organogenesis

• embryonic development period

  first 8 weeks after fertilisation
• fetal development period

  from week 9 until birth
• cellular and molecular events of development are regulated by molecular interactions and signalling in local environment
• lateral folding


  in week 4, establishes 3D tube shape of embryo, gut tube, neural tube, and heart tube. right and left sides move ventrally, then medially, and fuse with the opposite side on the midline
• cephalocaudal folding

  in week 4, folding at cranial and caudal end to form head fold and tail fold
• the embryo and placenta develop within the endometrium of one wall of the uterus
• ectoderm

  nervous system and epidermis of skin
• mesoderm

  muscle tissues, connective tissue, much of urinary and reproductive system, as well as serous membranes of body cavities
• endoderm

  epithelium of digestive and respiratory systems, many endocrine glands, and parts of the reproductive system
• Ectoderm near midline forms the neural plate → neural folds & neural groove → neural tube → central nervous system (CNS)
• neural crest cells become peripheral nervous system ganglia, Schwann cells, Merkel cells, and melanocytes
• cranial end forms:


  cerebral hemispheres, thalamus, hypothalamus, midbrain, pons and cerebellum, and medulla
• caudal end forms the spinal cord
• epidermis is mostly from ectoderm, melanocytes and merkel cells are from the neural crest of ectoderm. exception: langerhans cells are from mesoderm
• dermis is mostly from mesoderm
• Hair follicles, sebaceous glands, sweat glands, mammary glands develop as invaginations of epidermis (from ectoderm)
• skeletal system is from mesoderm, except for many skull bones (from ectoderm)
• vertebrae and ribs are from the paraxial mesoderm, which forms somites
• limb bones are from lateral plate somatic mesoderm
• skeletal muscle is from the mesoderm: somites and somitomeres
• cardiovascular system is almost entirely from mesoderm, a bit of the heart is from neural crest ectoderm
• in week 3, blood vessels begin to form
• in week 4, heart tube folds, and begins to contract to pump blood
• in weeks 5-8, heart chambers are separated by septa and valves
• Foramen ovale: right atrium to left atrium
• Ductus arteriosus: pulmonary trunk to aorta
• lymphatic system is mostly from mesoderm
• urinary system is mostly from mesoderm
• 3 sets of kidneys

  pronephros, mesonephros, metanephros (forms definitive kidney)
• ureteric bud forms ureter, renal pelvis, and collecting ducts
• reproductive system is mostly from intermediate mesoderm
• germ cells develop in the yolk sac, then migrate to intermediate mesoderm, which forms support cells and endocrine cells of the gonads
• Mesonephric ducts form male genital tract
• Paramesonephric ducts form female genital tract
• Lateral folding of embryo creates gut tube
• Epithelium of gut tube from endoderm
• Smooth muscle, connective tissue from mesoderm
• Enteric nervous system from neural crest from ectoderm
• Foregut: pharynx to proximal duodenum
• liver, gallbladder, pancreas develop as outgrowths of foregut at

  foregut-midgut junction