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. 2013 Jan 8;11:1.
doi: 10.1186/1741-7007-11-1.

The Songbird Syrinx Morphome: A Three-Dimensional, High-Resolution, Interactive Morphological Map of the Zebra Finch Vocal Organ

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Free PMC article

The Songbird Syrinx Morphome: A Three-Dimensional, High-Resolution, Interactive Morphological Map of the Zebra Finch Vocal Organ

Daniel N Düring et al. BMC Biol. .
Free PMC article

Abstract

Background: Like human infants, songbirds learn their species-specific vocalizations through imitation learning. The birdsong system has emerged as a widely used experimental animal model for understanding the underlying neural mechanisms responsible for vocal production learning. However, how neural impulses are translated into the precise motor behavior of the complex vocal organ (syrinx) to create song is poorly understood. First and foremost, we lack a detailed understanding of syringeal morphology.

Results: To fill this gap we combined non-invasive (high-field magnetic resonance imaging and micro-computed tomography) and invasive techniques (histology and micro-dissection) to construct the annotated high-resolution three-dimensional dataset, or morphome, of the zebra finch (Taeniopygia guttata) syrinx. We identified and annotated syringeal cartilage, bone and musculature in situ in unprecedented detail. We provide interactive three-dimensional models that greatly improve the communication of complex morphological data and our understanding of syringeal function in general.

Conclusions: Our results show that the syringeal skeleton is optimized for low weight driven by physiological constraints on song production. The present refinement of muscle organization and identity elucidates how apposed muscles actuate different syringeal elements. Our dataset allows for more precise predictions about muscle co-activation and synergies and has important implications for muscle activity and stimulation experiments. We also demonstrate how the syrinx can be stabilized during song to reduce mechanical noise and, as such, enhance repetitive execution of stereotypic motor patterns. In addition, we identify a cartilaginous structure suited to play a crucial role in the uncoupling of sound frequency and amplitude control, which permits a novel explanation of the evolutionary success of songbirds.

Figures

Figure 1
Figure 1
Location of song system components in the male zebra finch. (A) μCT-based volume rendering of the zebra finch skeleton. Boxed section shown enlarged in C. (B) Virtual sagittal section through a 3D high-field MRI dataset of the male zebra finch head. Visible are three nuclei of the song system: HVC (used as a proper name) and the robust nucleus of the arcopallium (RA) that both belong to the descending motor pathway, and the lateral portion of the magnocellular nucleus of the anterior nidopallium (LMAN), which belongs to the anterior forebrain pathway. Dotted lines indicate the virtual sections shown in E and F. (C) Virtual sagittal section through a 3D MRI dataset showing that the syrinx is located at the bifurcation of the trachea into the bilateral primary bronchi, close to the heart (red shaded outline) and lungs (green shaded outline). Note the dark seeds in the crop. The asterisks indicate intercostal muscles. (D) μCT-based volume rendering showing the hyoid bone (yellow) and larynx (blue), which are upper vocal tract modulators important in the filtering of sound properties [38,40]. (E,F) Virtual transversal sections through a 3D MRI dataset of the head as indicated in (B) showing the tongue, larynx, trachea (blue) and hyoid (yellow dotted line). Scale bars: 10 mm.
Figure 2
Figure 2
Ossified structural elements of the male and female zebra finch syrinx. (A) Ventral and (C) dorsal view of male syringeal skeleton surface rendering based on μCT datasets with 5 μm isotropic voxel resolution. (B) Ventral and (D) dorsal view of the female syringeal skeleton. Muscle attachment sites leave impressions on the surface of the tympanum (black arrowheads in A) and on bronchial half-rings B1 and B2 (insets). Abbreviations as listed in Table 1.
Figure 3
Figure 3
Selection of male and female syringeal skeletons based on volume rendered μCT datasets.
Figure 4
Figure 4
Overview of terminology describing ossified structural elements of the songbird syrinx. Alternative nomenclatures of previous authors [16,18,19,28,43,52,72,73,106,123-129]. The system that we adopt in this study is a combination of terminologies proposed by Häcker [129], Chamberlain and colleagues [125], and Warner [126]. Note in particular the approach proposed by Ames [19], which emphasizes the microstructure of individual syringeal bones that make up the tympanum. Abbreviations as listed in Table 1.
Figure 5
Figure 5
The internal bone structure of the male zebra finch demonstrates optimization by combining low weight with strength. (A) Ventral and (B) dorsal halves of a clipped bone surface rendering of non-contrasted μCT scan of the male syrinx, revealing the inside surface of the syrinx and cross-sectional views of the bronchial rings. The bronchial half-rings are hollow, laterally flattened, thin-walled bones fortified with trabeculae. The holes (asterisks) in the tympanum indicate lower X-ray attenuation values due to very thin walls or lower-density bone. The boxed inset shows a detailed view of bronchial half-rings B1 and B2 with trabeculae in bronchial half-ring B1. (C) Medial view of a semi-transparent volume rendering of the left hemisyrinx. Trabeculae can be seen as bright bars or dots, when seen on-axis, due to high density bone tissue (dashed circles). (D) Medial view of right hemisyrinx. Lateral flattening (ellipses) of bronchial half-rings increases their resistance to bending in the horizontal plane (dotted line and shaded plane) and therefore increases the maximal perpendicular force (F) that can be applied before mechanical failure occurs due to breaking [75,76]. The trabeculae prevent failure of the bones due to buckling [76]. Abbreviations as listed in Table 1.
Figure 6
Figure 6
Direct comparison of syringeal soft tissue imaging techniques reveals advantages of μCT. (A) Frontal section of a male zebra finch syrinx prepared with conventional histology and stained with hematoxylin and eosin. Muscles appear as bright red, cartilage as speckled blue/purple. (B) Virtual frontal section through a 3D MRI dataset of a Magnevist-contrasted male zebra finch syrinx with isotropic voxel resolution of 23 μm. (C) Virtual frontal section through a 3D μCT dataset of an iodine-contrasted male zebra finch syrinx with an isotropic voxel resolution of 5 μm. The non-destructiveness, high resolution and relatively short scanning times of contrasted samples made μCT the optimal technique for the construction of the syrinx morphome. (D) Volume rendering of an iodine-contrasted μCT scan. (E-G) Virtual horizontal sections through the 3D μCT dataset at different positions as shown in D. Special effort was made to fix and scan the syrinx in situ, with surrounding tissues remaining intact. Abbreviations as listed in Table 1. Scale bars: 1 mm.
Figure 7
Figure 7
The vibratory soft tissues in the primary bronchi. (A) Lateral view onto the lateral labium (LL, orange) of the male right hemisyrinx. Cartilaginous tissue (blue) extends from bronchial half-rings B2 and B3. The MVC (blue), a large cartilaginous pad, extends from B2 and bends medially. (B) Same structure as in A, but turned 180° around the vertical axis, providing a medial to lateral view onto the LL. The LL is connected to the medial side of bronchial half-ring B3 and is thickened in two bands (asterisks). (C) Same medial view as in B onto right LL of freshly dissected male right hemisyrinx (MVM removed). (D) Caudal view onto left LL of left hemisyrinx with left primary bronchus removed. (E) Lateral view on the medial vibratory mass (MVM, pink), the tissue continuum comprising ML and medial tympaniform membrane (MTM). Bronchial half-ring B3 is removed for an unobscured view onto the MVM, which attaches to the pessulus (PES) and forms the medial wall of the primary bronchi. (F) Same structure as in E, but turned 180° giving a medial to lateral view. The MVC is embedded in the ventral part of the MVM, as well as two small other cartilaginous pads: the medial dorsal cartilage (MDC) and the lateral dorsal cartilage (LDC). (G) Same view as in F of a freshly dissected right hemisyrinx with the musculature left intact. (H) Surface renderings of the ventral and (I) dorsal half of the internal bone structure of the syrinx with sound-producing labia. Abbreviations as listed in Table 1.
Figure 8
Figure 8
Syringeal muscles and their attachment sites in the male and female zebra finch syrinx morphome. (A) Ventral view of the male zebra finch syrinx morphome based on an iodine-stained μCT scan showing muscles, bones, cartilaginous pads, and sound-producing labia. (B) Ventro-lateral view with transparent right ventral muscles VS and SVTB, revealing the underlying DVTB. (C) Attachment sites of the syringeal muscles seen from ventral and (D) ventro-lateral. The dotted line indicates the location of the CASM on which several muscles insert. (E) Dorsal view of the male zebra finch syrinx morphome. (F) Ventro-lateral view with transparent right dorsal muscles SVTB and LDS, revealing the DDS and DVTB. (G) Attachment sites of the syringeal muscles seen from dorsal and (H) dorso-lateral. (I) Ventral view of the female zebra finch syrinx morphome showing muscles, bones, cartilaginous pads and sound-producing labia. The muscles VS, SVTB and DVTB on the right hemisyrinx are transparent. (J) Dorsal view with left MDS and DTB transparent. (K) Muscle attachment sites seen from ventro-lateral and (L) dorso-lateral view. Abbreviations as listed in Table 1. DDS: yellow; DTB: orange; DVTB: dark green; LDS: neon green; MDS: violet; ST: blue; STB: cyan; SVTB: light green; VS: purple.
Figure 9
Figure 9
Syringeal muscles of a freshly dissected male zebra finch syrinx. (A) Ventral view of the caudal part of the syrinx showing the ventral muscles VS and VTB, and the IBL. The IBL connects the primary bronchi and restricts their lateral movement. Dashed box is shown enlarged in B. (B) Detail of VTB attachment on bronchial half-ring B3. Syringeal muscles are organized in sheets of muscle fibers (arrowheads). (C) Close-up of the rostral attachment sites of the ventral muscles. The VS fibers end on the edge of the tympanum, while the SVTB fibers do not attach onto bone but end in the connective tissue of the CASM. Rostral to the CASM, the extrinsic TL attaches and runs along the trachea up to the larynx. (D) Lateral view of the right hemisyrinx showing the muscle attaching on B3. Some fibers of the STB attach directly on the ML that is located on the inside of B3. (E) Dorsal muscle attachment sites. (F) Lateral view of rostral attachment sites of SVTB and DTB clearly reveals the organization of syringeal muscles in sheets (arrowheads). Individual muscle fibers can be seen within the fiber bundle sheets. The edge of the CASM is indicated by the dashed black line. Abbreviations as listed in Table 1.
Figure 10
Figure 10
Two syringeal muscles exert direct motor control on the medial vibratory mass. (A) Ventral and (B) dorsal view of the right primary bronchus. The muscles VS and MDS insert directly on cartilaginous pads embedded in the MVM, hence controlling its tension. (C) Medial view of the MVM of the morphome and (D) of a fresh dissection. Contraction of the VS generates a force (FVS) that bends the MVC, thereby stretching (dotted lines) the ML towards the lateral dorsal cartilage (LDC). Energy stored within the MVC is released after VS contraction. This restores VS length back to resting length, and therefore also reduces the tension in the ML back to baseline. Contraction of the MDS generates a force (FMDS) that also increases tension in MVM, but with a different orientation. (E) The rostral view shows that the projected working lines (arrows) of the VS and MDS are aligned along the dorso-ventral axis. Contraction of these muscles therefore results in no or very little adduction of the ML into the bronchial lumen, and modulates tension only. Abbreviations as listed in Table 1. Colors as in previous figures.
Figure 11
Figure 11
Muscle actuation of bronchial half-rings B1-B3. (A) Rostro-lateral and (B) dorso-lateral view of the right primary bronchus showing the muscles DDS and LDS that attach directly onto bronchial half-ring B1. (C,D) Same views as in A and B, but now showing the muscles DTB and STB that attach directly onto bronchial half-ring B2. The large VS muscle is omitted for clarity. (E,F) The muscles SVTB and DVTB attach directly onto bronchial half-ring B3. Abbreviations as listed in Table 1. Colors as in previous figures.
Figure 12
Figure 12
The sternotracheal muscle stabilizes the syrinx during song. (A) Lateral view of a μCT-based volume rendering showing the position of the syrinx in the skeletal framework of the upper thorax. A spine (orange dashed line) projects ventro-caudally from the second thoracic vertebra, thereby providing an anchor point for the lungs (green) and a pivot point (white circle) for the syrinx. On its ventral side, the syrinx fits into a dorsally oriented protrusion of the sternum, the external spine (EXS, *). The force (Fst) exerted by contraction of the ST (blue) rotates the syrinx ventrally (arrow) into the external spine. (B) Lateral view of a dissected syrinx with the ST muscles (left ST, black arrowheads) and their attachments intact. The external spine is continuous with a collagenous band (CB) that connects to the CASM (dotted white line). Also visible are the arteria syringealis, which supplies blood to the syrinx (white arrow), and the left syringeal nerve (black arrows). (C) Caudal view of a μCT-based volume rendering looking up from the sternum showing the position of the syrinx (yellow circle) and the attachment sites of the ST muscles (blue lines) in the intact skeletal framework. The ST attaches to tracheal ring T1 and on two lateral protrusions of the sternum. Contraction of the ST muscles pulls the syrinx onto the EXS. (D) Virtual slice through a 3D MRI dataset showing the syrinx (yellow dotted line) and EXS. Abbreviations as listed in Table 1.

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References

    1. Hauser MD, Chomsky N, Fitch WT. The faculty of language: what is it, who has it, and how did it evolve? Science. 2002;298:1569–1579. doi: 10.1126/science.298.5598.1569. - DOI - PubMed
    1. Doupe AJ, Kuhl PK. Birdsong and human speech: common themes and mechanisms. Ann Rev NeuroSci. 1999;22:567–631. doi: 10.1146/annurev.neuro.22.1.567. - DOI - PubMed
    1. Woodgate JL, Mariette MM, Bennett ATD, Griffith SC, Buchanan KL. Male song structure predicts reproductive success in a wild zebra finch population. Anim Behav. 2012;83:773–781. doi: 10.1016/j.anbehav.2011.12.027. - DOI
    1. Collins S. In: Nature's Music: The Science of Birdsong. Marler PR, Slabbekoorn H, editor. San Diego, CA: Elsevier; 2004. Vocal fighting and flirting: the functions of birdsong; pp. 39–79.
    1. Sakata JT, Vehrencamp SL. Integrating perspectives on vocal performance and consistency. J Exp Biol. 2012;215:201–209. doi: 10.1242/jeb.056911. - DOI - PMC - PubMed

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