ATP production by oxidative phosphorylation happens in mitochondria, which have a double membrane—the outer membrane and a highly folded inner membrane known as cristae. The inner membrane houses the electron transport chain and the ATP synthase enzyme, and its folds provide a large surface area to maximize ATP production. The proton gradient generated across the inner membrane powers the synthesis of ATP as protons flow back through ATP synthase. Mitochondria also contain their own DNA and ribosomes, reflecting their distinct, energy-producing role. Other organelles have different functions and don’t carry out oxidative phosphorylation. The Golgi apparatus modifies and packages proteins, the peroxisomes handle fatty acid oxidation and detoxification, and lysosomes degrade macromolecules.

In bone tissue, osteocytes reside in small spaces called lacunae and stay connected through slender, hairlike channels known as canaliculi. These canaliculi form a network that links neighboring lacunae and reaches the central canal, enabling nutrients, wastes, and signals to move between cells and the blood supply in the center of the osteon. That network of narrow passages is what canaliculi describes. The central canal itself is the Haversian canal, and the whole repeating unit is the osteon; viscerocranium refers to facial bones and isn’t part of this microscopic connection.

The lumbosacral plexus is the network that primarily innervates the muscles of the buttock, pelvis, perineum, and lower limb. It is formed by ventral rami from the lumbar and sacral regions (roughly L1–S4) that come together and give rise to the major nerves of the lower body. For the buttock, nerves such as the gluteal nerves originate from this plexus and supply the gluteal muscles. The perineum receives innervation from nerves like the pudendal nerve, also part of this plexus, and pelvic organs are served by branches within the same network. The entire lower limb depends on nerves that branch from the lumbosacral plexus, including the sciatic nerve for the majority of the leg and foot. In contrast, the brachial plexus serves the upper limb, the lumbar plexus mainly handles the anterior thigh and abdominal wall, and the stratum basale is a skin layer with no neural plexus involvement.

Chromosome movement in cell division is driven by the spindle apparatus organized at the centrosome. The centrosome contains a pair of centrioles, each a cylindrical bundle of microtubule triplets. These two centrioles sit at opposite ends of the cell and organize the microtubules that attach to chromosomes at kinetochores, pulling sister chromatids toward opposite poles during mitosis. The centrioles duplicate before division so two poles form, creating the bipolar spindle necessary for accurate chromosome separation. The nucleolus is focused on ribosome production, not spindle formation; microtubules are the fibers that form the spindle but are not paired, bundle-like inclusions themselves; organelles is a broad term that does not specify the structure responsible for organizing the spindle.