An Investigation into the Sonic Hedgehog Gene
In the depths of the animal cell, genes are fragments of DNA that code for an animal’s biological traits. One such
fragment, discovered in 1980 via fruit flies of the Drosophila genus (Cecchetelli), is named the Sonic Hedgehog (SHH)
gene. In the cell, this gene is used to make messenger-RNA, which is then used as input by ribosomes attached to the
rough endoplasmic reticulum (Mao) to synthesize the Sonic Hedgehog (SHH) protein. In general, proteins are the body’s
“building blocks” (LaPelusa and Kaushik). Clifford J. Tabin, a Harvard researcher, says, “Individual genes” that code for
proteins “can …[guide] the formation of complex body parts” (Your Inner Fish). The SHH protein is one such building
block; Clifford says, it “tells different cells to do things in a particular order depending on how close they [are] to the
source of the signal” (YIF, 39:18), applying this especially to the embryonic development of limbs.
To find experimental evidence that supports this theory, Clifford and his team looked for a match between a
fruit-fly's SHH gene and a vertebrate homologous gene in chickens (Mossman). “After months of searching” (YIF,
39:35), they were able to identify the location of the SHH gene in chickens and isolate it to conduct studies on its
patterns in limb development. One such study entailed adding a “bead” of pure SHH genes to the opposite side of the
embryo’s growing digits, which was met with the emergence of “a second set of digits.” The SHH protein was somehow
“telling the digits where to form” (YIF, 37:30). Cliff concludes, “SHH shapes not just the wings of chicken, but [...] even
our own hands” (YIF, 40:50). The video also makes a bold claim, stating that increasing the effect of Sonic Hedgehog
results in “extra fingers” (YIF, 42:18) and vice versa.
The SHH protein “plays a role in shaping the body” (MedlinePlus) and “determining the placement of limbs and
organs in [a] developing embryo” (Perman). SHH begins by catalyzing the proliferation of mesenchyme cells in human
embryos (Tickle and Towers), causing the growth of a limb bud. Next, it causes cells along the anteroposterior axis (see
Image 1) to establish their “identity” – which differentiates an index from a thumb – by forming a concentration gradient
across the limb bud. This gradient also influences the apical ectodermal ridge (see Image 1), affecting growth along the
proximo-distal axis. Growing evidence also suggests the “effectiveness” of SHH – referring to enhancement of SHH
gene expression levels – has the potential to cause limb defects (Wolpert), such as oligodactyly and polydactyly (Chiang)
– examples shown in the video. Ultimately, I am very convinced about the presence and function of the SHH gene.
Scientists are yet to transform this knowledge into solutions to address SHH-related defects, which remains a possibility
for the near future.
Image 1 (Wolpert)
Word Count
Word count = 465
Excluding in-text citations = 465 - 38 = 429
In the depths of the animal cell, genes are fragments of DNA that code for an animal’s biological traits. One such
fragment, discovered in 1980 via fruit flies of the Drosophila genus (Cecchetelli), is named the Sonic Hedgehog (SHH)
gene. In the cell, this gene is used to make messenger-RNA, which is then used as input by ribosomes attached to the
rough endoplasmic reticulum (Mao) to synthesize the Sonic Hedgehog (SHH) protein. In general, proteins are the body’s
“building blocks” (LaPelusa and Kaushik). Clifford J. Tabin, a Harvard researcher, says, “Individual genes” that code for
proteins “can …[guide] the formation of complex body parts” (Your Inner Fish). The SHH protein is one such building
block; Clifford says, it “tells different cells to do things in a particular order depending on how close they [are] to the
source of the signal” (YIF, 39:18), applying this especially to the embryonic development of limbs.
To find experimental evidence that supports this theory, Clifford and his team looked for a match between a
fruit-fly's SHH gene and a vertebrate homologous gene in chickens (Mossman). “After months of searching” (YIF,
39:35), they were able to identify the location of the SHH gene in chickens and isolate it to conduct studies on its
patterns in limb development. One such study entailed adding a “bead” of pure SHH genes to the opposite side of the
embryo’s growing digits, which was met with the emergence of “a second set of digits.” The SHH protein was somehow
“telling the digits where to form” (YIF, 37:30). Cliff concludes, “SHH shapes not just the wings of chicken, but [...] even
our own hands” (YIF, 40:50). The video also makes a bold claim, stating that increasing the effect of Sonic Hedgehog
results in “extra fingers” (YIF, 42:18) and vice versa.
The SHH protein “plays a role in shaping the body” (MedlinePlus) and “determining the placement of limbs and
organs in [a] developing embryo” (Perman). SHH begins by catalyzing the proliferation of mesenchyme cells in human
embryos (Tickle and Towers), causing the growth of a limb bud. Next, it causes cells along the anteroposterior axis (see
Image 1) to establish their “identity” – which differentiates an index from a thumb – by forming a concentration gradient
across the limb bud. This gradient also influences the apical ectodermal ridge (see Image 1), affecting growth along the
proximo-distal axis. Growing evidence also suggests the “effectiveness” of SHH – referring to enhancement of SHH
gene expression levels – has the potential to cause limb defects (Wolpert), such as oligodactyly and polydactyly (Chiang)
– examples shown in the video. Ultimately, I am very convinced about the presence and function of the SHH gene.
Scientists are yet to transform this knowledge into solutions to address SHH-related defects, which remains a possibility
for the near future.
Image 1 (Wolpert)
Word Count
Word count = 465
Excluding in-text citations = 465 - 38 = 429
