BGDB Face and Ear - Trilaminar Embryo

From Embryology
BGDsmall.jpg
Practical 6: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities


Week 3

Gastrulation/Neuralation: In the third week, gastrulation establishes the 3 germ cell layers, endoderm, mesoderm and ectoderm. All 3 layers will later contribute specific components of the face and ear.

Also present from these early weeks is the buccopharyngeal membrane or oral membrane. The membrane forms in the midline towards the cranial end of of the trilaminar embryo. Initially as a small circular region during gastrulation where ectoderm and endoderm associate closely together, without mesoderm between the layers (like the cloacal membrane at the other end of the disc). Folding of the embryonic disc will change its relative position.


The primitive streak extends from the primitive node towards the connecting stalk end of the disc. The primitive node (Hensen's node) is also the beginning of an axial process that extends in the opposite direction within the mesoderm layer. The axial process can extend cranially only as far as the buccopharyngeal membrane an differentiates to form then notochord. The notochord regulates development, both differentiation and folding, in the surrounding tissues. The regulation is initially physical, as a dense column of cells (folding) and then by secretion of developmental factors (differentiation) that pattern the surrounding ectoderm, mesoderm and endoderm.

Germ Layer Contributions

Now consider how each of the germ layers will eventually contribute specific components of the face and ear. The list below is a simple overview, by the end of todays pracrical you should be able to identify the germ layer embryonic origin of many different components.

Endoderm

buccopharyngeal membrane pharynx tubotympanic recess endocrine

Mesoderm

unsegmented mesoderm somites

Ectoderm

neural tube neural crest epithelium

Embryonic Folding

As we discussed in gastrointestinal tract development, initial folding of the embryonic disc occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.

Folding in relation to the notochord:

  • Laterally (either side of the notochord) lies mesoderm.
  • Rostrally (above the notochord end) lies the buccopharyngeal membrane, above this again is the mesoderm region forming the heart.
  • Caudally (below the notochord end) lies the primitive streak (where gastrulation occurred), below this again is the cloacal membrane.
  • Dorsally (above the notochord) lies the neural tube then ectoderm.
  • Ventrally (beneath the notochord) lies the mesoderm then endoderm.

Carnegie Stage 11

Two other rapid growth events influence folding in the third to forth week and define the location of the early face:

Heart growth beneath the buccopharyngeal membrane, mesodermal heart tube growth defines the initial position of the foregut which will form the pharynx

Brain growth above the buccopharyngeal membrane, cranial neural tube growth folds the neural tube ventrally

BGDsmall.jpg
Practical 6: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities




BGDsmall.jpg

BGDB: Lecture - Gastrointestinal System | Practical - Gastrointestinal System | Lecture - Face and Ear | Practical - Face and Ear | Lecture - Endocrine | Lecture - Sexual Differentiation | Practical - Sexual Differentiation | Tutorial


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



Cite this page: Hill, M.A. (2024, March 28) Embryology BGDB Face and Ear - Trilaminar Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Face_and_Ear_-_Trilaminar_Embryo

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