ToB 9.1 Early Embryonic Development
Learning Outcomes
Describe formation of head fold, tail fold, and lateral body folds.
Describe differentiation of somites into dermatome, myotome, and sclerotome, and their fates.
Explain use of terms “dermatome” and “myotome” in describing skin innervation.
Describe differentiation of mesoderm into notochord, somites, intermediate mesoderm, lateral plate mesoderm, and their derivatives.
Describe formation and fate of the coelom and its regions.
List derivatives of each mesoderm region and their later development.
Describe formation and fate of the neural plate and neural tube.
Understand neurulation stages and key timing events.
Recognize neural crest formation and its derivatives.
Understand how folding relates to organ formation and body plan.
Recognize establishment of the nervous system and basic brain regionalization.
Appreciate neural tube defects (NTDs), their causes, consequences, and prevention.
Be familiar with basic “derivatives of the three germ layers” as a framework.
Recognize the basic timeline of events in weeks 3–4 of embryogenesis.
Terminology: Position, Direction, and Planes Used for Embryos
Cranial (cephalic): superior; used for the head end.
Anterior: front; synonymous with ventral in many contexts.
Posterior: back; synonymous with caudal.
Dorsal: back side.
Ventral: belly side.
Superior: above.
Inferior: below.
Body planes:
Sagittal (longitudinal) plane.
Frontal (coronal) plane.
Transverse section (axial plane).
Median section (midline sagittal section).
Early Development: Germ Layers, Gastrulation, and Germ-Layer Derivatives
Gamete to zygote progression (summary context):
Totipotent zygote $\rightarrow$ blastomeres $\rightarrow$ morula $\rightarrow$ blastocyst with trophoblast and embryoblast (ICM).
Trophoblast supports implantation and placentation; embryoblast forms the embryo.
Week 1–2 germ layers and gastrulation:
Bilaminar disc composed of epiblast and hypoblast; amnion and yolk sac establish.
Primitive streak forms and gives rise to ectoderm, mesoderm, and endoderm via epiblast migration.
Epiblast cells migrating through the node and primitive streak generate mesoderm and endoderm; ectoderm mainly from remaining epiblast.
Trophoblast and extraembryonic structures:
Trophoblast: nutrients, aids implantation, contributes to placenta.
Yolk sac, chorion, extraembryonic mesoderm.
Primary germ layers (major derivatives):
Ectoderm: nervous system, epidermis, neural crest derivatives, sensory organs, anterior pituitary.
Mesoderm: somites; axial skeleton; muscular system; dermis; circulatory system; kidneys and gonads; connective tissues of body wall.
Endoderm: lining of gut and respiratory tract; associated visceral organs (liver, pancreas).
Derivatives of the Three Germ Layers (High-Level Framework)
ECTODERM:
Neuroectoderm: neural tube $\rightarrow$ CNS; neural crest $\rightarrow$ PNS and many other derivatives (craniofacial structures, pigment cells, adrenal medulla).
Surface ectoderm: epidermis; hair and nails; cutaneous glands; lens and cornea components; various epithelial tissues.
Other ectodermal derivatives include components of the pituitary (anterior part arises from oral ectoderm).
MESODERM:
Paraxial mesoderm: somites $\rightarrow$ vertebrae (axial skeleton), skeletal muscle of body axis and limbs, dermis of back, plus associated connective tissue.
Intermediate mesoderm: urogenital system (gonads, ducts, and accessory glands).
Lateral plate mesoderm: body wall and limb muscles and connective tissue; serous membranes (pleura, pericardium, peritoneum); somatic (parietal) and splanchnic (visceral) mesoderm contribute to coelomic cavities; primordial heart; blood and lymphatic cells; spleen; adrenal cortex.
ENDODERM:
Epithelium of gastrointestinal tract and its derivatives (liver, pancreas).
Epithelium of respiratory system (trachea, bronchi, lungs).
Epithelium of urinary bladder and urachus; pharyngeal pouch derivatives (thyroid, thymus, parathyroid, tonsils, pharyngotympanic tube and middle ear epithelium).
Formation, Fate, and Patterning Role of the Notochord
Definition: A transient, rod-like structure ventral to the neural tube.
Functions:
Defines the embryonic axis and guides vertebral column development.
Maintains left-right asymmetry cues during development.
Induces neural plate formation from overlying ectoderm.
Transforms mesodermal cells in somites toward vertebral identities (vertebral body formation).
Stimulates dorsal pancreas development early in development.
Origin and Timing: Arises from axial mesoderm around day and is fully formed by the beginning of week 4.
Post-embryogenesis Note: In adults, the notochord persists as the nucleus pulposus within intervertebral discs.
Key Organizer Role:
Induces overlying ectoderm to form the neural plate.
Specifies identity of certain neural tube cells (floor plate).
Transitions certain somite cells into vertebral bone components (vertebral columns).
Stimulates early dorsal pancreatic development.
Neurulation: Neural Plate, Neural Tube, and Nervous System Establishment
Four major stages of neurulation:
Transformation of ectoderm into a thickened neural plate.
Shaping of the neural plate to become narrower and longer.
Elevation of neural folds to form neural folds and neural plate invagination into neural groove.
Fusion of the lateral neural folds to form the neural tube (autonomous formation).
Timeline highlights:
Day : neural plate invaginates to form neural groove.
End of Week 3: neural folds start to fuse along the midline, converting neural plate into neural tube.
Closure of the anterior (cranial) neuropore Day .
Closure of the posterior (caudal) neuropore Day .
Derivatives:
The neural tube gives rise to the CNS.
Neural crest cells arise at the borders of the neural plate and later migrate.
Neural Crest Derivatives (selected examples):
Melanocytes.
Dorsal root and sympathetic chain ganglia; adrenal medulla.
Schwann cells and other glia of the peripheral nervous system.
Pharyngeal arch cartilage derivatives (facial bones and structures).
Odontoblasts (dentition components).
Some cranial nerves and components of the eye and ear.
Cranial neural crest vs. trunk neural crest: distinct migratory paths and derivative sets.
Establishment of the nervous system:
Closure of the caudal neuropore marks progression toward a rudimentary spinal cord caudally and brain vesicles cranially.
Brain regionalization begins with a three-part brain: forebrain (prosencephalon), midbrain (mesencephalon), hindbrain (rhombencephalon).
Later subdivision of forebrain into telencephalon and diencephalon; rhombencephalon subdivides into metencephalon and myelencephalon.
Early brain patterning sets up gross organization of the adult brain.
Primary vesicle formation occurs as neural tube closes and expands cranially.
Subsequent secondary vesicle divisions refine regions that contribute to higher centers.
Neural Tube Defects (NTDs)
Cause: NTDs occur when neural tube closure fails to occur properly.
Types:
Cranial NTDs: failure to close cranially leads to severe brain malformations (anencephaly is a common example).
Spinal NTDs: failures to close in the cervical to lumbosacral regions; spina bifida is a common outcome, often in the lumbosacral region.
Prevention: Daily intake of folic acid reduces of NTDs.
Common examples mentioned: anencephaly, spina bifida.
Somites: Formation, Differentiation, and Function
Somite formation timeline:
Paraxial mesoderm organizes into somitomeres around week 3.
Somitomeres further segment into somites on each side of the neural tube; new somites appear cranially to caudally at .
By end of week 4 (and into week 5), somites are present.
Somite differentiation: regionalization into three compartments within each somite:
Sclerotome (ventro-medial): forms vertebral bones and vertebral column; chondrocytes form cartilage; bones of axial skeleton.
Dermamyotome (dorsolateral): gives rise to two components:
Dermatome: dermis of the back skin.
Myotome: skeletal muscle of axial and appendicular (limb) regions; myoblasts form muscle.
Somite innervation and resegmentation:
Somites influence neural crest cell migration and spinal nerve axon trajectories.
Re-segmentation: each sclerotome splits into cranial and caudal halves; the cranial half of a sclerotome fuses with the caudal half of the previous somite to form a vertebra.
Each myotome and dermatome retains segmental innervation from its originating spinal nerve root; patterning of body wall and limb innervation follows this segmental organization.
Dermatomes and Nerve Supply
Definition: Skin regions innervated by a single spinal nerve root.
Quantity: Typically 30 dermatomes distributed across cervical, thoracic, lumbar, sacral, and coccygeal levels.
Clinical Use: Dermatome mapping is used clinically to assess radiculopathy and to localize pathology in the spinal cord or nerve roots.
Embryo Folding and the Formation of the Body Plan
Timing: Embryo folding occurs around Day and progresses through Week 4.
Folding directions:
Longitudinal (cephalo-caudal) flexion forms cranial and caudal rims that curve ventrally.
Transverse (lateral) folding creates two lateral body folds.
Consequences of folding:
The embryo transitions from a flat trilaminar disc to a cylindrical, “two-tube” body plan with an outer ectodermal tube and an inner endodermal tube.
The space between the tubes is mainly filled by mesoderm-derived tissue.
The coelom (body cavity) forms within the lateral plate mesoderm compartments.
Formation of the Intraembryonic Coelom and Body Cavities
Lateral plate mesoderm splits into two layers:
Somatic (parietal) mesoderm – lines the body wall.
Splanchnic (visceral) mesoderm – covers the gut tube.
Coelom formation: Folded mesoderm meet and merge to form the intraembryonic coelom.
The intraembryonic coelom forms the major body cavities:
Pericardial cavity.
Pleural cavities.
Peritoneal cavity.
Role: The coelomic cavities ensue from embryo folding, contributing to the primitive heart and subsequent organ development.
Week 4 and Pharyngeal Arch Development (Overview)
Key events in Week 4:
Emergence of facial prominences from pharyngeal arch regions (mandibular, maxillary, hyoid).
Cardiac prominence develops as the heart begins to form and migrate to the thoracic position.
Auditory placode and nasal placodes appear; neural crest contributions begin to populate pharyngeal arches.
Early limb buds (mesenchymal cores) emerge from lateral plate mesoderm.
Pharyngeal arches (I–IV):
I (Mandibular) arch; II (Hyoid) arch; III arch; IV arch.
These arches give rise to facial bones, jaws, middle ear structures, etc., and contribute to pharyngeal-derived glands and blood vessels.
Overall context: This week also features organ primordia development and regional specification important for later morphogenesis.
Derivatives: Integration and Clinical Relevance
Germ-layer framework: Helps explain where tissues originate and how congenital anomalies arise.
Key processes: Notochord signaling, neural plate formation, and neural crest migration are central to proper neural and skeletal development.
Somites: Provide a vertebral and muscular blueprint, with segmentation guiding nervous system innervation and skeletal patterning.
Coelom formation and folding: Underlie the body’s major cavities and organ arrangement, enabling a connected yet compartmentalized body plan.
Additional Resources and References (Recommended)
Embryology resources and texts cited: UNSW Embryology (Sadler chapters 3–5), Moore et al. Before We Are Born, Larsen’s Human Embryology, Langman’s Medical Embryology, Carlson’s Human Embryology & Developmental Biology.
Video and online resources mentioned:
Embryology at a Glance (YouTube) – Embryology overview.
Alexander Tsiaras visuals (Conception to Birth visualization).
Further study: Consult the recommended resources listed in the slides for deeper explanations and figures.
Equations and Quantitative Notes
Somite formation rate: .
Closure timing in neurulation:
Anterior neuropore closure: .
Posterior neuropore closure: .
Body plan: The longitudinal folding and lateral folding produce the adult body plan in which the body cavity is subdivided into the pericardial, pleural, and peritoneal spaces via the intraembryonic coelom.