Muscle tissue is one of the four main types of tissues that allow the human body to function properly. It is composed of muscle cells, or myocytes, all of which share the fundamental property of contraction. There are three main types of muscles: skeletal, cardiac, and smooth.
Smooth muscle is non-striated and consists of histologically distinct cells. It is capable of synchronous contractions that are based on a similar mechanism to the one in skeletal muscle. However, smooth muscle has other properties in addition to contractility. This article will describe the histology of smooth muscle, together with its properties.
Characteristics & Organisation
Smooth muscle has several distinct characteristics and a particular histological structure. In routine hematoxylin and eosin (H&E) staining, the cells appear as elongated spindles, or fusiform-shaped, having tapered ends. Each myocyte contains a nucleus. The shape of the cell and nucleus varies, depending on whether the muscle is contracting or relaxing. In the relaxed state, the nucleus appears elongated and it is located centrally within the cytoplasm. To visualise it, image the nucleus as being ‘cigar-shaped’ with blunt ends. During the contracting phase, the cell appears a globular shape, while the nucleus becomes spiral-shaped. However, it is still maintains a central location.
The cytoplasm (sarcoplasm) is eosinophilic and contains anchoring points called focal densities. Their role is to provide one attachment point for the bundles of contractile proteins, which form a criss-cross pattern in smooth muscle. The contractile proteins are alpha actin and type II myosin, which form thick and thin filaments, respectively. Thick filaments do not occupy a fixed place within the sarcoplasm, but are rather scattered throughout. They are not typically seen in routine H&E staining because they are lost during tissue processing. Smooth muscle cells also contain intermediate filaments, called desmin, which insert into focal densities. The smooth muscle of blood vessels also contains vimentin, which is another type of intermediate filament.
In addition to the focal densities found within the cytoplasm, smooth muscle has a second set of proteins providing attachment points. Those second connections are provided by the focal adhesion densities, also known as dense bodies, which are located on the plasma membrane of myocytes. These densities represent the point of attachment and intersection between the contractile apparatus (actin-myosin), intermediate filaments, and sarcolemma. They usually exhibit a linear or branching configuration and are small, isolated, and electron-dense structures.
The ultrastructure of smooth muscle cells reveals distinct invaginations all around the plasma membrane. Some of them are irregularly shaped and it is believed they are involved in pinocytosis. In contrast, others have more regular shapes and are called caveolae. Scattered smooth endoplasmic reticulum (sER) fragments and cytoplasmic vesicles are located directly underneath and in close proximity to these caveolae. You can imagine these three structures to be the ‘T-system’ of the smooth muscle, similar to skeletal muscle. In reality, this muscle type does not posses such a system. In addition, adjacent cells form gap junctions, which allow small ions and molecules to pass from one cell to the next. This communication facilitates a synchronous smooth muscle contraction by allowing the spread of depolarization. Organelles are located at each end of the nucleus and consist of free ribosomes, glycogen granules, small Golgi apparatus, rough endoplasmic reticulum (rER), and many mitochondria.
When smooth muscle cells are bound together, they appear as bundles called fasciculi, which form an irregular, branching pattern. The arrangement of these fasciculi is highly varied from organ to organ. In addition, taken together, they can form structures called sheets.
Both individual cells and fasciculi are surrounded, hence separated, by collagenous tissue. In tubular viscera, like fallopian tubes, gut, and ureters, the fasciculi are arranged into a specific pattern. They occur in layers, each one being orientated perpendicularly relative to the adjacent ones. This particular arrangement facilitates a reduction in the tube’s lumen, allowing processes like peristalsis to take place.
The fasciculi are the functional contractile units of smooth muscle. Also, the lack of arrangement of contractile proteins inside myofibrils makes this muscle type non-striated. The key players which allow the smooth muscle to contract as a unit are the contractile proteins that criss cross the cells and are attached at focal densities. These densities transmit the contraction to the external lamina which surrounds the cells, facilitating its propagation. It is similar to an elastic net around the cell; constricting the net itself results in constricting the cell.
Compared to skeletal muscle, smooth muscle has three stimuli that can result in contraction:
- Mechanical stimuli, which involve passive stretching and subsequent activation of mechanosensitive ion channels.
- Electrical stimuli, which involve nervous impulses, together with the release of acetylcholine and norepinephrine.
- Chemical stimuli, such as vasopressin, thromboxane A2, and angiotensin II
Following either one of the above stimuli, calcium ions, which are normally sequestered inside the sER fragments, are released into the cytoplasm. They bind the protein calmodulin, which subsequently binds and activates the myosin light-chain kinase enzyme. The latter phosphorylates myosin, which can now bind to actin. The contraction now proceeds in a similar manner to the one of skeletal muscle, through the filament-sliding mechanism.
To stop the contraction, ATP-dependent calcium pumps are activated, which return the calcium ions back to the sER or into the extracellular space.
Compared to skeletal muscle which requires high amounts of adenosine triphosphate (ATP), smooth muscle can sustain a forceful contraction with a minimal concentration of ATP. This is because the rate of ATP hydrolysis by smooth muscle is one tenth of the rate in skeletal muscle, increasing the availability of energy.
A second mechanism, called ‘latching’, also helps in sustaining the contraction state of smooth muscle. This phenomenon is part of the ‘latch-bridge’ hypothesis of smooth muscle contraction. It occurs after the binding of the myosin head to the actin filament, when there is a decrease in ATPase activity. This temporarily prevents the detachment of the myosin head. In essence, the head is similar to a barbed wire fence. Once it grabs hold or latches onto your clothes, you struggle and contract more because it takes longer for you to free yourself.
This type of muscle can contract in two main ways: wavelike and extrusive. Wavelike contraction is called peristalsis, which occur in the gastrointestinal and male genital tracts. The extrusive contractions occur along the entire length of the muscle, such as those in the uterus, urinary bladder, and gallbladder. Smooth muscle is under involuntary control. Therefore, it is innervated by the three branches of the autonomic nervous system (ANS): sympathetic, parasympathetic, and enteric systems. According to the control of the innervation, smooth muscle can be divided into unitary (tonic) or phasic. Unitary smooth muscle generates its own rhythmic contraction, which is transmitted via gap junctions. In this case, the ANS only modulates the level of spontaneous contraction, rather than directly initiating it. However, in phasic smooth muscle the ANS directly kickstarts contraction. This subtype of smooth muscle can be found in the iris, certain large arteries, and the vas deferens.
When you think of smooth muscle, your mind is probably zooming directly to contraction. This is indeed their primary function. However, the cells also contain a particular set of organelles that offers smooth muscle cells secretory properties, hence giving them an ability to take on a secondary function, secretion, as ‘phenotypic plasticity’. They contain well developed rER and Golgi apparatus, which are located around the nucleus (perinuclear zone). Smooth muscle cells synthesize and secrete the following:
- Type III collagen
- Type IV collagen
- Multiadhesive glycoproteins
- Type I collagen and elastin, by those in the uterus and blood vessel walls
According to research, this form of plasticity is important in the development of various diseases, such as atherosclerosis and primary pulmonary hypertension. While the exact causes driving the changes of typical smooth muscle cells into secretory ones are unknown, histological observations clearly reveal the dramatic nature of the changes themselves.
Renewal & Repair
Smooth muscle is located at various sites that have a high rate of cellular turnover, caused by the continuous aging or damage suffered by the myocytes. As a result, new smooth muscle cells need to be continuously produced to replace the problematic ones.