Regenerative abnormalities in Notophthalmus viridescens induced by repeated amputations.

The fidelity of the regenerative response in the adult newt, Notophthalmus viridescens, was examined following repeated amputations at the level of the distal one-third of humerus. Three to four months following amputation, all regenerates were scored for gross morphology, reamputated, and stained with methylene blue for skeletal elements. The occurrence of abnormal regeneration with respect both to gross morphology and to skeletal structure was found to increase directly with the number of times the limb stumps were required to initiate dedifferentiation and repair. The initial amputation-regeneration process produced structurally normal replacement limbs in 91% of the cases examined. Reamputations of 4-digit regenerates (3--4 months after the previous transection) resulted in structurally abnormal regenerates in 28% of the cases following two amputations; 50% of the cases following three amputations; 65% of the cases following four amputations; and 81% of the cases following five amputations. The relationships between repeated dedifferentiation, proliferation, and redifferentiation and normal limb development are discussed.     

A model of NaCl and water flow through paracellular pathways of renal proximal tubules.

To explain how hydrostatic pressure differences between tubule lumen and interstitium modulate isotonic reabsorption rates, we developed a model of NaCl and water flow through paracellular pathways of the proximal tubule. Structural elements of the model are a tight junction membrane, an intercellular channel whose walls transport NaCl actively at a constant rate, and a basement membrane. Equations of change were derived for the channel, boundary conditions were formulated from irreversible thermodynamics, and a pressure-area relationship typical of thin-walled tubing was assumed. The boundary value problem was solved numerically. The principal conclusions are: 1) channel NaCl concentration must remain within a few mOsm of isotonic values for reabsorption rates to be modulated by transtubular pressure differences known to affect this system: 2) basement membrane and channel wall parameters determine reabsorbate tonicity; tight junction parameters affect the sensitivity of reabsorption to transmural pressure; 3) channel NaCl concentration varies inversely with transmural pressure difference; this concentration variation controls NaCl diffusion through the tight junction; 4) modulation of NaCl diffusion through the tight junction controls the rate of isotonic reabsorption; modulation of water flow can increase sensitivity to transmural pressure; 5) no pressure-induced change in permeability of the tight junction or basement membrane is needed for pressure to modulate reabsorption; and 6) system performance is indifferent to the distribution of active transport sites, to the numerical value of the compliance function, and to the relationship between lumen and cell pressures.     

Synthesis and storage of a neurohormone in insect brains in vitro

The build-up of neurosecretory material in the median neurosecretory cells and fibre tracts of cultured cockroach brains was demonstrated by staining and bioassay. Examination of the cultured brains by electron microscopy showed active production of neurosecretory granules after 3 days in vitro. The close correlation of the results obtained by these different methods of assay leaves little doubt that a neurohormone is being synthesized and stored.