In chapter 7, Shubin uses the analogy of a bridge to describe how our skeletons work (124). He details how the skeleton’s strength has to do with not just the size and shape of the bones, but their molecular properties, as cells are organized differently and have different characteristics. For example, some cells are separated by hydroxyapatite for strength, while others are separated by collagen. He further describes the skeleton by talking about how cartilage adds pliability to help make our joints run smoothly.
By making different kinds of molecules, it is possible for our bodies to create a mix between them to create a specific type of material. Why is it a selective advantage to be able to mix different ratios of hydroxyapatite, collagen, enamel, and proteoglycans to create different kinds of tissue? To close out the section, Shubin talks about how the cells in our body have to “stick together” to communicate, using molecular “rivets” (127). What kinds of rivets are Shubin talking about? What kind of signals do these cells communicate with? How would the body organize bone cells and other tissue cells using these methods of communication?
Eugene Bulkin (doubleaw002@gmail.com)
The skeleton is one of the most crucial elements of the human body. Without it, we would have no structure, our organs would collapse, and we’d most likely die. But for something so simply necessarily, the materials and mechanisms used to build our skeletons are extremely complex.
ReplyDeleteNumerous molecules conglomerate to build our bones. One such molecule is hydroxyapatite, which Shubin compares to concrete. Like concrete, hydroxyapatite is strong when compressed but week when bent or twisted (Shubin 125). Hydroxyapatite can be found between separated cells in bones which are structured to maximize compression and minimize twisting and bending. This ensures that bones are strong and stable. This is a selective advantage because firm bones allow for both steadiness and flexibility in the body. Collagen is another molecule found in bones that, like rope, is strong when pulled but weak when released (Shubin 125). Hydroxyapatite and collagen, along with a few other, less prevalent molecules, form strong bones. Think about the great inconveniences you encounter when you break a bone. Not only is it painful, but you are temporarily incapable of performing necessary functions, such as bending that bone or flexing the surrounding muscle (http://www.emedicinehealth.com/broken_arm/page3_em.htm). Over time, the development of our bone structure using hydroxyapatite and collagen, became more effective at preventing breakage. This gave animals with such a molecular make up an advantage for sturdier bone structure and decreased their vulnerability to predators who may be crippled by broken bones.
Collagen mixed with other molecules makes up cartilage, another important part of our skeleton. Cartilage serves as a protective cap for joints and works as a cushion when force is applied through physical activity. Essentially, cartilage is made up of a few cells separated by an interstitial filling of collagen and proteoglycan (126). Because cartilage needs to be more flexible and pliant than bones, its structure is quite different. Proteoglycan, which swells up with water, is wrapped around collagen which, like in bones, resembles a rope. This structural relationship between proteoglycan and collagen allows cartilage to remain flexible yet resistant to tension (Shubin 127). Cartilage furthers the body’s flexibility with its role in joint function. Cartilage protects joints such as the ball-and-socket joints, hinge joints, and pivot joints from wearing out (Campbell 1114). By preventing bones from directly grinding against one another, cartilage builds durability.
Cells in both bones and cartilage must also stick together. The molecular “rivets” that allow this binding can be found in various forms. Some rivets have two molecules each bind to the outside of a cell membrane, thus linking the two membranes together. Others only bind selectively to identical rivets, allowing cells to organize themselves into tissues and organs (Shubin 127). These rivets are often in the form of plasmodesmata - pieces of cellular cytoplasm that extend through cellular pores into other cell cytoplasms. More specifically, plasmodesmata can be seen in the form of spot desmosomes, tight junctions, and gap junctions (http://www.slideshare.net/musselburghgrammar/cell-molecular-biology). Spot desmosomes provide mechanical strength while tight junctions are a more selective type of rivet. Gap junctions provide transportation channels which may also play a role in cell communication. Without these rivets, multicellular organisms would not remain cohesive and would easily fall apart.
Cell communication is another crucial advantage for multicellular organisms. Cells communicate by exchanging molecules (Shubin 128). Simply, a cell might send a molecule to a receiving cell, where the molecule may cross multiple membranes to reach the nucleus where it can affect the genetic information being relayed. For example, hormones are sent not only from cell to cell, but from organ to organ, and they allow different organs and systems to collaborate. ... (see next post!)
Paracrine signaling is a form of cell communication that works for short-distance communication using local regulators. For example, animal growth factors stimulate cell growth and division to nearby cells. Gap junctions, as mentioned earlier, also aid in local cell communication by providing passageways for signaling molecules (http://kentsimmons.uwinnipeg.ca/cm1504/cellcommunication)
ReplyDeleteUsing rivets and cellular communication, cells may organize into distinct tissues. During embryonic development, growth, and tissue replenishment, various proteins travel through stem cells, to their nucleus where they program the cell to read only certain DNA. This specifies the cell’s function. When the proteins have programmed numerous cells, rivets may be used to bind cells similar in function (http://ts-si.org/biology/29261-cell-communities-self-organize-into-healthy-organ-tissue).
Thus, it is through molecular composition, rivets, and cellular communication, that we and many other animals are able to sustain a healthy skeleton. And, without our skeleton we would be a pile of slime rather than an upright, living human being.