Regen Med Week 1

What are the primary methods used to manufacture scaffolds for nerve regeneration?
Electrospinning, freeze-drying (lyophilization), 3D bioprinting, solvent casting & particulate leaching, and gas foaming are the main techniques used to create nerve scaffolds. Each method varies in terms of porosity, mechanical properties, and ability to integrate bioactive molecules.

How does electrospinning create scaffolds for nerve regeneration?
Electrospinning uses a high-voltage electric field to stretch a polymer solution into nanofibers, which are then collected onto a rotating drum. This process produces a nanofiber mesh that mimics the extracellular matrix, providing a high surface area for cell attachment and directional axonal growth.

What materials are commonly used for electrospun scaffolds?
Common materials for electrospinning include natural biomaterials such as collagen, fibrin, and silk fibroin, as well as synthetic polymers like polycaprolactone (PCL), polylactic acid (PLA), and poly(lactic-co-glycolic acid) (PLGA). These materials support Schwann cell migration and axonal regeneration.

Why is electrospinning advantageous for nerve regeneration?
Electrospinning produces scaffolds with nanofiber structures that closely resemble the extracellular matrix, providing a high surface area for cell attachment, excellent porosity for nutrient diffusion, and the ability to incorporate bioactive molecules like neurotrophic factors.

How does freeze-drying (lyophilization) create scaffolds?
Freeze-drying involves freezing a polymer solution and then sublimating the water content, leaving behind a highly porous structure. This method ensures interconnected pores for cell migration and can retain biological macromolecules such as collagen and growth factors.

Where are freeze-dried scaffolds used in nerve regeneration?
Freeze-dried scaffolds are commonly used for peripheral nerve regeneration. They provide a biocompatible structure for Schwann cell attachment and axonal growth, often incorporating fibrin-based hydrogels or collagen to support tissue repair.

How does 3D bioprinting create nerve scaffolds?
3D bioprinting uses a computer-controlled nozzle to deposit layers of biomaterials (bioinks), which can include live cells and growth factors. This method enables precise control over scaffold architecture, creating patient-specific implants for nerve repair.

What are the benefits of 3D bioprinting for nerve regeneration?
3D bioprinting allows for precise scaffold design, customization for patient-specific injuries, and direct incorporation of cells and growth factors into the printed structure. This technique is particularly useful for developing complex nerve conduits for spinal cord repair.

How does solvent casting & particulate leaching produce scaffolds?
In this method, a polymer is dissolved and mixed with a porogen (e.g., salt or sugar), followed by solvent evaporation. The porogen is then dissolved, leaving behind a highly porous scaffold structure suitable for cellular infiltration and axon regeneration.

Why is solvent casting & particulate leaching used for nerve scaffolds?
This method is cost-effective, easy to implement, and produces highly porous scaffolds that allow for Schwann cell migration and axonal growth. It is widely used in nerve conduits designed for peripheral nerve repair.

How does gas foaming create nerve scaffolds?
Gas foaming exposes a polymer to high-pressure CO₂, which is then rapidly depressurized, creating pores throughout the material. This technique eliminates the need for toxic solvents and produces biodegradable scaffolds with controlled porosity.

Where are gas-foamed scaffolds used in nerve regeneration?
Gas-foamed scaffolds are used in tissue-engineered nerve grafts to provide a biodegradable framework for Schwann cell support and axonal extension, particularly in synthetic nerve conduits.

What is the Peripheral Nervous System (PNS)?
The Peripheral Nervous System (PNS) consists of all the nerves and ganglia outside the Central Nervous System (CNS). It connects the CNS to the rest of the body, allowing for sensory input, motor output, and autonomic control.

What are the main functions of the Peripheral Nervous System?
The PNS transmits sensory information from the body to the CNS, carries motor commands from the CNS to muscles and glands, and regulates autonomic functions such as heart rate and digestion.

What are the main structural components of the Peripheral Nervous System?
The PNS is composed of peripheral nerves, ganglia, and supportive cells such as Schwann cells and fibroblasts.

How many cranial nerves are there, and where do they originate?
There are 12 pairs of cranial nerves, and they originate from the brainstem. They control sensory and motor functions in the head and neck, except for the vagus nerve, which extends to the thorax and abdomen.

What are spinal nerves, and how many pairs exist?
Spinal nerves are 31 pairs of nerves that originate from the spinal cord. They contain both sensory fibers (dorsal root) and motor fibers (ventral root).

What is the difference between sensory and motor nerves?
Sensory nerves carry information from sensory receptors to the CNS, while motor nerves transmit signals from the CNS to muscles and glands.

What is the functional difference between the dorsal and ventral roots of spinal nerves?
The dorsal root carries sensory information into the CNS, while the ventral root carries motor commands from the CNS to the body.

What are ganglia, and what are their types?
Ganglia are clusters of neuron cell bodies in the PNS. There are two main types: sensory ganglia (e.g., dorsal root ganglia) and autonomic ganglia (sympathetic and parasympathetic ganglia).

What role do Schwann cells play in the PNS?
Schwann cells produce myelin, which insulates axons and speeds up electrical conduction. They also promote axon regeneration after injury.

How does myelination differ in the PNS compared to the CNS?
In the PNS, Schwann cells wrap around individual axons, forming a single myelin sheath per axon, whereas in the CNS, oligodendrocytes myelinate multiple axons.

What are the three connective tissue layers in peripheral nerves?

Epineurium – Outer layer surrounding the entire nerve.
Perineurium – Encloses groups of nerve fibers into fascicles.
Endoneurium – Surrounds individual axons.

What is the difference between the Somatic and Autonomic Nervous System?
The Somatic Nervous System (SNS) controls voluntary movements of skeletal muscles, while the Autonomic Nervous System (ANS) regulates involuntary functions like heart rate and digestion.

What are the key differences between the sympathetic and parasympathetic divisions of the ANS?

Sympathetic ("Fight or Flight"): Increases heart rate, blood pressure, and pupil dilation. Uses norepinephrine as its neurotransmitter.
Parasympathetic ("Rest and Digest"): Slows heart rate, stimulates digestion, and constricts pupils. Uses acetylcholine as its neurotransmitter.

What is the Enteric Nervous System (ENS), and why is it called "The Second Brain"?
The ENS is a network of neurons that control gastrointestinal (GI) function. It operates independently of the CNS, regulating peristalsis and enzyme secretion, earning it the nickname "The Second Brain."

How do peripheral nerves regenerate after injury?
Peripheral nerves regenerate through Wallerian degeneration, where the distal axon breaks down, and Schwann cells clear debris while secreting growth factors that guide regrowth from the proximal stump.

At what rate do peripheral nerves regenerate?
Axonal regrowth in the PNS occurs at approximately 2-5 mm per day, though it is more effective in younger individuals.

Why can the PNS regenerate while the CNS cannot?
The PNS can regenerate because Schwann cells support axon regrowth. The CNS has inhibitory factors such as glial scars and myelin-associated inhibitors, which block axonal regeneration.

What happens to a peripheral nerve after an injury?
When a peripheral nerve is injured, the proximal segment swells, while the distal segment undergoes Wallerian degeneration due to protease activity. Schwann cells and macrophages help clear debris and create a growth-permissive environment for axonal regrowth.

What is Wallerian degeneration, and why is it important for nerve regeneration?
Wallerian degeneration is the process where the distal part of a severed axon degenerates while Schwann cells clear myelin debris and secrete neurotrophic factors to promote axonal regeneration.

How does the location of a nerve injury affect regeneration?
Nerve injuries closer to the spinal cord are more severe because they are more likely to cause neuronal death. Injuries in the distal periphery have a higher chance of successful regeneration.

How does axonal regeneration occur after peripheral nerve injury?
After injury, Schwann cells in the distal stump proliferate and release growth factors. Once debris is cleared, axons extend from the proximal end toward the distal stump at a rate of 2-5 mm per day.

What factors influence successful peripheral nerve regeneration?

Age: Younger individuals regenerate nerves more efficiently.
Injury size: Small gaps heal naturally; large gaps require intervention.
Schwann cell activity: Schwann cells guide axonal regrowth and remyelinate new axons.
Alignment of nerve fascicles: Correct matching of sensory and motor fibers is crucial for functional recovery.
Peripheral Nerve Repair Strategies (With and Without Scaffolds)

How are small peripheral nerve gaps repaired?
Small nerve gaps (<5 mm) can be treated by direct suturing of the nerve ends, aligning axons to allow regrowth without additional intervention.

How are large peripheral nerve gaps repaired?
For gaps >5 mm, a nerve graft or synthetic scaffold is implanted to bridge the defect and guide regenerating axons toward the distal segment.

What are nerve autografts, and when are they used?
Nerve autografts involve taking a nerve from a less essential part of the body (e.g., the sural nerve) and implanting it at the injury site. They are used when direct repair is not possible.

What are the limitations of using autografts for nerve repair?

Donor site morbidity: Removing a nerve from another body part can cause functional loss.
Mismatch of fascicles: Sensory and motor axons may not align properly.
Limited availability: Suitable donor nerves are scarce for large defects.

What are nerve allografts, and how do they work?
Nerve allografts are cadaveric nerve grafts that have been decellularized to remove immunogenic material while retaining the extracellular matrix. They provide a scaffold for axonal regeneration.

What are nerve conduits, and how do they function in PNI repair?
Nerve conduits are synthetic or natural tubes that enclose the damaged nerve, creating a protected environment where regenerating axons can grow toward the distal stump.

What are the major types of nerve conduits available?

Collagen-based conduits (e.g., NeuraGen™) – Biodegradable and supports Schwann cells.
Synthetic conduits (e.g., PLGA, PCL) – Tunable degradation rate, customizable structure.
Hydrogel-based conduits – Injectable, flexible, and bioactive.

Why are nerve conduits beneficial for nerve repair?
✔ Prevents axonal misrouting.
✔ Provides a controlled environment for axonal growth.
✔ Can be infused with growth factors (e.g., NGF, BDNF).
✔ Biodegradable options eliminate the need for removal after healing.

What are the challenges associated with nerve conduits?
❌ Limited to small-to-medium nerve gaps (~3-10 mm).
❌ Lacks native ECM proteins compared to natural grafts.
❌ May require bioactive modifications (e.g., Schwann cells, neurotrophic factors).

How are Schwann cells used in nerve regeneration?
Schwann cells are harvested, expanded in vitro, and then transplanted into nerve conduits or grafts to enhance regeneration by secreting growth factors and guiding axonal growth.

What are the difficulties in using Schwann cells for nerve repair?

Slow proliferation in vitro, requiring large-scale expansion.
Potential contamination by fibroblasts.
Risk of losing regenerative properties during culture.

How do hydrogels aid in nerve regeneration?
Hydrogels provide a soft, ECM-like scaffold for regenerating nerves and can be injected directly into injury sites. They also allow controlled release of neurotrophic factors.

What is the Critical Gap Length (Lc), and why is it important?
The Critical Gap Length (Lc) is the maximum nerve gap that can successfully regenerate without intervention. Beyond this length, axonal regeneration fails, requiring grafts or nerve guides.

How do matrices improve nerve regeneration?
Matrices mimic the natural ECM, providing biochemical and mechanical support for regenerating axons. They must have high water content and proper mechanical properties to avoid obstructing cell migration.

What are oriented matrices, and why are they beneficial?
Oriented matrices guide axonal growth in a specific direction using aligned fibers (e.g., collagen, fibrin). They improve regeneration more effectively than isotropic scaffolds.

How do Schwann cell grafts improve nerve repair?
Schwann cell grafts enhance axonal regeneration by secreting neurotrophic factors, improving myelination, and supporting neuronal survival.

How can gene therapy improve nerve regeneration?
Gene therapy involves modifying Schwann cells or neurons to overexpress growth factors (e.g., BDNF, NT-3), improving axonal regrowth and neuron survival.

What are neurotrophic factors, and how do they aid nerve regeneration?
Neurotrophic factors (e.g., NGF, BDNF, NT-3, GDNF) are proteins that promote neuron survival, axon extension, and synaptic function. They are often incorporated into hydrogels or scaffolds to enhance regeneration.

How do neurotrophic factor gradients improve nerve repair?
Neurotrophic factors released in a gradient (higher concentration near the injury, lower toward the target) guide axons more effectively than uniform distributions.

Why is peripheral nerve repair more successful than CNS repair?
Peripheral nerves regenerate more effectively because Schwann cells promote axonal regrowth, whereas CNS repair is hindered by glial scarring, myelin inhibitors, and limited neurogenesis.