Template:Week 4 Embryo Stages and Events table
|Week 4 - Human Embryo Stages and Events (GA week 6)|
|Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9|
Neural Crest - differentiation at spinal cord level from day 22 until day 26
Neural Crest - migration of vagal level neural crest cells begins (7-10 somite stage)
Neural - Brain rostral neural tube forms 3 primary brain vesicles (week 4)
Respire - Week 4 laryngotracheal groove forms on floor foregut.
|Heart - begins to beat in Humans by day 22-23, first functioning embryonic organ formed.|
Thyroid - thyroid median endodermal thickening in the floor of pharynx
Neural - rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently.
Optic ventricle appears
Pituitary - Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof
Neural - caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2)
Neural - secondary neurulation begins
Neural Crest - vagal neural crest enter the foregut (20-25 somite stage)
|Stage 13|| Neural - the neural tube is normally completely closed, ventricular system now separated from amniotic fluid. Neural crest at spinal level is segregating, and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop.
telencephalon cavity appears
Skin - 4 weeks simple ectoderm epithelium over mesenchyme
Skin - 1 to 3 months ectoderm - germinative (basal) cell repeated division of generates stratified epithelium; mesoderm - differentiates into connective tissue and blood vessels
|Note - the day timing of stages is only approximate, system names link to first page of that specific system, and events are based upon the literature cited below.|