Structural flexibility of the intestine of Burmese python in response to feeding

The small intestine of Burmese pythons, Python molurus bivittatus, undergoe s a remarkable size increase shortly after feeding. We studied the dynamics , reversibility and repeatability of organ size changes using noninvasive i maging techniques. We employed light and electron microscopy, flow cytometr y and immunohistology to study the cytological mechanisms that drive the si ze changes of the small intestine. Within 2 days of feeding, the size of th e small intestine increased to up to three times the fasting value. The siz e changes were fully reversible and could be elicited repeatedly by feeding . These enormous size changes were possible because the mucosal epithelium of the small intestine is a transitional epithelium that allows for conside rable size changes without cell proliferation. Histological evidence sugges ted that a fluid pressure-pump system (lymphatic, blood pressure) was the d riving force that inflated the intestinal villi. The rates of cell prolifer ation were not elevated immediately after feeding but peaked 1 week later w hen small intestine size was already declining. In contrast to the current paradigm, we suggest that the small intestine is not part of the previously proposed 'pay-before-pumping' model. Instead, the size of the python's sma ll intestine may be upregulated without major metabolic investment. It can occur even if the individual is energetically exhausted. An evolutionary pe rspective of the transitional epithelium mechanism suggests superior functi onality compared with the pay-before-pumping model because it allows for lo ng periods of fasting and depletion of energy reserves, while still enablin g the snake to digest prey and absorb nutrients.