BGDB Gastrointestinal - Trilaminar Embryo

From Embryology
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Practical 1: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Lecture | Quiz

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

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 1 Movie

Week 3

(GA week 5)

Trilaminar embryo.jpg

Trilaminar Embryo

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).

These layers are also called the "germ layers" as they form all the tissues and organs of the 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.

Endoderm cartoon.jpg

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.

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Week 4

(GA week 6) Carnegie stage 10, 23 day, 5-11 somite pairs

Stage10 sem10.jpg

Stage10 sem1.jpg

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.

Buccopharyngeal membrane

also called mouth or oral membrane

Carnegie stage 10 (21 days, 4-5 somite pairs, ventral sem)

Stage10 sem4.jpg


Carnegie stage 11 (25 days, 20 somite pairs)

Cloacal membrane - not clearly visible in the above section

Carnegie stage 12

Stage12 sem9 cloacal membrane.jpg

Splanchnic Mesoderm

Mesoderm-cartoon4.jpg 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 righthand side is shown, lefthand side would be identical).

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 adjacent 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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Practical 1: Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Lecture | Quiz

Gastrointestinal Tract Terms  
  • allantois - An extraembryonic membrane, endoderm in origin extension from the early hindgut, then cloaca into the connecting stalk of placental animals, connected to the superior end of developing bladder. In reptiles and birds, acts as a reservoir for wastes and mediates gas exchange. In mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface.
  • amnion - An extra-embryonic membrane, ectoderm and extraembryonic mesoderm in origin, also forms the innermost fetal membrane, that produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membane led to the description of reptiles, bird, and mammals as amniotes.
  • amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.
  • buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity).
  • bile salts - Liver synthesized compounds derived from cholesterol that function postnatally in the small intestine to solubilize and absorb lipids, vitamins, and proteins. These compounds act as water-soluble amphipathic detergents.
  • buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomodeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane.
  • cloacal membrane - Forms the external lower membrane limit (caudal end) of the early gastrointestinal tract (GIT). This membrane is formed during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane breaks down to form the initial "anal opening" of the gastrointestinal tract.
  • cholangiocytes - epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. These cells modify the hepatocyte-derived bile, and are regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules.
  • coelom - Term used to describe a space. There are extraembryonic and intraembryonic coeloms that form during vertebrate development. The single intraembryonic coelom will form the 3 major body cavities: pleural, pericardial and peritoneal.
  • crypt of Lieberkühn - (intestinal gland, intestinal crypt) intestinal villi epithelia extend down into the lamina propria where they form crypts that are the source of epithelial stem cells and immune function.
  • foregut - The first of the three part/division (foregut - midgut - hindgut) of the early forming gastrointestinal tract. The foregut runs from the buccopharyngeal membrane to the midgut and forms all the tract (esophagus and stomach) from the oral cavity to beneath the stomach. In addition, a ventral bifurcation of the foregut will also form the respiratory tract epithelium.
  • galactosemia - Metabolic abnormality where the simple sugar galactose (half of lactose, the sugar in milk) cannot be metabolised. People with galactosemia cannot tolerate any form of milk (human or animal). Detected by the Guthrie test.
  • gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers (Endoderm/ Mesoderm/Ectoderm) have just formed.
  • gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (Endoderm/ Mesoderm/Ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes. (More? Gastrulation)
  • Guthrie test - (heel prick) A neonatal blood screening test developed by Dr Robert Guthrie (1916-95) for determining a range of metabolic disorders and infections in the neonate. (More? Guthrie test)
  • hindgut - The last of the three part/division foregut - midgut - hindgut) of the early forming gastrointestinal tract. The hindgut forms all the tract from the distral transverse colon to the cloacal membrane and extends into the connecting stalk (placental cord) as the allantois. In addition, a ventral of the hindgut will also form the urinary tract (bladder, urethra) epithelium.
  • intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom.
  • neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4.
  • neural crest - region of cells at the edge of the neural plate that migrates throughout the embryo and contributes to many different tissues. In the gastrointestinal tract it contributes mainly the enteric nervous system within the wall of the gut responsible for peristalsis and secretion.
  • pharynx - uppermost end of gastrointestinal and respiratory tract, in the embryo beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches.
  • somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/4 hours (mouse 1 pair/90 min) until on average 44 pairs eventually form.
  • splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.
  • stomodeum - (stomadeum, stomatodeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch.
Other Terms Lists  
Terms Lists: ART | Birth | Bone | Cardiovascular | Cell Division | Gastrointestinal | Genetic | Hearing | Heart | Immune | Integumentary | Neural | Oocyte | Palate | Placenta | Renal | Respiratory | Spermatozoa | Ultrasound | Vision | Historic | Glossary

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.

Terms

  • allantois - An extraembryonic membrane, endoderm in origin extension from the early hindgut, then cloaca into the connecting stalk of placental animals, connected to the superior end of developing bladder. In reptiles and birds, acts as a reservoir for wastes and mediates gas exchange. In mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface.
  • amniotic cavity - The fluid-filled (amniotic fluid) extraembryonic coelom (cavity) formed initially by epiblast and then lined by ectoderm and surrounding extraembryonic mesoderm. In humans, it forms the innermost fetal membrane, produces amniotic fluid expanding to eventually fuse with the chorionic membrane during week 8 of development. This fluid-filled sac initially lies above the trilaminar embryo disc and with embryonic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membrane led to the description of reptiles, bird, and mammals as "amniotes".
  • buccopharyngeal membrane - (oral membrane; Latin, bucca = "cheek") A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomadeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane.
  • cloaca - (cloacal cavity) The term describing the common cavity into which the intestinal, genital, and urinary tracts open in vertebrates. Located at the caudal end of the embryo it is located on the surface by the cloacal membrane. In many species this common cavity is later divided into a ventral urogenital region (urogenital sinus) and a dorsal gastrointestinal (rectal) region.
  • cloacal membrane - Forms the external lower membrane limit (caudal end) of the early gastrointestinal tract (GIT). This membrane is formed during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane breaks down to form the initial "anal opening" of the gastrointestinal tract. The upper end of the gastrointestinal tract has a similar embryonic membrane, the buccopharyngeal membrane.
  • endoderm - (Greek, endo = inside + derma = skin) One of the initial 3 germ cell layers (ectoderm, mesoderm, endoderm) formed by the process of gastrulation. The endoderm forms as a cuboidal epithelium and contributes not only to the trilaminar embryo, but also lines the yolk sac. It will form the entire epithelial lining of the gastrointestinal tract (GIT), contribute to the accessory organs of GIT and also forms the epithelial lining of the respiratory tract. Note that in the GIT it contributes both epithelium and the associated epithelial glands. In humans, endoderm forms during week 3 of development.
  • enteric nervous system - (ENS) Gastrointestinal tract neural network located within the tract wall that locally controls and coordinates intestinal functions (motility, epithelial secretion and blood flow) derived from the neural crest. It forms part of the autonomic nervous system with parasympathetic and sympathetic inputs as well as afferent nerve fibres, through the vagus nerves and spinal afferent pathways. The two main networks in the gastrointestinal tract (gut) wall are the myenteric plexus (Auerbach's plexus) and the submucosal plexus (Meissner's plexus). In humans, there are and estimated 200-600 million neurons that form the adult enteric nervous system.
  • gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm, mesoderm, ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes.
  • germ layers - The first three cellular layers (ectoderm, mesoderm, and endoderm) that will form all tissues of the embryo. In humans, these layers begin to form during week 3 of development. Term should not be confused with germ cells, which are the oocyte and spermatazoa forming cells. Term originally used by Robert Remak (1815 - 1865), a German scientist and embryologist.
  • gestational age - (GA) The clinical term given in week to describe human development timed from the first day of the last menstrual period (LMP). For gestational age in assisted reproductive technology pregnancy 2 weeks are added to the fertilisation date. Age therefore differs by approximately two weeks from research materials timed from fertilisation (conceptional age), this term is generally not used clinically.
  • neural crest - A cell region at edge of neural plate, then atop the neural folds, that remains outside and initially dorsal to the neural tube when it forms. These paired dorsal lateral streaks of cells migrate throughout the embryo and can differentiate into many different cell types (= pluripotential). Those that remain on the dorsal neural tube form the sensory spinal ganglia (DRG), those that migrate ventrally form the sympatheitic ganglia. Neural crest cells also migrate into the somites and regions throught the entire embryo. In the GIT these cells form the enteric nervous system (myenteric and submucosal ganglia).
  • splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.
  • stomodeum - (stomatodeum) The primordial mouth region of the developing head. Initially a ventral surface depression on the early embryo the region lies between the forebrain bulge (cranially) and the heart bulge (caudally) and between the maxillary and mandibular components of the first pharyngeal arch. At the floor of this surface depression lies the buccopharyngeal membrane, which breaks down (Carnegie stage 11) opening the gastrointestinal/respiratory tract to the amniotic space and fluid.
  • yolk sac - An extraembryonic membrane which is endoderm origin and covered with extraembryonic mesoderm. Yolk sac lies outside the embryo connected initially by a yolk stalk to the midgut with which it is continuous with. The endodermal lining is continuous with the endoderm of the gastrointestinal tract. The extra-embryonic mesoderm differentiates to form both blood and blood vessels of the vitelline system. In reptiles and birds, the yolk sac has a function associated with nutrition. In mammals the yolk sac acts as a source of primordial germ cells and blood cells. Note that in early development (week 2) a structure called the "primitive yolk sac" forms from hypoblast, this is an entirely different structure.
  • yolk stalk - (vitelline duct, omphalomesenteric duct, Latin, vitellus = yolk of an egg) The endodermal connection between the midgut and the yolk sac. See vitelline duct.


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Cite this page: Hill, M.A. 2017 Embryology BGDB Gastrointestinal - Trilaminar Embryo. Retrieved November 18, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Gastrointestinal_-_Trilaminar_Embryo

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G