The promise of human embryonic stem cells in aging-associated diseases

Abstract

Aging-associated diseases are often caused by progressive loss or dysfunction of cells that ultimately affect the overall function of tissues and organs. Successful treatment of these diseases could benefit from cell-based therapy that would regenerate lost cells or otherwise restore tissue function. Human embryonic stem cells (hESCs) promise to be an important therapeutic candidate in treating aging-associated diseases due to their unique capacity for self-renewal and pluripotency. To date, there are numerous hESC lines that have been developed and characterized. We will discuss how hESC lines are derived, their molecular and cellular properties, and how their ability to differentiate into all three embryonic germ layers is determined. We will also outline the methods currently employed to direct their differentiation into populations of tissue-specific, functional cells. Finally, we will highlight the general challenges that must be overcome and the strategies being developed to generate highly-purified hESC-derived cell populations that can safely be used for clinical applications.

Figure 1.. Generation of pluripotent human embryonic stem cell lines.

Generation of human embryonic stem cell (hESC) lines involves several steps. Donor embryos are first obtained after in vitro fertilization or by egg activation (parthenogenetic embryos), and allowed to develop in vitro. Pluripotent cells are then isolated either from the inner cell mass of pre-implantation blastocysts or from 4, 8, or 16 -cell stage morulae. Finally, isolated cells are plated in defined hESC medium with or without feeder cell layers to propagate and select for pluripotent cell populations. These processes have resulted in hESC lines able to generate tissues from all three embryonic germ layers and the germline.

Figure 2.. Typical undifferentiated and differentiating hESCs in culture.

(A) A compact colony of proliferating pluripotent hESCs can be seen when cultured in defined medium on mouse embryonic fibroblasts. (B) Floating hEBs observed at 3 days after induction of differentiation. (C) Differentiating tissues, including cardiomyocytes, appear within adherent cultures at 48 hours after plating hEBs onto a gelatin-coated culture dish. Bar, 25 μm.

Figure 3.. Teratoma formation provides an in vivo assay of hESCs differentiation capacity.

Proliferating cultures of hESCs were used to form teratomas by renal capsule grafting using established methods [25-28]. (A) An explanted teratoma is shown. (B-F) Teratomas were sectioned and stained with hematoxylin and eosin to identify embryonic tissues. Representative tissues from all three embryonic germ layers can be seen, including mesoderm (B,C), endoderm (D) and ectoderm (E,F). (B) Nascent renal tubules and glomeruli within bed of primitive renal epithelium. (C) Cartilage surrounded by capsule of condensed mesenchyme. (D) Glandular intestinal structure. (E) Nascent neural tube. (F) Primitive squamous epithelium. Bar, 100 μm.