Hydra Model Organism: Regeneration Abilities and Anatomy

Hydra Model Organism

• Hydra is a freshwater cnidarian that lives in unpolluted freshwater ponds, lakes, and streams.
• They have a radial symmetric body, a few millimeters in length, and eat small aquatic invertebrates.
• They proliferate asexually by budding when well-fed.
• Environmentalists use the presence of Hydra as a test of pollution.

Hydra Anatomy

• Gland cells are responsible for digestion.
• Interstitial stem cells are responsible for stinging.
• Hypostome is the location of the mouth.
• Endodermal epithelial cells are located on the interior, ectodermal epithelial cells on the exterior, and mesoglea in between.

Cell Division and Apoptosis

• Cells are continuously undergoing mitosis.
• BrdU staining shows that new DNA is being made, but only in the animal's trunk, not in the tentacles or extremities.
• Apoptosis is necessary and sufficient for head regeneration.
• Apoptotic cells are seen in the regenerating head region.
• Inhibition of apoptosis leads to an increase in foot regeneration and a reduction in head regeneration.

Wnt Signaling

• Wnt3 expression and nuclear beta-catenin are apoptosis-dependent.
• Wnt signaling is necessary for the development of the proliferative zone.
• There is no proliferative staining in the head when Wnt or beta-catenin is knocked out.
• The pathway: Apoptosis -> Wnt/beta-catenin -> cell proliferation/head regeneration.

Head Inhibitor

• The inhibitor Sp5 is upregulated during head regeneration, prevents head formation, inhibits Wnt/beta-catenin signaling, and is activated by Wnt/beta-catenin.
• Blocking Sp5 results in an animal with multiple ectopic heads.
• Sp5 requires beta-catenin, and inhibiting beta-catenin results in a more normal phenotype.

Foot Regeneration

• Beta-catenin is involved in both head and foot regeneration.
• Inhibition of beta-catenin inhibits foot regeneration.
• Overexpression of beta-catenin results in too many head tentacles and ectopic patches of basal discs.
• Beta-catenin is expressed at the beginning of foot and head regeneration.

Hydra as a Simple Model for Regeneration

• Hydra has a single apical-basal axis.
• Hydra cells are continuously dividing, and there are three types of stem cells.
• Transplantation experiments identify activating/inhibitory gradients for both head and foot regeneration.
• Hypostome acts as a head organizer, and Wnt signaling appears to be the head activation signal.
• Sp5 functions as part of the head inhibitory signal.
• Apoptosis triggers the initial release of Wnt3 in head, but not foot regeneration.
• The Ectoderm has the capacity to initiate head regeneration.
• Beta-catenin regulates foot regeneration independent of Bra1.

Regeneration Capabilities of Hydra

• Hydra is capable of regenerating head, foot, and tentacles.
• Hydra is very plastic as an organism, and can reaggregate and form a new Hydra from a chunk of cells.
• Within 48-72 hours, if you cut the body column in half horizontally, the tentacle zone will begin to regrow.
• Graft experiments provided evidence of polarity gradients.

Polarity Gradients

• Hypostome grafted onto the host trunk budding region induces a secondary axis with another hypostome (head).
• Grafts of trunk tissue below the head have a reduced capacity to form a secondary axis.
• Basal disc grafted into the host trunk budding region induces a secondary axis with another tail.
• Opposing apical and basal gradients may determine where a new bud is formed.
• A combination of activating and inhibiting head and foot gradients ensures that there is only one head and one foot.

Hypostome as an Organizer

• Hypostome acts as an organizer, having inductive properties, and the body cavity has self-organizing properties.
• Transplanted tissue provides a molecular signal for growth, not the tissue for growth itself.
• Wnt is the apical organizer that maintains apical polarity.

Cell Types Required for Regeneration

• Endoderm, Ectoderm, and Interstitial stem cells (ISCs) are required for regeneration.
• Interstitial cell lineage is not required for regeneration.
• Ectoderm cells are required for head regeneration.
• Endodermal cells contribute to foot regeneration.

How is Regeneration Triggered?

• Regeneration is triggered by apoptosis, which leads to Wnt signaling and the development of the proliferative zone.