Chapter 23 Essays D
1) What is the relationship between cartilage and bone
a. Most bones originate from minute “scale models” formed out of cartilage in the embryo. Each scale model grows, and as new cartilage forms, the older cartilage is replaced by bone. The process is endochondral bone formation.
1) What else controls osteoclasts?
a. The osteoblasts that make the matrix also produce the signals that recruit and activate the osteoclasts to degrade it. Two proteionshave this role—Macrophage-CSF and TNF11, a member of the TNF family. The behavior o the osteoblasts in attracting their opponents may seem self-defeating, but it has the useful function of localizing osteoclasts in the tissue where they are needed.
1) What is done to prevent excessive degradation of matrix?
a. The osteoblasts secrete, along with MCSF and TNF11, another protein, osteoprotegerin, that tends to block the action of TNF11. The higher the level of Wnt activation in the osteoblasts, the more osteoproteregin they secrete and, consequently, the lower the level of osteoclast activation and the lower the rate of bone matrix degradation. The Wnt signaling pathway thus seems to have two distinct functions in bone formation: at early stages, it controls the initial commitment of cells to an osteoblast fate; later, it acts in the differentiated osteoblasts to help govern the balance between matrix deposition and matrix erosion.
1) What do circulating homromes do?
a. They affect bones throughout the body. No less important are local controls tha tallow bone o be deposited in one place while it is resorbed in another. Through such controls over the process of remodeling, bones are endowed with a remarkable abiity t adjust their structure in response to long-term variations in the load imposed on them.
1) What does fat-cell differentiation begin with?
a. It begins with the expression of two familie of gene regulatory proteins: the CEBP family and the PPAR family, especially PPAR-gamma. Like MyoD and MEF2 families of skeletal muscle development, the CEBP and PPAR-gamma proteins drive and maintain one another’s expression, trough various cross-regulatory and autoregulatory control loos. They work together to control the expression of the other genes characteristic of adipocytes.
1) What does the production of enzymes for import of fatty acids and glucose and for fat synthesis lead to ?
a. It leads to an accumulation of fat droplets, consitsting mainly of triaglycerol. These then coalesce and enlarge until the cell is hugely distended up with only a thin rim of cytoplasm around the mass of lipid. Lipases are made in the fat cell, giving it the capacity to reverse the process of lipid accumulation, by breaking down the triacylglyceroles into fatty acids that can be secreted for consumption by other cells. The fat cell can change its volume by a factor of a thousand as it accumulates and releases lipid.