1. MIGRATION

Migration (from Lat. migratio) is the active passage of cells from one site, e.g. site of their origin, into another place. This process is distinct from passive movement of .e.g. erythrocytes in the blood stream.

Examples of cell migration:

  • Cells of the intraembryonic mesoderm migrate from the primitive pit and groove between the layers of epiblast and hypoblast and for ma trilaminar embryonic disc.
  • Migration of neural crest cells in embryonic jaws → odontoblasts etc. (c. migrating in a new microenvironment are exposed to new inducing signals, which generate a new phenotype.
  • Primordial germ cells (primitive sex cells) migrate from the wall of the yolk sac, yolk stalk and allantois along the dorsal mesentery of the hindgut to the coelomic epithelium of the gonadal ridges.
  • Two successive migrations of neuroblasts during the development of the telencephalon:
    1) Cells from the ventricular zone migrate into the intemediate zone;
    2) Neuroblasts migrate from the intemediate zone into the marginal zone to give rise to the cortical plate.
  • Cells of ventromedial parts of somites migrate to the notochord to form the sclerotome.
Infiltration of the tissue by migratory cells is a determining condition for further successful development of a particular organ. Interruption of migration leads to malformations (polymicrogyria - ectopic neurons lying in the white matter). Migratory cells compete for reaching their destination (competition). Unsuccessful cells are eliminated by apoptosis.
After cells reach their destination, they lose the ability to migrate.
The process of migration is strictly controlled by variety of different mechanisms.

Cell migration is mediated by lots of factors:

  • Matrix glycoproteins in the embryonic body help to define cell migration pathways.
  • Chemoatractans – e.g. vascularization of organs is regulated with VEGF
  • Radial glia fibers (processes) guide neuroblasts migrating from the intermediate zone to the marginal zone.
  • Adhesion molecules - eg. astrotactin of migrating neuroblasts form the migration junction with the ligand expressed in radial glia; Endothelial cells enable transit of leukocytes from the blood stream into lymphatic organs.
  • Movement of migrating cells is performed by contractile cytoplasmic proteins (e.g. actin).
  • Proteinases make the passage through extracellular matrix possible.
Migration of synapses, neuritis is regulated with attractants and repelents (diffusible and membrane-bound molecules).
migrace.swf

Fig. 1. Formation of cortical plate. Primordium of the neocortex arises within the marginal zone after imigration of neuroblasts from the adjacetnt intermediate zone. Migration occurs in well-defined temporal intervals; the first neuroblasts enter the area of the future sixth (multiform) layer and their accumulation gives rise to the primordium ofthe cortical plate. Other migrating cells enter a layer above, i.e. the 5th layer (internal pyramidal layer). Subsequent migrations give rise to the 4th layer, after is formed the 3rd layer and then the 2nd layer. Finaly neuroblasts migrate into the remaining free area at the edge of the marginal zone - this way the first layer is created (molecular layer) that contains the youngests neuroblasts. Each time when migrating neurobalsts have to penetrate a previous layer the come in a close contact that is necessayr for creation of neuronal circuits between neurons of different layers. The esteblshed circuits form the basis of synaptoarchitectonics of cortical coluns.
m.l.e., membrana elastica externa, outer elastic membrane; m.l.i., membrana elastica interna, inner elastic membrane.