UNSW Embryology

During week 4 the heart has begun to contract and is the first functional organ within the developing embryo. (More? Heart Development).

Day

Stage

Event

22

Stage 10

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 (PMID: 17848161)

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

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

Pituitary Week 4 – hypophysial pouch, Rathke’s pouch, diverticulum from roof

GIT - Liver – septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae (PMID: 9407542)

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 (PMID: 17848161)

26

27

28

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. (PMID: 3354839)

GIT - Liver – epithelial cord proliferation enmeshing stromal capillaries (PMID: 9407542)

Sense - Smell – Crest comes from the nasal plates (PMID: 15604533)

Skin – 4 weeks - simple ectoderm epithelium over mesenchyme

Skin – 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels