Quantitative characterization of mesenchymal cell aggregation

Abstract

The underlying principle for the formation of multicellular organism from single cells is one of the fundamental questions in understanding evolution, embryonic development, tissue engineering and tumorigenesis. As the transition from unicellular organism to multicellular organism happened in the deep past and all the transitional forms have been extinct, this question cannot be resolved by studying evolution alone.

I attempted to understand the mechanism for formation of multicellular organism using a physiological cell self-organization process. Due to the complexity of the self-organization process, I started with an autonomous cell aggregation process in my study. Although it can be relatively simple, it is one of the fundamental ways for cell to form basic multicellular structure, it has been demonstrated to be involved in various physiological and pathological processes, such as embryonic development (e.g. vertebrate, heart and kidney), blood flow and cancer metastasis.

Mesenchymal cell aggregation (or named ‘condensation’ in developmental biology) is one of the typical cell aggregate formation processes which has been observed and investigated for years. Although the mechanisms of the mesenchymal cell aggregate formation is still not clear yet, scientists have found ways to create the mesenchymal cell aggregates in vitro for tissue engineering applications. Mesenchymal cell aggregation with proper size and timing is the key to the efficiency of subsequent differentiation and generation of functional cells. Nevertheless, there is still little knowledge on the general principles controlling the aggregate formation and the determination of their size and time.

The research aim of this study was to quantitatively characterize the mesenchymal cell aggregation process in order to identify its governing principles behind. To this end, an in vitro mesenchymal cell aggregation system was established and the biological properties were characterized. A method to tune the cell aggregation process was also discovered.

Although the cells did facilitate their pattern formation by secreting large biomolecules, the Turning or Keller-Segel mechanisms were ruled out. I discovered that the cells relied on cell spreading and random motion to induce instability, a novel mechanism for natural occurring pattern. I postulated a three-step model, with cell spreading and motility, cell-cell adhesion and cell contraction. The hypothesis was validated by experimental, modeling investigation as well as automatic large scale data analysis. I established quantitative methodologies to independently quantify the three steps. Particularly, I developed a novel analytic method to directly measure the effective cell-cell attraction "force". With these quantitative assessment, I tested a number of factors known to be secreted by the cells, and identified several orthogonal factors stimulating separate aspects of the process. A counter intuitive effect of cell motion heterogeneity on the improvement of the uniformity of the aggregate formation was also demonstrated.

This study provided a unique underlying mechanism for the mesenchymal cell aggregation based on quantitative analysis. It also provided clues for further analysis of pattern formation in biological systems, as well as postulation of the origin of multicellularity.