Apoptosis generates mechanical forces that close the lens vesicle in the chick embryo

Abstract

During the initial stages of eye development, optic vesicles grow laterally outward from both sides of the forebrain and come into contact with the surrounding surface ectoderm (SE). Within the region of contact, these layers then thicken locally to create placodes and invaginate to form the optic cup (primitive retina) and lens vesicle (LV), respectively. This paper examines the biophysical mechanisms involved in LV formation, which consists of three phases: (1) lens placode formation; (2) invagination to create the lens pit (LP); and (3) closure to form a complete ellipsoidally shaped LV. Previous studies have suggested that extracellular matrix deposited between the SE and optic vesicle causes the lens placode to form by locally constraining expansion of the SE as it grows, while actomyosin contraction causes this structure to invaginate. Here, using computational modeling and experiments on chick embryos, we confirm that these mechanisms for Phases 1 and 2 are physically plausible. Our results also suggest, however, that they are not sufficient to close the LP during Phase 3. We postulate that apoptosis provides an additional mechanism by removing cells near the LP opening, thereby decreasing its circumference and generating tension that closes the LP. This hypothesis is supported by staining that shows a ring of cell death located around the LP opening during closure. Inhibiting apoptosis in cultured embryos using caspase inhibitors significantly reduced LP closure, and results from a finite-element model indicate that closure driven by cell death is plausible. Taken together, our results suggest an important mechanical role for apoptosis in lens development.

Rho kinase-dependent apical constriction counteracts M-phase apical expansion to enable mouse neural tube closure

“…Tissue-level integration of cellular force-generating mechanisms is necessary to achieve coordinated morphogenetic shape change. Force-generating mechanisms often act in opposing ways; for example, both cell apoptosis (Oltean and Taber, 2018) and regional cell proliferation (Hosseini et al, 2017; Peeters et al, 1998) can change tissue shape despite having opposing effects on cell number. This mutual antagonism may also be true of the interplay between apical constriction and INM.…”

How mechanical forces shape the developing eye

“…Details of the models and simulations for each process are described in the following sections. Specified distributions of growth, contraction, stiffness, etc., are based on previous 2-D models developed in our lab (Hosseini et al 2014, Oltean et al 2016, Oltean & Taber 2017). In some cases, however, aspects of the models have been modified somewhat to obtain better agreement with experimental data.…”

“…For example, as discussed later in section 5.1, hoop stress makes closing a circular hole in a spherical shell considerably more difficult than closing an opening in a cylindrical model for a cross section of the sphere. Thus, understanding the mechanics of closing the relatively spherical lens vesicle requires a 3-D model (Oltean & Taber 2017). Here, we present a 3-D model for OV morphogenesis.…”

“…In the embryo, an actomyosin ring forms around the wound margin and contracts to close the wound (Redd et al 2004). Actin staining, however, reveals no such ring, and modeling suggests that apical contraction alone cannot generate enough force to overcome large circumferential compressive stresses that develop as the perimeter of the lens pit decreases toward zero (Oltean & Taber 2017).…”

“…The current state of knowledge is reviewed, and the physical plausibility and limitations of proposed morphogenetic mechanisms are examined using computational models. These models, which are based on the fundamental laws of solid mechanics, extend to three dimensions our recent two-dimensional models for OV, OC, and LV morphogenesis (Hosseini et al 2014, Oltean et al 2016, Oltean & Taber 2017). The results illustrate how relatively simple processes can produce the complex morphological changes observed in the embryonic eyes.…”