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5. Muscle.pdf

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McGill University
Anatomy & Cell Biology
ANAT 261
Craig Mandato

Naveen Sooknanan McGill Fall 2011 Muscle: Muscular tissues structure is closely related to its function. Morphological features are related to individual molecules. For example, sarcomeres of skeletal muscle give rise to characteristic striations In multicellular organisms, movement is dependent on muscle fibers (specialized cells). They contract upon stimulation and are elongated in shape o Muscle cells are elongated and contain a great deal of cytoplasmic filaments Muscle originates from the mesoderm o Differentiation by the ectoderms produces the skin nervous system and annexes o Differentiation by the endoderm forms the digestive tract o Differentiation of the mesoderm forms CT and muscular tissue Differentiation occurs mainly by a process of cell lengthening and synthesis of myofibrillar proteins There is specialized terminology in muscles for things we have already seen: o Sarcolemma means muscle cell membrane o Sarcoplasm means muscle cell cytoplasm o Sarcoplasmic reticulum means muscle ER There are three different types of muscle: smooth muscle, striates skeletal muscle and striated cardiac muscle Smooth muscle is mainly found in the wall of blood vessels and organs (e.g. in arteries, veins and the walls of the digestive tract) o They have an elongated cell shape with a central, round nucleus o The cytoplasmic myofilaments are under the power of LM, so we do not see them o Spindle shaped cells will be observes in longitudinal section Striated skeletal muscle (or simply skeletal muscle) contains cross striations (stripes) which run perpendicular to the fiber o One fiber is one long, multinucleated cell, and it can be as long as 5-30cm Therefore they are very elongated These long cells are produced because stem cells differentiate during development and fuse together to form one long fiber o The nuclei arte squished and are located at the periphery of the cell o The myofilaments have a special configuration which allows us to see them in LM in cross section They appear as small dots within the cytoplasm o The contain organized bands which we will talk about later Striated cardiac muscle looks a lot like skeletal muscle, but has some important differences o The stem cells do not fuse together, meaning each cell only has one nucleus 1 Naveen Sooknanan McGill Fall 2011 o The nucleus is centrally located, and there is a pale staining halo around the nucleus o The same bands are visible here as in skeletal muscle o The cells are squished together, but separating them are intercalated discs This stains dark because it contains the 2 PMs of the two adjacent cells o The myofibrils are visible here as well o The cytoplasm also becomes branched, unlike skeletal muscle which remain in packed fibers Plane of section is very important for these cells, because 90% of the sections we will see are oblique and can sometimes be hard to distinguish Smooth muscle, as we stated before, has spindle shapes cell and a centrally located nucleus: Central nucleus nucleus in cross section In longitudinal section, this spindle shape, or fusiform (wider middle and tapered edges) is really seen well. The nucleus is centrally located and is an oval shape o There are no striations in smooth muscle, the cytoplasm appears clear, as no myofilaments are visible o These cells have only one nucleus each In cross section, a variety of circular shapes can be seen, all differing in size. This is because each cell is cut at a different location o Some cells may be cut at the ends, which means we wont see the nucleus o Other will be cut in the center, and appear bigger, and will have a nucleus o All cells are roughly the same size, and have a central, round nucleus in cross section o No myofibrils are present, which means the cytoplasm is clear o There are no halos around the nuclei (unlike cardiac muscle) 2 Naveen Sooknanan McGill Fall 2011 Cross (left) vs. Longitudinal (right) section of smooth muscle Spindle shaped cells Circular cells Elongated central nucleus Central, round nucleus No striations No grains in cytoplasm Striated skeletal muscle is made of up very long cells called muscle fibers: Myofibrils give grainy A band I band Z line appearance Capillary Fibrocyte Nucleus In longitudinal section, the A (dark) and I (light) bands can be seen very well o The nuclei are flattened and elongated, and pushed to the periphery of the cell Any nuclei outside of the fibers may be the nuclei of fibrocytes. These nuclei are much darker o These cells are multinucleated, so many nuclei will be seen in the longitudinal section of one fiber o There are visible striations in the muscle because of sarcomeres In cross section, the cytoplasm will have tiny dots which are visible myofilaments o The nuclei are small and round, and pushed to the edges of the cell Sometimes, you will see more than one nucleus per cell o Sometimes, blood vessels such as capillaries will be in between the fibers They will be much smaller and tube shaped, and only have one nucleus o The cells themselves appear almost round 3
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