Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 1 – Introduction
1.1. Eukaryote multicellular organisms
All living beings share some characteristics. In particular, all modern organisms are
comprised of cells, which are present in two distinct types. Eukaryote cells have a
nucleus enclosing the genetic material, while prokaryote cells lack of such nucleus. [1]
Similarly to prokaryotes, eukaryotes are surrounded by membrane and contain
ribosomes, organelles responsible for the synthesis of proteins. However, the
eukaryotic cellular structure is more complex and contains various other organelles,
such as mitochondria, where various metabolic reactions occur. An important
characteristic of cells is their capacity to generate copies of their own genetic material
resulting in identical cells. Organisms based on prokaryote cells are always unicellular
with very simple structure and include bacteria such as Escherichia coli, while
eukaryotic living beings can be formed by one or more cells and their simplest
representatives are yeasts. Multicellular organisms started to emerge when some
unicellular eukaryotes, to survive the difficult environmental conditions on Earth,
formed multicellular aggregates of only one cell type, such as the modern algae
Volvox. [2] The oldest fossil findings of primitive living life, i.e. the oldest cellular
aggregates, date back to 3.5 billion years ago (Figure 1). [3, 4] There is no evidence of
life in earlier times, i.e. during the prebiotic era, but hypotheses have been formulated
and experimentally verified in various experiments (Figure 1). [4–7] When cellular
aggregates composed by different cellular types, having various functions, could
mutually cooperate with each other (Figure 1), truly complex multicellular organisms
arose. In such organisms cells are physically held together by a molecular and elastic
framework and can communicate with each other in various ways, influencing on
each other’s fate. [1] Later, the continuous cellular specialization and division of labor
amongst the cells in an organism led to the complexity and diversity of modern
multicellular organisms, i.e. plants and animals. Each aggregate of specialized (i.e.
differentiated) cells, or combinations of them, forms a solid mass of interconnected
cells supported by structural macro–molecules. Such masses, performing diverse
functions, are referred to as tissues. Groups of mutually cooperating tissues then
compose various body parts that, together, will form what we call a ‘living being’.
1.2. The body tissues – matrices of molecules and differentiated cells
Every tissue in multicellular organisms is formed by three–dimensional collagen
structures, [8, 9] which provide supporting frameworks for cell adhesion, migration and
polarity (Figure 2). The functions of such collagenous matrices have been
demonstrated when heart tissue was decellularized and it provided a matrix that could
support seeded cardiac and endothelial cells. [10] Collagen in tissues varies per type
according to the tissue functions. [8, 9] When collagen type I is the most present, it
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