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CHAPTER 11 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.
Transiently amplifying cells
Transiently amplifying cells undergo proliferative cell mitosis and produce cells that are equally determined. At some stage, and as a result of an inductive stimulus, these cells will enter a quantal cycle that culminates in a quantal mitosis. This will result in an increase in the restriction of their progeny, which continue to undergo proliferative mitoses at a progressive level of determination. The quantal mitosis corresponds to the time of binary choice when the commitment of the progeny is different from that of the parent.
Progenitor cells
Progenitor cells are already determined along a particular pathway. They may individually follow that differentiation pathway, or may proliferate and produce large numbers of similarly determined progenitor cells that subsequently differentiate; neuroblasts or myoblasts are examples of progenitor cells.
Stem cells
Either individually or as a population, stem cells can both produce determined progeny and reproduce themselves. It is generally believed that stem cells undergo asymmetric divisions, in which one daughter cell remains as a stem cell, while the other proceeds along a differentiation pathway; in marked contrast, proliferative cell division may be symmetrical, producing derived cells with an identical determination. Human embryonic stem cells (hESC) are pluripotent cells that can be derived from the inner cell mass of human blastocysts in vitro; or obtained surplus to in vitro fertilization fertility programmes; or created from oocytes donated and fertilized for that purpose. Although not yet achieved, it is hoped that hESCs can be coaxed down particular pathways under appropriate pharmaceutical conditions to produce differentiated cells that will be effective in reversing some degenerative diseases (e.g. dopamine producing neurones for Parkinson’s disease, insulin producing islet cells for diabetes), or to replace acutely damaged tissues (e.g. motor neurones for acute spinal cord injury; cardiomyocytes in acute myocardial infarction). Proof of principle has been demonstrated in some animal models, and multipotent haematopoietic progenitor stem cells from human umbilical cord blood are now used as an alternative treatment to bone marrow transplantation for the treatment of some inherited genetic disorders (thalassaemia) and blood malignancies (leukaemias).
Terminally differentiated cells
By virtue of their extreme specialization, terminally differentiated cells can no longer divide, e.g. erythrocytes and neurones. Apoptosis is a particular variety of terminal differentiation in which the final outcome is the death of the individual cells or cell populations. It occurs in the developing limb, where cells die along the pre- and postaxial limits of the apical ectodermal ridge, limiting its extent, and also between the digits, permitting their separation.
There are two types of cell and tissue interaction, namely, permissive and instructive.
Grays Anatomy The Anatomical Basis of Clinical Practice
