Examples of mechanisms that could account for changes in organ shape.

<p>In isotropic growth, the tissue grows equally in all directions. In anisotropic growth, there is more growth in some directions than others (in this case more growth occurs vertically, resulting in an elongated shape). Anisotropic growth can result from organ-extrinsic or intrinsic mechanisms. Extrinsic: the tissue elongates in response to directional forces imposed from outside. Intrinsic mechanisms result from two types of cell behavior: non-directional and directional. Non-directional: in non-uniform proliferation rate the orientation of divisions is random, but more divisions occur in some places in the tissue than others. Cell size can also change non-uniformly, resulting in different cell densities in different parts of the tissue. Directional: cell migration translocates cells in a preferred direction. Oriented cell division occurs when the spatial allocation of the daughter cells is directionally biased, resulting in elongated “clones” of cells. (The red and green cells are given for reference).</p

Geometry and patterning of the limb bud.

<p>A) Geometry of the limb bud. Yellow - ectodermal layer. Blue – dorsal. Green - ventral mesenchyme. Red line indicates the dorsoventral boundary (solid - posterior, dashed - anterior). The thick grey lines represent the flank, with the dots indicating the points of cross-section between the posterior and the dorsal view. B) Expression domains of patterning signals. The AER - apical ectodermal ridge, expresses FGF encoding genes (<i>Fgf8</i>, <i>Fgf4</i>, <i>Fgf9</i>, <i>Fgf17</i>). The ZPA-zone of polarizing activity, is the source of Sonic Hedgehog (Shh). Bone morphogenetic protein (BMP4) is expressed in a broad domain, which is progressively restricted in time. Gremlin1 is a BMP antagonist. BMP, Gremlin, Shh, and FGF are interlinked in signaling feedback loops, which causes their domains of expression and activity to change over time (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000421#pbio.1000421-Zeller1" target="_blank">[37]</a>). Wnt5a is expressed in a proximo-distal gradient, with highest levels at the tip of the limb bud <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000421#pbio.1000421-Summerhurst1" target="_blank">[38]</a>.</p

Heterozygous (DEL-90-C2) embryo E17.5

Heterozygous deletion of the genomic region (DEL-90-C2) analyzed at embryonic day E17.

Col2a1-CreERT2/R26DTA mouse embryo E15.5

Col2a1-CreERT2/R26DTA mouse embryo injected with 2.5 mg tamoxifen at E12.5 and analysed at embryonic day E15.

mouse control embryo E17.5

mouse control embryo analyzed at embryonic day E17.

Shh hypomorph mouse embryo E17.5

Shh hypomorph mouse embryo analyzed at embryonic day E17.

Homozygous INV(-500-C1) E17.5

Homozygous inversion of the genomic region INV(-500-C1) analyzed at embryonic day E17.

Nkx2.2-Cre/Shhfloxed/floxed embryos E15.5

Nkx2.2-Cre/Shhfloxed/floxed mouse embryo analyzed at embryonic day E15.

Col2a1-CreERT2/R26DTA mouse embryo E17.5

Col2a1-CreERT2/R26DTA mouse embryo injected with 2.5 mg tamoxifen at E12.5 and analysed at embryonic day E17.

(INV6b-C2)/(DEL-90-C2) E17.5

Inversion of the genomic region (INV6b-C2) and Deletion of the genomic region (DEL-90-C2) analyzed at embryonic day E17.