October 9, 2014

As Tom and Jerry have proven time and time again, repulsive forces are serious business and highly entertaining. Today’s image is from a paper describing how different cell types repel one another to help create boundaries between tissues. 

The study of how cells adhere to or repel one another is an important field of study in developmental biology. Ephrin ligands and their respective Eph receptors trigger repulsive cues between cells of different types. Many different tissue types express the same ephrins and Eph receptors, yet only those cells at the tissue interface repel one another. A recent study tests how these signals are integrated to provide repulsion at only the tissue interface, and not between cells of the same tissue. Rohani and colleagues used the dorsal ectoderm-mesoderm boundary of early frog embryos to find Eph-ephrin pairs that are expressed in complementary tissues. The cells at the boundary of the tissues have a combined Eph-ephrin repulsive signal that is sufficient for a repulsive force, suggesting a simple model of repulsion based on relative concentrations and binding affinities of Eph receptors and ephrins at tissue boundaries. The image above shows the higher concentration of EphB receptors (green) at the ectoderm-mesoderm boundary.

Rohani, N., Parmeggiani, A., Winklbauer, R., & Fagotto, F. (2014). Variable Combinations of Specific Ephrin Ligand/Eph Receptor Pairs Control Embryonic Tissue Separation PLoS Biology, 12 (9) DOI: 10.1371/journal.pbio.1001955

February 19, 2013

No matter how long you’ve been with your partner, sometimes he or she reveals a hidden talent that you’re just amazed to witness for the first time.  Maybe it’s his or her plate-spinning routine, amazing juggling, or a surprise skill at carving flowers out of radishes.  One day I will surprise my husband with my own hidden talent, once I find it, and he will wonder if it’s normal for one person to love another so much.  Today’s image is from a paper showing microtubules branching from existing microtubules…branching!  Is it normal for one person to love microtubules so much?!

Microtubules are dynamic filaments required in countless cellular processes, so their nucleation and growth has always been a point of interest for many biologists.  Microtubule nucleation from centrosomes is the best understood mechanism for microtubule nucleation, yet centrosomes are not always necessary for microtubule growth within a mitotic spindle.  A recent paper shows direct evidence for microtubule nucleation from existing microtubules in meiotic frog extracts, through the use of TIRF microscopy.  Petry and colleagues show that these new daughter microtubules nucleate from existing microtubules at a low “branch” angle.  In addition, daughter microtubules have the same polarity as mother microtubules, which can help maintain mitotic spindle integrity.   Branching microtubule nucleation requires γ-tubuin and augmin, a protein that increases microtubule density.  RanGTP, which is required for chromatin-mediated microtubule nucleation, and its effector protein TPX2 both stimulate branching microtubule nucleation.  In the images above, microtubules branch from existing microtubules after the addition of both RanGTP and TPX2, resulting in fan-like microtubule structures.  Lower panel is an enlarged view of the area marked with the asterisk.  Long arrows point to daughter microtubules nucleating at a clear branched angle, while short arrows point to daughter microtubules growing along the length of mother microtubules.

BONUS!!  Here's a mesmerizing movie of branching microtubules after the addition of RanGTP and TPX2, similar to the above image.

Petry, S., Groen, A., Ishihara, K., Mitchison, T., & Vale, R. (2013). Branching Microtubule Nucleation in Xenopus Egg Extracts Mediated by Augmin and TPX2 Cell, 152 (4), 768-777 DOI: 10.1016/j.cell.2012.12.044 
Copyright ©2013 Elsevier Ltd. All rights reserved.

April 26, 2012

One of my favorite analogies in cell biology revolves around cupcakes and asymmetric cell division, which happen to be two of the most wonderful things in the world. If you cut a cupcake in half down the center, you have two equal pieces with both icing and cake. Or, you can cut the cupcake in half across the center, and have one piece with just icing and one piece with just cake. Today’s image is from a paper describing how a cell divides to result into two equal icing-and-cake cells.

During development, the orientation of cell division within an epithelial sheet helps to drive tissue shape changes. Symmetric cell division, during which the mitotic spindle is parallel to the plane of the sheet, leads to tissue growth and elongation, while asymmetric division, during which the spindle is perpendicular to the epithelial sheet, causes tissue thickening and stratification. Most research on mitotic spindle orientation has focused on asymmetric cell division, but a recent paper describes interesting results on how a spindle is positioned during symmetric division. Woolner and Papalopulu looked at epithelial tissue in early frog embryos to test possible mechanisms of spindle positioning in symmetric cell divisions. As seen in the image above (left), the spindle is positioned precisely in the plane of the epithelial sheet. Woolner and Papalopulu found that a basally-directed force (pushing down, into the sheet) is provided by microtubules and myosin-10, while an apically-directed force is provided by actin filaments and myosin-2. Both of these forces are required to position the spindle in the plane of the epithelium, and at its proper position along the apical-basal axis. In the middle image above, the spindle is positioned near the apical cell surface after astral microtubules were disrupted. After actin-filament disruption (right image), spindles moved toward the basal cell surface.

Woolner, S., & Papalopulu, N. (2012). Spindle Position in Symmetric Cell Divisions during Epiboly Is Controlled by Opposing and Dynamic Apicobasal Forces Developmental Cell, 22 (4), 775-787 DOI: 10.1016/j.devcel.2012.01.002

Copyright ©2012 Elsevier Ltd. All rights reserved.

 

August 29, 2011

It’s easy to be overwhelmed when learning about all that goes on during development. Then, you learn about the regularity and geometry seen throughout development (and biology in general) and that anxiety is washed away. I love seeing regular patterns and shapes throughout the biological world, and today’s image is a nice example of precision during development.

During development of a frog’s skin, there are two layers of cells. The top layer is made of cells that secrete mucus, and the bottom layer is made of cells that have cilia, or small hair-like projections. During skin development, these two layers intercalate and the resulting organization of mucus-secreting and cilia-containing cells is precise and predictable. A recent paper identified a role for a receptor protein called dystroglycan (Dg for short) in skin development. Dg is a critical receptor of basement membranes, which are regions that underlie tissues to provide signals and mechanical support. According to Sirour and colleagues, Dg gene expression is found in the bottom layer of cells in the developing skin of frog embryos. Loss of Dg function results in the disruption of skin development, specifically cell intercalation, cell-cell adhesion, and organization of the underlying matrix. As seen in the images above, the skin of control frog embryos have a precise flower-like arrangement of cells—a cilia-containing cell is surrounded by mucus-secreting cells (top). When Dg levels were reduced (bottom), that organization is disrupted.

Sirour, C., Hidalgo, M., Bello, V., Buisson, N., Darribere, T., & Moreau, N. (2011). Dystroglycan is involved in skin morphogenesis downstream of the Notch signaling pathway Molecular Biology of the Cell, 22 (16), 2957-2969 DOI: 10.1091/mbc.E11-01-0074

April 5, 2010

Morphogenesis is the arrangement of cells into tissue during development, and often requires complex coordination of several different cellular processes. A recent paper shows the coordination of endocytosis, the uptake of material into a cell, with contraction of the actin-myosin network, in order to drive cell shape changes. Image is of bottle cells in the embryo of the frog, Xenopus laevis, with microtubules in green and newly internalized vesicles in red.

Reference: Jen-Yi Lee and Richard M. Harland. Current Biology 20, 253-258. ©2010 Elsevier Ltd All rights reserved. Paper can be found here.