Dynamic Development
CONTENTS

Main Page Dynamic Development

The Foundations of Developmental Biology

Gametogenesis

From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity


Dynamic Development at a Glance


Learning Resources

Research Resources

The Developmental Biology Journal Club

Developmental Biology Tutorial

Cell Motility

How do cells move?

Cell motility has been studied extensively in cultured fibroblasts. Few embryonic cells are as amenable to investigation as are cells in culture. The deep cells of the fish blastula are an exception. The fish embryo is transparent, and the behaviors of individual cells can be tracked. They have been studied most extensively by Dr. J.P. Trinkaus. These studies have revealed that the fibroblast model provides a good approximation of embryonic cell motility.

Cell motility is generated by contractile elements of the cytoskeleton. It is unfortunate that the term "cytoskeleton" implies rigidity, because the cytoskeleton is dynamic.

Microfilaments play key roles in generation of motility. Microfilaments are organized in one of two ways in fibroblasts. Microfilaments form a meshwork in the leading edge (lamellipodium), whereas transient bundles of microfilaments (stress bundles) extend along the length of cells.

(See Browder et al., 1991, Figs. 9.17, 9.18A; Gilbert, 1997, Fig. 3.29)

The bundles of microfilaments are attached to focal contacts, which anchor the bundles to the plasma membrane. Focal contacts facilitate attachment of the cell to the substratum, and allow cells to exert tension on the substratum, which is necessary for cell migration along the substratum.

(See Browder et al., 1991, Fig. 9.8; Shostak, 1991, Fig. 20.6; Wolpert et al., Box 8B)

There are three major processes in fibroblast motility (Alberts et al., 1994; see Fig. 16-80):

Protrusion of lamellipodia or filopodia, which is thought to be due to actin polymerization at the leading edge. Other possible mechanisms for generating the force of extension include a local increase in osmotic pressure, an increase in hydrostatic pressure caused by contraction of the cortex elsewhere in the cell or, alternatively, myosin-I motors attached to the plasma membrane that would drive the cell forward by walking along the actin filaments.

(See Browder et al., Fig. 9.5; Gilbert, 1997, Fig. 3.29a; Kalthoff, 1996, Figs. 2.18, 2.19, 2.20; Wolpert et al., Fig. 8.33)

Formation of focal contacts
, which anchor the lamellipodium in its new position, permitting net forward progress.

Retraction of the trailing edge. This retraction involves both contraction of the cytoskeleton and passive recoil due to the elasticity of the cytoskeleton and plasma membrane.

(See Browder et al., Fig. 9.6)

The main molecules that mediate cell anchorage to the substratum are members of a family of transmembrane linker proteins known as integrins. Integrins traverse the cell membrane, anchoring the actin microfilaments on the inside and binding to the fibronectin of the extracellular matrix.

(See Alberts et al., 1994, Fig. 16-75; Gilbert, 1997, Fig. 3.29b)

Linkage of integrin to the microfilaments is indirect and involves a number of attachment proteins. The cytoplasmic domain of integrin binds to talin, which binds to vinculin, which binds to alpha-actinin, which binds to actin filaments. Each integrin molecule is a dimer of alpha and beta subunits. There is a diversity of alpha and beta integrins (over 24 members of the integrin family are known); different combinations of alpha and beta subunits provide different cell types with different affinities for the extracellular matrix. For example, alpha2 beta1 binds to collagen and laminin, whereas alpha4 beta1 binds to fibronectin (Gilbert, 1997).

The significance of integrins during development is demonstrated by mutations of genes encoding integrins in Drosophila, which either cause developmental abnormalities or death. A number of correlations have been drawn between changes in the expression of integrins and developmental processes. For example, a single layer of keratinocytes gives rise to mature human epidermal tissue through a process of stratification, which is associated with changes is the types and locations of integrin expression. During muscle development, the fusion of individual myoblasts to form myotubes is a key step in the differentiation process. Fusion is correlated with a change in the expression of 1 integrins on the surface of the cells. (For a review of integrins in development, see Adams and Watt, 1993.)

One of the most prevalent components of the extracellular matrix is fibronectin. All known cellular receptors for fibronectin belong to the integrin family. The significance of the fibronectin/integrin interaction is exemplified by knockouts of the genes encoding either fibronectin or two fibronectin receptors (integrins alpha4 and alpha5) in mice, which cause developmental defects that are severe enough to cause embryonic lethality (George et al., 1993; Yang et al., 1993, 1995). Fibronectin molecules are dimers having domains with distinct binding properties. One domain binds to collagen, one binds to heparin and another binds to cell surfaces. The cell surface binding domain is characterized by the presence of an essential Arg-Gly-Asp (RGD) tripeptide. The RGD sequence is so critical for binding that short peptides having the RGD sequence will compete with fibronectin for binding and inhibit the attachment of cells to a fibronectin matrix. The RGD sequence is also found in other extracellular matrix (ECM) proteins. The RGD sequence is recognized by members of the integrin family, although it is not sufficient to account for the specificity of binding between integrins and ECM molecules; other regions of the molecule must provide that specificity.

(See Alberts et al., Fig. 19-51)

In addition to their roles in fibroblasts, integrins are expressed in epithelial cells. In fact, virtually every cell type of multicellular organisms expresses integrins, mediating their interactions with the extracellular matrix. Integrins not only play a structural role; they are also involved in communicating signals from the extracellular matrix into the cell. The clustering of integrins at the sites of contact with the matrix can activate several intracellular signaling pathways. The signals that the integrins initiate appear to influence such cell properties as differentiation, proliferation, survival, and gene expression (Sheppard, 1996).

The cytoplasmic domains of the integrin subunits have no intrinsic enzymatic activity. Thus, they do not function like kinase enzymes that pass a signal on to a receptor molecule by phosphorylating it. Instead, they function by initiating the assembly of a functional signaling complex that contains catalytic signaling proteins, such as protein kinases (Clark and Brugge, 1995).

Control of Directional Migration

To study this topic, you should go to The Molecular Basis of Migratory Specificity in Zygote.


Learning Objectives

  • What are focal contacts?
  • What is thought to be responsible for protrusion of lamellipodia?
  • What are the functions of integrins?
  • What are the properties of integrins?
  • How has the significance of integrins been demonstrated in Drosophila melanogaster?
  • Discuss the interactions between integrins and fibronectin and describe an experiment to demonstrate the significance of these interactions.
  • Define the following phenomena and give an example of each phenomenon:
    chemotaxis
    haptotaxis
    galvanotaxis
    contact guidance
    contact inhibition of movement

Digging Deeper:

Link to Related Material

Dynamics of Thin Filopodia

Recent Literature

Chen, C.S., Mrsich, M., Huang, S., Whitesides, G.M. and Ingber, D.E. 1997. Geometric control of cell life and death. Science 276: 1425-1428..

Ruoslahti, E. 1997. Stretching is good for a cell. Science 276: 1345-1346.

Wallingford, J.B., Sater, A.K., Uzman, J.A. and Danilchik, M.V. 1997. Inhibition of morphogenetic movement during Xenopus gastrulation by injected sulfatase: implications for anteroposterior and dorsoventral axis formation. Develop. Biol. 187: 224-235.

Zipkin, I.D., Kindt, R.M. and Kenyon, C.J. 1997. Role of a new Rho family member in cell migration and axon guidance in C. elegans. Cell 90: 883-894.


References

Adams, J.C. and Watt, F.M. 1993. Regulation of development and differentiation by the extracellular matrix. Development 117: 1183-1198.

Alberts, B. et al. 1994. Molecular Biology of the Cell. Third edition. Garland. New York.

Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third edition. Saunders College Pub. Philadelphia.

Clark, E.A. and J.S. Brugge. 1995. Integrins and signal transduction pathways: the road taken. Science 268: 233-239.

George, E.L., Georges-Labouesse, E.N., Patel-King, R.S., Rayburn, H. and Hynes, R.O. 1993. Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin. Development 119: 1079-1091.

Gilbert, S.F. 1997. Developmental Biology. Fifth edition. Sinauer. Sunderland, MA.

Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New York.

Sheppard, D. Epithelial integrins. BioEssays 18: 655-660.

Shostak, S. 1991. Embryology. An Introduction to Developmental Biology. HarperCollins. New York.

Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E. 1998. Principles of Development. Current Biology. London.

Yang, J.T., Rayburn, H. and Hynes, R.O. 1993. Embryonic mesodermal defects in alpha 5 integrin-deficient mice. Development 119: 1093-1105.

Yang, J.T., Rayburn, H. and Hynes, R.O. 1995. Cell adhesion events mediated by alpha 4 integrins are essential in placental and cardiac development. Development 121: 549-560.


Dynamic Development at a Glance
Main Page Dynamic Development

Dynamic Development is a Virtual Embryo learning resource

This material may be reproduced for educational purposes only provided credit is given to the original source.
Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Monday, August 4, 1998