Cellular & Molecular Biology Basics for Orthopaedics

The eukaryotic cell compartmentalises its work. The nucleus stores DNA and is the site of transcription; the rough endoplasmic reticulum, studded with ribosomes, synthesises secreted and membrane proteins; the smooth ER handles lipids and calcium; the Golgi apparatus modifies, sorts and packages proteins; mitochondria generate ATP and release factors that trigger intrinsic apoptosis; and lysosomes degrade material with acid hydrolases. In musculoskeletal tissue these are specialised: chondrocytes and osteoblasts have prominent rough ER and Golgi to manufacture and secrete the collagen-rich extracellular matrix, while the osteoclast is a multinucleated cell that resorbs bone using lysosomal enzymes (and acid) delivered at its ruffled border.

Dividing cells progress through interphase - G1 (growth), S (DNA synthesis/replication), G2 (growth/preparation) - and then M (mitosis); non-dividing cells rest in G0. Progression is driven by cyclins and cyclin-dependent kinases (CDKs) and policed at checkpoints (principally G1/S and G2/M) that verify DNA integrity and replication before the cell commits. The tumour suppressors p53 (the 'guardian of the genome', which arrests the cycle or triggers apoptosis when DNA is damaged) and Rb are central. Loss of cell-cycle control - through oncogene activation or tumour-suppressor loss - is the basis of oncogenesis, relevant to bone and soft-tissue tumours.

Genetic information flows DNA -> RNA -> protein. In the nucleus, a gene's DNA is transcribed by RNA polymerase into messenger RNA (mRNA), which is processed (splicing, capping, poly-A tail) and exported to the cytoplasm. There, ribosomes translate the mRNA codons into a chain of amino acids (translation), using transfer RNA. The polypeptide then folds and undergoes post-translational modification into a functional protein. A mutation at any point yields an abnormal protein and disease - the molecular basis of many orthopaedic conditions: COL1A1/COL1A2 (osteogenesis imperfecta), FGFR3 (achondroplasia), COMP (pseudoachondroplasia/MED), EXT1/2 (multiple hereditary exostoses).

Collagen is the dominant structural protein of the musculoskeletal system and the best orthopaedic example of protein synthesis and post-translational modification:

1. Transcription/translation of pro-alpha chains on the rough ER, each with the repeating glycine-X-Y sequence (X often proline, Y often hydroxyproline).
2. Hydroxylation of proline and lysine residues - which requires VITAMIN C as a cofactor (its lack in scurvy impairs collagen and causes bleeding and poor healing).
3. Glycosylation and assembly of three chains into the triple helix (procollagen).
4. Secretion, then cleavage of the propeptides to form tropocollagen.
5. Fibril assembly and CROSS-LINKING (lysyl oxidase) for tensile strength. There are 28 collagen types; the fibrillar collagens (types I, II, III, V, XI...) build the load-bearing frameworks. Type I predominates in bone, tendon and skin; type II in hyaline cartilage.