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Hydra are small, invertebrate, predatory animals that live in water. They're tubular, radially symmetric and up to 10 mm long, with a head (mostly a mouth), a single, adhesive foot, and tentacles.

In a study in the journal PRX Life, researchers investigated how technical forces and feedbacks on a Hydra might affect its body plan.

They choose Hydra because they are notable for being able to regenerate, as most of their body cells are stem cells, which can continually divide and then differentiate into any of the body's cell types. In fact, Hydra are so good at it that do not appear to age and , constantly regenerating whatever cells they need, even from an initial small piece of tissue.

All animals share a common body plan because all come from a common ancestor, including bilateral symmetry, segmented bodies and a digestive system. Over billions of years, evolution has modified their shapes to create the enormous variety of body morphologies observed in the animal kingdom. But this biological pattern formation is still not well understood.

Morphogenesis is the biological process that causes a cell, tissue, or organism to develop its shape. It involves the differentiation of cells, tissues, and organs, leading to the creation of order in the developing organism.

Morphogenesis is a fundamental aspect of developmental biology, alongside tissue growth control and cellular differentiation. But what if an organism is constrained in some way due to external forces?

In this study, a team of researchers from Israel and Germany led by Yonit Maroudas-Sacks of the Technion–Israel Institute of Technology in Haifa, confined Hydra into a narrow cylindrical channel. The channel constrained the morphology of the animal—the form and structure of an organism, and particular features of its structure.

In the group's earlier work, they focused on the role of multi-cellular arrays of actomyosin fibers in guiding and stabilizing the body axis of the Hydra as they regenerated. (Actomyosin is a complex formed by two interacting proteins, actin and myosin. It plays crucial roles in muscle contraction and cell movement, with the myosin motor protein pulling the actin filaments into place.)

Hydra have parallel actomyosin fibers that contract, and previous work by the same group found that the body axis of Hydra regenerated when tissue segments were aligned with the inherited body axis of the parent.

They decided to investigate how the orientation field of the actomyosin fibers, which contained locally disordered regions called topological defects, is relevant to the body plan of Hydra morphogenesis, which was still unknown.

They developed a methodology to confine regenerating Hydra in an anisotropic manner—on an axis other than the Hydra's parallel fibers. This required a method of confinement that did not damage the organism's tissue or regenerating capacity over the course of several days. They also needed high resolution live imaging over the entire time of regeneration.

The confinement was in a glass capillary tube, equipped with small cylindrical channels on its inner surface, 120 to 300 microns wide, made of a stiff gel between the spherical tissue samples and the glass wall.

When the Hydra tissue was introduced into the resulting channel, while a softer gel was pushed into the channel cavities on the edges to create a width available to the Hydra, care was taken not to tear the tissue during the soft gel insertion.

This reduced the movement of the tissue along the cylinder axis, with about 20 to 50 cells along the circumference of the cavity (a typical cell size is 20 microns), while allowing the spherical tissue to unfold and regenerate into an elongated, ellipsoidal shape.

After some time, the regenerating tissue fills the channel available to it, then forms a mouth and tentacles as the body column becomes narrower than the channel, and the animal separates from the channel walls.

In this way, an angle develops between the constrained body axis and the inherited body axis. The relative angle between the inherited body axis and the channel axis depends on the orientation in which the Hydra tissue spheroid enters the channel, with its inherited axis parallel or perpendicular to the channel's axis.

The constraint imposed on the tissue geometry by the channel walls affects the patterns of mechanical stress experienced by the Hydra tissue, from both the hydrostatic pressure gradient across the tube and the frequent muscle contractions that take place.

The group found there was a strong preference of the body axes and the actomyosin fiber to come into alignment with the "easy-axis" of the channel, with one head and one foot along the channel axis. But different body plans developed if the initial tissue was perpendicular to the channel axis.

They wrote, "samples that are initially oriented with their primary fiber alignment perpendicular to the channel direction often regenerate into multiaxial morphologies."

But if the animals that were confined in length, perpendicular to the channel axis, they consisted mostly of animals with, amazingly, two heads, and often more than one foot. These multiple morphological features are not arranged along a single axis, but rather at junctions between axes with particular topological defects in the fiber organization.

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