Template:Week 4 Embryo Stages and Events table

From Embryology
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
Day
Stage
Event
22
Stage 10 Stage10 dorsal1.jpg Stage10-dorsal2.jpg

Neural Crest - differentiation at spinal cord level from day 22 until day 26

Neural - neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest - cells are arising mainly from the neural ectoderm

Neural Crest - trigeminal, facial, and postotic ganglia components visible[1]

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.

23
  Heart - begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
24
Stage 11 Stage11 bf1c.jpg

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

25
Stage 12 Stage12 bf1c.jpg

Pituitary - Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof

Liver Development septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae[2]

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 - cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus[3]

Neural Crest - vagal neural crest enter the foregut (20-25 somite stage)

26
   
27
   
28
Stage 13 Stage13 bf1c.jpg 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.[4]

telencephalon cavity appears

Liver Development epithelial cord proliferation enmeshing stromal capillaries[5]

Sense - Smell Crest comes from the nasal plates[6]

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.
References
  1. <pubmed>17848161</pubmed>
  2. <pubmed>9407542</pubmed>
  3. <pubmed>17848161</pubmed>
  4. <pubmed>3354839</pubmed>
  5. <pubmed>9407542</pubmed>
  6. <pubmed>15604533</pubmed>