Morphogenesis and the mechanics of living tissues

6 problems

• Predict embryo-scale tissue flow from measured motor activity

  Partially resolved

  During early development, sheets of cells flow across the embryo as motor proteins pull on them. A test of understanding is to predict these flows, and the resulting shapes, from the measured pattern of motor activity alone.

• Mechanism of morphogen gradient scaling in growing tissue

  Contested

  The signaling molecule Dpp forms a concentration gradient across the fly wing, and its decay length grows in proportion to the wing as it grows. Which chemistry keeps this proportion is disputed.

• How cell-level chirality sets left-right body asymmetry

  Open

  Animals that look symmetric outside place organs such as the heart asymmetrically inside. In several species this traces to handedness in individual cells, and how that small bias becomes an organ-scale asymmetry is not understood.

• How a growing organ knows when to stop

  Open

  Organs such as a fly's wing grow to the right size and then stop, even when their cells divide at different rates. Whether the stop signal comes from chemical gradients, mechanical stress, or both is unknown.

• Does cell shape predict whether a tissue flows or holds?

  Contested

  Computer models of tightly packed cells predict that a tissue switches from solid-like to fluid-like once cells become elongated beyond a critical shape. Experiments disagree on whether this shape rule works in real tissues.

• Are periodic tissue patterns produced by Turing reaction-diffusion?

  Partially resolved

  In 1952 Turing showed that two diffusing chemicals, one activating and one inhibiting, can make spots and stripes from a uniform start. Whether real fingers, hair follicles and feather buds form this way, or by cells pulling on each other, is settled for few systems.