Tendon's ultrastructure

Abstract

The structure of a tendon is an important example of complexity of ECM three-dimensional organization. The extracellular matrix (ECM) is a macromolecular network with both structural and regulatory functions. ECM components belong to four major types of macromolecules: the collagens, elastin, proteoglycans, and noncollagenous glycoproteins. Tendons are made by a fibrous, compact connective tissue that connect muscle to bone designed to transmit forces and withstand tension during muscle contraction. Here we show the ultrastructural features of tendon's components.

Keywords: collagen; extra-cellular matrix; tendon; ultrastructure.

Figure 1

Ultrastructure features of ECM in tendon. A: Collagen I is the mainly constituent of tendons and creates highly aligned fibrils organized as fibers oriented longitudinally, transversally and also crossing each others, thus conferring a great resistance to different forces (O.M. ×7900). B: Collagen I fibrils are in contact to thin fibrils of collagen V and to proteoglycans and glycoproteins (O.M. ×12500).

Figure 2

Immunolocalization of type V collagen in different tissues. Immunostaining with a specific monoclonal antibody against collagen type V and colloidal gold conjugated goat anti-mouse (O.M. × 11500, ×7900, ×18000, × 45000 respectively). A: desmoplasia in breast carcinoma; B: fibroblasts tissue culture; C-D: Degenerative tendon scar

Figure 3

Ultrastructure features of tenocytes and tenoblasts. The cellular population of tendon is mainly represented by tenocytes: tenocytes are very elongated cells with ovoid and very long nuclei, occupying the entire length of the cell, with a high nucleus-to-cytoplasm ratio. The cellular process are long and thin and in direct contact with extracellular matrix components (A). In the cytoplasm of tenoblasts is often visible a well developed rough endoplasmic reticulum with dilated cisternae (B), few mitochondria, primary and secondary lysosomes, vacuoles, phagosomes and glycogen (C) (O.M. × 3900, ×7900, ×3900).

Figure 4

Ultrastructure features of epitenon. Epitenon is a delicate, loose connective-tissue sheath containing blood and lymphatic vessels. A–B: Endothelial cells show cell membrane protrusion and several granules and pinocytic vesicles in the cytoplasm. Endothelial cells are surrounded by pericytes and show a well constituted basal membrane (O.M. ×4900, ×3900).

Figure 5

Ultrastructure features of nerves in the epitenon. Unmyelinated (A–B) and myelinated (C) fibers surrounded by cytoplasmic elongation of Schwann cell (O.M. ×3900, ×5500, ×7500).