Fig. 2. Graphic drawing showing a proposed model for cocoon formation in African lungfish.

Free-swimming lungfish skin is characterized by a columnar mucosal epithelium. (A) Large numbers of multipotent stem cells with alkaline phosphatase activity (fig. S1) can be observed at the interphase between the epidermis and the dermis. Granulocyte deposits in the tissue reservoirs of free-swimming lungfish become mobilized to the skin via peripheral circulation when lungfish begin to sense lack of food and water. (B) Skin remodeling begins, with increasing numbers of granulocytes infiltrating the dermal and epidermal layers resulting in loosening of the basal membrane and inflammation. (C) The cocoon then starts to form by detachment and shedding of the inflamed epidermis. Many granulocytes are part of the cocoon, and they produce ETs in response to the high microbial load. Epithelial cells, goblet cells, and antimicrobial peptides (AMPs) are also present in the cocoon. The pool of stem cells starts to regenerate the epidermis, while granulocytes continue to arrive from reservoirs maintaining an inflammatory state. (D) In the next stages of estivation (end of the induction phase), the lungfish skin shows complete flattening of the epidermis and goblet cell exhaustion. The cocoon has several layers derived from multiple rounds of epidermal shedding and regeneration, and stem cell numbers are severely reduced. Granulocytes in the cocoon continue to undergo ETosis and are still elevated in the epidermis and dermis compared to free-swimming controls. It is unknown whether the cocoon continues to thicken beyond 2 weeks after terrestrialization. This illustration was created in BioRender. eDNA, extracellular DNA.