Week 4

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Introduction

Key Events of Human Development during the fourth week (week 4) following fertilization or clinical gestational age GA week 6, based on last menstrual period.

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.


Week 4 Links: Placodes | Lecture - Mesoderm | Lecture - Ectoderm | Lecture - Early Vascular | Category:Week 4
Carnegie Stages: 10 | 11 | 12 | 13
Week Links: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Timeline

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

Week 4 Movies

Note that many of the movies start in week 4 and continue on through later embryonic development.

Neural

Neuralplate 001 icon.jpg Neuraltube 001 icon.jpg
Neural Plate Neural Tube

Mesoderm

Mesoderm 001 icon.jpg Somite 001 icon.jpg Vertabra 003 icon.jpg
Mesoderm Somite Structures Vertebra

Heart

Heart1 looping icon.jpg Heart1 realign icon.jpg Heart1 atrium icon.jpg Heart1 ventricle icon.jpg
Heart Looping Heart Realign Heart Atrial Septation Heart Outflow Septation

References

  1. Ronan O'Rahilly, Fabiola Müller The development of the neural crest in the human. J. Anat.: 2007, 211(3);335-51 PMID:17848161
  2. G Godlewski, R Gaubert-Cristol, S Rouy, M Prudhomme Liver development in the rat and in man during the embryonic period (Carnegie stages 11-23). Microsc. Res. Tech.: 1997, 39(4);314-27 PMID:9407542
  3. Ronan O'Rahilly, Fabiola Müller The development of the neural crest in the human. J. Anat.: 2007, 211(3);335-51 PMID:17848161
  4. F Müller, R O'Rahilly The development of the human brain from a closed neural tube at stage 13. Anat. Embryol.: 1988, 177(3);203-24 PMID:3354839
  5. G Godlewski, R Gaubert-Cristol, S Rouy, M Prudhomme Liver development in the rat and in man during the embryonic period (Carnegie stages 11-23). Microsc. Res. Tech.: 1997, 39(4);314-27 PMID:9407542
  6. F Müller, R O'Rahilly Olfactory structures in staged human embryos. Cells Tissues Organs (Print): 2004, 178(2);93-116 PMID:15604533



Week Links: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline


Glossary Links

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols


Cite this page: Hill, M.A. (2014) Embryology Week 4. Retrieved August 21, 2014, from https://php.med.unsw.edu.au/embryology/index.php?title=Week_4

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© Dr Mark Hill 2014, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G