5.2 Atypical Cells and Endosymbiosis

Skeletal muscle fibres form when precursor cells fuse, producing long syncytial cells containing many nuclei. Their cytoplasm is packed with contractile myofibrils and mitochondria. A single nucleus could not efficiently govern gene expression across the entire fibre, so the multinucleate condition supports a very large specialized cell.

Many fungal hyphae grow as branching tubes. In coenocytic fungi, nuclei share a continuous cytoplasm without complete cross walls; in septate fungi, perforated septa allow cytoplasmic continuity. The extensive surface helps secrete digestive enzymes and absorb products through the material in which the fungus grows.

Mature mammalian erythrocytes expel their nuclei and lose most organelles, creating more space for haemoglobin and preventing the cell from consuming the oxygen it transports by aerobic respiration. The trade-off is limited repair and a finite lifespan. Their biconcave shape increases surface area and shortens diffusion distance.

Xylem vessel elements are dead at maturity. Loss of end walls and contents creates continuous low-resistance tubes, while lignified walls resist collapse under the tension generated by transpiration. Phloem sieve-tube elements remain alive but lose the nucleus and many organelles; adjacent companion cells provide metabolic support and load or unload solutes.