Lec 09 Dev Bio

The Ectoderm and Its Derivatives

Overview of Ectoderm Differentiation

The ectoderm is one of the three primary germ layers in the early embryo. Its differentiation is initiated by neurulation, a process that leads to the formation of the neural plate and subsequently the neural tube. The neural crest forms at the boundary between the neural plate and the prospective epidermis.

• Neurulation: The process by which the neural plate forms and gives rise to the neural tube.

• Neural Crest: Specialized cells at the junction of the neural plate and epidermis that migrate and differentiate into various cell types.

Major Ectodermal Derivatives

• Surface ectoderm: Epidermis, hair, nails, glands.

• Neural tube: Central nervous system (brain and spinal cord).

• Neural crest: Peripheral nervous system, pigment cells, facial cartilage, and more.

Neurulation

Primary Neurulation

Primary neurulation involves the transformation of the neural plate into the neural tube through a series of morphogenetic movements.

• Formation of Neural Plate: Ectodermal cells thicken to form the neural plate.

• Folding and Elevation: The neural plate folds, elevating the neural crest at its edges.

• Convergence and Closure: Neural folds move toward the dorsal midline, adhere, and merge to form the closed neural tube.

Neural Tube Closure

Neural tube closure is a critical step in central nervous system development. In birds, closure starts at the future midbrain and proceeds bidirectionally. In humans, there are three distinct sites of closure.

• Anterior and Posterior Neuropores: These are the last regions to close during neurulation.

Failures of Neural Tube Closure

Defects in neural tube closure can result in severe congenital conditions.

• Anencephalus: Failure to close the anterior neuropore, leading to degeneration of the forebrain.

• Spina bifida: Failure to close the posterior neuropore, with varying severities.

Spina Bifida: Types and Severities

Type	Description
Spina bifida occulta	Cleft in vertebral arch; spinal cord and meninges unaffected
Spina bifida aperta	Spinal cord and/or meninges affected; includes meningocele, myelomeningocele, myeloschisis

• Meningocele: Meninges form a cyst outside the vertebral canal; spinal cord and nerves not affected.

• Myelomeningocele: Spinal cord and/or nerves are inside the cyst.

• Myeloschisis: Spinal cord or nerves are not covered.

Secondary Neurulation

Secondary neurulation forms the posterior neural tube through a different mechanism.

• Mesenchymal cells condense to form a solid chord.

• Cavitation: Cavities arise within the chord and unite to form a hollow tube.

• Anterior neural tube: Formed by primary neurulation.

• Posterior neural tube: Formed by secondary neurulation.

Cell Adhesion in Neural Tube Closure

Role of Cadherins

Cell adhesion molecules regulate the separation of neural and epidermal tissues during neurulation.

• E-cadherin: Expressed by all ectodermal cells initially.

• N-cadherin: Neural tube cells switch to N-cadherin, allowing separation from epidermal cells.

• Experimental evidence: Forced expression of N-cadherin in epidermal cells impedes separation.

Differentiation of the Neural Tube Wall

Formation of Nervous System Regions

The neural tube wall differentiates into distinct regions that give rise to the spinal cord, cerebellum, and cerebral cortex.

• Spinal cord: Central canal, intermediate zone, marginal zone.

• Cerebellum: Ventricular, intermediate, Purkinje, and marginal layers.

• Cerebral cortex: Ventricular, intermediate, marginal layers.

Dorsal-Ventral Polarity of the Neural Tube

Signaling Pathways

Dorsal-ventral patterning of the neural tube is established by gradients of signaling molecules.

• Ventral signal: Sonic hedgehog (Shh) from the notochord induces the floor plate, which also secretes Shh.

• Dorsal signal: TGF-β proteins (BMP4, BMP7) from the dorsal epidermis induce the roof plate, which secretes BMP4.

• Gradients: Shh and TGF-β gradients specify neuronal identities along the dorsal-ventral axis.

Experimental Evidence

• Implantation of a second notochord induces a second floor plate and set of motoneurons.

Development of the Brain

Brain Vesicle Formation

The brain develops from the anterior neural tube, forming primary and secondary vesicles.

• 3 primary vesicles: Forebrain (prosencephalon), midbrain (mesencephalon), hindbrain (rhombencephalon).

• 5 secondary vesicles: Telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon.

Adult Human Brain Regions

• Telencephalon: Cerebral hemispheres

• Diencephalon: Thalamus, hypothalamus

• Mesencephalon: Midbrain

• Metencephalon: Pons, cerebellum

• Myelencephalon: Medulla oblongata

Mechanisms of Brain Enlargement

• Growth is primarily due to expansion of brain cavities (ventricles), not tissue proliferation.

• Na+/K+-ATPase activity establishes an osmotic gradient, causing water influx into ventricles.

• Deficiency in Na+/K+-ATPase prevents ventricle formation.

Development of the Vertebrate Eye

Optic Vesicle and Lens Formation

The vertebrate eye develops as an outgrowth of the diencephalon, interacting with the surface ectoderm.

• Optic vesicle: Protrusion from diencephalon.

• Lens placode: Thickening of epidermis upon contact with optic vesicle.

• Invagination: Lens placode forms the lens; optic vesicle forms the optic cup.

• Optic cup layers: Outer layer becomes pigmented epithelium; inner layer becomes neural retina.

Eye Field Formation and Separation

• Noggin: Promotes Otx2 expression, inhibits ET.

• Pax6: Key transcription factor for eye field; its expression is regulated to form two optic vesicles.

• Sonic hedgehog: Secreted by prechordal plate, inhibits Pax6 in midline, leading to separation of eye field.

Disorders of Eye Field Separation

• Cyclopia: Failure to separate eye field due to lack of Sonic hedgehog signaling.

• Excess Sonic hedgehog: Can also disrupt eye development, as seen in blind cave fish (Astyanax mexicanus).

Development of the Skin and Its Appendages

Skin Structure

• Epidermis: Derived from ectoderm.

• Dermis and subcutis: Derived from mesoderm.

Epidermal Development

• BMPs: Stimulate p63 transcription factor, promoting keratinocyte proliferation and differentiation.

• Melanocytes: Derived from neural crest, migrate into epidermis.

Hair Follicle Development

• Epidermal cells: Form hair follicles but require mesodermal (dermal fibroblast) signals.

• Wnt10 and Dickkopf (Dkk): Secreted by dermal cells, regulate patterning of hair follicle placodes.

• High Wnt induces follicle formation; Dkk inhibits.

The Neural Crest

Formation and Migration

The neural crest forms during neurulation at the contact site of neural plate and epidermis. It undergoes epithelial-mesenchymal transition (EMT) and migrates to various destinations.

• Induction: Controlled by Wnt and BMP signaling.

• Migration: Neural crest cells migrate and differentiate into multiple cell types.

Neural Crest Derivatives

Derivative	Cell Type or Structure Derived
Peripheral nervous system (PNS)	Neurons, glial cells, Schwann cells
Endocrine and pigment cells	Adrenal medulla, melanocytes
Facial cartilage and bone	Facial skeleton, connective tissue
Connective tissue	Teeth, dermis, smooth muscle

Cell Lineages

• Neural crest cells are pluripotent stem cells that differentiate into neurons, glia, melanocytes, cartilage, and bone.

Regional Organization

• Cranial neural crest: Forms craniofacial mesenchyme, cartilage, bone, neural and glial cells.

• Cardiac neural crest: Forms melanocytes, neurons, cartilage, connective tissue of large arteries.

• Trunk neural crest: Forms melanocytes, dorsal root ganglia, sympathetic ganglia, adrenal medulla.

• Vagal and sacral neural crest: Forms parasympathetic ganglia of the gut.

Migration Pathways

• Dorsolateral pathway: Neural crest cells migrate between ectoderm and somites to form melanocytes.

• Ventral pathway: Cells migrate through somites to form sensory and sympathetic ganglia.

Key Questions in Migration

• Signals initiating migration

• Competence of migrating cells

• Directional cues and destination recognition

The Mesoderm and Somite Formation

Mesodermal Subdivisions

• Chordamesoderm: Forms notochord, induces neural tube.

• Paraxial (somitic) mesoderm: Forms somites, muscle, connective tissue of the back.

• Intermediate mesoderm: Forms urogenital system, adrenal cortex.

• Lateral plate mesoderm: Forms heart, blood vessels, body cavity linings, limb components.

Somite Formation and Differentiation

• Somites form periodically at the anterior end of presomitic mesoderm.

• Number of somites is species-specific (human: 33, chick: 50, mouse: 65, snake: 500).

• BMP antagonists (e.g., Noggin) induce paraxial mesoderm specification.

Somite Differentiation

• Ventral-medial cells become mesenchymal, forming the sclerotome (vertebrae, tendons, joints).

• Remaining epithelial cells form the dermamyotome, which differentiates into myotome (muscle) and dermatome (dermis).

Inductive Events and Vertebrae Formation

• Surrounding tissues (neural tube, notochord, epidermis, intermediate mesoderm) provide paracrine signals for somite specification.

• Sclerotome cells migrate around the notochord to form vertebrae; notochord is displaced by developing vertebral body.

Structure of Intervertebral Discs

• Each vertebra consists of material from two somites.

• Intervertebral discs are formed from notochordal remnants and sclerotome-derived tissue.

Additional info: These notes expand on the original lecture slides by providing definitions, context, and structured explanations suitable for cell biology students preparing for exams.