Xenopus Muscle Development

This illustration is of a frog galvanoscope, the first instrument used to detect electrical nerve conduction by the scientist Luigi Galvani in the eighteenth century. Source image: (Armin6 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)])

Xenopus spp. exhibit biphasic development. This means there are two primary phases of development, both an embryonic/larval stage as well as metamorphosis. This page will mostly discuss embryonic myogenesis, wherein muscle tissue first begins to take shape.

When does muscle first appear in Xenopus? What about humans?

It is challenging to define a time in which a whole tissue type first emerges. Instead, it may be easier to discuss various ‘defining’ moments controlling the formation of muscle. In Xenopus, muscle-specific genes first become activated sometime between a stage 9 blastula and stage 10 pre-embryo, occurring approximately 7 hours and 9 hours post-fertilization (at 23 degrees C), respectively. Expressed soon afterwards is alpha-cardiac actin, the first gene transcribed in vertebrates that dictates muscle structure. Notably, the myogenic regulatory factors (MRFs), which control myogenesis, are expressed in three myogenic ‘waves’ – the first beginning at around stage 10.5. As for when muscle cells themselves emerge, the differentiation of the first muscle fibers begins at the end of gastrulation (stage 12.5-13), and the somites that form the first myotome lineage appear at about stage 17 (18 hours 45 minutes post-fertilization). At ~96 hours post fertilization, muscles begin responding to stimuli, providing a perhaps decent (if crude) indicator of whole muscle tissue development.

In humans, the muscle-specific glycolytic enzyme b-enolase has been reported as among the earliest markers of myogenic differentiation and is first expressed at three-week-old embryos in the heart and four-week-old embryos in somites of the myotomal compartment. The first human organ to form is indeed the heart, which begins beat sometime during the fourth week (compared to only ~4-5 days in the frog). Comparing directly, the human heart takes about three weeks to develop, while the frog (tadpole) heart beats in just five days. Placed side-by-side like this, we can begin to see why the frog is a useful model organism for biological research.

What are some similarities and differences in frog and human muscle?

Cartoon depiction of sarcomeres. It is believed that frogs only have the tension-bearing support of costameres at the Z-line, while humans have additional costameres positioned at M-lines.

Compared to murine myofibers, frog myofibers are twice the diameter. Meanwhile, frog sarcolemmae are ‘weaker’ (sustain lower surface tensions) than mammalian myofibers. This is perhaps due to differences in frog versus mammalian muscle in the structure of costameres, which link the cell-surrounding sarcolemma to cell-internal sarcomeres. Frog costameres appear to link sarcomeres only at Z-discs, while mammalian costameres additionally link at M-bands, providing human sarcolemma with additional longitudinal resistance to surface tension and allowing greater force generation per myofiber at a given sarcomere length. However, it is unclear how these insights would directly translate to comparisons with human muscle tissue. Notably, frogs differ from humans in that they do not have a diaphragm, but can respire both i) through the skin while underwater, ii) readily through a lining on the mouth surface, and also iii) by breathing much like humans, pulling air into the nostrils by expanding the throat, then forcing air into the lungs by contracting the mouth floor. The steps outlined in example (iii) are mediated by the contraction of skeletal muscle.