10.5Induction and Morphogenetic Movement
Induction is a phenomenon that strongly influences the fate of adjacent cells and tissues through its action on them. Induction occurs throughout the course of embryonic development.
When induction (which is found in various phases of the developmental process) is considered at the cellular level, a number of common mechanisms are found. The three main methods by which one cell influences and changes the fate of other cells are via secretory substances, molecules on the plasma membrane surface, and gap junctions (Fig. 10-8). Whichever method is taken, one cell can influence many adjacent cells.
In triploblastic animals, whose bodies consist of the three basic layers of endoderm, mesoderm and ectoderm (including many types of animal from planarians to humans), the embryo is roughly divided into three regions in the early stages of development. This determines the fate of each region. By way of example, the respiratory and digestive organs are formed from the endoderm, muscle and connective tissues are formed from the mesoderm, and the central nervous system and skin are formed from the ectoderm. Induction plays a role in the differentiation into the three germ layers. This mechanism is discussed below using mesoderm induction in frogs as an example.
In mesoderm induction, the mesoderm is formed in a certain area of the embryo (the equatorial region) as a result of influence from various other parts of the embryo (Fig. 10-9). This induction is caused by many types of substance secreted from the organizer on the dorsal side, the animal pole, the vegetal pole and ventral side. Through their interaction, the mesoderm region is induced in the intermediate area of the embryo (the equatorial region).
Fig.10-8. Induction in adjacent cells
A) There are three methods of causing induction in adjacent cells: a) influence via secretory substances such as growth factors and hormones; b) influence through cell adhesion via proteins and carbohydrate chains that exist on the surface of the plasma membrane; and c) influence from the formation of gap junctions between the cell and adjacent cells.
B) Methods of transmitting induction to surrounding adjacent cells: a) through the diffusion of secretory substances; b) by extending cell protrusions and attaching them to surrounding cells; and c) intercellular transmission via gap junctions.
→ indicates the direction of induction.
Fig.10-9. Mesoderm induction in frogs
Mesoderm induction in frogs is caused by the action of inducers secreted from the surrounding area.
→ indicates the direction of induction from each of the four regions. As a result of the combined induction effects, the mesoderm region is induced in the central area.
After the determination of the rough arrangement of embryonic regions - such as the prospective mesoderm and ectoderm areas - following the formation of the organizer (the center of embryogenesis), major morphogenetic movement is seen. First, the formation of three germ layers and morphogenetic movement occurs to create archenterons, which later become the gastrointestinal tract. Second, further morphogenetic movement occurs to create the neural tube. The distribution of embryonic cells is rearranged through these morphogenetic movements, thereby allowing the interaction of embryonic cells that previously existed separately.
Mesoderm formation, gastrulation and neurulation all take place through a number of basic cell movements (Fig. 10-10). Since the early embryo is made of epithelial tissue (see Chapter 11), morphogenetic movement during this stage mainly involves the movement and deformation of epithelial cells. This includes the bending movement caused by contraction on one side of epithelial cells, invagination movement in which bent epithelial tissues extend inward in the embryo, locomotive movement caused by cells separated from epithelial cells, and extension movement caused by the flattening and rearrangement of epithelial cells.
Among these movement types, one that plays a particularly important role in subsequent organogenesis is the large-scale movement of mesodermal cells. This movement in vertebrates is so great that the cells move to the opposite side of the embryo. Taking the chicken embryo as an example, cells that break away from the epithelium of the ectoderm enter the embryo, form mesodermal cells and move to the sides and the anterior part between the ectoderm and endoderm (Fig. 10-11). The prospective heart mesoderm regions, which later become the heart, are located at the tip of the moving mesoderm on both sides of the embryo.
Fig. 10-10. Morphogenetic movement
A) Morphogenetic movement occurring in the early embryo. This takes place mainly through the movement and deformation of the epithelial cells that constitute the embryo, and includes: a) the movement of cells that break away (or migrate) from epithelial tissues; b) the bending or invagination movement of the epithelium; and c) the extension movement caused by the rearrangement or flattening of epithelial cells.
B) Examples of morphogenetic movement in sea-urchin and chicken embryos: a) bending and invagination movement in the sea-urchin embryo, and b) locomotion of mesodermal cells in the chicken embryo. In chicken embryos, cells that separate from the central part of the ectoderm move inwardly in the embryo and become mesodermal cells, which then move in the direction of the arrows between the ectoderm and endoderm. The part from which cells break away appears as a streak, which is referred to as a primitive streak.
Fig.10-11. Movement of mesodermal cells in the chicken embryo
A schematic diagram showing the movement of mesodermal cells, seen from the dorsal side (the ectoderm is not shown). The prospective heart mesoderm region is located at the tip of the moving mesoderm. → indicates the direction of movement of mesodermal cells.
After mesoderm induction, neural induction occurs. By this process, the neural tube (which later becomes the brain and the spinal cord) is created from the ectoderm. Neural induction is an important event in the development of animals, since the neural tube plays the central role in the construction of the body and forms the brain and the spinal cord - the most important components in the bodies of animals.
Neural induction is brought about by the organizer and the mesoderm. First, the organizer sends induction signals to the ectoderm. Then, induction signals are sent from the mesoderm, which has moved below the ectoderm, to the ectoderm itself. Both induction effects are exerted by substances secreted from cells. In response to this induction, which destines the ectoderm to become nerve tissue, the neural tube - the origin of the brain and the spinal cord - is formed from the ectoderm. The brain is created from the anterior part of the tube, and the spinal cord is created from the posterior part (Fig. 10-12).
Fig. 10-12. Neural induction in a frog embryo
Neural induction exerted by the mesoderm to the ectoderm is shown here. Inducers are secreted from the anterior and posterior parts of the mesoderm that have moved inside the embryo, which respectively induce the formation of the brain and the spinal cord from the ectoderm.