The developing inner ear of the teleost, Brachydanio rerio, provides an opportunity for observing an epithelial fusion between the apical surfaces of apposed epithelia in a vertebrate embryo in vivo. The developing otocyst was filmed for periods up to 4 days in unanesthetized embryos, and specimens were fixed at intervals and processed for light microscopy, TEM, and SEM. The semicircular canals are formed as a consequence of the union between the tips of three cylindrical projections from the wall of the otocyst, which grow toward corresponding bulges of a projection from the lateral wall. The epithelial cells covering the projections contain extensive rough endopasmic reticulum, exhibit apical junctional complexes, and are not underlain by a basal lamina. The core of each projection contains large amounts of flocculent and fibrillar extracellular material. After a period of growth and elongation, the tip of each projection contacts, and adheres to, the appropriate bulge to create a circular, flattened, bilayered, epithelial plate. Small, focal junctions form between the apposed apical cell surfaces within the plate during this period, but they are not numerous. Junctional complexes do develop, however, between apposed cells at the periphery of the plate. After 1-2 hours, the basal surface of the plate exhibit considerable alteration in contour. Adjacent cells within the plate then separate to allow continuity of the connective tissue components of the two structures. The observations of this study indicate that following an initial period of contact and adhesion, cellular reorientation and changes in junctional contacts between adjacent cells within the epithelial plate, rather than cell degeneration, are responsible for perforation of the plate.

Nine-month-old rats were injected with 5 microCi 3H-thymidine (3H-Tdr) and allowed to survive for 20 days. In light-microscopic radioautographs, labeled cells were found in the granule cell layer of the hippocampus. Analysis of electron micrographs of the labeled cells, taken from re-embedded 1.5 micron radioautographic sections, clearly demonstrated their neuronal nature with synapses along their cell bodies and dendrites. Our results indicate that 0.025% of the granule neurons are heavily labeled in the dorsal hippocampus. Electron microscopy of re-embedded light-microscopic radioautographic sections confirms that granule neurons in the rodent are newly formed up until 9 months after birth.

Our field study did not support anecdotal claims alleging pathogenicity on the part of P. cylindraceus in starlings. Within-clutch analysis of nestling starling weights (n = 25) over time showed that P. cylindraceus had no effect on position in clutch relative to siblings. Parasitized nestlings tended to weigh more than control siblings. Within-sex analysis of wild adult starling weight (n = 103) showed no difference between animals with acanthocephalans and uninfected animals and weight was not related to intensity of infection. Host response to the proboscis was limited to immediately-surrounding tissue (90 microns). Other observations made in this study included the following: (1) no evidence for deleterious intraspecific interaction among adult P. cylindraceus in starlings; (2) no apparent interspecific interactions between P. cylindraceus and (unidentified) cestode species in starlings; and (3) copulatory caps on male P. cylindraceus may be a response to environmental deterioration in dead hosts.