Structures in the Model

1. Duodenum (tan/yellowish-brown C-shape)

2. Pancreas (dark reddish-brown, leaf-shaped mass)

• Exocrine: Secretes digestive enzymes (amylase, lipase, proteases) into the duodenum
• Endocrine: Secretes hormones (insulin, glucagon) into the bloodstream

3. Gallbladder (bright green oval at the bottom)

4. Biliary Duct System (bright green branching tubes)

• The cystic duct leaving the gallbladder
• The common hepatic duct coming down from the liver
• These two joining to form the common bile duct (CBD), which runs down toward the duodenum
• A small green connection at the top showing the pancreatic duct or Ampulla of Vater - the point where both bile and pancreatic juices enter the duodenum together

5. Blue Patch (light blue with branching lines)

Why These Structures Are Shown Together

1. The liver produces bile → stored in the gallbladder
2. When food enters the duodenum, the hormone cholecystokinin (CCK) triggers the gallbladder to contract
3. Bile flows through the CBD and joins pancreatic enzymes at the Ampulla of Vater
4. Both secretions enter the duodenum to digest fats and proteins

What Is Being Shown

The Embryonic Disc (Bilaminar → Trilaminar)

Structure-by-Structure Breakdown

What Is Gastrulation?

1. Ectoderm (blue - the surface shown) → gives rise to skin, brain, spinal cord
2. Mesoderm → gives rise to muscle, bone, heart, blood vessels
3. Endoderm → gives rise to gut lining, lungs, liver

Clinical Relevance

• If the primitive streak fails to regress fully, it can leave remnant cells in the sacrococcygeal region, causing a sacrococcygeal teratoma - the most common tumor in newborns
• The primitive node is responsible for establishing body symmetry - defects here cause conditions like situs inversus (organs mirrored to the opposite side)

Layer-by-Layer Breakdown

🔴 Red - Anterior 2/3 of the Tongue

• Formed by the tuberculum impar (median swelling) and two lateral lingual swellings
• All arise from Pharyngeal Arch 1 (mandibular arch)
• Sensory supply: CN V3 (mandibular branch of trigeminal) - general sensation; CN VII (chorda tympani) - taste

🟡 Yellow/Brown Speckled - Posterior 1/3 of the Tongue

• Formed by the copula / hypobranchial eminence
• Derived primarily from Pharyngeal Arch 3
• The V-shaped groove at the red-yellow boundary is the sulcus terminalis
• The small pit at the apex of the V = the foramen cecum - the site where the thyroid gland originates and descends into the neck
• Sensory supply: CN IX (glossopharyngeal)

🟢 Green - Epiglottis

• Formed by the posterior hypobranchial eminence
• Derived from Pharyngeal Arch 4
• Sensory supply: CN X (superior laryngeal nerve)

⬛ Dark V-shaped slit at the bottom - Laryngeal Orifice (Aditus)

• The entrance to the larynx, flanked by the arytenoid swellings
• Derived from Pharyngeal Arch 6
• Supply: CN X (recurrent laryngeal nerve)

What the Numbers 1, 2, 3, 4 Mean

Why This Matters Clinically

• Thyroglossal duct cysts arise from incomplete descent of the thyroid from the foramen cecum - can appear anywhere along the midline of the neck
• The tongue has a split nerve supply (V3, VII, IX, X) because it is built from multiple different arch contributions - this is why referred pain from different regions can be complex
• Ankyloglossia (tongue-tie) is a developmental failure of the tongue to separate from the floor of the mouth

Region-by-Region Breakdown

🔵 Blue Upper Portion - Posterior 1/3 of the Tongue (Root)

• Derived from Pharyngeal Arches 3 & 4 (hypobranchial eminence / copula)
• The black speckles along the midline represent developing lingual papillae or mucosal texture
• Adult innervation: CN IX (glossopharyngeal) for general sensation and taste

🟡🟢 Central Transition Zone (Green/Yellow gradient at the waist)

• Marks the Sulcus Terminalis - the V-shaped boundary between anterior and posterior tongue
• The midline apex of this V = the Foramen Cecum, the embryological origin point of the thyroid gland
• The thyroid descends from here down the neck via the thyroglossal duct

🩷 Pink/Salmon Lower Portion - Anterior 2/3 of the Tongue (Body)

• Derived from Pharyngeal Arch 1 (mandibular arch) via:
  • Median tuberculum impar
  • Two lateral lingual swellings (which fuse and overgrow the tuberculum impar)
• Adult innervation: CN V3 (general sensation) + CN VII (taste via chorda tympani)

⚪ White Bead-like Structures on Both Sides

• These represent the developing tooth buds / dental lamina and the alveolar ridges of the mandibular arch flanking the tongue
• The reddish-pink outlines show the gingival (gum) margin

Summary Table

Clinical Pearls

• Thyroglossal duct cyst - if the thyroid fails to fully descend from the foramen cecum, a cyst can form anywhere along the midline of the neck
• Lingual thyroid - thyroid tissue remaining at the base of the tongue (foramen cecum area) - the most common ectopic thyroid location
• The split nerve supply of the tongue (two different nerves for the anterior vs posterior) directly reflects this dual embryological origin - a classic exam question

What Is Being Shown: The Notochord

Region-by-Region Breakdown

🔵 Blue Upper Portion - Cranial End of the Embryonic Disc

• Represents the cranial (head) end of the embryonic disc
• Blue = ectoderm (the dorsal surface layer)
• The black midline streak running through the blue region = the notochordal plate / notochord forming along the craniocaudal axis
• The notochord grows from the primitive node cranially toward the prechordal plate

🟡🟢 Central Transition / Waist Region - The Notochord at the Level of the Primitive Node

• The constriction represents the primitive node (Hensen's node) - the organizer center of the embryo
• The green/yellow gradient marks where notochordal cells are being actively secreted and intercalating into the endoderm
• This is the region of notochordal process formation

🩷 Pink Lower Portion - Caudal End of the Embryonic Disc

• Represents the caudal (tail) end
• Pink coloring = transition to mesoderm/endoderm territory at the caudal region
• Contains the primitive streak running along the midline (the dark central line)

⚪ White Bead-like Structures on Both Sides

• Represent the somites forming on either side of the notochord
• Somites are blocks of paraxial mesoderm that form in a cranial-to-caudal sequence
• They eventually give rise to vertebrae, ribs, skeletal muscle, and dermis
• The paired, segmented appearance is the hallmark of somite formation

The Notochord - Key Facts

Clinical Relevance

• Chordoma - a rare malignant tumor arising from notochordal remnants, typically at the base of skull (clivus) or sacrococcygeal region
• Nucleus pulposus herniation (disc prolapse) - the NP is the adult remnant of the notochord; when it herniates it compresses nerve roots
• Notochord defects can cause vertebral segmentation anomalies and spina bifida

What Is Being Shown

Structure-by-Structure Breakdown

🟤 The U-shaped Intestinal Loop (tan/clay color)

• Cranial limb (upper part) - gives rise to the distal duodenum, jejunum, and proximal ileum
• Caudal limb (lower part) - gives rise to the distal ileum, cecum, appendix, ascending colon, and proximal 2/3 of transverse colon
• The bulbous swelling near the bottom = the cecal bud, the future cecum and appendix

🩷 The Branching Red/Pink Structure - Superior Mesenteric Artery (SMA)

• The axis of rotation around which the entire midgut rotates
• Supplies blood to all midgut derivatives
• The loop always rotates around the SMA - this is why the SMA is at the center of the model

🔵 Blue Background

• Represents the extraembryonic coelom (the space inside the umbilical cord where the herniated gut sits temporarily)

The 270° Counterclockwise Rotation

Final Adult Positions After Rotation

• Duodenum gets pushed to the right and retroperitoneal
• Cecum/appendix ends up in the right iliac fossa (right lower quadrant)
• Transverse colon crosses anterior to the duodenum

Clinical Relevance - Rotation Defects

Color-by-Color Breakdown

🟢 Green Chain/Beaded Structures - Trophoblast Layers

• Outer dark green (with clear cell outlines) = Cytotrophoblast - individual mononucleated cells forming the inner cellular layer of the villus covering
• Inner light green/teal speckled mass = Syncytiotrophoblast - a multinucleated, fused layer with no cell boundaries (the dots represent scattered nuclei). This layer directly invades the maternal endometrium and erodes maternal blood vessels

🔴 Red Areas - Maternal Blood Lacunae / Intervillous Spaces

🔵🔴 Blue and Red Thin Winding Lines - Fetal Blood Vessels

• Blue lines = fetal vessels carrying deoxygenated blood and waste away from the embryo
• Red lines = fetal vessels carrying oxygenated, nutrient-rich blood back to the embryo
• Where they loop near the trophoblast = villous capillary loops, the site of actual exchange

🟡 Beige/Cream Background - Extraembryonic Mesoderm (Villous Stroma)

The Three Stages of Villus Development

The Placental Membrane (Barrier between fetal & maternal blood)

1. Syncytiotrophoblast
2. Cytotrophoblast (thins out later in pregnancy)
3. Villous basement membrane
4. Fetal capillary endothelium

Clinical Relevance