Embryonic induction and cell division

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CHAPTER 11 Embryonic induction and cell division

EMBRYONIC INDUCTION AND CELL DIVISION

Cell populations within the embryo interact to provide the developmental integration and fine control necessary to achieve tissue-specific morphogenesis. In the early embryo, such interactions may occur only if particular regions of the embryo are present, e.g. signalling centres or organizers. As the embryo matures, some interactions tend to occur between adjacent cell populations, e.g. epithelium and mesenchyme, and later between adjacent differentiating tissues, e.g. between nerves and muscle, or between muscle and skeletal elements. The interactions between adjacent epithelia and underlying connective tissue continue throughout embryonic and fetal life and extend into postnatal life. In the adult, these interactions also permit the metaplastic changes that tissues can undergo in response to local environmental conditions.

Tissue interactions result in changes or reorganization of one or both tissues, which would not have occurred in the absence of the tissue interactions. The process of tissue interaction is also called induction, i.e. one tissue is said to induce another. The ability of a tissue to respond to inductive signals is called competence, and denotes the ability of a cell population to develop in response to the environments present in the embryo at that particular stage. After a cell population has been induced to develop along a certain pathway, it will lose competence and become restricted. Once restricted, cells are set on a particular pathway of development; after a number of binary choices (further restrictions) they are said to be determined. Determined cells are programmed to follow a process of development that will lead to differentiation. The determined state is a heritable characteristic of cells, and is the final step in restriction. Once a cell has become determined, it will progress to a differentiated phenotype if the environmental factors are suitable.

The process of determination and differentiation within embryonic cell populations is reflected by the ability of these populations to produce specific proteins. Primary proteins (colloquially termed housekeeping proteins) are considered essential for cellular metabolism, whereas proteins synthesized as cells become determined, those specific to the state of determination, are termed secondary proteins; for example, liver and kidney cells, but not muscle cells, produce arginase. Fully differentiated cells produce tertiary proteins, which no other cell line can synthesize, e.g. haemoglobin in erythrocytes.

As populations of cells become progressively determined, they can be described within a hierarchy of cellular development as transiently amplifying cells, progenitor cells, stem cells and terminally differentiated cells.