UC Berkeley

The cellular and molecular mechanisms generating the diversity of animal morphologies are still a relatively little explored subject within developmental biology. While changes in cell number help with the growth of tissue, the orientation of cell divisions, migrations, cell rearrangements or changes in cell shape can explain how anisotropies in tissue shape are formed during development. It remains unclear whether the current set of described behaviors can account for all of the diverse morphologies we see in extant metazoan species.

To better understand the mechanisms underlying tissue morphogenesis, my Ph.D. dissertation focused on the development of the ellipsoid egg in Drosophila melanogaster as a new, emerging model system to study tissue elongation. Developing egg chambers (follicles) in Drosophila originate as a sphere and initially grow isotropically but elongate during oogenesis to form an ellipsoid egg. The cellular and molecular mechanisms underlying follicle elongation were not known, although genetic evidence suggested interactions between the follicle epithelium and the surrounding extracellular matrix (ECM) are somehow involved.

To elucidate how tissue elongation occurs in the Drosophila ovary, I utilized live imaging of developing follicles ex vivo and discovered that the follicle unexpectedly undergoes several revolutions of polarized, circumferential global tissue rotation - relative to the surrounding ECM - during the major elongation phase of oogenesis. Follicles with epithelia mutant for an Integrin receptor or Collagen IV, an ECM molecule, fail to rotate and result in round eggs. We found that Collagen IV fibrils become circumferentially planar polarized during the rotation phase but become misoriented in non-rotating `round egg' mutants. Furthermore, acute degradation of Collagen IV rounds previously elongated follicles, suggesting that follicle rotation polarizes a fibrillar matrix that constrains the growing egg in a `molecular corset', generating its ellipsoid shape. Global tissue rotation is thus a novel morphogenetic behavior, using polarized cell motility to propagate planar polarity information in synchrony to the ECM to control tissue shape.

Many new questions developed from the discovery of global tissue rotation. It is unclear how the follicle epithelium responds to a revolving tissue and a mechanically constraining ECM as the tissue grows and elongates. Do conventional behaviors of tissue elongation like polarized cell intercalation, cell elongation or cell division occur in the Drosophila follicle? Chapter 3 aims to further our understanding of the cellular basis of follicle elongation by developing and performing preliminary quantitative morphometric analysis on the follicle epithelium during the elongation phase. In addition, I examine other known round egg mutants to determine whether they too regulate global tissue rotation or whether these genes regulate additional morphogenetic behaviors necessary for egg elongation.

My final chapter investigates how planar cell polarity (PCP) and global tissue rotation are established in the Drosophila follicle. How does molecular planar polarity arise? Does this occur before the onset of polarized follicle rotation? Is there an activation signal to initiate global tissue rotation and from where does this signal originate? I perform an in silico enhancer trap screen to identify developmental signaling pathways and other genes that may coincide with the major elongation phase and has led to the identification of the Notch/Delta signaling pathway as a putative regulator of follicle PCP and global tissue rotation, potentially providing an activation signal from the germline.

The findings from my Ph.D. dissertation provide a new framework to think about the cellular and molecular mechanisms underlying tissue elongation of the developing egg, but moreover, challenge current perspectives on metazoan morphogenesis. Global tissue rotation is a novel polarized morphogenetic behavior required for tissue elongation, but the cellular output of this movement remains ambiguous, perhaps because of the follicle's closed topology as an epithelial chamber. It is the first collective cell migration with an obvious individual cell polarity and tissue polarity, but with no obvious collective cell polarity with leader and follower cells. It also raises the possibility that different mechanisms of PCP establishment and propagation may occur in different tissues. These and other issues indicate that continued studies of Drosophila egg elongation will provide novel and exciting insights to our understanding of tissue polarity, morphogenesis and development in a variety of organisms.