UNSW Embryology

Introduction

During week 4 a series of paired mesodermal structures either side of the notochord begin to form in the body region. These structures form balls of mesoderm known as "somites" and are the basis of our segmented axial skeleton. Each pair of somites is added regularly in sequence rostro-caudally (head to tail) down the length of the embryo this means that the rostral somites are both older and more "mature" than caudal somites. Each somite pair will contribute essential components of the future musculoskeletal system of the body and limbs (More? Somite Development).

In the midline of the mesodermal layer is the notocord (also called axial mesoderm) and either side of this is paraxial mesoderm. It is segmentation of paraxial mesoderm into blocks and then "balls" of mesoderm that forms somites. The stage 10 and 11 embryos show the midline paired somites. Note that somites begin to form before the neural groove closes. The process of somite formation is called "Somitogenesis".

Cells migrate through the primitive streak to form mesodermal layer. Extraembryonic mesoderm lies adjacent to the trilaminar embryo totally enclosing the amnion, yolk sac and forming the connecting stalk.

Paraxial mesoderm accumulates under the neural plate with thinner mesoderm laterally. This forms 2 thickened streaks running the length of the embryonic disc along the rostrocaudal axis. In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds.

Segmentation of the paraxial mesoderm into somites continues caudally at 1 somite/90minutes and a cavity (intraembryonic coelom) forms in the lateral plate mesoderm separating somatic and splanchnic mesoderm.

Note intraembryonic coelomic cavity communicates with extraembryonic coelom through portals (holes) initially on lateral margin of embryonic disc.

Somites continue to form. The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up cranially and caudally and the neural crest migrates into the mesoderm.

Next Somite Development

Mesoderm beside the notochord (axial mesoderm) thickens, forming the paraxial mesoderm as a pair of strips along the rostro-caudal axis. Paraxial mesoderm towards the rostral end, begins to segment forming the first somite. Somites are then sequentially added caudally. The somitocoel, is a cavity forming in early somites, which is lost as the somite matures.

Cells in the somite differentiate medially to form the sclerotome (forms vertebral column) and laterally to form the dermomyotome. The dermomyotome then forms the dermotome (forms dermis) and myotome (forms muscle).

Neural crest cells migrate beside and through somite.

The myotome differentiates to form 2 components dorsally the epimere and ventrally the hypomere, which in turn form epaxial and hypaxial muscles respectively. The bulk of the trunk and limb muscle coming from the Hypaxial mesoderm. Different structures will be contributed depending upon the somite level.

Note these are very simplified figures.

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