Week 4

Key Events of Human Development during the fourth week (week 4) following fertilization or Clinical week 6 (LMP).

These notes cover the fourth week of embryonic development, which is the beginning of organogenesis, (specific tissues and systems are beginning to differentiate) from the trilaminar embryo.

On the embryo surface sensory placodes and limb buds appear. Sensory placodes (otic, lens, nasal) will form specific components of the ear, eye and nose. Limb buds form from ectoderm and mesoderm (somite) components and are the "paddle-like" projections from the trunk which will form all the upper and lower limb components.

Within the embryo, this period of organogensis is usually extended to cover until 8 weeks of development. Folding of the embryo continues and the earliest functioning organ is the heart. Other systems such as the circulatory, digestive, urogenital and nervous system begin to all take shape.

As systems are beginning to develop, each page in this section is only a brief summary with additional links to specific notes covering these systems/tissues. Note that most tissue development during week 4 will also be covered in the system development notes.

Day	Stage	Event
22	Stage 10	File:File:Stage10 dorsal1.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 PMID: 17848161 Neural Crest migration of vagal level neural crest cells begins (7-10 somite stage) 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	File:CST11.gif 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	File:CST12.gif [endocrine7.htm Pituitary ]Week 4 hypophysial pouch, Rathke‚Äôs 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 Neural Crest vagal neural crest enter the foregut (20-25 somite stage)
26
27
28	Stage 13	File:CST13.gif 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 telencephalon cavity appears GIT - Liver epithelial cord proliferation enmeshing stromal capillaries PMID: 9407542 Sense - Smell Crest comes from the nasal platesPMID: 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

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Cite this page: Hill, M.A. (2024, April 20) Embryology Week 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Week_4

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