A muscle cell is also known as a myocyte when referring to either a
cardiac muscle cell (cardiomyocyte), or a
smooth muscle cell as these are both small
cells.
A
skeletal muscle cell is long and threadlike with
many nuclei and is called a
muscle fiber.
Muscle cells (including myocytes and muscle fibers) develop from embryonic
precursor cell
In cell biology, a precursor cell, also called a blast cell or simply blast, is a partially differentiated cell, usually referred to as a unipotent cell that has lost most of its stem cell properties. A precursor cell is also known as a proge ...
s called
myoblasts.
Myoblasts
fuse to form
multinucleated skeletal muscle cells known as
syncytia in a process known as
myogenesis.
Skeletal muscle cells and cardiac muscle cells both contain
myofibrils and
sarcomeres and form a
striated muscle tissue
Striations means a series of ridges, furrows or linear marks, and is used in several ways:
* Glacial striation
* Striation (fatigue), in material
* Striation (geology), a ''striation'' as a result of a geological fault
* Striation Valley, in Anta ...
.
Cardiac muscle cells form the
cardiac muscle in the walls of the heart chambers, and have a single central
nucleus. Cardiac muscle cells are joined to neighboring cells by
intercalated discs, and when joined in a visible unit they are described as a ''cardiac muscle fiber''.
Smooth muscle cells control involuntary movements such as the
peristalsis contractions in the
esophagus and
stomach. Smooth muscle has no myofibrils or sarcomeres and is therefore non-striated. Smooth muscle cells have a single nucleus.
Structure
The unusual
microscopic anatomy of a muscle cell gave rise to its own terminology. The
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
in a muscle cell is termed the
sarcoplasm; the
smooth endoplasmic reticulum of a muscle cell is termed the
sarcoplasmic reticulum; and the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...
in a muscle cell is termed the
sarcolemma.
The sarcolemma receives and conducts stimuli.
Skeletal muscle cells
Skeletal muscle cells are the individual contractile cells within a muscle, and are more usually known as muscle fibers because of their longer threadlike appearance.
A single muscle such as the
biceps brachii in a young adult human male contains around 253,000 muscle fibers.
Skeletal muscle fibers are the only muscle cells that are
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coord ...
d with the
nuclei usually referred to as
myonuclei. This occurs during
myogenesis with the
fusion of
myoblasts each contributing a nucleus to the newly formed muscle cell or ''myotube''.
Fusion depends on muscle-specific proteins known as
fusogens called ''myomaker'' and ''myomerger''.
A striated muscle fiber contains
myofibrils consisting of long protein chains of
myofilaments. There are three types of myofilaments: thin, thick, and elastic that work together to produce a
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
.
The thin myofilaments are filaments of mostly
actin
Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
and the thick filaments are of mostly
myosin and they
slide over each other to shorten the fiber length in a muscle contraction. The third type of myofilament is an elastic filament composed of
titin, a very large protein.
In
striations of muscle bands, myosin forms the dark filaments that make up the
A band
The A Band are a British musical collective formed in Nottingham in the late 1980s. In 2005, The Wire compared the band's importance to that of AMM and SME, and noted their lack of musical training.
The line-up of the group has never been ...
. Thin filaments of actin are the light filaments that make up the
I band. The smallest contractile unit in the fiber is called the sarcomere which is a repeating unit within two
Z bands. The sarcoplasm also contains
glycogen
Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body.
Glycogen functions as one o ...
which provides energy to the cell during heightened exercise, and
myoglobin, the red pigment that stores oxygen until needed for muscular activity.
The
sarcoplasmic reticulum, a specialized type of
smooth endoplasmic reticulum, forms a network around each myofibril of the muscle fiber. This network is composed of groupings of two dilated end-sacs called terminal cisternae, and a single
T-tubule (transverse tubule), which bores through the cell and emerge on the other side; together these three components form the
triads that exist within the network of the sarcoplasmic reticulum, in which each T-tubule has two terminal cisternae on each side of it. The sarcoplasmic reticulum serves as reservoir for calcium ions, so when an action potential spreads over the T-tubule, it signals the sarcoplasmic reticulum to release calcium ions from the gated membrane channels to stimulate a muscle contraction.
In skeletal muscle, at the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers at the
myotendinous junction.
Within the muscle fiber pressed against the sarcolemma are multiple flattened
nuclei; embryologically, this
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coord ...
condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus.
Cardiac muscle cells
The cell membrane of a
cardiac muscle cell has several specialized regions, which may include the
intercalated disc, and
transverse tubules. The cell membrane is covered by a lamina coat which is approximately 50 nm wide. The laminar coat is separable into two layers; the
lamina densa and
lamina lucida. In between these two layers can be several different types of ions, including
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
.
Cardiac muscle like skeletal muscle is also striated and the cells contain myofibrils, myofilaments, and sarcomeres as the skeletal muscle cell.
The cell membrane is anchored to the cell's
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is comp ...
by anchor fibers that are approximately 10 nm wide. These are generally located at the Z lines so that they form grooves and transverse tubules emanate. In cardiac myocytes this forms a scalloped surface.
The cytoskeleton is what the rest of the cell builds off of and has two primary purposes; the first is to stabilize the topography of the intracellular components and the second is to help control the size and shape of the cell. While the first function is important for biochemical processes, the latter is crucial in defining the surface to volume ratio of the cell. This heavily influences the potential electrical properties of
excitable cell
Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
s. Additionally deviation from the standard shape and size of the cell can have negative prognostic impact.
Smooth muscle cells
Smooth muscle cells are so-called because they have neither myofibrils nor sarcomeres, and therefore no
striations. They are found in the walls of hollow
organs, including the
stomach,
intestines,
bladder and
uterus
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
, in the walls of
blood vessel
Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from ...
s, and in the tracts of the
respiratory,
urinary, and
reproductive systems. In the
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
s, the
ciliary muscle
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, uvea (vascular layer). It controls accommodation for viewing ob ...
s dilate and contract the
iris and alter the shape of the
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'' ...
. In the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
, smooth muscle cells such as those of the
arrector pili cause
hair to stand erect in response to
cold temperature or
fear
Fear is an intensely unpleasant emotion in response to perceiving or recognizing a danger or threat. Fear causes physiological changes that may produce behavioral reactions such as mounting an aggressive response or fleeing the threat. Fear ...
.
Smooth muscle cells are spindle-shaped with wide middles, and tapering ends. They have a single nucleus and range from 30 to 200
micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of ...
s in length. This is thousands of times shorter than skeletal muscle fibers. The diameter of their cells is also much smaller which removes the need for
T-tubules found in striated muscle cells. Although smooth muscle cells lack sarcomeres and myofibrils they do contain large amounts of the contractile proteins actin and myosin. Actin filaments are anchored by
dense bodies (similar to the
Z discs in sarcomeres) to the sarcolemma.
Development
A
myoblast is an embryonic
precursor cell
In cell biology, a precursor cell, also called a blast cell or simply blast, is a partially differentiated cell, usually referred to as a unipotent cell that has lost most of its stem cell properties. A precursor cell is also known as a proge ...
that
differentiates to give rise to the different muscle cell types. Differentiation is regulated by
myogenic regulatory factors, including
MyoD,
Myf5,
myogenin, and
MRF4.
GATA4 and
GATA6 also play a role in myocyte differentiation.
Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore are cells with
multiple nuclei, known as
myonuclei, with each
cell nucleus originating from a single myoblast. The fusion of myoblasts is specific to skeletal muscle, and not
cardiac muscle or
smooth muscle
Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
.
Myoblasts in skeletal muscle that do not form muscle fibers
dedifferentiate back into
myosatellite cells. These satellite cells remain adjacent to a skeletal muscle fiber, situated between the sarcolemma and the basement membrane of the
endomysium
The endomysium, meaning ''within the muscle'', is a wispy layer of areolar connective tissue that ensheaths each individual muscle fiber, or muscle cell. It also contains capillaries and nerves. It overlies the muscle fiber's cell membrane: the sa ...
(the connective tissue investment that divides the muscle fascicles into individual fibers). To re-activate myogenesis, the satellite cells must be stimulated to differentiate into new fibers.
Myoblasts and their derivatives, including satellite cells, can now be generated in vitro through
directed differentiation of
pluripotent stem cells.
[ ]
Kindlin-2 plays a role in developmental elongation during myogenesis.
Function
Muscle contraction in striated muscle
Skeletal muscle contraction
When
contracting
A contract is a legally enforceable agreement between two or more parties that creates, defines, and governs mutual rights and obligations between them. A contract typically involves the transfer of goods, services, money, or a promise to tran ...
, thin and thick filaments slide with respect to each other by using
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms ...
. This pulls the Z discs closer together in a process called sliding filament mechanism. The contraction of all the
sarcomeres results in the contraction of the whole muscle fiber. This contraction of the myocyte is triggered by the
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
over the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...
of the myocyte. The action potential uses
transverse tubules to get from the surface to the interior of the myocyte, which is continuous within the cell membrane.
Sarcoplasmic reticula are membranous bags that transverse tubules touch but remain separate from. These wrap themselves around each sarcomere and are filled with Ca
2+.
Excitation of a myocyte causes depolarization at its synapses, the
neuromuscular junctions, which triggers action potential. With a singular neuromuscular junction, each muscle fiber receives input from just one somatic efferent neuron. Action potential in a somatic efferent neuron causes the release of the neurotransmitter
acetylcholine
Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...
.
When the acetylcholine is released it diffuses across the
synapse
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses fr ...
and binds to a receptor on the
sarcolemma, a term unique to muscle cells that refers to the cell membrane. This initiates an impulse that travels across the sarcolemma.
When the action potential reaches the sarcoplasmic reticulum it triggers the release of Ca
2+ from the Ca
2+ channels. The Ca
2+ flows from the sarcoplasmic reticulum into the sarcomere with both of its filaments. This causes the filaments to start sliding and the sarcomeres to become shorter. This requires a large amount of ATP, as it is used in both the attachment and release of every
myosin head. Very quickly Ca
2+ is actively transported back into the sarcoplasmic reticulum, which blocks the interaction between the thin and thick filament. This in turn causes the muscle cell to relax.
There are four main types of muscle contraction: twitch, treppe, tetanus and isometric/isotonic. Twitch contraction is the process in which a single stimulus signals for a single contraction. In twitch contraction the length of the contraction may vary depending on the size of the muscle cell. During treppe (or summation) contraction muscles do not start at maximum efficiency; instead they achieve increased strength of contraction due to repeated stimuli. Tetanus involves a sustained contraction of muscles due to a series of rapid stimuli, which can continue until the muscles fatigue. Isometric contractions are skeletal muscle contractions that do not cause movement of the muscle. However, isotonic contractions are skeletal muscle contractions that do cause movement.
Cardiac muscle contraction
Specialized
cardiomyocytes in the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approxi ...
generate electrical impulses that control the
heart
The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...
rate. These electrical impulses coordinate contraction throughout the remaining heart muscle via the
electrical conduction system of the heart. Sinoatrial node activity is modulated, in turn, by nerve fibres of both the
sympathetic and
parasympathetic nervous systems. These systems act to increase and decrease, respectively, the rate of production of electrical impulses by the sinoatrial node.
Evolution
The
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary origin of muscle cells in
animals
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
is highly debated. One view is that muscle cells evolved once and thus all muscles cells have a single common ancestor. Another view is that muscles cells evolved more than once and any
morphological or structural similarities are due to convergent evolution and genes that predate the evolution of muscle and even the
mesoderm—the
germ layer that gives rise to vertebrate muscle cells.
Schmid and Seipel argue that the origin of muscle cells is a
monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...
trait that occurred concurrently with the development of the digestive and nervous systems of all animals and that this origin can be traced to a single metazoan ancestor in which muscle cells are present. They argue that molecular and morphological similarities between the muscles cells in
cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter.
Their distinguishing feature is cnidocyt ...
and
ctenophora are similar enough to those of
bilateria
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
ns that there would be one ancestor in metazoans from which muscle cells derive. In this case, Schmid and Seipel argue that the last common ancestor of bilateria, ctenophora, and cnidaria was a
triploblast or an organism with three germ layers and that
diploblasty, meaning an organism with two germ layers, evolved secondarily due to their observation of the lack of mesoderm or muscle found in most cnidarians and ctenophores. By comparing the morphology of cnidarians and ctenophores to bilaterians, Schmid and Seipel were able to conclude that there were
myoblast-like structures in the tentacles and gut of some species of cnidarians and in the tentacles of ctenophores. Since this is a structure unique to muscle cells, these scientists determined based on the data collected by their peers that this is a marker for
striated muscles similar to that observed in bilaterians. The authors also remark that the muscle cells found in cnidarians and ctenophores are often contests due to the origin of these muscle cells being the
ectoderm rather than the mesoderm or mesendoderm. The origin of true muscles cells is argued by others to be the
endoderm portion of the
mesoderm and the endoderm. However, Schmid and Seipel counter this skepticism about whether or not the muscle cells found in ctenophores and cnidarians are true muscle cells by considering that cnidarians develop through a medusa stage and polyp stage. They observe that in the hydrozoan medusa stage there is a layer of cells that separate from the distal side of the ectoderm to form the striated muscle cells in a way that seems similar to that of the mesoderm and call this third separated layer of cells the ectocodon. They also argue that not all muscle cells are derived from the mesendoderm in bilaterians with key examples being that in both the eye muscles of vertebrates and the muscles of spiralians these cells derive from the ectodermal mesoderm rather than the endodermal mesoderm. Furthermore, Schmid and Seipel argue that since myogenesis does occur in cnidarians with the help of molecular regulatory elements found in the specification of muscles cells in bilaterians that there is evidence for a single origin for striated muscle.
In contrast to this argument for a single origin of muscle cells, Steinmetz et al. argue that molecular markers such as the
myosin II protein used to determine this single origin of striated muscle actually predate the formation of muscle cells. This author uses an example of the contractile elements present in the porifera or sponges that do truly lack this striated muscle containing this protein. Furthermore, Steinmetz et al. present evidence for a
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
origin of striated muscle cell development through their analysis of morphological and molecular markers that are present in bilaterians and absent in cnidarians, ctenophores, and bilaterians. Steimetz et al. showed that the traditional morphological and regulatory markers such as
actin
Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...
, the ability to couple myosin side chains phosphorylation to higher concentrations of the positive concentrations of calcium, and other
MyHC elements are present in all metazoans not just the organisms that have been shown to have muscle cells. Thus, the usage of any of these structural or regulatory elements in determining whether or not the muscle cells of the cnidarians and ctenophores are similar enough to the muscle cells of the bilaterians to confirm a single lineage is questionable according to Steinmetz et al. Furthermore, Steinmetz et al. explain that the orthologues of the MyHc genes that have been used to hypothesize the origin of striated muscle occurred through a gene duplication event that predates the first true muscle cells (meaning striated muscle), and they show that the MyHc genes are present in the sponges that have contractile elements but no true muscle cells. Furthermore, Steinmetz et al. showed that the localization of this duplicated set of genes that serve both the function of facilitating the formation of striated muscle genes and cell regulation and movement genes were already separated into striated myhc and non-muscle myhc. This separation of the duplicated set of genes is shown through the localization of the striated myhc to the contractile vacuole in sponges while the non-muscle myhc was more diffusely expressed during developmental cell shape and change. Steinmetz et al. found a similar pattern of localization in cnidarians with except with the cnidarian ''N. vectensis'' having this striated muscle marker present in the smooth muscle of the digestive track. Thus, Steinmetz et al. argue that the pleisiomorphic trait of the separated orthologues of myhc cannot be used to determine the monophylogeny of muscle, and additionally argue that the presence of a striated muscle marker in the smooth muscle of this cnidarian shows a fundamentally different mechanism of muscle cell development and structure in cnidarians.
Steinmetz et al. continue to argue for multiple origins of striated muscle in the metazoans by explaining that a key set of genes used to form the troponin complex for muscle regulation and formation in bilaterians is missing from the cnidarians and ctenophores, and of 47 structural and regulatory proteins observed, Steinmetz et al. were not able to find even on unique striated muscle cell protein that was expressed in both cnidarians and bilaterians. Furthermore, the Z-disc seemed to have evolved differently even within bilaterians and there is a great deal diversity of proteins developed even between this clade, showing a large degree of radiation for muscle cells. Through this divergence of the
Z-disc, Steimetz et al. argue that there are only four common protein components that were present in all bilaterians muscle ancestors and that of these for necessary Z-disc components only an actin protein that they have already argued is an uninformative marker through its pleisiomorphic state is present in cnidarians. Through further molecular marker testing, Steinmetz et al. observe that non-bilaterians lack many regulatory and structural components necessary for bilaterians muscle formation and do not find any unique set of proteins to both bilaterians and cnidarians and ctenophores that are not present in earlier, more primitive animals such as the sponges and
amoebozoans. Through this analysis the authors conclude that due to the lack of elements that bilaterians muscles are dependent on for structure and usage, nonbilaterian muscles must be of a different origin with a different set regulatory and structural proteins.
In another take on the argument, Andrikou and Arnone use the newly available data on
gene regulatory networks to look at how the hierarchy of genes and morphogens and other mechanism of tissue specification diverge and are similar among early deuterostomes and protostomes. By understanding not only what genes are present in all bilaterians but also the time and place of deployment of these genes, Andrikou and Arnone discuss a deeper understanding of the evolution of myogenesis.
In their paper Andrikou and Arnone argue that to truly understand the evolution of muscle cells the function of transcriptional regulators must be understood in the context of other external and internal interactions. Through their analysis, Andrikou and Arnone found that there were conserved
orthologues of the gene regulatory network in both invertebrate bilaterians and in cnidarians. They argue that having this common, general regulatory circuit allowed for a high degree of divergence from a single well functioning network. Andrikou and Arnone found that the orthologues of genes found in vertebrates had been changed through different types of structural mutations in the invertebrate deuterostomes and protostomes, and they argue that these structural changes in the genes allowed for a large divergence of muscle function and muscle formation in these species. Andrikou and Arnone were able to recognize not only any difference due to mutation in the genes found in vertebrates and invertebrates but also the integration of species specific genes that could also cause divergence from the original gene regulatory network function. Thus, although a common muscle patterning system has been determined, they argue that this could be due to a more ancestral gene regulatory network being coopted several times across lineages with additional genes and mutations causing very divergent development of muscles. Thus it seems that myogenic patterning framework may be an ancestral trait. However, Andrikou and Arnone explain that the basic muscle patterning structure must also be considered in combination with the
cis regulatory elements present at different times during development. In contrast with the high level of gene family apparatuses structure, Andrikou and Arnone found that the cis regulatory elements were not well conserved both in time and place in the network which could show a large degree of divergence in the formation of muscle cells. Through this analysis, it seems that the myogenic GRN is an ancestral GRN with actual changes in myogenic function and structure possibly being linked to later coopts of genes at different times and places.
Evolutionarily, specialized forms of skeletal and
cardiac muscles predated the divergence of the
vertebrate
Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...
/
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
evolutionary line.
This indicates that these types of muscle developed in a common
ancestor sometime before 700
million years ago (mya). Vertebrate smooth muscle was found to have evolved independently from the skeletal and cardiac muscle types.
Invertebrate muscle cell types
The properties used for distinguishing fast, intermediate, and slow muscle fibers can be different for invertebrate flight and jump muscle.
To further complicate this classification scheme, the mitochondria content and other morphological properties within a muscle fiber can change in a
tsetse fly with exercise and age.
See also
*
List of human cell types derived from the germ layers
References
External links
*
Structure of a Muscle Cell
{{Use dmy dates, date=November 2019
Contractile cells
Animal cells
Non-terminally differentiated (blast) cells
*