BGDB Face and Ear - Trilaminar Embryo

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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.

Gastrulation forms the trilaminar embryo


  • buccopharyngeal (oral) membrane
  • pharynx
  • tubotympanic recess
  • endocrine


  • unsegmented mesoderm (somitomeres)
  • somites


  • neural tube - central nervous system, posterior pituitary
  • neural crest - (ectomesenchyme) connective tissues of head, and other tissues.
  • sensory placodes
  • epithelium
Neural Plate Neural Groove Neural Tube and Neural Crest
Mesoderm-cartoon1.jpg [[File::Mesoderm-cartoon2.jpg|250px]] Mesoderm-cartoon4.jpg

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.

Week 4

Stage11 bf2.jpg Stage11 sem5.jpg Human embryo - Week 4 (GA week 6) Carnegie stage 11

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

Brain growth

Heart growth

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

Trilaminar Embryo Interactive Component

Attempt the Quiz - Trilaminar Embryo 

Here are a few simple Quiz questions that relate to Trilaminar Embryo from the lecture and practical.


1 Which of the following best describes the term "ectomesenchyme":

ectoderm derived from mesoderm
epithelium derived from connective tissue
sensory placodal induction by mesoderm
connective tissue derived from neural crest
none of the above is correct

2 Which of the following best describes the buccopharyngeal membrane structure:

a ventral surface indentation in the early embryo head
endoderm layer of the foregut
ectoderm layer of the epidermis
ectoderm and endoderm
none of the above is correct

Practical 6: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities

Practical 6: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities


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

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Cite this page: Hill, M.A. (2024, May 21) Embryology BGDB Face and Ear - Trilaminar Embryo. Retrieved from

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