No. 44–Question Answered
Those who follow this column with regularity will recall a question I raised a while back regarding the ability of sand dollars (Mellita quinquiesperforta) to heal themselves, the following is a reply received from Professor Emeritus, Charles Biernbaum of Grice Marine Laboratory, College of Charleston.
“I’m a whole-animal invert person and, so I can’t even pretend to know what goes on at the cellular level, but when I think about healing in Mellita, I recall, of course, the significant capability for healing and regeneration of lost structures in the asteroids, ophiuroids and, following virtual total loss of internal organs, in many holothuroids. With those familiar examples in mind, it doesn’t surprise me that the echinoids have significant powers of healing, provided their body/test isn’t crushed beyond repair. I, to, have come have come across tests of Mellita that have significant portions missing, but healed over. I suspect that connective tissue cells responsible for ossicle formation in the animal (sclerocytes, which, of course, are continuously enlarging ossicles as the animal ages) simply secrete the necessary calcite to heal the test’s fracture, with the epidermis and associated tissues healing the wound.”
With a point in the right direction from Professor Biernbaum I have revisited the Ruppert, Fox and Barnes invertebrate zoology book cited in the previous columns on echinoderms and found his logic basically substantiated in the text.
The asteroids referred to by Professor Biernbaum are the 1,500 living species of sea stars world wide, brittle stars which are found in both the estuary and local near and offshore waters are ophiuroids, and holothuroids are the sea cucumbers which, if sufficiently traumatized, will throw up their guts to gross out an attacker. They are able to regenerate their internal organs.
Ossicles are the basic sub dermal structural components used in the construction of asteroids. They are the calcite re-bar, if you will, forming a three dimensional lattice called stereom that may be configured as plates (sand dollars), spines (sea urchins) and other structural elements found in a wide variety of configurations among the asteroids.
To follow the re-bar analogy, sclerocytes are the cement trucks as they process and deliver calcite to the construction site. In Mellita they, along with other connective tissue cells are found within the dermis (as are the ossicles) and are responsible for both creating and enlarging the skeletal plates (ossicles) one sees on close observation of a sand dollar test. The honeycombed space of the connected ossicles, the stroma, facilitates the entry of sclerocytes for both enlarging the individual ossicles and for the attachment of ligaments that suture the ossicles together to create the skeletal framework.
In light of the fact that sclerocytes are the prime contractor responsible for structural integrity in new construction, I would naturally follow that they would also be called upon for structural repair.
As I waded through some pretty complicated physiology and an awful lot of Latin to better understand the subject of this column and be able to convey the information in an understandable way, I couldn’t help but be struck by the elegant complexities underlying the structural appearance of a creature as seemingly simple as a sand dollar. The plethora of inherent physiological mechanisms both shared and differentiated across species approaches the sublime. The brief and sometimes mind numbing venture into the biological minutia of creatures I have superficially observed over the years has awakened a curiosity of another level. I have a head full of questions on par with the one now resolved which, on occasion, will appear as of topics of future columns.