form) states the following.
(1) The essential genetic component of early pattern formation in morphogenesis comes from the existence and temporal response to inductive signals of those regulatory genes and structural genes that determine the appearance and function of morphoregulatory molecules. The epigenetic component comes from the topobiological response of cells to these molecules. Tissue differentiation, by contrast results from the action of historegulatory genes, the sequences of which are under the control of selector genes (table 8.1). In general morphoregulatory genes act independently of historegulatory and selector genes.
(2) The link between the epigenetic and the genetic components of development is in the inductive signal paths arising from adhesion-dominated collectives giving rise to signals that go directly back from these collectives to genes governing morphoregulatory molecules. Embryonic induction proceeds by signaling from groups (or collectives) of cells to other groups of cells not via single cells. The key suggestion is that the signals do not arise until at least one such collective out of a pair of mutually inducing collectives is actually formed under the action of morphoregulatory molecules and their genes. Histogenesis occurs within this framework as a result of similarly conditioned signals to historegulatory genes-those controlling the structural genes responsible for individual cell metabolism and for the structure, shape, and movement of particular cells. Historegulatory genes are essential to the life of the organism and govern all primary processes except adhesion but do not directly govern any products yielding cell-cell or cell-substrate binding. Nonetheless, the control of these genes by a set of selector genes akin to homeotic genes determines pathways of histogenetic expression. We will consider these selector genes later.
(3) The place-dependent coordinate activity of these three kind of genes ─ morphoregulatory, historegulatory, and selector genes ─ leads to overall from and pattern. This activity depends on a sequence of epigenetic events leading to the formation of inducing collectives. Such a dependence therefore results in a temporal sequence of cycles of gene expression for at least some morphoregulatory molecules, as is reflected, for example, in CAM rules and SAM modulatory networks. As we will see in the next chapter,the particular combinations of the types and the response activities of morphoregulatory and historegulatory genes lead to form in a given species because those combinations have led to morphologies that enhance fitness. The bounds of morphoregulatory pattern are therefore determined by evolution, not by development.
These statements together constitute the morphoregulator hypothesis but do not give a detailed mechanistic account of linked genetic and epigenetic expression. We may now embody these ideas in several very specific models based on the CAMs; as we will indicate later similarly appropriate models can be constructed for SAMs and CJMs. Examining such models should help relate the morphoregulator hypothesis more tightly to the detailed facts that form its basis. |
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