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:
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.
(Animation based upon images from Human Embryology, Larsen)
Facts: 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)
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
Cloacal membrane - not clearly visible in the above section
(Images from The Developing Human, Moore and Persaud)
This second animation is similar to the first, this time though the position of the embryo, chorionic space (white), amniotic space (blue) and yolk sac (yellow) with forming GIT.
(Animation based upon images from Human Embryology, Larsen)
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.
Finally, below are listed some links to Embryo Images by Drs. Kathleen K. Sulik and Peter R. Bream Jr. which has a series of scanning electron microscope images related to GIT development.
If you have some time before we go on to the next page in the Practical, look at the 3 dimensional embryo shape and the cavities associated with early GIT development
Link to next page in this GIT Practical - Early Embryo
Looks at the appearance of the GIT at Carnegie stage 13/14, an early embryonic stage.
Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.
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. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)
amnion - An extraembryonic membrane ectoderm and extraembryonic mesoderm in origin and forms the innermost fetal membrane, 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. (More? Placenta Notes | Week 2 Notes)
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). (More? GIT Notes)
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. (More? GIT Notes | Face Notes | Head Notes)
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. (More? buccopharyngeal membrane | GIT Notes)
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. (More? Coelom Notes)
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. (More? Gastrointestinal Tract - Stomach | Gastrointestinal Tract Notes | Respiratory Notes)
gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers have just formed. (More? Gastrulation)
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)
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. (More? Gastrointestinal Tract Notes | Urogenital Notes)
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. (More? Coelom Notes | Week 3 Notes)
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. (More? Neural Notes | Neural Crest Notes)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. (More? Head Notes | GIT Notes | Carnegie stage 13 pharynx)
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/90 minutes until on average 44 pairs eventually form. (More? Week 3 - Somitogenesis | Musculoskeletal Notes)
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. (More? Week 3 | Gastrointestinal Tract Notes | Heart Notes )
stomadeum - (stomadeum) 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. (More? Gastrointestinal Tract Notes | Head Notes)
On this second Practical page are a number of images of the whole week 4 (stage 11) embryo and sections through this embryo to see the simple internal structure and cavities at this time.
There are also animations to help with the concept of folding, these can be played back and forth by dragging the control at the bottom of the movie.
The additional resources section showing scanning electron micrographs will not be covered in the Practical and is for self-directed learning.
These notes and linked materials have been prepared for Educational purposes only.
Next Mark Hill Practical:
Sexual Differentiation covers early stages of male female development differences.
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)