**1. Introduction**

Neuromuscular diseases (NMDs) refer to a set of conditions that affect motor units, which are functional units comprising individual spinal motor neurons, their axons, the axon's terminal nerve branches, neuromuscular junctions (NMJ), and the skeletal muscle fibers connected to these junctions (**Figure 1**). One of the most the well-known NMDs is amyotrophic lateral sclerosis (ALS) (**Figure 2**). ALS is a neurodegenerative disorder characterized by the degeneration of upper and lower motor neurons, leading to a progressive loss of motor function and ultimately resulting in death, often due to respiratory failure [1, 2]. Unfortunately, the disease is generally fatal within 3 to 5 years after diagnosis [3]. It typically appears in mid-adulthood, with the average age of onset being 55 years, although it can begin as early as the first or second decade of life or even develop later in life [4, 5]. ALS has an annual diagnosis rate of 1–2 individuals per 100,000 in most countries [1, 2]. In the United States and the United Kingdom, ALS is responsible for more than 1 in 500 deaths in adults, indicating that over 15 million people currently alive may eventually succumb to this disease [1]. Another significant motor neuron disease that is typically classified as an NMD is spinal muscular atrophy (SMA). The global occurrence of SMA is approximately 1 in 40–60 [6, 7]. SMA involves the disruption of the motor unit, leading to the degeneration of proximal motor axons, loss of synaptic inputs to cell bodies, and, ultimately, the death of motor neuron cell bodies [6].

ALS and SMA share a common symptom, and current approaches to managing these NMDs is centered around addressing symptoms, such as preserving weakened muscle function, rather than tackling the root cause of the disease. The paucity of effective treatments for NMDs such as ALS and SMA has led to a recent effort to develop more predictive preclinical models with which to model these conditions and evaluate novel therapeutic efficacy.

Historically, the *in vitro* study of motor neurons has relied on sourcing cells from spinal cord tissue derived from embryonic chicks and rodents [8–14]. Such neurons are more similar to human cells than to worm-like animals and arthropods, but they still have different developmental patterns and morphologies compared to humans, particularly in the distribution of motor nerve terminals and their size and conformation [15]. Animal models are the current "gold standard" preclinical method for

#### **Figure 1.**

*Motor unit. Motor units are composed of lower motor neurons, neuromuscular junctions (NMJs), and skeletal muscle fiber.*

*Exploring the Potential for Biomaterials to Improve the Development of Spinal Motor Neurons… DOI: http://dx.doi.org/10.5772/intechopen.113275*

#### **Figure 2.**

*Neuromuscular disease (NMD). NMDs encompass cellular disorders of the motor unit. Each unique disorder affects different aspect of the motor unit.*

evaluating the efficacy of novel ALS therapeutics. However, the last 20 years have seen rigorous animal tests on multiple ALS-targeted drugs that prolonged life in the animal but failed to elicit a therapeutic benefit in humans [16]. This has fueled a recent interest in developing alternative, human-based preclinical assays to better inform patient responses to compound exposure. The establishment of such assays is dependent on the establishment of robust models of human motor neurons.

To address this need, researchers have turned to human-induced pluripotent stem cell (iPSC) technology to derive motor neurons that more accurately reflect the cells present in patients. The advantage of human iPSC-derived motor neurons is that they have unlimited expansion potential and can generate large homogeneous populations of neurons for downstream studies. This makes them suitable for studies requiring a large number of neurons for repetitive tasks, such as drug screening, proteomics, and biochemistry [17]. Additionally, human iPSC-derived motor neurons retain patientspecific gene mutations, making it possible to study individual patient genotype–phenotype relationships *in vitro* [18].

Despite those efforts, a significant challenge in using human iPSC-derived motor neurons is their relative immaturity compared to primary motor neurons. This is because the culture process typically used to differentiate human iPSCs into motor neurons does not encapsulate the complexity of the developing spinal cord *in vivo* and is conducted on a timescale (days to weeks) that is far more rapid than native embryogenesis and subsequent postnatal development (months to years). As such, some concerns remain that human iPSC-derived motor neurons may not exhibit all of the same physiological properties as primary motor neurons.

As a result, it is essential to improve the maturity of human iPSC-derived motor neurons to increase their suitability for use in next-generation disease modeling and/ or drug screening applications. This maturation process is crucial for ensuring that

*Motor Neurons – New Insights*

the findings obtained using human iPSC-derived motor neurons are reliable and accurately represent the relevant biological processes as they occur *in vivo*. Achieving more mature cultured human motor neurons is particularly important to the endeavors to model NMDs such as ALS that exhibit symptomatic onset at stages of life that are far later than embryogenesis and early postnatal development.
