June 30, 2014

Which came first, the primordial germ cell or the gamete? Unlike the old chicken or egg philosophical dilemma, we know for certain that the primordial germ cell came first. And, thanks to a recent paper about primordial germ cells in sea urchins, we now know that they can migrate across the urchin embryo.

During development, germ cells produce gametes (eggs or sperm). In many organisms, including mammals, primordial germ cells (PGCs) are born far from the eventual location of gametes and must migrate across the embryo while dividing. In sea urchins, small cells called micromeres are PGCs and precisely segregate along the left-right axis of the embryo. A recent paper by Campanale and colleagues describes the use of live-cell imaging of small micromeres in urchin embryos to test whether the precise segregation of these eight micromeres is due to passive translocation or active migration. Images show that the micromeres are, in fact, motile cells with features such as cortical blebs and filopodia that direct migration across the sea urchin embryo, towards the coelomic pouches. In the images above, sea urchin embryos express micromere (red) and apical membrane (green) markers before (left) and during (middle, right) gastrulation.

Campanale, J., Gökirmak, T., Espinoza, J., Oulhen, N., Wessel, G., & Hamdoun, A. (2014). Migration of sea urchin primordial germ cells Developmental Dynamics, 243 (7), 917-927 DOI: 10.1002/dvdy.24133

September 20, 2012

Once an egg finds its dance partner, the incredible changes that take place have fascinated biologists for centuries. These changes are kicked off immediately, and thanks to modern microscopy we get to see them up close. Today’s image is from a paper describing actin-mediated changes at the membrane of an egg shortly after fertilization.

Shortly after fertilization, actin assembly and dynamics drive changes in membrane physiology in the egg. One of these changes is an increase in efflux from the egg, or transport out of the egg, at the same time that short actin-based protrusions called microvilli are assembled around the egg. A recent paper investigates this link between efflux and microvilli after fertilization of sea urchin eggs. Whalen and colleagues found that a protein called ABCB1a translocates to the tips of microvilli shortly after fertilization. ABCB1a is an ABC transporter, a family transmembrane proteins that drive movement of various material into and out of many cell types. By using structured illumination microscopy (3D-SIM), Whalen and colleagues captured high-resolution images of actin filaments shortly after fertilization, and found movement of ABCB1a along growing actin filaments. Both efflux activity and movement of ABCB1a to microvilli tips were inhibited after actin polymerization was blocked. The images above show actin filaments (green) and ABCB1a (red) at different time points shortly after fertilization. By 60 minutes after fertilization, ABCB1a moved from below microvilli to the tips of the protrusions. 

Kristen Whalen, Adam M. Reitzel, & Amro Hamdoun (2012). Actin polymerization controls the activation of multidrug efflux at fertilization by translocation and fine-scale positioning of ABCB1 on microvilli Molecular Biology of the Cell, 23 (18), 3663-3672 DOI: 10.1091/mbc.E12-06-0438

June 20, 2011

As you all vacation this summer at the beach, throw a “Thank you!” out to the sea urchins for their amazing contributions to cell and developmental biology research. Today’s image is from a paper showing a role for a major polarity protein in cilia formation in sea urchin embryos.

After a sea urchin’s initial stages of embryogenesis, it hatches out of its eggshell to become a swimming resident of the marine world. It swims using cilia, which beat to push water around. The cilia on this swimming embryo reside in the outer layer of epithelial cells, and emanate from basal bodies just below each cell’s surface. A recent paper shows a role for the polarity protein aPKC (atypical protein kinase C) in the formation of cilia in the sea urchin embryo. aPKC is a member of a protein complex important in asymmetric cell divisions, and Prulière and colleagues found that it has a very asymmetric localization during ciliogenesis. Images above show cilia (red) of sea urchin embryos in the absence (top left) or presence of an aPKC inhibitor. As the levels of the inhibitor increases (concentrations are indicated), the length of cilia decreases.

Pruliere, G., Cosson, J., Chevalier, S., Sardet, C., & Chenevert, J. (2011). Atypical protein kinase C controls sea urchin ciliogenesis Molecular Biology of the Cell, 22 (12), 2042-2053 DOI: 10.1091/mbc.E10-10-0844