Origin of smooth muscle

Origin of smooth muscle

Smooth muscle cells arise from undifferentiated mesenchymal cells. These cells differentiate first into mitotically active cells, myoblasts, which contain a few myofilaments. Myoblasts give rise to the cells which will differentiate into mature smooth muscle cells.

Types of smooth muscle

Two broad types of smooth muscle can be distinguished on the basis of the type of stimulus which results in contraction and the specificity with which individual smooth muscle cells react to the stimulus:

1. The multiunit type represents functionally independent smooth muscle cells which are often innervated by a single nerve terminal and which never contract spontaneously (e.g. smooth muscle in the walls of blood vessels).
2. The visceral type represents bundles of smooth muscle cells connected by GAP junctions, which contract spontaneously if stretched beyond a certain limit (e.g. smooth muscle in the walls of the intestines).

Suitable Slides

Sections of the intestines (duodenum, jejunum, ileum or colon) - H&E

Jejunum, baboon - H&E
The outer part of the tube forming the intestines consists of two layers of smooth muscle - one circular layer and one longitudinal layer. If you look at the tissue close to the border between the two layers of smooth muscle, you will be able to see both longitudinally sectioned smooth muscle cells and transversely sectioned smooth muscle cells. The smooth muscle cells are much longer than their nuclei. Transversely sectioned smooth muscle cells may not have their nuclei in the plane of the section.
Occasionally you will find small nerves between the two muscle layers, and, if you are lucky and/or patient, you will also see some very large nuclei in this region. These nuclei belong to peripheral nerve cells (ganglion cells of the myenteric plexus), which regulate the contraction of the muscle around the gastrointestinal tract.
Draw a small area which contains both longitudinally sectioned and transversely sectioned smooth muscle at high magnification.

The only tissues which perhaps could be confused with smooth muscle are dense regular connective tissues and peripheral nerves. Both the number of nuclei and their shapes clearly distinguish smooth muscle from dense regular connective tissues. Nuclei are much more frequent and larger in smooth muscle, and they are very elongated if cut longitudinally. Peripheral nerves will be surrounded by a capsule of cells and connective tissue - the perineurium. The thickness of longitudinally cut nerve fibres is constant while smooth muscle cells are spindle shaped. Also, axon and nodes of Ranvier should be visible in peripheral nerves

Skeletal Muscle

• Skeletal muscle consists of very long tubular cells, which are also called muscle fibres.
  The average length of skeletal muscle cells in humans is about 3 cm (sartorius muscle up to 30 cm, stapedius muscle only about 1 mm). Their diameters vary from 10 to 100 µm.
• Skeletal muscle fibres contain many peripherally placed nuclei.
  Up to several hundred rather small nuclei with 1 or 2 nucleoli are located just beneath the plasma membrane.
• Skeletal muscle fibres show in many preparations characteristic cross-striations. It is therefore also called striated muscle.
• Skeletal muscle is innervated by the somatic nervous system.
• Skeletal muscle makes up the voluntary muscle.

Structure of skeletal muscle

Muscle fibres in skeletal muscle occur in bundles, fascicles, which make up the muscle. The muscle is surrounded by a layer of connective tissue, the epimysium, which is continuous with the muscle fascia. Connective tissue from the epimysium extends into the muscle to surround individual fascicles (perimysium). A delicate network of loose connective tissue composed of fine collagenous and reticular fibres (endomysium) is found between the muscle fibres of a fascicle. Finally, each muscle fibre is surrounded by a basement membrane.

Origin of skeletal muscle

The myoblasts of all skeletal muscle fibres originate from the paraxial mesoderm. Myoblasts undergo frequent divisions and coalesce with the formation of a multinucleated, syncytial muscle fibre or myotube. The nuclei of the myotube are still located centrally in the muscle fibre. In the course of the synthesis of the myofilaments/myofibrils, the nuclei are gradually displaced to the periphery of the cell.
Satellite cells are small cells which are closely apposed to muscle fibres within the basal lamina which surrounds the muscle fibre. Their nuclei are slightly darker than those of the muscle fibre. Satellite cells are believed to represent persistent myoblasts. They may regenerate muscle fibres in case of damage.

 
Suitable Slides

Sections of skeletal muscle, tongue or upper esophagus - H&E

Tongue, Skeletal Muscle, human - H&E Skeletal muscle in the tongue is arranged in bundles which typically run at right angles to each other. Both longitudinally and transversely cut skeletal muscle fibres are present. In both section planes you can see that the nuclei are located in the periphery of the muscle fibre. Myofibrils may be visible as very fine dots in some of the transversely muscle fibres. Striations formed by the A- and I-Bands of the sarcomeres are visible in longitudinally cut fibres. Z-lines and H-bands can be identified in well-preserved tissue. Details of the sarcomeres stand out more clearly if you close the iris diaphragm of the microscope. Remember to open the diaphragm after you have seen the striations clearly ! In the connective tissue between the muscle fibres, the endomysium, numerous capillaries supply the muscle with oxygen and nutrients. Draw a small section of longitudinal and transversely cut skeletal muscle at high magnification. The muscle surrounding the upper one-third of the oesophagus is skeletal muscle. Smooth muscle surrounds its lower one-third. In section of the middle of the esophagus it is usually possible to identify both muscle types and their appearances can be compared.