As many students are new to embryology the majority of the Practical class time has been spent introducing general concepts of early human development and combining this with the early formation of GIT structures. While the fetal period is substantially longer, there will be much less content to be covered.
Fetal developmental features include: the growth and rotation of intestines initially herniated outside the ventral body wall; changes in mesenteries; development of the blood supply and tract wall.
Finally consider the initial functions of the tract with amionic fluid swallowing and the accumulation of both secretions and swallowed components within the large intestine.
The Embryonic period involved transient structures to establish body and placental tissues, folding and form. The long human Fetal period (4x the embryonic period) is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period. (More? See Fetal Notes)
Firstly, look at the change in actual size from the early embryo we saw on Page 3 (week 5) to the the end of the embryonic period (week 8) and just 2 weeks later in the early fetal period.
When measuring fetal growth, there is also a separation in timing of the greatest changes in length and weight.
Fetal length change is greatest in the middle period (second trimester).
Fetal weight change is greatest in the final weeks of development (third trimester).
(More? Graph of Fetal Growth)Begin by just looking at the fetal anatomy in sections taken through the above 10 week female fetus (which is approximately 40 mm in size) and see how this compares with what you already know of adult GIT anatomy.
There are 4 sections taken in the parasagittal and sagittal plane (moving from the right at Plane A towards the midline at Plane D). Click on the small images (or the text below) to open the linked large image pages.
(More? 10 Week Plane A | 10 Week Plane B | 10 Week Plane C | 10 Week Plane D)
In the above week 10 fetus the midgut is still herniated (lying outside the ventral body wall) at the umbilicus. We saw in earlier pages that this heriation began back in the embryonic period (week 5) and the initial loop undergoes a series of rotations through embryonic and early fetal periods which position the midgut in its correct adult anatomical locations. These rotations are around the superior mesenteric artery (which supplies this region) and the associated mesentry. (Note this rotation is complicated and explained differently in different texts)
One week later (week 11) continued fetal body wall growth (and other factors) returns the midgut herniation returns to the abdominal cavity.
Cartoon of Midgut Rotation
View from left hand side of embryo facing left. Sequence spans approx 6-11 weeks of development.
There is an initial 90 degree rotation followed by 2 more 90 degree rotations to give 270 degrees total. The upper GIT is shader light green, the red vessel is the superior mesenteric artery off the dorsal aorta.
In the second trimester, the ventral and dorsal mesenteries associated with the stomach are still anatomically different from the newborn. The figure shows a lateral view of this process comparing the early second trimester arrangement with the newborn structure. Ventral Mesogastrium Attached to the superior end of the stomach will form the lesser omentum. This structure will connect the lesser curvature of the stomach to the liver as a ligamentous structure. Dorsal Mesogastrium Attached to the inferior end of the stomach initially as an extended fold, this later fuses as a single "apron-like" structure, the greater omentum. Fusion will also incorporate the transverse colon part of the large intestine. This will also contribute the gastrosplenic ligament (gastrolienal ligament). (More? Stomach Development) |
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Each region of the GIT has a specific arterial supply which historically defined the 3 GIT regions.
Foregut- celiac artery (Adult: pharynx, esophagus, stomach, upper duodenum, respiratory tract, liver, gallbladder pancreas) Midgut- superior mesenteric artery (Adult: lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, half transverse colon) Hindgut- inferior mesenteric artery (Adult: half transverse colon, descending colon, rectum, superior part anal canal) |
Lateral view of developing GIT 3 main arteries |
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Ventral view of intestinal vascularization |
Ventral view of stomach blood supply |
Week 11 - villi begin to appear in small intestine, goblet cells present Week 16 - villi apparent in entire intestine Adult small intestine will be divided into 3 regions with the same basic histological organization.
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Week 20 - Peyer's patches appear in small intestine The adult intestinal immune system includes:
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Peyer's Patch, adult |
You may also like to review all prenatal GIT development in relation to other systems by looking at the timeline of human development
Amniotic Fluid Swallowing In early embryonic development both the buccopharyngeal and cloacal membranes degenerated, allowing the GIT to be filled with amniotic fluid. Towards the end of the fetal period the fetus is swallowing approximately 500 ml of amniotic fluid / day.
This swallowed amniotic fluid moves through the GIT from esophagus, to stomach, to small intestine, stopping at the large bowel. In the large bowel the majority of fluid (water) is absorbed, along with electrolytes, glucose, urea and hormones. This process may contribute to fetal nutrition and prepare the GIT for its postnatal function. The process of swallowing amniotic fluid has been suggested to also help regulate fluid volume.
Polyhydramnios (or hydramnios) refers to abnormally high amniotic fluid levels. This can be caused by a range of different abnormalities: byesophageal atresia, duodenal atresia, anencephaly, hydrops fetalis, achondroplasia, Beckwith-Wiedemann syndrome, diaphragmatic hernia, gastroschisis, multiple gestations (twins or triplets) or gestational diabetes.
Fetal Meconium (green fecal material) is the mixture of substances (debris, glandular secretions, fatty material and bile pigments) that accumulate in the large bowel. The mixture will form the neonatal meconium which is the first (usually within 24h to 48h) postnatal excretion from the GIT. If no discharge (bowel motion) is observed in this period it can be indicative of an abnormality of the GIT.
Meconium Aspiration can occur near term or at delivery, if meconiumis discharged into the amiotic fluid (meconium stained amniotic fluid) and then injested by the fetus as it swallows amiotic fluid. This can then lead to meconium aspiration syndrome (MAS), meconium drawn into the fetal/newborn lungs, causing inflammation, cell death and potentially perinatal death.
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Link to next page in this GIT Practical - Postnatal This next page will consider early postnatal changes in the GIT. |
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.
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 |
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.
bowel - see intestine.
fetal length - The measurement of crown to rump length of the developing fetus. The greatest growth in length occurs in the middle second trimester, of human development. There are a number of other growth parameters that can be measured, commonly determined by ultrasound, during the fetal period. (More? Fetal | Ultrasound Measurements | Ultrasound Movies)
fetal weight - The measurement of the weight of the developing fetus. The measurement is obtained by ultrasound calculation or clinically estimated by palpatation. The greatest addition of fetal weight occurs during and towards the end of the third trimester. (More? Fetal Weight | Fetal Notes | Ultrasound Measurements | Ultrasound Movies)
gastrosplenic ligament - (gastrolienal ligament, ligamentum gastrosplenicum) derived from the dosal mesogastrium (mesentery) connects the spleen to the stomach as part of the greater omentum.
herniated - The discription of the process of protrusion of an organ through a weak spot in the surrounding tissue. In normal development, herniated midgut, describes the gastrointestinal tract growth outside the abdominal wall prior to body wall growth. In abnormal development, abnormal protrusion of organs in the diaphragm, abdominal or groin areas (hiatal hernias or inguinal hernias). Occurs normally in the development of the gastrointestinal tract when the midgut is initially herniated at the umbilicus during embryonic development.
intestine - (bowel) Term used to describe the midgut and hindgut portion of the gastrointestinal tract running from after the stomach to the anus. This can also be anatomically subdivided into the small intestine (small bowel) and the large intestine (large bowel). (More? Gastrointestinal Tract Notes)
intestinal immune system - System consists of gastrointestinal tract functional regions and cells including: Peyer's patches, isolated lymphoid follicles, cryptopatches and mesenteric lymph nodes. (More? Gastrointestinal Tract Notes | Immune Notes)
large intestine - gastrointestinal tract between the end of the ileum (cecum) and anus. The large intestine can be further subdivided into parts: cecum, colon (ascending, transverse, descending, and sigmoid), and rectum. Functions in the resorption of water, vitamin K metabolism, and the formation of faeces. (More? Gastrointestinal Tract Notes)
lesser omentum - The stomach has 2 peritoneal folds attached to each of the stomach curvatures. The lesser omentum is the peritoneal fold extending from lesser curvature of the stomach to liver. The greater omentum is the peritoneal fold extending from the greater curvature of the stomach to the colon, and hanging down over the small intestine. (More? GIT Notes)
meconium The gastrointestinal contents that accumulate in the intestines during the fetal period. This material is a mixture of liver bile and glandular secretions, amniotic fluid, and cellular debris. Meconium is also used to describe the first postnatal rectal discharge from the neonate. Fetal stress in the third trimester or at parturition can lead to premature meconium discharge, into the amniotic fluid and ingestion by the fetus (meconium aspiration syndrome) and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur. (More? Birth | Gastrointestinal Tract Notes | Respiratory Notes)
meconium aspiration syndrome (MAS) Fetal stress in the third trimester, prior to/at/ or during parturition can lead to premature meconium discharge into the amniotic fluid and sunsequent ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur.
mesentery - The tissue fold attaching gastrointestinal tract to posterior abdominal wall in which blood vessels, lymph and nerves run. Developmentally derived from lateral plate mesoderm forming splanchnic mesoderm which then forms the posterior mesogastrium. (More? mesogastrium | GIT Notes)
omphalocele - a congenital malformation with herniation of abdominal contents through the umbilicus, which can be covered by a membrane. Not to be confused with gastroschisis (para-umbilical hernia) or hypoplasia of abdominal muscles, skin-covered umbilical hernia. (More? GIT Abnormalities)
outer submucous plexus - (OSP, Meissner’s plexus, plexus submucosus externus , Schabadasch plexus) A gastrointestinal tract intrinsic neral network, that is one of the two (inner and outer) lying within the submucosa involved in intestinal secretion and gut motility in large mammals. Smaller animals have only a single submucous plexus. (More? Gastrointestinal Tract Notes)
Peyer's patches - Gut-Associated Lymphoid Tissue (GALT) located in the ileum and appendix (lying between the lamina propria to the submucosa). These immune system structures are the first line of protection against ingested infectious agents. (More? Gastrointestinal Tract Notes)
soft palate - (velum, muscular palate) The muscular posterior portion of the palate forming the roof of the oral cavity. The bony anterior portion of the palate is called the hard palate, formed by maxillary and palatine bones. (More? Head 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 )
trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal. The second and third trimester correspond to the fetal period of growth in size (second trimester) and weight (third trimester), as well as continued differentiation of existing organs and tissues. (More? Embryo Stages | Human Fetal Period | Development Week by Week)
villi - Plural of villus, which is a thin projection from a surface.
On this fifth Practical page are a number of images of sections through the whole week 10 (early) female fetus to see the internal structure, organs and cavities that have now developed.
The first section (A) is away the midline and subsequent sections (B, C and D) move toward the midline. Use these slices to construct a 3 dimensional image in your mind. Clicking on regions of the fetus (head, body) will open enlarged images of those regions. Try and link the terms that surround the fetus with specific structures in the section.
Note also that we will be revisiting these images in the Sexual Differentiation practical.
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)
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