A majority of larval cells fundamentally transform in metamorphosis

Amazing stuff!

"In brief

• A new study revealed that the majority of larval cells in the marine worm “Schizocardium californicum” appeared to fundamentally transform in metamorphosis, against prior understanding of the process.
• Genetic sequencing of thousands of cells showed evidence of this cellular reprogramming, with larval nerve and gut cells taking on different functions in the adult.
• The findings are the strongest evidence yet of developmental reprogramming in bilaterally symmetrical animals, where one side of the body mirrors the other.

...

Unlike humans, about 80% of animal species undergo metamorphosis, a stepped development from egg to larva to adult, but how it works at a cellular level is not well understood. Some theories and prior research suggested that the original cells in the larva die and are replaced with newly generated adult cells. Other work pointed to cells growing into their same function – for example, larval skin cells would become adult skin cells.

Instead, a ... [new] study has found strong evidence in an acorn worm called Schizocardium californicum that most larval cells were reprogrammed, with even neurons taking on a new role in the adult organism. ...

Cellular reprogramming is thought to happen after an injury or in some species that regenerate organs or whole limbs – but not as a feature of normal development. ..."

From the abstract:

"A major gap in our understanding of animal development is how adult body plans arise in animals with indirect development, where adults emerge from the transformation of a distinct larval form during metamorphosis.

We address this question by examining cellular changes in the enteropneust hemichordate Schizocardium californicum, a species with a complex lifecycle and dramatic metamorphosis.

Employing whole-body single-cell RNA sequencing, we chart the cellular composition and transcriptional dynamics of larval, metamorphic, and adult stages. Our tissue-level atlas reveals that ectodermal and endodermal cell types in larvae and adults occupy distinct transcriptional spaces, showing greater similarity to other cell types within the same life stage than to their counterparts in the opposite stage.

In contrast, mesodermal cell types from both larvae and adults cluster closely together, indicating conserved transcriptional profiles.

These findings demonstrate that the extensive morphological reorganization during metamorphosis is accompanied by broad shifts in transcriptional identity and reveal life-history stage as a major organizing axis of cellular state during the larva-to-adult transition."

Study reshapes understanding of metamorphosis | Stanford Report "Many cells in an acorn worm reprogram into different cell types instead of dying or staying the same, the first evidence of this process in bilaterally symmetrical animals."

Distinct cell states define larval and adult body plans in a hemichordate (open access)

Fig. 1: Indirect developing hemichordates as a model to study the transition between larval and adult body plans.

Fig. 2: Single-cell sequencing reveals the broad diversity of hemichordate cell types.

Fig. 5: Genetic makeup of cell type diversity across life history stages.