BGDB Gastrointestinal - Trilaminar Embryo

Introduction

Begin by very briefly covering the first 3 weeks of development. Our story of GIT development begins in the third week with the formation of the 3 germ cell layers, one layer the endoderm will form the lining of the entire gastrointestinal tract and also contibute the respiratory tract and other organs.

Note that the description below, and throughout the practical, has been substantially simplified.

Week 1 and 2

Fertilization/Blastocyst Formation: an egg released into the uterine tube has been fertilized by a single sperm to form the first diploid cell (zygote). This cell then undergoes rapid division to form first a solid ball of cells (morula) and then a hollow ball (blastocyst) with an outer cell layer, an inner cell mass and a fluid filled cavity. All this has occurred in the uterine tube (horn, oviduct, fallopian tube) prior to implantation in the uterus.

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Week 3

Gastrulation/Neuralation: The inner cell mass forms a flat sheet of cells and cells migrate through a specific region of the sheet (primitive streak) turning the single layer into first 2 then 3 layers (trilaminar embryo).

Folding

The next process to follow is the folding of the embryonic disc which will form the "tube" of the GIT. Forming the ends of this tube are the 2 membranes which form the upper and lower limits of the GIT.

Note that in addition to gastrulation, neuralation (forming the early neural tube that makes the nervous system) and somitogenesis (segmentation of the mesoderm forming the axial skeleton) are the other major processes occuring in week 3 to 4.

Folding of the embryonic disc occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.

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.

The ventral endoderm (shown yellow) has grown to line a space called the yolk sac. Folding of the embryonic disc "pinches off" part of this yolk sac forming the first primative GIT.

Week 4

Carnegie stage 11 Week 4, 23 - 26 days, 2.5 - 4.5 mm, Somite Number 13 - 20

View: This is a dorsolateral view of embryo. Amniotic membrane removed.

View: Embryo cross-section through the trunk region (low power inset).

Identify: neural groove and neural folds, the mesoderm, which segments beside the neural groove to form somites but extends laterally to margin of embryonic disc lateral plate mesoderm, where it merges with the covering extraembryonic mesoderm.

The intra-embryonic coelom will develop later in the middle of the lateral plate mesoderm. Note amniotic ectoderm covered by extramebryonic mesoderm (empty spaces above and below the mesoderm are artefacts, as are the lateral folds in the ectoderm).

View: Embryo cut longitudinally (opposite orientation to first image; rostral to left, caudal right)

Since all embryos are twisted, the "longitudinal" section actually cuts obliquely through the brain and neural tube, showing where these structures open into the amniotic sac, i.e., the cranial and caudal neuropores respectively.

Identify: Somites, Embryonic Heart (cut in three places)

GIT - foregut (lying between the brain and the heart), midgut ("roof" region of the yolk sac) and hindgut.

(Note: The foregut appears to be separated from the midgut by a dense column of tissue, but this is a section through one of the umbilical veins crossing to the heart obliquely from the side of the embryo)

Transverse septum (cut in cross-section, inferior to the heart, site where liver and diaphragm arise)

Membranes

During the process of gastrulation the embryonic disc formed 3 layers, except in 2 specific membrane regions where ectoderm and endoderm have no mesoderm between them: buccopharyngeal membrane and cloacal membrane. These will form the upper and lower extend of the GIT.

(Quicktime movie in new window)

Buccopharyngeal membrane - also called mouth or oral membrane

Cloacal membrane - not clearly visible in the above section

Splanchnic Mesoderm

The cartoon above is a section through the trunk of the trilaminar embryo showing the further development of the 3 layers and the space (coelom) that forms in the mesoderm (only the righhand side is shown).

Within the embryonic disc lateral plate mesoderm a space (coelom) forms, it lies within the embryo and so is called the intraembryonic coelom. This single "horseshoe-shaped" space will form the 3 major body cavities: pericardial (around the heart), pleural (around the lungs) and peritoneal (around the GIT and visceral organs).

The mesoderm adjacennt to the endoderm is now called the splanchnic mesoderm which forms the connective tissue and muscular wall of the GIT.

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

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Cite this page: Hill, M.A. (2024, April 16) Embryology BGDB Gastrointestinal - Trilaminar Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Gastrointestinal_-_Trilaminar_Embryo

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