Understanding Human Lung Development through In Vitro Model Systems

An abundance of information about lung development in animal models exists; however, comparatively little is known about lung development in humans. Recent advances using primary human lung tissue combined with the use of human in vitro model systems, such as human pluripotent stem cell-derived tissue, have led to a growing understanding of the mechanisms governing human lung development. They have illuminated key differences between animal models and humans, underscoring the need for continued advancements in modeling human lung development and utilizing human tissue. This review discusses the use of human tissue and the use of human in vitro model systems that have been leveraged to better understand key regulators of human lung development and that have identified uniquely human features of development. This review also examines the implementation and challenges of human model systems and discusses how they can be applied to address knowledge gaps.

In Vitro and In Vivo Development of the Human Airway at Single-Cell Resolution

Bud tip progenitor cells give rise to all murine lung epithelial lineages and have been described in the developing human lung; however, the mechanisms controlling human bud tip differentiation into specific lineages are unclear. Here, we used homogeneous human bud tip organoid cultures and identified SMAD signaling as a key regulator of the bud tip-to-airway transition. SMAD induction led to the differentiation of airway-like organoids possessing functional basal cells capable of clonal expansion and multilineage differentiation. To benchmark in vitro-derived organoids, we developed a single-cell mRNA sequencing atlas of the human lung from 11.5 to 21 weeks of development, which revealed high degrees of similarity between the in vitro-derived and in vivo airway. Together, this work sheds light on human airway differentiation in vitro and provides a single-cell atlas of the developing human lung.

A Purification Technique for Adipose-Derived Stromal Cell Cultures Leads to a More Regenerative Cell Population

It has been demonstrated that adipose-derived stromal cells (ASCs) are a regenerative cell population with potential uses for bone and cartilage regeneration. However, the biomarker expression and heterogeneity of the population has not been thoroughly characterized. By analyzing biomarker expression, we aimed to understand the composition of ASC populations extracted using a common extraction technique in comparison to ASC populations given an additional purification step. Human adipose tissue samples were collected, and ASCs were extracted from these samples using a common, published extraction technique (primary extraction). These cells were cultured and half were given an additional purification. The primarily-extracted and purified cell populations were analyzed for biomarkers that correspond to specific cell types. The addition of the purification technique reduced the number of cells expressing hematopoietic and endothelial biomarkers and did not cause the yield of mesenchymal stem cell biomarker-expressing cells to decrease. Biomarkers corresponding to erythrocytes and lymphocytes were lost during the primary extraction, and biomarkers corresponding to most granulocytes and progenitor cells were lost during the additional purification. Biomarkers identifying dendritic cells, monocytes/macrophages, neutrophils, vascular endothelial cells, smooth muscle cells, and pericytes were upregulated in purified cell populations while those identifying fibroblasts and adipocytes were downregulated. Pluripotency biomarkers were more highly expressed in purified cell populations. These results demonstrate that the most commonly utilized adipose tissue recovery and ASC extraction technique leads to a heterogeneous cell population in which further purification of this population, as described in this manuscript, isolates a cell subset that has more regenerative potential.

Increased numbers of microchimeric cells of fetal origin are associated with dermal fibrosis in mice following injection of vinyl chloride

OBJECTIVE

To develop a murine model for use in examining the role of microchimeric cells and certain chemical exposures in the pathogenesis of systemic sclerosis (SSc).

METHODS

Female BALB/cJ retired breeder mice were bled before and after vinyl chloride injection. The DNA from their white blood cells was obtained, and the number of microchimeric cell equivalents was determined by quantitative polymerase chain reaction using DNA primers specific for the H-2Kb gene, a sequence not found in BALB/cJ mice. Skin was obtained at autopsy, embedded in paraffin, sectioned, and stained with Masson's trichrome. Hydroxyproline analyses were performed on 4-mm skin biopsy samples.

RESULTS

Microchimeric cells were identified and quantitated before and after 20 daily intraperitoneal injections of vinyl chloride. The number of microchimeric cells in the peripheral blood increased an average of 48-fold after treatment with vinyl chloride. Histologic examination of the skin of these same mice (which had an increased number of microchimeric cells) showed inflammation, with abundant fibroblasts and a heavy mononuclear infiltration in the dermis. The collagen fibers appeared densely packed and disorganized. Histologic examination of the skin of untreated retired breeder mice and treated virgin mice appeared normal. Quantitative assays to determine the collagen content of skin biopsy samples obtained from treated microchimeric mice compared with nontreated microchimeric or with treated nonmicrochimeric mice showed a 2-3-fold increase in collagen content in the treated microchimeric mice. Extraordinary splenomegaly was present in the vinyl chloride-treated microchimeric mice, accompanied by cellular infiltration and fibrosis.

CONCLUSION

The results suggest that vinyl chloride injections into BALB/cJ retired breeder mice lead to activation of microchimeric cells, which causes the cells to divide and multiply. The correlation between the 48-fold increase in microchimeric cells and the appearance of dermal inflammation and fibrosis similar to that of graft-versus-host disease suggests that activated microchimeric cells may be a necessary factor in the pathogenesis of autoimmune diseases such as SSc.