Ultrastructure of the atrioventricular node of the big brown bat, <i>Eptesicus fucus</i>

Abstract

This investigation describes the ultrastructure of the atrioventricular node of Eptesicusfuscus. Two conducting cell types (nodal and transitional) are identified which differ in location, myofibrillar content, and types of intercellular junctions. Centrally located nodal cells display variable staining intensity and contain disorganized myofibrils which rarely form sarcomeres. Desmosomes and nexus-like junctions connect the nodal cells. Transitional cells, situated peripherally, exhibit distinct sarcomeres and are attached to the adjacent cells through desmosomes and underdeveloped intercalated discs. Longitudinal arrangement of the mitochondrial cristae is frequently seen in both cell types. In the connective tissue stroma, numerous capillaries (with micropinocytotic vesicles), axons, and possibly axonal terminals, some filled with vesicles, are observed. A large ganglionated nerve trunk is present on the nodal periphery. True nexuses and neuromuscular junctions are not observed. It is suggested that nodal cell types previously reported in different vertebrates under various names are merely simple variations of the two basic types of conducting cells – nodal and transitional. No interspecffic differences are observed between these cells of the big brown bat compared with those in other vertebrates.

The Ultrastructure of the Sinu-Atrial Node of the Bat

“…The SAN of P.subflavus is richly inner vated, consistent with reports for other mammalian species [Colborn and Carsey, 1972;James et al, 1966;Cheng, 1971;Kawamura et al, 1978;Kikuchi, 1976;Bhatnagar and Spoonamore, 1979], We observed two types of nerve terminals, one with clear vesicles the other with larger dense-core vesicles. We did not observe specialized neuromuscular junctions, such as have been reported in the monkey [Colborn and Carsey, 1972], the rat [Cheng, 1971], and the mole [Kikuchi, 1976], consisting of a nerve terminal lying in a groove in the nodal cell.…”

“…9). Such common junctions are typical of mammalian SAN [Kawamura and James, 1971] and were also seen in the bat atrioventricular node [Bhatnagar and Spoonamore, 1979], However, unlike the atrioventricular node, the zone of contact contained no nexuses, but rather either desmosomes, fasciae ad hérentes or undifferentiated regions.…”

“…In these reports the synaptic gap varied from 20 to 200 nm, and occasionally the postsynaptic membrane was electron dense [Kikuchi, 1976], In our study the distance between nerve terminals and nodal cells varied from 20 to 100 nm, with a mean of 43.2 nm, but no synaptic grooves were seen. No true neuromuscular junctions have been reported in the atrioventricular node of the bat [Bhatnagar and Spoonamore, 1979] and, even in those species where true junctions are present, they are rarely seen. Kikuchi [1976] described two types of neuromuscular contacts in the mole SAN, the remote type and the close type of synapse.…”

Morfologia da junção atrioventricular em Iguana iguana (Reptilia-Iguanidae)

“…An abundant amount of connective tissue surrounded the AV node in iguana. In chiropteras, there also is considerable amount of connective tissue in this region as other mammals 35,41 .…”