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The examination of cell lineages is an important step towards understanding the developmental events that specify the various cell types in the organism. The mechanisms that control which cell types are formed, their locations, and their numbers remain unknown. Analyses of cell lineage in the frog neural retina have revealed that individual precursors are multipotent and are capable of producing almost any combination of cell types. In addition to giving rise to a wide range of phenotypes, the precursors can give rise to a wide range of clone sizes. Cell lineage studies in other systems indicate that some precursors are multipotent, like those in the retina, while others appear to produce a more restricted range of descendants, perhaps even a single phenotype. These differences in the developmental potential of precursor cells suggest that the nervous system uses several strategies for producing its many cell types. Investigation of these strategies, at the cellular and molecular level, requires more than a description of the normal cell lineages. We are now exploiting the frog neural retina to perform the experimental manipulations needed to elucidate these strategies.
Fig. 1. Rhodamine dextran as a lineage tracer in the frog
neural retina. (A) A single precursor cell is filled with
rhodamine dextran by microinjection. This cell, labelled at
St 23, is located in the optic vesicle (OV); its elongated
morphology is characteristic of retinal precursor cells.
(B) After several days of development, the descendants of
an injected precursor are identified by the rhodamine label.
As shown in this example, precursors in the optic vesicle
frequently produced clones that spanned all three layers of
the neural retina (the ONL, INL, and GCL). Since
different cell types are located in each of these layers, the
multilaminar distribution of the descendants indicates that
their precursor produced multiple cell types. Most of the
descendants are aligned in a radial column, indicating that
there is little cell mixing during retinal development.
Abbreviations: Ep, epidermis; GCL, ganglion cell layer;
INL, inner nuclear layer; L, lens; ONL, outer nuclear
layer; OV, optic vesicle; PRE, pigmented retinal
epithelium. Scale bars=20 microns.
Fig. 2. Histograms of the frequency distributions of clone
sizes for precursors labelled at the optic vesicle (St 22) and
the optic cup (St 30) stages. Precursors labelled at St 30
(top histogram) produce more small clones and fewer large
clones than do precursors labelled at St 22 (bottom
histogram). This difference between St 30 and St 22 is not
surprising, because precursors at St 30 have less time to
proliferate before the end of the birthdate period (at St
37). Precursors at both stages produce a relatively wide
range of clone sizes; this wide range suggests that the
regulation of proliferation might involve a stochastic
process.
