Lecture 3.2 Notes: Developing the Human Body Plan, Part 2

Gastrulation: overview and trilaminar germ layer formation

• Bilaminar to trilaminar transition focused on the epiblast and hypoblast layers.
  • Bilaminar cell layer that migrates medially and ventrally: the epiblast (cells migrate through the primitive streak).
  • Region where gastrulation starts: the primitive streak (caudal neural axis region).
  • Structure created by this migration: the trilaminar germ disc (endoderm, mesoderm, and ectoderm).
• Trilaminar germ layers and their order of formation
  • First layer formed: endoderm (formed when epiblast cells move inward displacing the hypoblast).
  • Second layer formed: mesoderm (formed as epiblast cells migrate between endoderm and ectoderm).
  • Last layer formed: ectoderm (the remaining epiblast becomes the ectoderm).

Organization of the Human Body: trans-segmental vs segmental structures

• Trans-segmental structures: run continuously along the axis without distinct repeating blocks; visible in cross-sections along most; not tied to discrete somites.
• Segmental structures: arranged as a series of repeated blocks corresponding to somites; visible in cross-sections in a segmental pattern.

Formation of the Notochord

• Mesoderm cells migrate to the midline to form the notochord.
  • How/why: mesodermal cells converge at the midline as part of gastrulation and neurulation to establish axial support and signaling centers.
  • Becomes: the notochord.
  • Patterning: trans-segmental structure (notochord runs along the midline; it is not organized into somite blocks).
• Note: In the slide, the notochord is contrasted with cranial to caudal orientation and mesoderm localization.

Neurulation: formation of the dorsal hollow nerve tube

• Neurulation is the development of the first body system: the central nervous system.
• Key event: formation of the dorsal hollow nerve tube from neural ectoderm.
  • 1) Ectoderm differentiates into epidermal ectoderm and neural ectoderm.
  • 2) Lateral edges of neural ectoderm thicken to form neural folds; neural groove forms.
  • 3) Neural folds fuse to create the dorsal hollow nerve tube (neuroectoderm-lined CNS).
  • 4) Neural crest cells migrate ventrally and laterally throughout the embryo.
• The neural tube gives rise to the brain and spinal cord.
• Neural crest derivatives appear as they migrate widely (see below).
• Epidermal ectoderm forms a continuous outer sheet (the epidermis).
• Final note: neural ectoderm, epidermal ectoderm, and neural crest contribute to multiple tissues; the neural tube is trans-segmental in terms of axial extent, not a somite-derived structure.

Neurulation: key tissues and their derivatives

• 1) Dorsal hollow nerve tube: formed from neural ectoderm; becomes the brain and spinal cord.
• 2) Epidermal ectoderm forms the epidermis (outer skin layer).
• 3) Neural crest cells migrate ventrally and laterally; derivatives include peripheral nerves, dorsal root ganglia, sympathetic ganglia, adrenal medulla chromaffin cells, melanocytes, and craniofacial cartilage/bone components, among others.
• 4) Neural crest derivatives: become multiple tissues including peripheral nervous system components and facial mesenchyme.

Neurulation: labeling in the cross-section schematic

• Neural tube and surrounding tissues arrangement (simplified):
  • Dorsal hollow nerve tube
  • Neural ectoderm (neural tube precursor)
  • Epidermal ectoderm (external surface)
• Note: In the schematic, the neural tube is the central structure; epidermal ectoderm remains on the surface.

Neurulation: cranial-caudal progression

• The dorsal hollow nerve tube begins forming in the middle of the embryo and expands both cranially and caudally as neurulation proceeds.

Clinical implications: spina bifida and anatomy relevance

• Spinal bifida occurs when the neural tube fails to close properly, typically in the caudal region and most often near the lumbar or sacral area.
• Potential complications: neuro deficits, motor/sensory impairment, hydrocephalus, and other CNS-related issues depending on the level and extent of malformation.
• Source for clinical context: Mayo Clinic overview of spina bifida.

Development of the Mesoderm: somites and segmentation

• Neurulation-related timing: mesoderm lateral to the dorsal hollow nerve tube and notochord differentiates into somites.
• Somites are segmental blocks along the anteroposterior axis; they develop cranial to caudal in sequence.
• Somites differentiate into three core components:
  • Dermatome: forms the dermis of the dorsal body region.
  • Myotome: forms trunk and limb musculature; subdivides into epaxial (dorsal) and hypaxial (ventral) muscle groups; also contributes to limb connective tissues.
  • Sclerotome: forms vertebrae and ribs; provides the precursors to the axial skeleton.
• Differentiation of the somite is segmental in organization and outcome.

Humans have segmental structures

• Dermis and spinal nerves are classic segmental structures; each dermatome corresponds to a spinal nerve segment, enabling clinical localization of nerve damage.
• Other segmental structures include muscles of the torso and skeletal components of the torso linked to somite derivatives.

Development of the Mesoderm: intermediate mesoderm

• Lateral to the somites, intermediate mesoderm forms the urogenital system:
  • Becomes kidneys and gonads (and associated ducts).
  • The transformation is segmental in origin, but the structures themselves are not arranged in repeating somite blocks; they are derived from intermediate mesoderm.

Development of the Mesoderm: lateral plate mesoderm and coelom

• Lateral plate mesoderm differentiates into two sublayers:
  • Somatic (parietal) mesoderm: lines the body wall and contributes to limb bone/muscle structures, and the parietal serosa.
  • Splanchnic (visceral) mesoderm: covers organs and contributes to heart and dorsal aorta; forms visceral splanchnic structures.
• The coelom is a fluid-filled space formed between the somatic and splanchnic layers of the lateral plate mesoderm.
• The coelom eventually gives rise to body cavities and mesenteries; the intraembryonic coelom partitions into pericardial, pleural, and peritoneal compartments.

Development of the Gut Tube and coelomic interactions

• The gut tube formation occurs concomitantly with lateral plate mesoderm development.
• The somatopleure (somatic mesoderm + overlying ectoderm) lifts away from the splanchnopleure (splanchnic mesoderm + endoderm), creating the coelom.
  • The lateral edges move toward the ventral surface.
  • Endoderm rolls into the gut tube, establishing the primitive gut.
• The gut tube becomes the foregut, midgut, and hindgut regions (within the endoderm-derived tube), while the surrounding mesoderm supports and vascularizes this tube.
• The coelom surrounds the gut tube and forms the intraembryonic body cavities.
• Mesenteries anchor the gut tube in place and contribute to dorsal and ventral mesenteries (connections between gut tube and body wall).

Trans-segmental vs segmental structures: recap

• Segmental structures: aligned with somites (dermatome, myotome, sclerotome, vertebrae, ribs, segmented spinal nerves).
• Trans-segmental structures: extend along many segments and are not restricted to a single somite; include the gut tube, coelomic cavities, and many visceral structures.
• Understanding these concepts helps locate structures in the embryo and in the adult.

Embryonic tissues and major structures: mapping by germ layer (summary from slide)

• ECTODERM

  • Origin: Ectoderm and neural ectoderm; epiblast-derived as neurulation proceeds.
  • Major derivatives:
  • Epidermis, hair, nails, glands of skin
  • Brain and spinal cord (CNS)
  • Neural crest and derivatives: cranial, spinal, and sympathetic ganglia; adrenal medulla chromaffin cells; pigment cells of the skin; craniofacial cartilage/bone precursors, and other neural crest derivatives

• MESODERM

  • Major derivatives include:
  • Notochord
  • Somites -> dermatome, myotome, sclerotome
  • Intermediate mesoderm -> kidneys and gonads (urogenital structures)
  • Lateral plate mesoderm -> somatic (parietal) mesoderm and splanchnic (visceral) mesoderm
  • Coelom (the intraembryonic cavity) forms between somatic and splanchnic layers

• ENDODERM

  • Major derivatives:
  • Epithelial lining and glands of the digestive and respiratory tracts

• Additional notes on specific labeled items in the cross-sections:

  • Dorsal hollow nerve cord: CNS tube formed from neural ectoderm; located dorsally in the embryo.
  • Notochord: mesoderm-derived axial support; gives signals for patterning and becomes nucleus pulposus in intervertebral discs in adults.
  • Coelom: intraembryonic cavity between somatic and splanchnic mesoderm; later partitions into pericardial, pleural, and peritoneal cavities.
  • Somite derivatives: dermis (dermatome), skeletal muscle (myotome), vertebrae/ribs (sclerotome).
  • Neurulation: dorsal hollow nerve tube closure and neural crest migration; neural crest derivatives populate multiple regions and systems.

Check-your-understanding prompts (summary questions from slides)

• What is the notochord and what embryonic tissue does it come from?
  • Derived from mesoderm; forms the notochord along the midline of the embryo.
• What is the function of the notochord?
  • Provides axial support and signals to pattern surrounding tissues (especially the neural tube and somites).
• Is the notochord present in humans after birth? If so, what does it become?
  • It regresses; its remnant becomes the nucleus pulposus of intervertebral discs.
• Describe the process of neurulation. What will this process create?
  • Neurulation forms the dorsal hollow neural tube, from neural ectoderm, giving rise to the brain and spinal cord.
• Where does neurulation start and end?
  • It begins in the mid-embryo and progresses cranially and caudally along the length of the neural plate to close the tube.
• What will the ectoderm, mesoderm, and endoderm become as the embryo develops? Be specific.
  • Ectoderm: epidermis and neural tissues (CNS from neural ectoderm; neural crest derivatives); surface ectoderm forms epidermis.
  • Mesoderm: notochord, somites (dermis, muscle, vertebrae), intermediate mesoderm (kidneys, gonads), lateral plate mesoderm (somatic and splanchnic), coelom formation.
  • Endoderm: epithelial lining and glands of digestive and respiratory tracts.
• Where does neural crest come from? Why is this tissue important?
  • Arises from the border of neural and non-neural ectoderm (neural crest); gives rise to numerous structures including peripheral nerves, ganglia, pigment cells, adrenal medulla components, and craniofacial skeletal elements.
• Describe the events that occur when the mesoderm becomes more complex. What other embryonic events are occurring at the same time?
  • Somites form along the body axis; differentiation into dermatome, myotome, and sclerotome; simultaneous neurulation and neural crest migration; formation of intermediate and lateral plate mesoderm; coelom formation.
• Where are somites located in the developing embryo? What do they give rise to later in the embryo and the adult?
  • Somites are segmented blocks along the future trunk; they give rise to dermis (dermatome), trunk/limb musculature (myotome), and vertebrae/ribs (sclerotome).

Quick-reference cross-section labeling (core structures listed for study)

• Dorsal hollow nerve cord
• Notochord
• Coelom
• Somatic mesoderm
• Splanchnic mesoderm
• Somatopleure (somatic mesoderm + ectoderm)
• Splanchnopleure (splanchnic mesoderm + endoderm)
• Dorsal mesentery
• Ventral mesentery
• Gut tube (endoderm-derived tube)
• Dorsal aorta
• Intermediate mesoderm
• Somite
• Myotome
• Dermatome
• Sclerotome

Labeling and diagrams (study tips)

• Page 21: Structures to label on cross-sections include the dorsal hollow nerve cord, notochord, coelom, somatic and splanchnic mesoderm, somatopleure/splanchnopleure, dorsal and ventral mesenteries, gut tube, dorsal aorta, intermediate mesoderm, somite, myotome, dermatome, and sclerotome.
• Page 22-23: Practice: Label the cylindrical embryo and the adult cross-section; become familiar with anatomy positions relative to the dorsal-ventral axis and left-right symmetry.

Notes on terminology and study strategy

• Remember: segmental refers to structures aligned with somite boundaries; trans-segmental refers to structures that extend across multiple segments or are not confined to a single somite.
• The coelom’s formation is central to body cavity development; understanding its partitioning helps locate pericardial, pleural, and peritoneal cavities in the adult.
• Notochord and neural tube are foundational axial structures; many later developments (notochord signaling, somite patterning) are directed by these early events.
• Practical relevance: knowledge of dermatomes and myotomes aids localization of nerve injuries and planning of regional anesthesia or surgical approaches.

Quick check: key terms to memorize

• Primitive streak, epiblast, hypoblast
• Endoderm, Mesoderm, Ectoderm
• Notochord, nucleus pulposus
• Neural tube, neural crest, epidermal ectoderm
• Somites: dermatome, myotome, sclerotome
• Intermediate mesoderm, kidneys, gonads
• Lateral plate mesoderm: somatic vs splanchnic
• Coelom, ventral/dorsal mesenteries
• Gut tube (foregut/midgut/hindgut)
• Segmental vs trans-segmental structures

End of notes from slides