Embryonic development begins from the third week age of gestation through the eighth week after conception. During thus period, all major body organs are already completed.
Week 3 (CRL = 1.5 mm)
By the third week most women miss their first menstrual period. It is also this time that the embryonic disk develops into three layers called germ layers. The following developments take place third week after conception.
- The three germ layers, ectoderm, mesoderm and endoderm, gives rise to major organ systems of the body.
- The ectoderm, the outermost germ layer, develops into the: brain and the spinal cord, peripheral nervous system, pituitary gland, sensory epithelium of the eye, ear and nose, epidermis, hair, nails, subcutaneous glands, mammary glands and tooth enamel.
- The mesoderm, the germ layer situated at the middle area, develops into the: cartilage, bone, connective tissue, muscle tissue, heart, blood vessels, blood cells, lymphatic system, spleen, kidneys, adrenal cortex, ovaries (females), testes (males), reproductive system and the lining membranes of the pericardial, pleural and peritoneal spaces.
- The endoderm, the innermost germ layer, develops into the lining of the gastrointestinal and respiratory tracts, tonsils, thyroid, parathyroid, thymus, liver, pancreas, lining of the urinary bladder and urethra and lining of the ear canal.
- CNS: Flat neural plate begins to close to form the neural tube. At this period, the neural tube is still open at each end.
- CV and RESPI: The heart now consists of two parallel tubes that fuse into a single tube. Contractions of the heart tube begin as early as week 3 (22 to 23 days) and the chorionic villi of early placenta connect with the heart.
- DIGESTIVE: the endoderm layer will become the digestive tract.
- MUSCULOSKELETAL: Paired, cube-shaped swellings (somites) appear and will form most of the head and trunk skeleton. The muscle bone, and cartilage develop from the mesoderm.
- INTEGUMENTARY: The outer layer of the skin, epidermis, will develop from the ectoderm. The dermis and connective tissues will develop from the mesoderm.