A method for TEM visualization of the extracellular matrix three-dimensional organization in tissues

Portrayal of the Notch system in embryonic stem cell-derived embryoid bodies

We portrayed the Notch system in embryonic stem cell (ESC)-derived embryoid bodies (EBs) differentiating under the standard protocols used to assess yolk sac (YS) hematopoiesis in vitro. Notch receptors and Notch ligands were detected in virtually all cells throughout EB development. Notch 1 and Notch 2, but not Notch 4, were visualized in the nucleus of EB cells, and all these receptors were also observed as patent cytoplasmic foci. Notch ligands (Delta-like 1 and 4, Jagged 1 and 2) were immunodetected mostly as cytoplasmic foci. Widespread Notch 1 activation was evident at days 2-4 of EB differentiation, the time window of hemangioblast generation in this in vitro system. EBs experienced major spatial remodeling beyond culture day 4, the time point coincident with the transition between primitive and multilineage waves of YS hematopoiesis in vitro. At day 6, where definitive YS hematopoiesis is established in EBs, these exhibit an immature densely packed cellular region (DCR) surrounded by a territory of mesodermal-like cells and an outer layer of endodermal cells. Immunolabeling of Notch receptors and ligands was usually higher in the DCR. Our results show that Notch system components are continuously and abundantly expressed in the multicellular environments arising in differentiating EBs. In such an active Notch system, receptors and ligands do not accumulate extensively at the cell surface but instead localize at cytoplasmic foci, an observation that fits current knowledge on endocytic modulation of Notch signaling. Our data thus suggest that Notch may function as a territorial modulator during early development, where it may eventually influence YS hematopoiesis.

Cell migration strategies in 3-D extracellular matrix: differences in morphology, cell matrix interactions, and integrin function

Cell migration in extracellular matrix is a complex process of adhesion and deadhesion events combined with cellular strategies to overcome the biophysical resistance imposed by three-dimensionally interconnected matrix ligands. Using a 3-D collagen matrix migration model in combination with computer-assisted cell tracking for reconstruction of migration paths and confocal microscopy, we investigated molecular principles governing cell-matrix interactions and migration of different cell types. Highly invasive MV3 melanoma cells and fibroblasts are large and highly polarized cells migrating at low speed (0.1-0.5 microm/min) and at high directional persistence. MV3 melanoma cells utilize adhesive migration strategies as characterized by high beta1 integrin surface expression, beta1 integrin clustering at interactions with matrix fibers, and beta1 integrin-mediated adhesion for force generation and migration. In contrast, T lymphocytes and dendritic cells are highly mobile cells of lower beta1 integrin expression migrating at 10- to 40-fold higher velocities, and directionally unpredictable path profiles. This migration occurs in the absence of focal adhesions and largely independent of beta1 integrin-mediated adhesion. Whereas cell-matrix interactions of migrating tumor cells result in traction and reorientation of collagen fibers, partial matrix degradation, and pore formation, leukocytes form transient and short-lived interactions with the collagen lacking structural proteolysis and matrix remodeling. In conclusion, the 3-D extracellular matrix provides a spatially complex and biomechanically demanding substrate for cell migration, thereby differing from cell migration across planar ligands. Highly adhesive and integrin-dependent migration strategies detected in morphologically large and slowly migrating cells may result in reorganization of the extracellular matrix, whereas leukocytes favor largely integrin-independent, rapid, and flexible migration strategies lacking typical focal adhesions and structural matrix remodeling.

Rat kidney glomerular basement membrane visualized in situ by embedment-free sectioning and subsequent platinum-carbon replication

Following the removal of polyethylene glycol (PEG) from thin sections, and viewing through the endothelial fenestrae and/or the interpedicel spaces, the rat renal glomerular basement membrane in situ was revealed to consist of meshworks and to be electron-transparent when examined at right angles to the plane of the membrane. By subsequent platinum replication of the embedment-free sections, the lamina densa of the basement membrane appeared as a veil composed of rather closely packed particles. The architecture of the split diaphragm and the surface morphology of the endothelial cell membrane were also clearly revealed. The present results indicate that the PEG method, with or without replication, can provide valuable information on basement membrane morphology.