Investigating cellular interactions in tissues

Examples of our research and methods include:

Total synthesis of complex self-assembling tissues

Researchers here are developing new ways to rapidly and precisely build detailed, complex, three-dimensional human tissues in vitro, including structures that self-assemble with programmed composition, thus controlling which cell-types interact and the spatial arrangement of groups of cells for experimental alteration and analysis.

Such tissue building is comparable to total synthesis, the process in organic chemistry in which complex biologically active molecules are built from more basic chemical building blocks.  Just as creating physically similar but slightly altered molecules (structural analogs) helps reveal structure-function relationships and new chemical principles, so constructing tissues from specific cell types should yield new insights into tissue organization and cellular interactions.

Using the human mammary gland as a model, department scientists engineer self-assembling tissues with programmed cell-type composition. Initial connectivity is directed by coating cell surfaces with short segments of DNA strands with complementary base pairs. The results of these initial combinations are then grown in gel matrices to provide a physiologically relevant 3-D spatial context.

[A] Normal cells (tweaked to make proteins that glow red) are combined via complementary single strands of DNA with abnormal ones (glowing green) that express more of a mutant protein. To program cell-type composition, researchers assemble aggregates [B] thru a 50:1 normal/abnormal mix, then use fluorescence-activated cell sorting to select 5:1 aggregates [C] for further growth in cultures [D].

Researchers here are working toward building a functional human mammary gland. They are starting with modular substructures, such as the lining of the milk ducts, made of two cell types in layers, and working up a hierarchy of complexity incorporating the ducts with blood vessels, immune cells, and connective tissue.

These models will provide new insight into how normal human tissues assemble during development, and conversely, how they break down in diseases such as breast cancer. Indeed, the synthesized models will use healthy cells and, for certain studies, their malignant derivatives, in structures controlled down to the positioning of individual cells. This will enable analyses of how cellular heterogeneity, injury and/or disturbances in the relay of signals among different cell types contribute to breast cancers.

Our applications of such cellular engineering include:

Building breast tissue to study effects of cell heterogeneity

Department scientists build mammary microtissues with programmed cellular heterogeneity in the cancer-related Ras protein to investigate the effect of such differences on coordinated cellular processes such as tissue structural formation, i.e., morphogenesis. (A hallmark of most tumors is increased cellular heterogeneity.)

A study found that in such growing mosaics (in which about one in five of the cells expressed over-activated Ras) abnormal cells were extruded, protruded, or exited from the tissues, exhibiting behaviors commonly associated with invasive tumors. Homogenous tissues, even comprised of all abnormal cells, did not. Thus heterogeneity in cells’ Ras activity, rather than total Ras activity, appears to be necessary in the development of some of the abnormal behaviors.

24-hour time-lapse images of mammary microtissues with green-glowing cells in lower two phenotypes expressing over-activated Ras protein. Middle row: In 20 to 30 percent of microtissues, abnormal Ras cells are extruded. Bottom row: In another 20 to 30 percent, abnormal Ras cells lead multi-cellular protrusions, a behavior seen in normal tissue growth but also in invasive cancers.