Primitive streak. 2. Gastrulation
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1. Primitive streak
The primitive streak is the first visible sign of gastrulation. It appears at the beginning of week 3 as a thickened linear band of epiblast in the midline at the caudal end of the embryonic disc.
Formation and parts
- Epiblast cells proliferate and migrate toward the midline, producing the primitive streak.
- Its cranial end forms the primitive node.
- The center of the node contains the primitive pit.
- A longitudinal depression in the streak is the primitive groove.
Functions
- Initiates gastrulation.
- Establishes the embryonic body axes:
- Cranial-caudal
- Dorsal-ventral
- Right-left
- Provides the site through which epiblast cells migrate inward to form definitive endoderm and intraembryonic mesoderm.
- The primitive node contributes to formation of the notochordal process and subsequently the notochord.
Fate and clinical importance
The primitive streak normally regresses and disappears by the end of the fourth week. Persistence of remnants may produce a sacrococcygeal teratoma, a tumor containing derivatives of multiple germ layers. This association is also described in the NCBI embryology review.
The Developing Human: Clinically Oriented Embryology, pp. 190-191.
2. Gastrulation
Gastrulation is the process during the third week in which the bilaminar embryonic disc is converted into a trilaminar embryonic disc. It creates the three primary germ layers and establishes the basic body plan.
Sequence of events
- Primitive streak forms in the epiblast.
- Epiblast cells move toward the streak, detach, and migrate inward, a process called ingression.
- The first wave of ingressing cells displaces the hypoblast and forms the definitive endoderm.
- The next wave spreads between ectoderm and endoderm to form the intraembryonic mesoderm.
- Epiblast cells remaining on the surface become ectoderm.
Result: three germ layers
| Germ layer | Main derivatives |
|---|---|
| Ectoderm | Epidermis, central and peripheral nervous systems, eyes, internal ear, neural crest derivatives |
| Mesoderm | Muscle, bone, cartilage, connective tissues, dermis, blood and vessels, cardiovascular system, kidneys, gonads, serous membranes |
| Endoderm | Epithelial lining of gastrointestinal and respiratory tracts, and parenchyma/epithelium of associated glands such as liver and pancreas |
Importance
- Begins morphogenesis, the development of body form.
- Establishes embryonic axes.
- Produces the germ layers from which all tissues and organs develop.
- Sets up the midline through formation of the notochordal process and notochord.
One-line summary:
Primitive streak is the gateway for gastrulation; gastrulation transforms a bilaminar disc into a trilaminar disc consisting of ectoderm, mesoderm, and endoderm.
Primitive streak is the gateway for gastrulation; gastrulation transforms a bilaminar disc into a trilaminar disc consisting of ectoderm, mesoderm, and endoderm.
The Developing Human: Clinically Oriented Embryology, pp. 187-190.
Notochord: Development, functions and clinical aspects
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Notochord
The notochord is a temporary, midline rod of axial mesoderm formed during the third week. It lies between the developing neural tube dorsally and endoderm ventrally. It defines the embryonic midline, induces neural development, and guides formation of the axial skeleton.
Development
1. Formation of notochordal process
- Cells migrate inward through the primitive node and primitive pit.
- These prenotochordal cells migrate cranially in the midline, between ectoderm and endoderm.
- They form a cellular cord called the notochordal process.
- A lumen develops within it, the notochordal canal.
- The process grows toward the prechordal plate, close to the future oropharyngeal membrane.
2. Formation of notochordal plate
- The floor of the notochordal canal fuses temporarily with the underlying endoderm.
- This creates a flattened layer of cells called the notochordal plate.
- For a short period, the amniotic cavity communicates with the yolk sac through the neurenteric canal.
3. Formation of definitive notochord
- The notochordal plate folds inward.
- Its margins fuse and separate from the endoderm.
- A solid midline rod, the definitive notochord, is formed.
The definitive notochord extends from the prechordal region cranially to the primitive node caudally. Formation is dynamic: cranial parts form first, while more caudal segments are added as the primitive streak regresses.
The Developing Human: Clinically Oriented Embryology, pp. 197-201.
Langman's Medical Embryology, pp. 78-79.
Langman's Medical Embryology, pp. 78-79.
Functions of the notochord
1. Establishes the embryonic midline and body axis
It is the central axial structure of the embryo and helps organize surrounding mesoderm and ectoderm.
2. Induces neurulation
The notochord induces the overlying ectoderm to thicken into the neural plate, the precursor of the brain and spinal cord.
It releases morphogenetic signals including:
- Sonic hedgehog (SHH)
- Noggin
- Chordin
These signals help pattern the neural tube, especially its ventral portion. The NCBI gastrulation review describes the notochord as both a structural midline landmark and an induction center for neuroectoderm.
3. Organizes axial skeleton formation
- Sclerotomal cells migrate around the notochord and neural tube.
- These cells form the vertebral bodies and other parts of the vertebral column.
- Thus, the notochord acts as a central template around which the vertebral column develops.
4. Contributes to intervertebral discs
Most of the notochord degenerates as vertebral bodies form.
Its persistent remnant forms the nucleus pulposus of each intervertebral disc. The annulus fibrosus, in contrast, is derived from sclerotome. This origin is summarized in the NCBI vertebral-development review.
Clinical aspects
1. Chordoma
A chordoma is a rare, locally aggressive malignant tumor showing notochordal differentiation. It is typically located along the axial skeleton:
- Skull base, particularly the clivus
- Vertebral column
- Sacrum
Possible manifestations depend on site and include persistent spinal pain, radiculopathy, myelopathy, cranial-nerve deficits, headache, dysphagia, or bowel and bladder symptoms. The NCBI chordoma review notes that MRI is preferred to define tumor extent, while CT helps assess bony involvement.
2. Neurenteric cyst and neurenteric fistula
Abnormal persistence of the temporary connection between the amniotic cavity and yolk sac, or defective separation of notochord from endoderm, can produce:
- Neurenteric cysts
- Neurenteric fistulae
- Associated vertebral and spinal-cord abnormalities
These lesions may be associated with vertebral segmentation defects and, in severe cases, communication between gastrointestinal and neural structures.
3. Vertebral anomalies
Defective notochordal signaling or abnormal interaction with surrounding paraxial mesoderm may contribute to abnormalities of vertebral development, including:
- Hemivertebrae
- Fused vertebrae
- Vertebral segmentation defects
- Scoliosis associated with congenital vertebral malformations
4. Persistent notochordal remnants
Notochordal remnants are normal in the nucleus pulposus, but remnants may also occur elsewhere along the axial skeleton. They are relevant because of their relationship to chordoma pathogenesis.
Exam summary
| Heading | Key point |
|---|---|
| Origin | Prenotochordal cells migrating through primitive node and pit |
| Time | Third week of development |
| Sequence | Notochordal process → notochordal plate → definitive notochord |
| Position | Midline, between ectoderm/neural tube and endoderm |
| Major roles | Midline organizer, neural induction, axial skeleton patterning |
| Main adult remnant | Nucleus pulposus of intervertebral disc |
| Clinical relevance | Chordoma, neurenteric anomalies, vertebral malformations |
Memory line:
Node cells form the notochord, which induces the neural plate, guides vertebral formation, and persists only as the nucleus pulposus.
Node cells form the notochord, which induces the neural plate, guides vertebral formation, and persists only as the nucleus pulposus.
2. Pharyngeal arches
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Pharyngeal arches
Pharyngeal arches, also called branchial arches, are paired embryonic swellings that form much of the face, neck, pharynx, larynx, and associated vessels and nerves.
They begin to appear in the 4th week. Four arches are visible externally: 1st, 2nd, 3rd, and 4th. The 5th arch is rudimentary and disappears; the 6th arch is present but not externally visible.
The Developing Human: Clinically Oriented Embryology, p. 461.
Basic structure
Each pharyngeal arch has:
- External ectoderm
- Internal endoderm
- A mesenchymal core derived from mesoderm and neural crest cells
- A cartilage component
- A muscular component
- A cranial nerve
- An arterial component, linked to an aortic arch
Neural crest cells largely form the skeletal and connective-tissue derivatives, while mesoderm forms the muscles.
Derivatives of pharyngeal arches
| Arch | Nerve | Skeletal/cartilage derivatives | Muscular derivatives | Arterial derivative |
|---|---|---|---|---|
| 1st - mandibular arch | Trigeminal nerve, mainly V3 | Malleus, incus, mandible, maxilla, zygomatic bone, part of temporal bone, sphenomandibular ligament | Muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini | Maxillary artery |
| 2nd - hyoid arch | Facial nerve, VII | Stapes, styloid process, stylohyoid ligament, lesser horn and upper body of hyoid | Muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric | Stapedial artery, mostly disappears |
| 3rd arch | Glossopharyngeal nerve, IX | Greater horn and lower body of hyoid | Stylopharyngeus | Common carotid artery and proximal internal carotid artery |
| 4th arch | Vagus nerve, X via superior laryngeal branch | Contributes to laryngeal cartilages, particularly thyroid cartilage | Cricothyroid, levator veli palatini, pharyngeal constrictors | Left: part of arch of aorta. Right: proximal right subclavian artery |
| 6th arch | Vagus nerve, X via recurrent laryngeal branch | Laryngeal cartilages, especially cricoid, arytenoid, corniculate, and cuneiform cartilages | Intrinsic muscles of larynx except cricothyroid | Proximal pulmonary arteries; left distal portion becomes ductus arteriosus |
Fifth arch: rudimentary and has no important adult derivatives.
Important individual arches
1st pharyngeal arch
The first arch divides into:
- Maxillary prominence: maxilla, zygomatic bone, part of temporal bone
- Mandibular prominence: mandible and related structures
Its cartilage is called Meckel cartilage. Most of it disappears, but its dorsal portions form the malleus and incus.
Key memory:
1st arch = muscles of mastication = trigeminal nerve (V3).
1st arch = muscles of mastication = trigeminal nerve (V3).
2nd pharyngeal arch
The cartilage of the second arch is called Reichert cartilage.
It forms:
- Stapes
- Styloid process
- Stylohyoid ligament
- Lesser horn and upper body of hyoid bone
Key memory:
2nd arch = facial expression = facial nerve (VII).
2nd arch = facial expression = facial nerve (VII).
3rd pharyngeal arch
The third arch forms:
- Greater horn of hyoid
- Lower part of body of hyoid
- Stylopharyngeus muscle
Key memory:
3rd arch = stylopharyngeus = glossopharyngeal nerve (IX).
3rd arch = stylopharyngeus = glossopharyngeal nerve (IX).
4th and 6th pharyngeal arches
These form structures in the larynx and pharynx.
- 4th arch: cricothyroid muscle and pharyngeal constrictors, supplied by the superior laryngeal nerve.
- 6th arch: intrinsic laryngeal muscles except cricothyroid, supplied by the recurrent laryngeal nerve.
Key memory:
- 4th = superior laryngeal
- 6th = recurrent laryngeal
Relation to pharyngeal pouches and clefts
Do not confuse arches with pouches or clefts:
- Arches: mesenchymal swellings that form muscle, cartilage, arteries, and nerves.
- Pouches: internal endoderm-lined outpocketings.
- Clefts/grooves: external ectoderm-lined depressions.
The first pharyngeal cleft forms the external auditory meatus. The other clefts normally disappear when the second arch overgrows them and creates a temporary cervical sinus. The NCBI embryology review summarizes this distinction and the main adult derivatives.
Clinical aspects
1. First-arch syndromes
Abnormal neural crest migration or differentiation in the first arch can cause mandibulofacial abnormalities.
Examples:
- Treacher Collins syndrome: malar and mandibular hypoplasia, down-slanting palpebral fissures, external-ear anomalies, and possible conductive hearing loss.
- Pierre Robin sequence: micrognathia, posterior displacement of the tongue, and airway obstruction, often with cleft palate.
- Hemifacial microsomia / oculo-auriculo-vertebral spectrum: asymmetric underdevelopment of facial structures and ear anomalies.
2. Branchial cyst, sinus, or fistula
These usually result from persistence of the cervical sinus or incomplete obliteration of pharyngeal clefts. A typical branchial cyst is a painless lateral neck swelling, often near the anterior border of the sternocleidomastoid muscle.
3. DiGeorge syndrome
DiGeorge syndrome is primarily a pharyngeal pouch developmental defect, particularly involving derivatives of the third and fourth pouches, rather than an isolated arch defect.
Typical features include:
- Thymic hypoplasia or aplasia, causing T-cell immunodeficiency
- Hypocalcemia from parathyroid hypoplasia
- Conotruncal cardiac defects
- Characteristic facial features
4. Aortic-arch abnormalities
Abnormal remodeling of aortic arch derivatives may produce:
- Right-sided aortic arch
- Double aortic arch
- Aberrant right subclavian artery
- Patent ductus arteriosus or ductus-related anomalies
Quick recall
| Arch | Main nerve | One classic muscle |
|---|---|---|
| 1st | V3 | Masseter |
| 2nd | VII | Facial-expression muscles |
| 3rd | IX | Stylopharyngeus |
| 4th | X, superior laryngeal | Cricothyroid |
| 6th | X, recurrent laryngeal | Intrinsic laryngeal muscles |
Mnemonic:
“Chew, smile, stylopharyngeus, swallow, speak”
“Chew, smile, stylopharyngeus, swallow, speak”
- 1st: Chew
- 2nd: Smile
- 3rd: Stylopharyngeus
- 4th: Swallow
- 6th: Speak
The Developing Human: Clinically Oriented Embryology, pp. 461-462.
Langman’s Medical Embryology, pp. 305-306.
Langman’s Medical Embryology, pp. 305-306.
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