2010 BGD Practical 6 - Week 4

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Introduction

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Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Quiz


Introduction

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.


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


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Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Quiz


Embryo Stages and Events

Day Stage Event
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Stage 10 Stage10 dorsal1.jpg Stage10-dorsal2.jpg

Neural

  • neural folds begin to fuse near the junction between brain and spinal cord
  • rostral neural tube forms 3 primary brain vesicles (week 4)

Neural Crest

  • differentiation at spinal cord level from day 22 until day 26
  • cells are arising mainly from the neural ectoderm
  • trigeminal, facial, and postotic ganglia components visible PMID: 17848161
  • migration of vagal level neural crest cells begins (7-10 somite stage)

Respire

  • Week 4 - laryngotracheal groove forms on floor foregut
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  Heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
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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

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Stage 12 Stage12 bf1c.jpg

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

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

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27
   
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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. 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

Glossary Links

Glossary: 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 | Term Link
2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12

Cite this page: Hill, M.A. (2024, March 29) Embryology 2010 BGD Practical 6 - Week 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Practical_6_-_Week_4

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G