The University of Alabama at Birmingham
Edit This Page

Morphogenesis

Morphogenesis is the creation of form during development.
  • This is accomplished at the cellular level whereby changes in cells lead to the formation of functional tissues and organs of the body.
  • Embryos have two major types of cells.
    • The epithelial cells form sheets or tubes through their tight interconnections.
    • The other cell type is referred to as mesenchymal cells.
        These cells are not connected to each other and operate as individual units.
There are several cellular processes that produce morphogenesis in these cell types.
    1) Direction and number of cell divisions

    2) Cell shape changes

    3) Cell movement or motility

    4) Cell growth

    5) Cell death

    6) Cell membrane or extracellular matrix changes

Mesenchymal and epithelial cells accomplish these processes in different ways.
    The different types of processes are shown as examples for mesenchymal and epithelial cells.
      • For instance, cell division is one of the processes carried out by these cells.
          Somatic cells undergo mitosis to produce more cells in all directions to produce hyperplasia as seen in mesenchymal cells or can occur in rows such as seen for epithelial cells in gastrulation.
      • Cell shape changes can also occur as shown for condensation of mesenchymal cells in cartilage or in neurulation in epithelial cells.
      • Cell movement involves the movement of cells at particular times and places such as in heart mesenchyme or intercalation or migration of epithelial cells in gastrulation and the chick endoderm.
          Epithelial cells may also disperse as seen in Mullerian duct degeneration.
      • Cell growth of mesenchymal cells also involves hypertrophy as in the example of fat cells or for epithelial cells in neurulation.
      • Interdigital mesenchyma is an example of the fifth process involving cell death of mesenchymal cells and delamination of the chick hypoblast is also a form of cell death.
      • The last process, matrix secretion and degradation occurs in cartilage mesenchyme or, for the case of epithelial cells, synthesis or removal of the extracellular layer in organ formation.

    Thus, the two main groups of cells in the embryo are involved in numerous cellular processes that contribute to the morphogenesis of the organism.

    Morphogenesis also involves communication between cells which occurs in two major ways:

      1) through diffusible substances such as hormones, growth factors, or morphogens

      2) surface contact between cells

    • In a later class we will discuss the role of hormones in development.
    • The second type of communication (surface contact) is the most important in morphogenesis.
        Here we see cells collectively reorganizing other cells as evidenced by the intercalation of epithelial cells in gastrulation.
    • They can also adhere to other cells as occurs in cartilage mesenchyme.
    • In addition, cells can migrate as evidenced by the dispersal of epithelial cells in Mullerian duct dispersal.
    Molecular changes at the cell surface mediate the selective recognition of cells and the formation of tissues and organs.
      This occurs through a process referred to as differential cellular affinity.
        This is illustrated in experiments fusing the endoderm and ectoderm from the amphibian blastula.
        • When these two different cell types are placed together in an artificial environment, they initially fuse but then separate until only a narrow strip of tissue joins the two cell types.
        • The sorting out of cells of different types is the result of cell movement and differential adhesiveness.
            In general, if the adhesion between like cells is greater than the adhesion between different cell types, the cells will segregate according to cell type.

            Thus, differential cell adhesion can stabilize the boundaries between tissues.

        Cellular affinities are further illustrated by disaggregating cells of the presumptive epidermis and neural plate of an amphibian neurula.
        • The cells are then mixed together allowing them to reaggregate.
        • The cells of the mixed cell mass exchange neighbors and move.
        • The effect is that the epidermal cells are eventually found on the outer surface of the aggregate with the neural cells on the interior.

        • Thus, like cells are in contact with each other.
        • This is also an example of differential cell adhesion.
        It is also interesting that the final positions of reaggregated cells reflect their embryonic positions.
        • The mesoderm, when mixed with the epidermal cells, is found in the middle.
        • The mesoderm also is in the middle when mixed with endodermal cells.
        • When the three germ cells are mixed together, the epidermis is on the outside and the endoderm is surrounded by the mesoderm.
        • After rearrangement, the ectoderm is on the surface, the endoderm is internal, and the mesoderm is between them.

        • This is brought about because the ectoderm has greater affinity for the mesoderm than the endoderm.
        • However, the mesoderm has affinity for the ectoderm and the endoderm.

        • Thus, for development to occur, cells must interact differently with other cell populations at specific times.
        • In the blastula, all cells have the same affinity for one another.
        • By the time gastrulation occurs, however, the cells acquire different cell affinities and massive reorganization occurs.
        Cells rearrange themselves into the most thermodynamically stable position.
        • This sorting out is brought about by differences in the adhesion strengths of different cells.
        • The very early embryo is considered in a state of equilibrium with similar genes expressed and the cell surface containing very similar adhesiveness.
          • However, as embryogenesis proceeds, different cells express different genes and many of these genes encode molecules which are deposited on the surface of the cell.
          • Movements of the cells begin and the tissues are rearranged in an ordered fashion.
        Several classes of adhesion molecules are important in development.
          The most important of these molecules are the cadherins.
          • About 30 different types of cadherins have been identified in vertebrates.
          • Cadherins bind to each other through one or more binding sites located within the extracellular amino-terminal 100 amino acids.
          • In general, cadherin binds only to another cadherin of the same type but they can also bind to some other molecules.
          • As cells approach and touch, the cadherins cluster at the point of contact.
          • Thus, sorting out occurs at the cell surface.
          • The amount of cadherin on the surface can also be important.
              This is illustrated by mixing experiments where two cells which contained different amount of the same cadherin on their surface were mixed.
                The cells containing the most cadherin formed on the inside of the mixture.
        Therefore the rearrangement of tissues during morphogenesis depends not only on the type of molecules that are expressed on their surface, but also the amount of these molecules present on their surface.

        Many of these rearrangements are important in the early development of the embryo.

Print This Page