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Pl. V · Iunctio tendinea — the myotendinous junction

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Pl. V · FAM. IUNCTIDAE

Iunctio tendinea var. mytilus

the splice  ·  the myotendinous junction

The plate

The specimen, drawn

Pl. V

Hand-drawn field-guide plate: a blue mussel in colored pencil, its wedge-shaped shell standing point-down on a rock, with a fan of amber byssal threads spreading from the gape and each thread ending in a flat, spread plaque rather than a point. One thread has snapped and lies frayed, its plaque still stuck to the rock. Below, a schematic of the myotendinous junction at ×400: the end of a striated muscle fiber drawn as five deep interlocking fingers, with tendon collagen bundles filling every gap and flattened fibrocyte nuclei in rows between them. A circular inset at ×40 shows muscle giving way to tendon — the fibers taper out and wavy parallel collagen takes over, with no visible seam between them.

The junction as you will meet it on the slide.

Habitat & range

Where it lives

Body map

Hand-drawn body map: a human figure in broken pencil outline with cross-hatched fill, carrying five numbered markers. Where it lives — 1, the rotator cuff, into the shoulder; 2, the biceps, at both ends of the belly; 3, the hamstrings, high at the back; 4, the calf, where it becomes the Achilles; 5, and every other muscle, at both ends.

Where this junction is found.

Range

  • Both ends of every skeletal muscle you have. A muscle does not attach to bone — its tendon does, and the muscle attaches to the tendon.
  • The calf, where gastrocnemius and soleus hand off to the Achilles. The junction sits well above the heel, and that is where a torn calf tears.
  • The hamstrings, high at the back of the thigh. The long head of biceps femoris runs its junction some twenty centimetres down into the muscle belly, and that is the most-torn junction in sprinting. The rotator cuff tears too, but usually at the bone rather than here.
  • The biceps, at both ends of the belly. An aponeurosis counts too; it is a tendon that has been flattened into a sheet.
  • Nowhere in cardiac or smooth muscle. Neither one ends in a tendon, because neither one pulls on a bone.

Collection record

  • Add your slide numbers here.

Niche

What it does for a living

It is a splice, and the problem it solves is that muscle is soft and tendon is not. Butt the two together and the whole force crosses a flat seam in tension, which is the worst way there is to load a bond. So the end of each muscle fiber is thrown instead into deep finger-like folds that interlock with the collagen coming the other way, multiplying the contact area by ten or twenty and turning a straight pull into a shear spread across all of it at once. Inside the fiber, the last sarcomere anchors its actin into a dense plaque bolted through the membrane to the collagen outside, so the force passes protein to protein and never crosses a gap. It is still the weakest link in the chain — a strain tears here, not in the middle of the belly — but it is far stronger than the flat seam it replaced.

field collected specimen

How to know it on sight

  • Pink muscle on one side, pinker and more fibrous tendon on the other, with a ragged boundary between them rather than a straight one.
  • The muscle fibers taper as they approach, and the striations fade out before the fiber ends.
  • On the tendon side the collagen bundles run parallel and slightly wavy, with flattened dark nuclei squeezed into rows between the bundles — never inside them.
  • There is no line you can point to and call the join. The two tissues interleave over a distance, and that is the entire trick.
A whole skeletal muscle with tendons at each end, below it a bundle of striped fibers (a fascicle), and below that one striped fiber, each labeled with the connective-tissue wrapping around it.

Specimen 1

Photomicrograph of dense regular connective tissue: pink collagen fibers running in parallel bundles, with small dark elliptical fibroblast nuclei lying in rows in the gaps between the bundles.

Dense regular connective tissue — here from the esophagus, but the same tissue a tendon is made of. Collagen in parallel, and the only nuclei are flattened fibroblasts lying in rows in the gaps. Compare that with the muscle side, where the nuclei sit inside the fiber, pressed against its edge.

Specimen 2

Skeletal muscle histology slide showing epimysium, perimysium, and endomysium stained pink.

The three wrappings, cut across — endomysium around each fiber, perimysium around each bundle, epimysium around the whole muscle. All three run on past the end of the muscle and become the tendon. The junction is where the fibers stop, not where the connective tissue starts.

Specimen 3

Microscopic view of skeletal muscle tissue arranged in a longitudinal section. The muscle fibers appear as elongated, pink-stained bands running horizontally across the image. Darkly stained oval nuclei are visible within the muscle fibers. The striated, or striped, appearance of the muscle fibers is also clearly visible.

Muscle in longitudinal section, well away from the join. Note how blunt and square the fiber ends look here. At the junction they do not end like this at all; they taper and fray into fingers you will only resolve at high power.

Two attachments: a biceps brachii on the arm bones, narrowing into a rope-like tendon at each end; and the back of a head with the two bellies of the epicranius joined by a flat pale sheet over the skull.

People keep calling her by the wrong name

Look-alikes

Two plates. Read the line underneath each one before you decide.

Dense irregular connective tissue

Photomicrograph of dense irregular plantar skin at high magnification. The dominant visual element is dense, eosinophilic (pink) collagen fibers arranged in irregular orientation throughout the field of view. Small areas of lighter staining or white space are interspersed between fiber bundles. A small portion of skin tissue is visible that shows skin cells and vessels. Illustrates histologic features of tissue strength and support.
Dense irregular connective tissue from plantar skin: the same collagen and the same cells as a tendon, but woven from every angle.

Tell them apart by whether the bundles agree with one another. A tendon is dense regular — every bundle running the same way, because it is only ever loaded one way. Dense irregular, as in the dermis, is woven from every angle because it is pulled from every angle. The cells in the two are the same cells.

Fibrocartilage

High magnification photomicrograph of fibrocartilage: coarse collagen bundles sweeping across the field, with chondrocytes sitting in lacunae between the bundles, each rounded cell in a space of its own.
Fibrocartilage, intervertebral disc, H&E ×40. The same sweeping collagen bundles, but with chondrocytes sitting in lacunae between them.

Tell them apart by what sits between the bundles. Fibrocartilage has the same sweeping collagen, but its cells are chondrocytes in lacunae — rounded, each in a little space of its own — rather than flattened fibroblasts squeezed into rows. Where a tendon meets bone it often turns into fibrocartilage for a short distance, which is why both can turn up on one slide.

Examine it yourself

The virtual slide

This slide is labelled muscle tendon insertion, and it carries both tissues. Dense regular connective tissue runs across the top; the junction itself is at the bottom. Find the boundary, then try to draw it — and notice that you cannot put it in any one place.

Virtual slide courtesy of Histology Guide.

YOur missions

Field Notes for Future Missions

Mission 1: find the join

Mission 2: quick check

Field notes

Why this organism

A mussel has the same problem a muscle has: she is soft, the thing she must hold is hard, and the surf does not care. Her answer is the byssus — a fan of threads from a single stem, each one ending not in a point but in a flat plaque spread wide on the stone, and each one graded along its length from soft where it leaves the animal to stiff where it meets the rock, so there is no sudden step for a crack to start at. The threads are also meant to give: pull hard enough and one snaps and is spun again while the animal stays put. That is the myotendinous junction, problem and answer both, worked out independently by a bivalve with no bones in it at all.

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In the wild

The organism, photographed

Close-up photograph of a dense cluster of live blue-black mussels packed shell to shell on a rock, almost every shell encrusted with pale barnacles.

Blue mussels (Mytilus edulis) packed shell to shell on a rock in Galicia, with barnacles filling every gap between them. Every one of those shells is held on by threads it spun itself. Photograph: Javier Balseiro / Pexels.

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