web contact: pietsch@indiana.edu RETINOIC ACID:
IT INDUCES MIRROR-IMAGE MUSCULATRES IN SALAMANDER LIMB REGENERATES
Retinoic acid (RA) is known to have
profound effects on differentiation in the salamander limb regenerate,[1] sometimes inducing the development of
supernumerary limbs.[2] Occasionally, the extra
members are nearly mirror images of each other, not only in external form, but also
internally, as manifest in the specific pattern of the musculatures. Thus
blastema cells on the right amputation stump possess the generally hidden
potentiality for making organs of the left side of the body, and vice versa.
<--- Here are 2 mirror-image hands, wrists and forearms regenerated from a left sided, upper
arm (humeral level) stump. Amputation (on 02 Jan 1987) was through the head of the humerus. Thus virtually everthing you see here had to have been regenerated by what remained of the left arm.
The animal at the outset of the experiment was a 3 cm long Ambystoma opacum larva. It received an
injection of RA into its abdominal cavity (coelom) on 12 Jan 1987. On 11 Apr
1987 it was put to sleep (in MS 222) and, while unconscious, transferred to
Bouin's fixative, which killed and preserved it. After this picture was taken, the specimen was prepared for
paraffin sectioning. Orientation of the specimen in the block, carried out
under stereoscopic microscope, was such that serial sections could be made of
the long axes of the back-to-back limbs. The specimens were stained
with Mallory's trichrome procedure, which renders connective tissue (collagen)
bright blue and muscle, ruby red. Two of the serial sections are shown below.
I was able to orient my analysis of the muscles by first identifying the small
but constant ulno-carpalis muscle, which runs almost the entire length of the
forearm and wrist, in its course parallelling the ventral side of the ulna and, in the wrist, the
ulnare carpal. The two proximally fused members were slightly out of register, but for a
considerable extent, I simultaneously traced the ulno-carpalis muscles in both (arrows) members,
first as a minute but fleshy, ruby-colored cylinder, then as a blue-staining tendon. With all
other muscles, those in one, were always matched by a mirror-image
contralogue a few serial sections away in the other member.
The set on the left is closer to the elbow than the one on the right. The arrows (white) point to the ulno-carpalis muscles. The upper member of each section is the distal wrist showing 4 carpals (they're made of cartilage -- gristle-- in the salamander larva) . The lower member on the left set is just at the junction of the forearm and wrist and shows the distal end of the ulna, so labeled; on the right, just a few sections distally, the lower member shows the proximal wrist. To visualize what's happened here, imagine folding open a book (go here for a flipped version of the left picture to assist the vis-à-vis imagery.) These serial sections, remember, come from the specimen shown above. {Click either photo for an enlargement!}
[Use your back command to return to your place in the dialogue.]
[Go here for the start of the Zook dialog on the blastema.]
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web contact: pietsch@indiana.edu