Myogenesis is the formation of muscular tissue during embryonic development. Muscle fibers form the fusion of myoblasts into multi-nucleated fibers called myotubes. In the early development of an embryo, myoblasts can either proliferate, or differentiate into a myotube. What controls this choice in vivo is unclear. If placed in cell culture, most myoblasts will proliferate if enough fibroblast growth factor or another growth factor is present in the medium surrounding the cells; when the growth factor runs out, the myoblasts cease division and undergo terminal differentiation into myotubes. Myoblast differentiation proceeds in stages; the first stage, involves the commencement of expression of certain genes. The second stage of differentiation involves the alignment of the myoblasts with one another. Studies have shown that rat and chick myoblasts can recognise and align with one another, suggesting evolutionary conservation of the mechanisms involved; the third stage is the actual cell fusion itself.
In this stage, the presence of calcium ions is critical. In mice, fusion is aided by a set of metalloproteinases called meltrins and a variety of other proteins still under investigation. Fusion involves recruitment of actin to the plasma membrane, followed by close apposition and creation of a pore that subsequently widens. Novel genes and their protein products that are expressed during the process are under active investigation in many laboratories, they include: Myocyte enhancer factors. Serum response factor plays a central role during myogenesis, being required for the expression of striated alpha-actin genes. Expression of skeletal alpha-actin is regulated by the androgen receptor. Myogenic regulatory factors: MyoD, Myf5, Myf6 and Myogenin. There myogenesis; each stage has various associated genetic factors lack of. Associated Genetic Factors: PAX3 and c-Met Mutations in PAX3 can cause a failure in c-Met expression; such a mutation would result in a lack of lateral migration. PAX3 mediates the transcription of c-Met and is responsible for the activation of MyoD expression—one of the functions of MyoD is to promote the regenerative ability of satellite cells.
PAX3 is expressed at its highest levels during embryonic development and is expressed at a lesser degree during the fetal stages. Mutations in Pax3 can cause a variety of complications including Waardenburg syndrome I and III as well as craniofacial-deafness-hand syndrome. Waardenburg syndrome is most associated with congenital disorders involving the intestinal tract and spine, an elevation of the scapula, among other symptoms; each stage has various associated genetic factors without. Associated Genetic Factors: c-Met/HGF and LBX1 Mutations in these genetic factors causes a lack of migration. LBX1 is responsible for the development and organization of muscles in the dorsal forelimb as well as the movement of dorsal muscles into the limb following delamination. Without LBX1, limb muscles will fail to form properly. A lack of c-Met disrupts secondary myogenesis and—as in LBX1—prevents the formation of limb musculature, it is clear that c-Met plays an important role in delamination and proliferation in addition to migration.
PAX3 is needed for the transcription of c-Met. Associated Genetic Factors: PAX3, c-Met, Mox2, MSX1, Myf5, MyoD Mox2 plays an important role in the induction of mesoderm and regional specification. Impairing the function of Mox2 will prevent the proliferation of myogenic precursors and will cause abnormal patterning of limb muscles. Studies have shown that hindlimbs are reduced in size while specific forelimb muscles will fail to form. Myf5 is required for proper myoblast proliferation. Studies have shown that mice muscle development in the intercostal and paraspinal regions can be delayed by inactivating Myf-5. Myf5 is considered to be the earliest expressed regulatory factor gene in myogenesis. If Myf-5 and MyoD are both inactivated, there will be a complete absence of skeletal muscle; these consequences further reveal the complexity of myogenesis and the importance of each genetic factor in proper muscle development. Associated Genetic Factors: Myf5 and MyoD One of the most important stages in myogenesis determination requires both Myf5 and MyoD to function properly in order for myogenic cells to progress normally.
Mutations in either associated genetic factor will cause the cells to adopt non-muscular phenotypes. As stated earlier, the combination of Myf5 and MyoD is crucial to the success of myogenesis. Both MyoD and Myf5 are members of the myogenic bHLH proteins transcription factor family. Cells that make myogenic bHLH transcription factors are committed to development as a muscle cell; the simultaneous deletion of Myf5 and MyoD results in a complete lack of skeletal muscle formation. Research has shown. Meanwhile, Myf5 expression is regulated by Sonic hedgehog, Wnt1, MyoD itsel
Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. It is a thin layer of single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells. Vascular endothelial cells line the entire circulatory system, from the heart to the smallest capillaries; these cells have unique functions in vascular biology. These functions include fluid filtration, such as in the glomerulus of the kidney, blood vessel tone, neutrophil recruitment, hormone trafficking. Endothelium of the interior surfaces of the heart chambers is called endocardium. Endothelium is mesodermal in origin. Both blood and lymphatic capillaries are composed of a single layer of endothelial cells called a monolayer. In straight sections of a blood vessel, vascular endothelial cells align and elongate in the direction of fluid flow.
The foundational model of anatomy makes a distinction between endothelial cells and epithelial cells on the basis of which tissues they develop from, states that the presence of vimentin rather than keratin filaments separate these from epithelial cells. Many considered the endothelium a specialized epithelial tissue. Endothelial cells are involved in many aspects of vascular biology, including: Barrier function - the endothelium acts as a semi-selective barrier between the vessel lumen and surrounding tissue, controlling the passage of materials and the transit of white blood cells into and out of the bloodstream. Excessive or prolonged increases in permeability of the endothelial monolayer, as in cases of chronic inflammation, may lead to tissue edema/swelling. Blood clotting; the endothelium provides a non-thrombogenic surface because it contains, for example, heparan sulfate which acts as a cofactor for activating antithrombin, a protease that inactivates several factors in the coagulation cascade.
Inflammation Formation of new blood vessels Vasoconstriction and vasodilation, hence the control of blood pressure Repair of damaged or diseased organs via an injection of blood vessel cells Angiopoietin-2 works with VEGF to facilitate cell proliferation and migration of endothelial cells Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular diseases, is regarded as a key early event in the development of atherosclerosis. Impaired endothelial function, causing hypertension and thrombosis, is seen in patients with coronary artery disease, diabetes mellitus, hypercholesterolemia, as well as in smokers. Endothelial dysfunction has been shown to be predictive of future adverse cardiovascular events, is present in inflammatory disease such as rheumatoid arthritis and systemic lupus erythematosus. One of the main mechanisms of endothelial dysfunction is the diminishing of nitric oxide due to high levels of asymmetric dimethylarginine, which interfere with the normal L-arginine-stimulated nitric oxide synthesis and so leads to hypertension.
The most prevailing mechanism of endothelial dysfunction is an increase in reactive oxygen species, which can impair nitric oxide production and activity via several mechanisms. The signalling protein ERK5 is essential for maintaining normal endothelial cell function. A further consequence of damage to the endothelium is the release of pathological quantities of von Willebrand factor, which promote platelet aggregation and adhesion to the subendothelium, thus the formation of fatal thrombi. Anatomy photo: Circulatory/vessels/capillaries1/capillaries3 - Comparative Organology at University of California, Davis, "Capillaries, non-fenestrated" Histology image: 21402ooa – Histology Learning System at Boston University Endothelium Journal of Endothelial Cell Research, Informa Healthcare Endothelium and inflammation Platelet Activation, University of Washington
The bilateria, bilaterians, or triploblasts, are animals with bilateral symmetry, i.e. they have a head and a tail as well as a back and a belly. The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm and ectoderm. Nearly all are bilaterally symmetrical, or so. Except for a few phyla, bilaterians anus; some bilaterians lack body cavities, while others display primary body cavities or secondary cavities. Some of the earliest bilaterians were wormlike, a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have a head end and a tail end as well as a back and a belly. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.
The body stretches back from the head, many bilaterians have a combination of circular muscles that constrict the body, making it longer, an opposing set of longitudinal muscles, that shorten the body. They have a gut that extends through the cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; the hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups, while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla have lost body cavities secondarily; the first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago. Earlier fossils are controversial.
Fossil embryos are known from around the time of Vernanimalcula, but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, are believed to be at least 585 million years old. There are superphyla, of Bilateria; the deuterostomes include the echinoderms, chordates, a few smaller phyla. The protostomes include most of the rest, such as arthropods, mollusks, so forth. There are a number of differences, most notably in. In particular, the first opening of the embryo becomes the mouth in protostomes, the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes and Spiralia; the arrow worms have proven difficult to classify. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial and the tree has changed between 2000 and 2010, it is indicated when clades radiated into newer clades in millions of years ago.
The original bilaterian is hypothesized to have been a bottom dwelling worm with a single body opening. It may have resembled the planula larvae of some cnidaria. Embryological origins of the mouth and anus Tree of Life web project — Bilateria University of California Museum of Paleontology — Systematics of the Metazoa
Cell signaling is part of any communication process that governs basic activities of cells and coordinates all cell actions. The ability of cells to perceive and respond to their microenvironment is the basis of development, tissue repair, immunity, as well as normal tissue homeostasis. Errors in signaling interactions and cellular information processing are responsible for diseases such as cancer and diabetes. By understanding cell signaling, diseases may be treated more and, artificial tissues may be created. Systems biology studies the underlying structure of cell signaling networks and how changes in these networks may affect the transmission and flow of information; such networks are complex systems in their organization and may exhibit a number of emergent properties including bistability and ultrasensitivity. Analysis of cell signaling networks requires a combination of experimental and theoretical approaches including the development and analysis of simulations and modeling. Long-range allostery is a significant component of cell signaling events.
Cell signaling has been most extensively studied in the context of human diseases and signaling between cells of a single organism. However, cell signaling may occur between the cells of two different organisms. In many mammals, early embryo cells exchange signals with cells of the uterus. In the human gastrointestinal tract, bacteria exchange signals with each other and with human epithelial and immune system cells. For the yeast Saccharomyces cerevisiae during mating, some cells send a peptide signal into their environment; the mating factor peptide may bind to a cell surface receptor on other yeast cells and induce them to prepare for mating. Cell signaling can be classified as either mechanical or biochemical based on the type of the signal. Mechanical signals are the forces exerted on the forces produced by the cell; these forces can both be responded to by the cells. Biochemical signals are the biochemical molecules such as proteins, lipids and gases; these signals can be categorized based on the distance between responder cells.
Signaling within and amongst cells is subdivided into the following classifications: Intracrine signals are produced by the target cell that stay within the target cell. Autocrine signals are produced by the target cell, are secreted, affect the target cell itself via receptors. Sometimes autocrine cells can target cells close by if they are the same type of cell as the emitting cell. An example of this are immune cells. Juxtacrine signals target adjacent cells; these signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells adjacent. Paracrine signals target cells in the vicinity of the emitting cell. Neurotransmitters represent an example. Endocrine signals target distant cells. Endocrine cells produce hormones. Cells communicate with each other via direct contact, over short distances, or over large distances and/or scales; some cell–cell communication requires direct cell–cell contact. Some cells can form gap junctions.
In cardiac muscle, gap junctions between adjacent cells allows for action potential propagation from the cardiac pacemaker region of the heart to spread and coordinate contraction of the heart. The notch signaling mechanism is an example of juxtacrine signaling in which two adjacent cells must make physical contact in order to communicate; this requirement for direct contact allows for precise control of cell differentiation during embryonic development. In the worm Caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide; the choice of which cell continues to divide is controlled by competition of cell surface signals. One cell will happen to produce more of a cell surface protein that activates the Notch receptor on the adjacent cell; this activates a feedback loop or system that reduces Notch expression in the cell that will differentiate and that increases Notch on the surface of the cell that continues as a stem cell.
Many cell signals are carried by molecules that are released by one cell and move to make contact with another cell. Endocrine signals are called hormones. Hormones are produced by endocrine cells and they travel through the blood to reach all parts of the body. Specificity of signaling can be controlled. Paracrine signals such as retinoic acid target only cells in the vicinity of the emitting cell. Neurotransmitters represent another example of a paracrine signal; some signaling molecules can function as a neurotransmitter. For example and norepinephrine can function as hormones when released from the adrenal gland and are transported to the heart by way of the blood stream. Norepinephrine can be produced by neurons to function as a neurotransmitter within the brain. Estrogen can be released by the ovary and function as a hormone or act locally via paracrine or autocrine signaling. Active species of oxygen and nitric oxide can act as cellular messengers; this process is dubbed redox signaling. In a multicellular organism, signaling between cells occurs either through release into the extracellular space, divided in paracrine signaling and endocrine signaling (ove
In the developing chordate, the neural tube is the embryonic precursor to the central nervous system, made up of the brain and spinal cord. The neural groove deepens as the neural folds become elevated, the folds meet and coalesce in the middle line and convert the groove into the closed neural tube. In humans, neural tube closure occurs by the fourth week of pregnancy; the ectodermal wall of the tube forms the rudiment of the nervous system. The centre of the tube is the neural canal; the neural tube develops in two ways: secondary neurulation. Primary neurulation divides the ectoderm into three cell types: The internally located neural tube The externally located epidermis The neural crest cells, which develop in the region between the neural tube and epidermis but migrate to new locationsPrimary neurulation begins after the neural plate forms; the edges of the neural plate start forming the neural folds. The center of the neural plate remains allowing a U-shaped neural groove to form; this neural groove left sides of the embryo.
The neural folds pinch in towards the midline of the embryo and fuse together to form the neural tube. In secondary neurulation, the cells of the neural plate form a cord-like structure that migrates inside the embryo and hollows to form the tube; each organism uses secondary neurulation to varying degrees. Neurulation in fish proceeds only via the secondary form. In avian species the posterior regions of the tube develop using secondary neurulation and the anterior regions develop by primary neurulation. In mammals, secondary neurulation begins around the 35th somite. Mammalian neural tubes close in the head in the opposite order. In the head:Neural crest cells migrate Neural tube closes Overlying ectoderm closesIn the trunk:Overlying ectoderm closes Neural tube closes Neural crest cells migrate Four neural tube subdivisions each develop into distinct regions of the central nervous system by the division of neuroepithelial cells: the forebrain, the midbrain, the hindbrain and the spinal cord.
The prosencephalon further goes on to develop into the diencephalon. The mesencephalon stays as the midbrain; the rhombencephalon develops into the myelencephalon. For a short time, the neural tube is open both caudally; these openings, called neuropores, close during the fourth week in humans. Improper closure of the neuropores can result in neural tube defects such as anencephaly or spina bifida; the dorsal part of the neural tube contains the alar plate, associated with sensation. The ventral part of the neural tube contains the basal plate, associated with motor control; the neural tube patterns along the dorsal-ventral axis to establish defined compartments of neural progenitor cells that lead to distinct classes of neurons. According to the French flag model of morphogenesis, this patterning occurs early in development and results from the activity of several secreted signaling molecules. Sonic hedgehog is a key player in patterning the ventral axis, while bone morphogenic proteins and Wnt family members play an important role in patterning the dorsal axis.
Other factors shown to provide positional information to the neural progenitor cells include fibroblast growth factors and retinoic acid. Retinoic acid is required ventrally along with Shh to induce Pax6 and Olig2 during differentiation of motor neurons. Three main ventral cell types are established during early neural tube development: the floor plate cells, which form at the ventral midline during the neural fold stage; these cell types are specified by the secretion of the Shh from the notochord, from the floor plate cells. Shh acts as a morphogen, meaning that it acts in a concentration-dependent manner to specify cell types as it moves further from its source; the following is a proposed mechanism for how Shh patterns the ventral neural tube: A gradient of Shh that controls the expression of a group of homeodomain and basic Helix-Loop-Helix transcription factors is created. These transcription factors are grouped into two protein classes based on. Class I is inhibited by Shh; these two classes of proteins cross-regulate each other to create more-defined boundaries of expression.
The different combinations of expression of these transcription factors along the dorsal-ventral axis of the neural tube are responsible for creating the identity of the neuronal progenitor cells. Five molecularly distinct groups of ventral neurons form from these neuronal progenitor cells in vitro; the position at which these neuronal groups are generated in vivo can be predicted by the concentration of Shh required for their induction in vitro. Studies have shown that neural progenitors can evoke different responses based on the length of exposure to Shh, with a longer exposure time resulting in more ventral cell types. At the dorsal end of the neural tube, BMPs are responsible for neuronal patterning. BMP is secreted from the overlying ectoderm. A secondary signaling center is established in the roof plate, the dorsal most structure of the neural tube. BMP from the dorsal end of the neural tube seems to act in the same concentration-dependent manner as Shh in the ventral end; this was shown using zeb
Histogenesis is the formation of different tissues from undifferentiated cells. These cells are constituents of three primary germ layers, the endoderm and ectoderm; the science of the microscopic structures of the tissues formed within histogenesis is termed histology. A germ layer is a collection of cells, formed during animal and mammalian embryogenesis. Germ layers are pronounced within vertebrate organisms. Animals with radial symmetry, such as cnidarians, produce two layers, called the ectoderm and endoderm. Therefore, they are diploblastic. Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic. Germ layers will give rise to all of an animal’s or mammal's tissues and organs through a process called organogenesis; the endoderm is one of the germ to penes layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm; the endoderm consists of flattened cells, which subsequently become columnar.
The mesoderm germ layer forms in the embryos of animals and mammals more complex than cnidarians, making them triploblastic. During gastrulation, some of the cells migrating inward to form the endoderm form an additional layer between the endoderm and the ectoderm. A theory suggests that this key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals; the formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom can move and develop independently of the body wall while fluid cushions and protects them from shocks; the ectoderm is the start of a tissue. It forms from the outermost of the germ layers; the proceeding graph represents the products produced by the three germ layers. HistologyList of human cell types derived from the germ layers Derivatives of the Ectoderm Derivatives of the Endoderm and Mesoderm
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell; the zygote possesses half the DNA from each of its two parents. In plants and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism; the result of this process is an embryo. In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. First attested in English in the mid-14c; the word embryon itself from Greek ἔμβρυον, lit. "young one", the neuter of ἔμβρυος, lit. "growing in", from ἐν, "in" and βρύω, "swell, be full". In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula and organogenesis; the blastula stage features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells called blastomeres.
In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are referred to as embryos throughout development. During gastrulation the cells of the blastula undergo coordinated processes of cell division, and/or migration to form two or three tissue layers. In triploblastic organisms, the three germ layers are called endoderm and mesoderm; the position and arrangement of the germ layers are species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", is thought to have been an important novelty in the evolution of head structures. During organogenesis and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.
For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, myogenesis and many others. In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves and root, as well as one or more cotyledons. Once the embryo begins to germinate—grow out from the seed—it is called a seedling. Bryophytes and ferns produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated; the inner wall of the archegonium lies in close contact with the "foot" of the developing embryo. The structure and development of the rest of the embryo varies by group of plants; as the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.
Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for use; the potential of embryonic stem cell research, reproductive cloning, germline engineering are being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for conditions. Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material; the embryos of Arabidopsis thaliana have been used as a model to understand gene activation and organogenesis of seed plants. In regards to research using human embryos, the ethics and legalities of this application continue to be debated. Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells; this method will help scientists to more study the first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases.
Fossilized animal embryos are known from the Precambrian, are found in great numbers during the Cambrian period. Fossilized dinosaur embryos have been discovered; some embryos do not survive to the next stage of development. When this happens it is called spontaneous abortion or miscarriage. There are many reasons; the most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants. In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources; this can disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedl