Intraorganismal Convergence

By Mike Gene, 3/19/05


Intraorganismal Convergence

The manner in which human skeletal muscle and peripheral nerves are organized is strikingly similar.

Consider the following:

  • In nerves, the individual axons are wrapped by a delicate connective tissue known as endoneurium. In muscles, individual muscle fibers are wrapped by a delicate connective tissue known as endomysium.
  • In nerves, the axons/endoneurium is bundled into fascicles by a connective tissue known as perineurium. In muscle, the muscle fibers/endomysium is bundled into fascicles by a connective tissue known as perimysium.
  • The entire nerve is surrounded by a connective tissue sheath called the epineurium. The nerve is thus a bundle of fascicles and blood vessels. The entire muscle is surrounded by a connective tissue sheath called the epimysium. The muscle is thus a bundle of fascicles and blood vessels.
  • With spinal nerves, the epineurium fuses with the dura mater of the spinal meninges. The dura mater is composed of dense irregular connective tissue. With skeletal muscle, the epimysium extends beyond the belly of the muscle to become the tendon (composed of dense regular connective tissue). The tendon connects to the periosteum of the bone, composed of dense irregular connective tissue.

    Are these striking structural similarities explained by common descent, where some structure in a simple metazoan shared this basic organization and served as both nerve and muscle? Or might this be an example of convergence within the same organism?

    Of course, both skeletal muscle and nerves conduct electrical impulses. Perhaps the concentric arrangement optimizes the transmission of these electrical impulses. Yet nerves and muscle are more than neurons and muscle fibers. To construct the structures as shown above, a developmental pathway must be tapped to arrange epithelial, connective, nervous, and muscle tissue in the same form.

    This would all add to the plausibility of a designer front-loading evolution. That is, unless we add other players to the picture, the mere function of a neuron or muscle cell can go a long way in guiding selection to unpack a developmental routine to form something as complex as a nerve or muscle.

    [Source of figures of nerve and muscle: Here

    ID THINK