Limits...
Development and organization of the larval nervous system in Phoronopsis harmeri: new insights into phoronid phylogeny.

Temereva EN, Tsitrin EB - Front. Zool. (2014)

Bottom Line: Phoronid larvae demonstrate some "deuterostome-like" features, which are, in fact, have to be ancestral bilaterian characters.Our new results and previous data indicate that phoronids have retained some plesiomorphic features, which were inherited from the last common ancestor of all Bilateria.It follows that phoronids should be extracted from the Trochozoan (=Spiralia) clade and placed at the base of the Lophotrochozoan stem.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Invertebrate Zoology, Biological faculty, Moscow State University, Moscow 119992, Russia. temereva@mail.ru.

ABSTRACT

Background: The organization and development of the nervous system has traditionally been used as an important character for establishing the relationships among large groups of animals. According to this criterion, phoronids were initially regarded as deuterostomian but have more recently been regarded as protostomian. The resolving of this conflict requires detailed information from poorly investigated members of phoronids, such as Phoronopsis harmeri.

Results: The serotonin-like immunoreactive part of the P. harmeri nervous system changes during larval development. These changes mostly concern the nervous system of the hood and correlate with the appearance of the median and two marginal neurite bundles, the frontal organ, and the sensory field. The apical organ has bilateral symmetry. The tentacular neurite bundle passes under the tentacles, contains several types of perikarya, and gives rise to intertentacular bundles, which branch in the tentacle base and penetrate into adjacent tentacles by two lateroabfrontal bundles. There are two groups of dorsolateral perikarya, which exhibit serotonin-like immunoreactivity, contact the tentacular neurite bundle, and are located near the youngest tentacles. Larvae have a minor nerve ring, which originates from the posterior marginal neurite bundle of the hood, passes above the tentacle base, and gives rise to the mediofrontal neurite bundle in each tentacle. Paired laterofrontal neurite bundles of tentacles form a continuous nerve tract that conducts to the postoral ciliated band.

Discussion: The organization of the nervous system differs among the planktotrophic larvae of phoronid species. These differences may correlate with differences in phoronid biology. Data concerning the innervation of tentacles in different phoronid larvae are conflicting and require careful reinvestigation. The overall organization of the nervous system in phoronid larvae has more in common with the deuterostomian than with the protostomian nervous system. Phoronid larvae demonstrate some "deuterostome-like" features, which are, in fact, have to be ancestral bilaterian characters. Our new results and previous data indicate that phoronids have retained some plesiomorphic features, which were inherited from the last common ancestor of all Bilateria. It follows that phoronids should be extracted from the Trochozoan (=Spiralia) clade and placed at the base of the Lophotrochozoan stem.

No MeSH data available.


Related in: MedlinePlus

Ultrastructure of the main tentacular neurite bundle in competent larvae of Phoronopsis harmeri. Thin cross sections. (A) A panorama of the two dorsolateral groups of perikarya (indicated by boxes), which are located near the youngest tentacles. The dorsal side of the larva is at the top. (B) Ultrastructure of a portion of the dorsolateral group of perikarya, which includes sensory (blue) and nonsensory (pink) cells. (C) One of the sensory cells (blue), which are scattered along the tentacular neurite bundle. (D) Details of the organization of the nonsensory perikaryon, which is located between neurites of the main tentacular neurite bundle on the lateral side of the larva. (E) Two large nonsensory prikarya associated with the tentacular neurite bundle. The synaptic contacts are indicated by arrowheads. Abbreviations: bb – basal body; bc – blastocoel; bl – basal lamina; G – Golgi apparatus; mc – muscle cell; mi – microvilli; n – nucleus; nf – nerve fibers; p – perikaryon; sr - striated rootlet.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC3924620&req=5

Figure 9: Ultrastructure of the main tentacular neurite bundle in competent larvae of Phoronopsis harmeri. Thin cross sections. (A) A panorama of the two dorsolateral groups of perikarya (indicated by boxes), which are located near the youngest tentacles. The dorsal side of the larva is at the top. (B) Ultrastructure of a portion of the dorsolateral group of perikarya, which includes sensory (blue) and nonsensory (pink) cells. (C) One of the sensory cells (blue), which are scattered along the tentacular neurite bundle. (D) Details of the organization of the nonsensory perikaryon, which is located between neurites of the main tentacular neurite bundle on the lateral side of the larva. (E) Two large nonsensory prikarya associated with the tentacular neurite bundle. The synaptic contacts are indicated by arrowheads. Abbreviations: bb – basal body; bc – blastocoel; bl – basal lamina; G – Golgi apparatus; mc – muscle cell; mi – microvilli; n – nucleus; nf – nerve fibers; p – perikaryon; sr - striated rootlet.

Mentions: Dorsolateral perikarya form two large groups in the base of the youngest tentacles (Figure 9A). Each group contains numerous perikarya of two main types: sensory cells and cells that do not contact the surface of the epidermis. The nucleus of sensory cells is small and contains a lot of peripheral chromatin. Sensory cells form several basal processes, which contain synaptic vesicles and surround the non-sensory perikarya (Figure 9B). The perikarya of the second type have large nuclei, which lack peripheral chromatin, and numerous small mitochondria and synaptic vesicles (Figure 9B). Both types of perikarya are scattered along the tentacular neurite bundle but do not form large aggregations (Figure 9C-E). Projections of the first type of neurons contact the basal lamina (Figure 9C). Each non-sensory perikaryon bears a large nucleus with a nucleolus (Figure 9E) and contains the basal body with striated rootlet and Golgi apparatus (Figure 9D). In sagittal sections of the larvae, the tentacular neurite bundle is cut transversally and consists of several groups of neurites, which are accompanied by different types of perikarya (Figure 10A). There are three types of neurites: those with dense-core synaptic vesicles, clear synaptic vesicles, and medium-dense synaptic vesicles. Synaptic contacts occur between the neurites (Figure 10B). The minor nerve ring is also formed by several bundles, most of which contain synaptic vesicles with medium-dense content (Figure 10C). In sagittal sections of larvae, most neurites of the minor nerve ring are oriented longitudinally and form a large mediofrontal bundle that extends into each tentacle.


Development and organization of the larval nervous system in Phoronopsis harmeri: new insights into phoronid phylogeny.

Temereva EN, Tsitrin EB - Front. Zool. (2014)

Ultrastructure of the main tentacular neurite bundle in competent larvae of Phoronopsis harmeri. Thin cross sections. (A) A panorama of the two dorsolateral groups of perikarya (indicated by boxes), which are located near the youngest tentacles. The dorsal side of the larva is at the top. (B) Ultrastructure of a portion of the dorsolateral group of perikarya, which includes sensory (blue) and nonsensory (pink) cells. (C) One of the sensory cells (blue), which are scattered along the tentacular neurite bundle. (D) Details of the organization of the nonsensory perikaryon, which is located between neurites of the main tentacular neurite bundle on the lateral side of the larva. (E) Two large nonsensory prikarya associated with the tentacular neurite bundle. The synaptic contacts are indicated by arrowheads. Abbreviations: bb – basal body; bc – blastocoel; bl – basal lamina; G – Golgi apparatus; mc – muscle cell; mi – microvilli; n – nucleus; nf – nerve fibers; p – perikaryon; sr - striated rootlet.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3924620&req=5

Figure 9: Ultrastructure of the main tentacular neurite bundle in competent larvae of Phoronopsis harmeri. Thin cross sections. (A) A panorama of the two dorsolateral groups of perikarya (indicated by boxes), which are located near the youngest tentacles. The dorsal side of the larva is at the top. (B) Ultrastructure of a portion of the dorsolateral group of perikarya, which includes sensory (blue) and nonsensory (pink) cells. (C) One of the sensory cells (blue), which are scattered along the tentacular neurite bundle. (D) Details of the organization of the nonsensory perikaryon, which is located between neurites of the main tentacular neurite bundle on the lateral side of the larva. (E) Two large nonsensory prikarya associated with the tentacular neurite bundle. The synaptic contacts are indicated by arrowheads. Abbreviations: bb – basal body; bc – blastocoel; bl – basal lamina; G – Golgi apparatus; mc – muscle cell; mi – microvilli; n – nucleus; nf – nerve fibers; p – perikaryon; sr - striated rootlet.
Mentions: Dorsolateral perikarya form two large groups in the base of the youngest tentacles (Figure 9A). Each group contains numerous perikarya of two main types: sensory cells and cells that do not contact the surface of the epidermis. The nucleus of sensory cells is small and contains a lot of peripheral chromatin. Sensory cells form several basal processes, which contain synaptic vesicles and surround the non-sensory perikarya (Figure 9B). The perikarya of the second type have large nuclei, which lack peripheral chromatin, and numerous small mitochondria and synaptic vesicles (Figure 9B). Both types of perikarya are scattered along the tentacular neurite bundle but do not form large aggregations (Figure 9C-E). Projections of the first type of neurons contact the basal lamina (Figure 9C). Each non-sensory perikaryon bears a large nucleus with a nucleolus (Figure 9E) and contains the basal body with striated rootlet and Golgi apparatus (Figure 9D). In sagittal sections of the larvae, the tentacular neurite bundle is cut transversally and consists of several groups of neurites, which are accompanied by different types of perikarya (Figure 10A). There are three types of neurites: those with dense-core synaptic vesicles, clear synaptic vesicles, and medium-dense synaptic vesicles. Synaptic contacts occur between the neurites (Figure 10B). The minor nerve ring is also formed by several bundles, most of which contain synaptic vesicles with medium-dense content (Figure 10C). In sagittal sections of larvae, most neurites of the minor nerve ring are oriented longitudinally and form a large mediofrontal bundle that extends into each tentacle.

Bottom Line: Phoronid larvae demonstrate some "deuterostome-like" features, which are, in fact, have to be ancestral bilaterian characters.Our new results and previous data indicate that phoronids have retained some plesiomorphic features, which were inherited from the last common ancestor of all Bilateria.It follows that phoronids should be extracted from the Trochozoan (=Spiralia) clade and placed at the base of the Lophotrochozoan stem.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Invertebrate Zoology, Biological faculty, Moscow State University, Moscow 119992, Russia. temereva@mail.ru.

ABSTRACT

Background: The organization and development of the nervous system has traditionally been used as an important character for establishing the relationships among large groups of animals. According to this criterion, phoronids were initially regarded as deuterostomian but have more recently been regarded as protostomian. The resolving of this conflict requires detailed information from poorly investigated members of phoronids, such as Phoronopsis harmeri.

Results: The serotonin-like immunoreactive part of the P. harmeri nervous system changes during larval development. These changes mostly concern the nervous system of the hood and correlate with the appearance of the median and two marginal neurite bundles, the frontal organ, and the sensory field. The apical organ has bilateral symmetry. The tentacular neurite bundle passes under the tentacles, contains several types of perikarya, and gives rise to intertentacular bundles, which branch in the tentacle base and penetrate into adjacent tentacles by two lateroabfrontal bundles. There are two groups of dorsolateral perikarya, which exhibit serotonin-like immunoreactivity, contact the tentacular neurite bundle, and are located near the youngest tentacles. Larvae have a minor nerve ring, which originates from the posterior marginal neurite bundle of the hood, passes above the tentacle base, and gives rise to the mediofrontal neurite bundle in each tentacle. Paired laterofrontal neurite bundles of tentacles form a continuous nerve tract that conducts to the postoral ciliated band.

Discussion: The organization of the nervous system differs among the planktotrophic larvae of phoronid species. These differences may correlate with differences in phoronid biology. Data concerning the innervation of tentacles in different phoronid larvae are conflicting and require careful reinvestigation. The overall organization of the nervous system in phoronid larvae has more in common with the deuterostomian than with the protostomian nervous system. Phoronid larvae demonstrate some "deuterostome-like" features, which are, in fact, have to be ancestral bilaterian characters. Our new results and previous data indicate that phoronids have retained some plesiomorphic features, which were inherited from the last common ancestor of all Bilateria. It follows that phoronids should be extracted from the Trochozoan (=Spiralia) clade and placed at the base of the Lophotrochozoan stem.

No MeSH data available.


Related in: MedlinePlus