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Review
. 2022 Nov 22;14(22):9338-9383.
doi: 10.18632/aging.204361. Epub 2022 Nov 22.

Organotypic cultures as aging associated disease models

Affiliations
Review

Organotypic cultures as aging associated disease models

Martina M Sanchez et al. Aging (Albany NY). .

Abstract

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

Keywords: intestine; organotypic; skeletal muscle; skin; tissue engineering.

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Conflict of interest statement

CONFLICTS OF INTEREST: The authors declare no conflicts of interest related to this study.

Figures

Figure 1
Figure 1
Organotypic models of skin aging. (A) Simplified skin anatomy and aging phenotypes. Skin can be separated into epidermal, dermal, and hypodermal layers. The epidermis is composed of Stratum Basale, Spinosum, Granulosum, and Corneum, composed of increasingly differentiated epidermal cells. The dermal-epidermal junction (DEJ) connects the basement membrane of the Stratum Basale to the upper (papillary) dermis, and is characterized by small dermal extensions (or papilla) into the epidermis. The DEJ flattens with age. The dermis is a collagen rich tissue supported by dermal fibroblasts. The subdermis (or hypodermis) is an important adipose compartment that contributes to overall metabolic function; this tends to thin with age. Both the dermis and subdermis are highly vascularized, important for thermal regulation; in age vascularization is reduced. The above schematic is simplified to focus on the level of current organotypic models, nerves, melanocytes, immune cells, and other components of in vivo skin are not pictured. (B) Organotypic skin models, also referred to as Human Skin Equivalents (HSE), typically consist of a dermal/subdermal culture grown on a permeable culture support (left), followed by seeding and differentiation of epidermis at the air-liquid interface (ALI). Benefits of this style is the accessibility of the culture format, ready customization of the specific cell populations (both immortalized or primary, patient specific, or transgenic disease models), and customization of the matrix and media formulations.
Figure 2
Figure 2
Organotypic models of gut aging. (A) Simplified gut anatomy and aging, focusing on the most commonly modeled components. A mixed epithelial population, described in the text, forms a simple cuboidal epithelial layer with both secretory and absorptive epithelium. A layer of mucus inside the gut lumen supports the host/microbiome interaction. The stroma underneath the epithelium, the submucosa, is host to nerves (not shown) blood vessels, fibroblasts, and immune cells important for gut function. Smooth muscle is required for gut peristalsis. In aging, the macrostructure of villi degrades, with villi becoming shorter and broader. Immune cell populations are disrupted, and reduced epithelial barrier integrity can lead to increased microbial infiltration into the submucosa and vasculature. (B) Organotypic models of the gut typically only model a small subset of these features, and are typically adapted to aspects that are relevant to specific questions. For example, epithelial and immune populations may be co-cultured to study intercellular interactions in a simple format. To study the influence of villous structures, soft lithography can be used to recreate the villi/crypt geometry. Microbiome co-cultures can be included, and microfluidic organ-on-a-chip models have been used to mimic the oxygen gradient from the vascularized submucosa to the anaerobic lumen.
Figure 3
Figure 3
Organotypic models of skeletal muscle aging. (A) Simplified muscle anatomy and aging, focusing on the most commonly modeled components. The primary unit of muscle is the myofiber, a multinucleated cell responsible for contraction. Specialized matrix (endomysium, perimysium, and epimysium) support and organize the tissue. Satellite cells are an important stem cell population for the muscle, and the muscle is supported by a host of other cell types including nerves, fibroblasts, adipose, and vascular cells. In aged muscle, cross-sectional area (CSA) is reduced, in part due to myofiber atrophy, and decreasing capillary and satellite cell density. Conversely, there is increased infiltration of adipose and thickening of the connective tissues. At the molecular level, there is decreased expression of GLUT4, an important glucose transporter, and insulin resistance (IR) frequently develops. (B) Organotypic models of muscle have several unique challenges but have distinct advantages over other traditional models. Muscle cultures are contractile, and require anchoring to prevent collapse. Typical approaches include posts (although other methods are used) to provide points of resistance for the muscle to pull against. In order to study active contraction, researchers have used various stimulation methods, including electrical and optogenetic methods. Due to the high metabolic demand, the cultures are typically quite small, to allow nutrients and waste to diffuse more readily. As with other organotypic models, the matrix, cell population, and media can be customized for the research question.

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