Lungfish cocoon found to be living antimicrobial tissue

by Bob Yirka , Phys.org

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. 

Credit: DOI: 10.1126/sciadv.abj0829

A team of researchers from the University of New Mexico, the University of California and the University of Murcia has found that the cocoon created by lungfish living in dry lakebeds in Africa is made of living antimicrobial tissue. They've published the results of their study in the journal Science Advances.

Lungfish live in parts of Africa in small lakes that tend to go dry when it does not rain for a long time. When this happens, the lungfish create a cocoon around themselves using mucus. The purpose of the cocoon is to protect the lungfish from drying out as it waits for wetter conditions to return. In this new effort, the researchers have found that there is more to the cocoon than previously thought.

Until now, researchers believed the cocoon was simply a shell casing of sorts, with no purpose other than to prevent moisture from escaping under the hot African sun. Now, it appears the cocoon is not only alive, but is made of antimicrobial tissue.

To learn more about the lungfish and its cocoon, the researchers began an analysis of its makeup in 2018. They found granulocyte (white blood cell) markers that migrated during the time when the lungfish was waiting for water to return. More recently, the team has taken a closer look and found that the cocoon was chock full of granulocytes. They also found that they migrated from the skin into the cocoon on a slow, continual basis—a finding that showed the cocoon was much more than just dry mucus; it was a living part of the lungfish.

Imaging showed the granulocytes create traps that immobilize bacteria. When the researchers removed such traps from several specimens, they found the lungfish became susceptible to skin infections and circulating bacteria that are known to lead to septicemia. They also found that some of the infections led to hemorrhage. The researchers suggest that the cocoon protects the lungfish from more than just heat and sun—it also protects them from infections.

Fossil expands ancient fish family tree

More information: Ryan Darby Heimroth et al, The lungfish cocoon is a living tissue with antimicrobial functions, Science Advances (2021). DOI: 10.1126/sciadv.abj0829

Journal information: Science Advances 

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