| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
FJ
EXPRESS SUMMARY ARTICLE The Full-length version of this article is also available, published online November 15, 2002 as doi:10.1096/fj.02-0451fje. |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Children’s Memorial Institute for Education and Research, Department of Pediatrics, Northwestern University Medical School, Chicago, Illinois, USA;
* Department of Reproductive and Developmental Sciences, Clinical Biochemistry Section, University of Edinburgh, Royal Infirmary NHS Trust, Edinburgh, EH3 9YW, UK;
Molecular Physiology Group, University of Edinburgh Medical School, Edinburgh, EH8 9AG, UK; and
Oral Pathology Unit, The University of Birmingham, Birmingham, B4 6NN, UK
2Correspondence: Department of Pediatrics, Northwestern University Medical School, 2300 Children’s Plaza (204), Chicago, IL 60611, USA. E-mail:pmi{at}nwu.edu
SPECIFIC AIMS
During fetal development, generation of organ parenchyma in a precise and reproducible manner is critical for the normal formation of functional tissues. Recursive iteration of stereotypical cell division rules is a plausible mechanism by which this may be achieved. By studying mosaic patch patterns in livers of rat aggregation chimeras formed from two genetically distinguishable cell populations, we earlier showed that such iterated cell division rules can explain the development of liver parenchyma and that the fractal dimensions of these mosaic patches are determined by the nature of the cell division rules operating during liver parenchymal growth. The striped mosaic patch patterns displayed in the adrenal cortices of rat aggregation chimeras and transgenic mice and rats displaying mosaic transgene expression lead to the hypothesis that adrenocortical parenchyma development will be governed by a distinct but related set of iterated stereotypical cell division rules. Here we test a prediction of this hypothesis that the resulting adrenocortical tissue in mosaic animals will have a fractal patch geometry with distinct fractal dimension.
PRINCIPAL FINDINGS
1. Mosaic patches in adrenal cortex in aggregation chimeras and mosaic transgenics are arrayed as cords of clonally related cells
Examination of adult rat aggregation chimeras with histological markers showed patches in the adrenal cortex arrayed as centripetal cords of cells. Similar patterns of reporter gene expression have been observed in multiple lines of transgenic mice in which 6.4 kb of the mouse steroid 21-hydroxylase A gene 5'-flanking region are fused to a LacZ reporter gene. The striped nature of reporter gene expression implies that these patches are clonally related and represent patterns of cell lineage comparable to those seen in rat chimeras. Mosaic patches in both the adult rat and mouse adrenal cortex are arrayed in cords regardless of the method of production and thus appear as a conserved pattern of stripes (Fig. 1
). The striped patches extend from the outermost zone of the cortex in the subcapsular region to the innermost layer where the cortex abuts the medulla (a distinct organ with independent embryonic origin). This striped pattern of mosaic patches is distinct from that seen in the liver of similar animals, where patches are arrayed like islands of one cell lineage in a sea of the other cell lineage. Generation of mosaic patches with a striped pattern can be modeled mathematically by constraining the position of daughter cells after division. This model of adrenocortical growth predicts that the striped patches will be fractal and that their surface fractal dimension will be less than that of mosaic patches in the liver where daughter cell placement is unconstrained.
|
2. Patches were apparent on E16 and E18 of mouse fetal development
Mosaic patches were observed in fetal mosaic transgenic mice on embryonic day 16 (E16) and fetal patches on the outside of the cortex were arrayed in a distribution consistent with expansion of the organ. The same pattern persists at E18; the patches expand as the organ grows, with some cord-like patches observed. A cross section of the cord-like patches at the outside of the organ on E18 shows them to be symmetrical.
3. Adult and fetal patches are fractal
Surface fractal dimension of the adrenocortical patches was determined using the yardstick method. The length of the perimeter of the patch L(
) was measured using yardsticks of different length . Logarithmically spaced ruler sizes were used. An automated tracking algorithm determined the distance from an arbitrary starting point on the perimeter of the patch until the best fit on the perimeter for any given ruler size was found. The length of the patch L() was the number of rulers counted multiplied by the average ruler length. The value of surface fractal dimension D was calculated as
 |
) plotted against log (). We have previously shown there is excellent agreement between surface fractal dimensions determined by this method and those determined by box counting or dilation. Results were obtained from analysis of 153 patches in adrenal cortices of 2 independent lines of transgenic mice displaying variegated reporter gene expression. The fractal surface dimension was determined on 152 patches with the yardstick method with a ruler range of from 4 to 18 pixels using an average of 16 positions. The average fractal dimension was D = 1.22 ± .05. The control square test image has D = 1.008.
4. Surface fractal dimension from liver and adrenal cortex are different
The results establish that adult and fetal adrenal patches are fractal but have a lower surface fractal dimension than those in the adult liver (P<0.001). The fractal dimension of fetal adrenal patches is not significantly different from those of adult adrenal patches. The values comparing liver, adrenal, and fetal adrenal patches for this method are presented in Table 1
.
|
CONCLUSIONS AND SIGNIFICANCE
We have demonstrated that irrespective of the experimental method of production, the adrenal cortices of mosaic rats and mice show a conserved pattern of cord-like patches. This adrenocortical mosaic striped patch pattern that is apparent even in highly unbalanced chimeras is quite distinct from mosaic liver patches, which resemble "geographic" islands in a sea. Both of these patch patterns could result from repetitively applied stereotypical cell division rule sets that differ only in the constraints on the placement of daughter cells after cell division. A testable prediction from the distinct but related nature of these rule sets is that both will give rise to patch patterns possessing fractal geometry but possessing distinct fractal dimension. We have shown that adrenocortical mosaic patches are indeed fractal objects and that their surface fractal dimension is less than that observed for liver patches. This suggests that adrenocortical growth is governed by the repetitive application of a specific stereotypical cell division rule set and that adrenocortical patches are subjected to a constrained growth pattern during development (Fig. 2
). We have described a nearest neighbor mathematical model of growth in which a cell division rule set severely biases daughter cell placement toward unoccupied sites. That is, a force must be overcome to allocate to an occupied position and shuffle cells to make room for the daughter cell. This model causes growth to occur at the edges of a patch and generates contact inhibition to division within the "organ." Computer-generated patch patterns modeled using a constrained cell division algorithm closely resemble those seen in the adrenal cortex of experimentally produced mosaic animals. If the daughter cell placement was unbiased, patterns much more like the unconstrained liver pattern were obtained. The data presented here support the contention that dissimilar mosaic patterns (i.e., liver and adrenal cortex) can result from closely related algorithmic processes. The results inform an important question in the generation of organ parenchyma: Is the process deterministic or procedural? Can the process of cell division in itself determine the positions of organ parenchymal cells during development without requiring any guidance, positional information, or active cell migration (procedural development)?
|
During fetal development in the rat, DNA synthesizing cells are arrayed throughout the developing organ and proliferate without apparent movement. Thus, we might expect to see cell division throughout developing adrenal cortex up to and possibly including E16 (unconstrained clonal expansion of patches). Meanwhile, at E18 the zones have begun to form (and will continue to resolve both pre-and postnatally), perhaps leading to more constrained cell division.
The data inform the broader area of ways in which stereotypical iterated cell division programs can self-organize and illustrate a manner in which biological chimeras and genetic mosaics can be used to study these aspects of organogenesis. Such data obtained from liver and adrenal cortex model systems are likely to be relevant to growth and cell division in other organs and will help in understanding how organ parenchyma are generated and maintained from multipotent stem cell populations located in specific geographical locations within the organ. Ultimately, understanding the basis of stereotypical iterated cell division programs is likely to be essential in achieving organ regeneration in vivo or in vitro from multipotent or pluripotent stem cell populations.
FOOTNOTES
1 To read the full text of this article, go to http://www.fasebj.org/cgi/doi/10.1096/fj.02-ndash;0451fje; to cite this article, use FASEB J. (November 15, 2002) 10.1096/fj.02-0451fje 
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
