ABSTRACT
During the third week of human embryonic development, three basic germ layers are formed (ectoderm, mesoderm and endoderm). During the period of organogenesis (the fourth to eighth week of embryonic development) these three germ layers give rise to very specific tissues and organ systems, with the musculoskeletal system being derived from mesoderm. Ectoderm derivatives include the central and peripheral nervous system, epidermis (including hair, nails), glands (pituitary,
sweat and mammary), tooth enamel and sensory epithelium of the eye, ears and nose. Mesoderm derivatives include cartilage, bone, muscle (smooth and striated), connective tissue, vascular structures and cells (arteries, veins, lymph system, heart and blood cells), and the urogenital system except the bladder, spleen and suprarenal glands. Endoderm derivatives include the epithelial linings (respiratory tract, gastrointestinal tract, tympanic cavity, Eustachian tube and urinary bladder), and parenchyma of the tonsils, thyroid, parathyroid, thymus, liver and pancreas. Alterations in musculoskeletal development can occur and are often traced to congenital (e.g. autosomal dominant point mutation in chromosome 4
producing achrondroplasia dwarfism) or environmental (e.g. radiation, drugs (thalidomide producing limb abnormalities)) factors interfering with the process of organogenesis.
