Summary• The main characteristics of the symbiosis of tagasaste ( Chamaecytisus proliferus ssp. proliferus var. palmensis ), a woody legume forming N 2 -fixing indeterminate nodules in response to infection by strains of Bradyrhizobium sp. ( Chamaecytisus ), are reported here.• The infection process in this legume was examined by bright field, phase contrast and transmission electron microscopy, and was found to be unlike any other previously described.• First steps in the infection process involve initiation of infection threads within deformed root hairs and induction of foci of host-cell divisions in the inner root cortex. However, infection of root hairs aborts early, and instead, the bacteria use the crack-entry mode of host infection, whereby they penetrate the periphery of the emerging nodule through an intercellular route, eventually infecting host nodule cells directly through altered cell walls. No successful infection threads were detected at any stage of primary-host infection or nodule invasion. Indeterminate nodules were mainly formed on unbranched areas of lateral roots.• This is the first description of such a combination of events in an infection process in the Rhizobium -legume root-nodule symbiosis.
The successful regrowth of retinal ganglion cell (RGC) axons after optic nerve (ON) axotomy in Gallotia galloti indicates a permissive role of the glial environment. We have characterised the astroglial lineage of the lizard optic pathway throughout its ontogeny (embryonic stage 30 [E30] to adults) by using electron microscopy and immunohistochemistry to detect the proliferation marker PCNA (proliferating cell nuclear antigen), the transcription factor Pax2 and the gliofilament proteins vimentin (Vim) and GFAP (glial fibrillary acidic protein). PCNA(+) cells were abundant until E39, with GFAP(+)/PCNA(+) astrocytes being observed between E37 and hatching. Proliferation diminished markedly afterwards, being undetectable in the adult optic pathway. Müller glia of the central retina expressed Pax2 from E37 and their endfeet accumulated Vim from E33 and GFAP from E37 onwards. Astrocytes were absent in the avascular lizard retina, whereas abundant Pax2(+) astrocytes were observed in the ON from E30. A major subpopulation of these astrocytes coexpressed Vim from E35 and also GFAP from E37 onwards; thus the majority of mature astrocytes coexpressed Pax2/Vim/GFAP. The astrocytes were ultrastructurally identified by their gliofilaments, microtubules, dense bodies, desmosomes and glycogen granules, which preferentially accumulated in cell processes. Astrocytes in the adult ON coexpressed both gliofilaments and presented desmosomes indicating a reinforcement of the ON structure; this is physiologically necessary for local adaptation to mechanical forces linked to eye movement. We suggest that astrocytes forming this structural scaffold facilitate the regrowth of RGCs after ON transection.
Spontaneous regrowth of the axons of retinal ganglion cells (RGC) occurs after unilateral optic nerve transection (ONT) in the lizard Gallotia galloti. We have performed an immunohistochemical and ultrastructural study of the conus papillaris (CP) of this lizard during ontogeny and after ONT in order to characterize its cell subpopulations, innervation and putative blood-brain barrier (BBB) and to evaluate changes occurring throughout regeneration. Proliferating PCNA(+) cells were abundant between embryonic stage 33 (E33) and hatching. From E33, we observed Pax2(+)/GS(+) glial cells in the primitive CP, which became increasingly pigmented and vascularised from E35. Conal astrocytes coexpressing Pax2 with vimentin and/or GFAP were identified from E37-E38. GluT-1(+)/LEA(+)/Pax2(-) endothelial cells (ECs) formed a continuous endothelium with tight junctions and luminal and abluminal microfolds. In adults, the peripheral blood vessels showed a thinner calibre, stronger GluT-1 staining and more abundant microfolds than those of the central CP indicating the higher specialization involved during transport within the former. Occasional pericytes, abundant Pax2(+) pigment cells, LEA(+) microglia/macrophages, unmyelinated Tuj1(+) nerve fibres and SV2(+) synaptic vesicles were also observed in the perivascular zone. After ONT, the expression of GluT-1 and p75(NTR) persisted in ECs, suggesting the preservation/early recovery of the BBB. Relevant ultrastructural alterations were observed at 0.5 months postlesion, although, by 3 months, the CP had recovered the ultrastructure of controls indicating tissue recovery. Abnormal newly formed blood vessels had developed in the CP-optic nerve junction. Thus, the CP is a central nervous system structure whose regenerating capacity might be key for the nutritional support of regenerating RGCs in G. galloti.
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