Diet-dependent production of calcium- and phosphorus-rich 'spheroids' along the intestine of Burmese pythons: identification of a new cell type?

Abstract

Burmese pythons, Python molurus bivittatus, digest the skeleton of their prey. It must result in a high amount of calcium and phosphorus passing through the intestinal lining. To determine how Burmese pythons can process this ion influx, the effects of different nutritional diets were examined in juveniles reared in captivity using three different diets: a normal diet with calcium and phosphorus provided from entire rodents; a low-calcium and phosphorus diet using rodents with no bones ('boneless prey'), and a calcium-rich diet composed of boneless rodents supplemented with calcium carbonate (CaCO3) through intraperitoneal injections inside the prey. The effect of these diets was analysed along the intestinal mucosa using light and electron microscopy techniques (alizarin red S and peroxidase staining, EDX analyses). Blood calcium and hormone levels (parathyroid hormone, PTH and calcitonin) were also analysed from fasting pythons and snakes repeatedly fed with either a normal prey or a low calcium and phosphorus diet (boneless rats). Results revealed the presence of specialised cells in the intestinal epithelium that are involved in the production of calcium and phosphorus particles in fed snakes. These cells have an apical crypt possessing a multi-layered particle made of calcium, phosphorus and iron-rich nucleation elements in the centre. In fasting snakes, this cell type has empty crypts. When snakes are fed with a boneless prey, particles are not produced by this cell type, although iron elements are located within the crypts. When calcium supplements are added to a boneless meal, large particles fill the crypts. When snakes are fed repeatedly with a low-calcium diet, blood calcium level drops while levels of calcitonin, and particularly of those of PTH, increase. Therefore, Burmese pythons possess a specialised intestinal cell type involved in excreting excess dissolved calcium and phosphorus that originate from the prey and are precipitated as particles that must accumulate in the faeces. This cell type is also found in other snake species eating vertebrates (some Boidae and a Colubrid) along with a lizard, the Gila monster, Heloderma suspectum. A broader analysis among vertebrates that ingest their prey whole and dissolve the prey skeleton would allow a thorough evolutionary analysis.