2 Tissues: a Social Life of Cells

One of the characteristics of multicellular organisms is the presence of tissues, the assemblies of cells of the same kind that reside together and have a specific function. Tissues emerged as a solution to the increase in a number of functions that eukaryotic cells gained through the acquisition of new genes or merging with other cells (think the microbial origin of mitochondria). So simply said, eukaryotic cells became too big and too complicated for their own good. The first solution to the problem was the introduction of internal compartments called organelles and streamlining the metabolic pathways. Check √

The other solution took the cells a good part of 3 billion years to develop, and it required going beyond their own membrane and splitting the functions between members of the group. Why do a little of everything if you can specialize in one particular function? It seems like a no-brainer, right? Welcome to the multicellular life!

Multicellular organisms are not large colonies of cells. The most significant difference that separates them from unicellular creatures, no matter how large a colony they form, is specialization; a delegation of function and most of all, dependence on one another. The cells in a multicellular organism are no longer identical, they can’t do everything. Even if they retain the entire genome, a specific set of genes are necessary for their respective functions; pancreatic cells express a gene (via production of protein) often called insulin, muscle cells express genes for contractile filaments, and neurons express genes for ligand-gated ion channels that allow them to react to stimuli. This differential gene expression has its price:  the dependency of highly specialized groups of cells on one another. If one group of cells fails, the entire organism suffers or dies. So it is in the case of type 1 diabetes, where the failure of pancreatic beta cells to produce insulin leads to the death of the organism within a few months. Luckily, the introduction of hormone replacement therapy with animals, and later recombinant insulin, now prolongs a patient’s life to full capacity. When the group of cells specialized in gas exchange, know as the lungs, are compromised in conditions such as emphysema or pulmonary edema, functions of all other groups of cells suffer.

Tissues have a higher level of complexity that allows them to gain new functions, not by evolving individual cells, but by combining large groups of them into precise and meaningful 3D arrangements and connections. Individual cells do not acquire new genes; they just build a more complex structure. Single skin cells can produce keratin and protect the underlying tissues from UV rays by producing melanin. But only sealing them with cell junctions gave them the new function of creating an impermeable barrier.

As life progressed, the next, even higher level of complexity, emerged. Several tissues came together to form organs, that in turn came together to build organ systems, making an organism a more powerful unit than any colony can be.

4.1 four basic types of tissues

A tissue is an assembly of cells and extracellular matrix that fills the spaces between the cells and keeps them together.  The extracellular matrix is produced by the cells and is a mixture of fibers and a gel-like substance filling the spaces between cells, binding them together and providing some level of organization to the entire group.

The proportion of cells to extracellular matrix in different tissues varies widely. Some tissues, mostly epithelial, are practically cells only, with just a thin layer of fibers forming a basal membrane underneath.  Other tissues, such as connective tissue, have few cells and lots of extracellular matrix filling the vast spaces between them.  Bone and cartilage, both examples of connective tissue, have exceptionally few cells and lots of fibers.

The composition of the extracellular matrix also has a significant influence on the structure and function of the tissues. More fibers make the tissues more elastic, while the background substance makes the tissues strong and resistant to the tension.  Tendons and ligaments–examples of dense connective tissue–have a lot of collagen fibers and they are incredibly stretchable, while cartilage–another type of connective tissue which has mainly a background substance–is very good at weight bearing and support.

Despite the many ways tissues can look and function (think liver, kidney, skin, etc.) there are many similarities between tissues that allow us to classify them into types. Based on these similarities; the arrangement of cells, the types of fibers, and where in the body they are located, all tissues can be assigned to one of four categories. They are either epithelial, connective, muscular, or neuronal.

Epithelial tissues are made of sheets of tightly bound cells (arrangement of cells) and are located on the surface of the organism or line the lumen of the organ from inside. They form sheets or roll into tubes that cover the ducts in the various structures in the body (pancreatic ducts, mammary ducts, sweat glands ducts) or cover the hollow organs from inside (urinary bladder, stomach, trachea or inside of moth cavity). Their function is to cover and protect, provide the selective barrier for entry and exit of substances into the body and to secrete substances such as sweat, mucus, or some hormones.

Connective tissues have fewer cells that are dispersed in a large quantity of extracellular matrix, are not necessarily bound together, and are located inside the organs, “gluing” all parts together (hence the name). They rarely rely on the organized arrangement of cells like epithelial sheets do. Their functions range from supporting and reinforcing the organs (the membranes around muscle fascicles or cartilage), to storage (fat is a connective tissue), to transport (blood is a fluid connective tissue).

The other two types of tissues, muscular and neuronal, are made of highly specialized cells that are optimized for a particular function. Muscular tissue is specialized in contraction and located in skeletal muscles and the walls of the organs that move the content inside them through contraction. Neuronal tissue specializes in the conduction of electrical impulses; is located in nerves, the spinal cord, and the brain, as well as special sensory organs such as the retina. Each tissue type will be more thoroughly discussed in subsequent chapters.

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