Embryonic Organogenesis and Body Formation in Amphibian Development
The fertilized egg is a single cell that forms an individual organism according to a built-in developmental program. During development, an organizer is produced in part of the gastrula. The organizer is the “center of the form” and triggers a progression
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Embryonic Organogenesis and Body Formation in Amphibian Development M. Asashima 1,2, A. Sogame 1, T. Ariizumi3, and T. Igarashi 1
15.1 Introduction
The fertilized egg is a single cell that forms an individual organism according to a built-in developmental program. During development, an organizer is produced in part of the gastrula. The organizer is the "center of the form" and triggers a progression of inductive signals to control the formation of various organs in predetermined places and the development of an overall morphology specific to each species. Over time, various phenomena follow, such as changes in the number and type of cells (cellular differentiation), morphogenetic movements, interactions between tissues, and embryonic induction. These processes advance the developmental program towards the goal of forming an integrated, individual organism. Life scientists today attempt to understand this ontogeny at the level of molecules (from a physical and chemical viewpoint). In recent years, the fields of organ engineering and regenerative medicine have advanced markedly through the use of embryonic stem (ES) cells and other mammalian stem cells. ES cells are pluripotent, that is, they have the potential to differentiate into multiple cell types (e.g. hemopoietic stem cells, myocardial cells, neural stem cells, osteoclasts, and insulin-secreting cells) depending on the culture conditions. This potential, however, is confined to cell differentiation and does not seem to extend to the differentiation of organs (e.g. heart, brain, and pancreas) that have three-dimensional shapes and functions. In this chapter, we describe how amphibian early embryos contain multipotent cell groups (animal caps) that can differentiate into organs. It is now possible to generate various mesoderm tissues (blood cells, muscle and notochord), central nervous system organs (brain and spinal cord), sensory organs (eye and ear vesicle) and other organs (heart, kidney, pancreas, etc.) in vitro (Table 15.1, Fig. 15.8). Since studies using mammalian stem cells have been described in
1 Department of Life Sciences (Biology), Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902, Japan
2 SORST, Japan Science and Technology Corporation OST), The University of Tokyo, Tokyo, Japan 3 Department of Experimental Nursing, Faculty of Nursing, Fukuoka Prefectural University, 4395 Ita, Tagawa-shi, Fukuoka 825-8585, Japan
H. Grunz (ed.), The Vertebrate Organizer © Springer-Verlag Berlin Heidelberg 2004
M. Asashima, A. Sogame, T. Ariizumi, and T. Igarashi
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Table 15.1. Tissues and organs induced from animal caps by activin and other factors in vitro. Tissues and organs
Inducers
Note
Ectodermal
Forebrain+eye
Con A (1 mg/ml)
Cotreatment of activin or RA with Con A results in the induction of posterior neural tissues.
Hindbrain+ear
Con A (1 mg/mll +RA (10-6Ml
Hindbrain and ear are also induced by Con A (1 mg/mll and 0.5 ng/mi of activin.
Spinal cord
Con A (1 mg/mll +activin (1 ng/mll
B
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