ANAT2341 Lab 5 - Fetal

From Embryology
Lab 5: Introduction | Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment

Fetal Development

Fetal liver histology

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)

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 as meconium.

Early Fetus

Week 10 Fetus

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.

Related Images

Human- fetal week 10 planes icon.jpg

Fetus (week 10) Planes A (most lateral), B (lateral), C (medial) and D (midline) from lateral towards the midline.

Image Source: UNSW Embryology, no reproduction without permission.

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.

Midgut Herniation

Week 10

In the week 10 fetus (right) 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.

Small intestine growth in length is initially linear (first half pregnancy to 32 cm CRL), followed by rapid growth in the last 15 weeks doubling the overall length to term. Growth continues postnatally but after 1 year slows again to a linear increase to adulthood. (More? Intestine Development)

Bailey299 300.jpg Bailey304.jpg

Simple cartoon of 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 (arising from the dorsal aorta).

Normal intestinal rotation cartoon.jpg

Normal intestinal rotation (note these are GA gestational age weeks).[1]

Stomach Mesentery


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).

Blood Supply

GIT blood supply.jpg

Each region of the gastrointestinal tract has a specific arterial supply which historically defined the 3 gastrointestinal tract regions (foregut, midgut and hindgut).

  1. Foregut - celiac artery (Adult: pharynx, esophagus, stomach, upper duodenum, respiratory tract, liver, gallbladder pancreas)
  2. Midgut - superior mesenteric artery (Adult: lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, half transverse colon)
  3. Hindgut - inferior mesenteric artery (Adult: half transverse colon, descending colon, rectum, superior part anal canal)

Fetal Tract Development

Small intestine villi and crypts
Adult Peyer's patch

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.

  1. duodenum (25-30 cm)
  2. jejunum (about first two-fifths of the rest)
  3. ileum

Week 20 - Peyer's patches appear in small intestine

The adult intestinal immune system includes:

  • Peyer's patches
  • isolated lymphoid follicles
  • cryptopatches - small clusters of lymphoid cells with an immature lymphocyte phenotype and dendritic cells.
  • mesenteric lymph nodes

Pancreas Development

Pancreatic buds and duct developing
  • Pancreatic buds - duodenal level endoderm, splanchnic mesoderm forms dorsal and ventral mesentery, dorsal bud (larger, first), ventral bud (smaller, later)
  • Pancreas Endoderm - pancreas may be opposite of liver
    • Heart cells promote/notochord prevents liver formation
    • Notochord may promote pancreas formation
    • Heart may block pancreas formation
  • Duodenum growth/rotation - brings ventral and dorsal buds together, fusion of buds
  • Pancreatic duct - ventral bud duct and distal part of dorsal bud, exocrine function
  • Islet cells - cords of endodermal cells form ducts, from which cells bud off to form islets

Pancreatic Islets

Human Pancreatic Islets (Islets of Langerhans)[2]
  • Islets of Langerhans - 4 endocrine cell types
  • Alpha - glucagon, mobilizes lipid
  • Beta - insulin, increase glucose uptake
    • Beta cells, stimulate fetal growth, continue to proliferate to postnatal, in infancy most abundant
  • Delta - somatostatin, inhibits glucagon, insulin secretion
  • F-cells - pancreatic polypeptide

Developing Pancreatic Islets

Model of endocrine cell and vessel organization in human islets[3]

Model of human pancreatic islet.jpg

A α-Cells (green) and β-cells (red) are organized into a thick folded plate lined at both sides with vessels (blue).
  • α-Cells are mostly at the periphery of the plate and in close contact with vessels.
  • β-Cells occupy a more central part of the plate and most of them develop cytoplasmic extension that runs between α-cells and reaches the surface of vessels.

B The plate with adjacent vessels is folded so that it forms an islet.

(Text based on original reference legend)

Pancreas Timeline

  • Week 7 to 20 - pancreatic hormones secretion increases, small amount maternal insulin
  • Week 10 - glucagon (alpha) differentiate first, somatostatin (delta), insulin (beta) cells differentiate, insulin secretion begins
  • Week 15 - glucagon detectable in fetal plasma

Links: Endocrine - Pancreas Development | Gastrointestinal Tract - Pancreas Development

Amniotic Fluid and Meconium

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 now 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.

Respiratory Development

Lung alveoli development cartoon.jpg

The sequence is most important rather than the actual timing, which is variable in the existing literature.

Human Lung Stages
Lung Stage Human Features Vascular
Embryonic week 4 to 5 lung buds originate as an outgrowth from the ventral wall of the foregut where lobar division occurs extra pulmonary artery then lobular artery
Pseudoglandular week 5 to 17 conducting epithelial tubes surrounded by thick mesenchyme are formed, extensive airway branching Pre-acinar arteries
Canalicular week 16 to 25 bronchioles are produced, increasing number of capillaries in close contact with cuboidal epithelium and the beginning of alveolar epithelium development Intra-acinar arteries
Saccular week 24 to 40 alveolar ducts and air sacs are developed alveolar duct arteries
Alveolar late fetal to 8 years secondary septation occurs, marked increase of the number and size of capillaries and alveoli alveolar capillaries
embryonic stage - pseudoglandular stage - canalicular stage - saccular stage - alveolar stage   Links: Species Stage Comparison | respiratory

Canalicular stage

  • week 16 - 24
  • Lung morphology changes dramatically
  • differentiation of the pulmonary epithelium results in the formation of the future air-blood tissue barrier.
  • Surfactant synthesis and the canalization of the lung parenchyma by capillaries begin.
  • future gas exchange regions can be distinguished from the future conducting airways of the lungs.

Saccular stage

Alveolar sac structure
  • week 24 to near term.
  • most peripheral airways form widened airspaces, termed saccules.
  • saccules widen and lengthen the airspace (by the addition of new generations).
  • future gas exchange region expands significantly.
  • Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin. May have a role in epithelial differentiation and control of surfactant secretion
  • The vascular tree also grows in length and diameter during this time.

  • in late fetal development respiratory motions and amniotic fluid are thought to have a role in lung maturation.
  • Development of this system is not completed until late fetal just before birth.
    • Therefore premature babies have difficulties associated with insufficient surfactant (end month 6 alveolar cells type 2 appear and begin to secrete surfactant).

Diaphragm Development

Adult Diaphragm
Five elements contribute to the diaphragm.
  1. septum transversum - central tendon
  2. 3rd to 5th somite - musculature of diaphragm (More? Somitogenesis)
  3. ventral pleural sac - connective tissue
  4. mesentry of oesophagus - connective tissue around oesophasus and IVC (More? Gastrointestinal Tract Development)
  5. pleuroperitoneal membranes - connective tissue around central tendon
Diaphragm components.jpg
Links: DiaphragmDevelopment
  1. <pubmed>20549505</pubmed>| PMC2908440
  2. <pubmed>18958289</pubmed>| PLoS ONE
  3. <pubmed>20185817</pubmed>| PMC2857900 | Diabetes.

Lab 5: Introduction | Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment

ANAT2341 Course Timetable  
Week (Mon) Lecture 1 (Mon 1-2pm) Lecture 2 (Tue 3-4pm) Practical (Fri 1-3pm)
Week 2 (1 Aug) Introduction Fertilization Lab 1
Week 3 (8 Aug) Week 1 and 2 Week 3 Lab 2
Week 4 (15 Aug) Mesoderm Ectoderm Lab 3
Week 5 (22 Aug) Early Vascular Placenta Lab 4
Week 6 (29 Aug) Gastrointestinal Respiratory Lab 5
Week 7 (5 Sep) Head Neural Crest Lab 6
Week 8 (12 Sep) Musculoskeletal Limb Development Lab 7
Week 9 (19 Sep) Renal Genital Lab 8
Mid-semester break
Week 10 (3 Oct) Public Holiday Stem Cells Lab 9
Week 11 (10 Oct) Integumentary Endocrine Lab 10
Week 12 (17 Oct) Heart Sensory Lab 11
Week 13 (24 Oct) Fetal Birth and Revision Lab 12

ANAT2341 2016: Moodle page | ECHO360 | Textbooks | Students 2016 | Projects 2016

ANAT2341Lectures - Textbook chapters  
Lecture (Timetable) Textbook - The Developing Human Textbook - Larsen's Human Embryology
Embryology Introduction Introduction to the Developing Human
Fertilization First Week of Human Development Gametogenesis, Fertilization, and First Week
Week 1 and 2 Second Week of Human Development Second Week: Becoming Bilaminar and Fully Implanting
Week 3 Third Week of Human Development Third Week: Becoming Trilaminar and Establishing Body Axes
Mesoderm Fourth to Eighth Weeks of Human Development Fourth Week: Forming the Embryo
Ectoderm Nervous System Development of the Central Nervous System
Early Vascular Cardiovascular System Development of the Vasculature
Placenta Placenta and Fetal Membranes Development of the Vasculature
Endoderm - GIT Alimentary System Development of the Gastrointestinal Tract
Respiratory Respiratory System Development of the Respiratory System and Body Cavities
Head Pharyngeal Apparatus, Face, and Neck Development of the Pharyngeal Apparatus and Face
Neural Crest Nervous System Development of the Peripheral Nervous System
Musculoskeletal Muscular System Development of the Musculoskeletal System
Limb Development of Limbs Development of the Limbs
Renal Urogenital System Development of the Urinary System
Genital Urogenital System Development of the Urinary System
Stem Cells
Integumentary Integumentary System Development of the Skin and Its Derivatives
Endocrine Covered through various chapters (see also alternate text), read head and neck, neural crest and renal chapters.
Endocrinology Textbook - Chapter Titles  
Nussey S. and Whitehead S. Endocrinology: An Integrated Approach (2001) Oxford: BIOS Scientific Publishers; ISBN-10: 1-85996-252-1.

Full Table of Contents

Heart Cardiovascular System Development of the Heart
Sensory Development of Eyes and Ears Development of the Eyes
Fetal Fetal Period Fetal Development and the Fetus as Patient
Birth and Revision
Additional Textbook Content - The following concepts also form part of the theory material covered throughout the course.
  1. Principles and Mechanisms of Morphogenesis and Dysmorphogenesis
  2. Common Signaling Pathways Used During Development
  3. Human Birth Defect

Glossary Links

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Cite this page: Hill, M.A. (2024, May 19) Embryology ANAT2341 Lab 5 - Fetal. Retrieved from

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