Correlative 3D‐imaging of Pipistrellus penis micromorphology: Validating quantitative microCT images with undecalcified serial ground section histomorphology

Abstract

Detailed knowledge of histomorphology is a prerequisite for the understanding of function, variation, and development. In bats, as in other mammals, penis and baculum morphology are important in species discrimination and phylogenetic studies. In this study, nondestructive 3D-microtomographic (microCT, µCT) images of bacula and iodine-stained penes of Pipistrellus pipistrellus were correlated with light microscopic images from undecalcified surface-stained ground sections of three of these penes of P. pipistrellus (1 juvenile). The results were then compared with µCT-images of bacula of P. pygmaeus, P. hanaki, and P. nathusii. The Y-shaped baculum in all studied Pipistrellus species has a proximal base with two club-shaped branches, a long slender shaft, and a forked distal tip. The branches contain a medullary cavity of variable size, which tapers into a central canal of variable length in the proximal baculum shaft. Both are surrounded by a lamellar and a woven bone layer and contain fatty marrow and blood vessels. The distal shaft consists of woven bone only, without a vascular canal. The proximal ends of the branches are connected with the tunica albuginea of the corpora cavernosa via entheses. In the penis shaft, the corpus spongiosum-surrounded urethra lies in a ventral grove of the corpora cavernosa, and continues in the glans under the baculum. The glans penis predominantly comprises an enlarged corpus spongiosum, which surrounds urethra and baculum. In the 12 studied juvenile and subadult P. pipistrellus specimens the proximal branches of the baculum were shorter and without marrow cavity, while shaft and distal tip appeared already fully developed. The present combination with light microscopic images from one species enabled a more reliable interpretation of histomorphological structures in the µCT-images from all four Pipistrellus species.

Testing hypotheses of bat baculum function with 3D models derived from microCT

“…In bats of the genera Pipistrellus (Herdina et al. ) and Plecotus (Herdina et al. ) our histological data also support the hypotheses that the baculum protects the urethra from compression during copulation (Dyck et al.…”

“…): not only the very end is connected, but also a broad area of the dorsal surface all along the bifurcated part of the proximal base of the baculum (see Herdina, ; Herdina et al. ).…”

“…; Herdina, ; Herdina et al. , ). One hypothesized function of the baculum is that it protects the distal urethra and urethral opening from compression during erection and copulation (Dixson, ).…”

“…During copulation this functional unit can transfer bending forces from the distal end of the glans penis to the corpora cavernosa, thereby increasing the flexural stiffness of the whole erect penis (Long & Frank, ; Kelly, ; Herdina et al. , ).…”

The Utility of DiceCT Imaging for High-Throughput Comparative Neuroanatomical Studies

“…As demonstrated by Herdina et al [2015b], using a bat phallus, destained specimens are amenable to subsequent staining under standard histological protocols (in this case, with Giemsa surface stain). Likewise, Hughes et al [2016aHughes et al [ , 2017 extended this approach for neural imaging with Nissl staining, using chameleon brains that had been previously stained for diceCT imaging but then subsequently destained.…”