The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery; the foregut arises from the endoderm, developing from the folding primitive gut, is developmentally distinct from the midgut and hindgut. Although the term “foregut” is used in reference to the anterior section of the primitive gut, components of the adult gut can be described with this designation. Pain in the epigastric region, just below the intersection of the ribs refers to structures in the adult foregut. Esophagus Respiratory tract Stomach Duodenum Liver Gallbladder Pancreas Spleen - The spleen arises from the mesodermal dorsal mesentery, but the spleen shares the same blood supply as many of the mature structures that arise from the foregut The enteric nervous system is one of the major divisions of the nervous system derived from neural crest cells. In mammals, it is composed of large number of interconnected ganglia that are arranged into two concentric rings embedded throughout the gut wall, beginning in the esophagus and ending in the anus.
The main function of the ENS is to control the secretory activity of the gastrointestinal glands and peristalsis of the gastrointestinal wall. A large number of organs derived from the developing foregut receive input from the vagus nerve, which works in tandem with the ENS to control gastrointestinal function; the foregut develops from a cranial region of endoderm created after the initial cephalocaudal folding of the embryo. Starting at the stomodeum, a rapid expansion of the primitive gut forms the esophagus, from which the respiratory bud branches off. During early foregut development, the esophagus lengthens reaching its proportional postnatal size; the stomach begins to expand in width dorsally and ventrally in an asymmetric manner. This asymmetric expansion creates two curvatures, with the ventral side creating the lesser curvature and the dorsal side creating the greater curvature; the expanding dorsal stomach wall rotates the on its transverse plane, pulling its caudal portion upward and forcing the upper duodenum into a C shape.
This rotation positions the left vagus nerve anteriorly and right vagus nerve posteriorly. While the hindgut and midgut are only attached dorsally to the body wall by a fold of peritoneum, the foregut has a ventral attachment, its two attachments are referred to as the dorsal mesogastrium and the ventral mesogastrium. As the stomach rotates during early development, the dorsal and ventral mesentery rotate with it. After this rotation the dorsal mesentery thins and forms the greater omentum, attached to the greater curvature of the stomach; the ventral mesentery forms the lesser omentum, is attached to the developing liver. In the adult, these connective structures of omentum and mesentery form the peritoneum, act as an insulating and protective layer while supplying organs with blood and lymph vessels as well as nerves. Arterial supply to all these structures is from the celiac trunk, venous drainage is by the portal venous system. Lymph from these organs is drained to the prevertebral celiac nodes at the origin of the celiac artery from the aorta.
In vertebrates, functional differentiation continues after birth, with the transformation into the adult phenotype occurring through epithelial-mesenchymal transition. Patterning events that determine tissue differentiation in vertebrates rely on several hox genes, the morphogen sonic hedgehog, transcription factors such as sox2 and sox9. Recent research has suggested that most foregut malformations are due to defects in these signalling pathways, with sonic hedgehog gene knockout mice showing phenotypes similar to those seen in patients with esophageal atresia/stenosis, tracheo-esophageal fistula, respiratory tract anomalies. Esophageal atresia is a congenital defect of the digestive system in which the continuity of the esophageal wall is interrupted. In most cases, the upper esophagus fails to stomach. Esophageal stricture is the narrowing of the esophagus resulting in swallowing difficulties. Pyloric stenosis is the thickening of the muscle that forms the pyloric sphincter, obstructing the passage of food.
Biliary atresia is a congenital defect where the common bile duct, which connects the small intestine to the liver, is obstructed or absent. Pancreatic disease exist as acquired diseases. Two of the well known types of congenital defect are: Pancreatic divisum, where the pancreatic duct fails to form, Annular pancreas, where extra pancreatic tissue grows and wraps around the duodenum leading to obstruction by constriction. Med.nyu.edu - embryology, Bogart, B web.duke.edu - anatomy www.indiana.edu - anatomy
The falciform ligament is a ligament that attaches the liver to the anterior body wall, separates the liver into the left medial lobe and left lateral lobe. The falciform ligament, from Latin, meaning'sickle-shaped', is a broad and thin fold of peritoneum, its base being directed downward and backward and its apex upward and backward; the falciform ligament droops down from the hilum of the liver. The falciform ligament stretches obliquely from the front to the back of the abdomen, with one surface in contact with the peritoneum behind the right rectus abdominis muscle and the diaphragm, the other in contact with the left lobe of the liver; the ligament stretches from the underside of the diaphragm to the posterior surface of the sheath of the right rectus abdominis muscle, as low down as the umbilicus. It is composed of two layers of peritoneum united together, its base or free edge contains, between its layers, the paraumbilical veins. It is a remnant of the embryonic ventral mesentery; the umbilical vein of the fetus gives rise to the round ligament of liver in the adult, found in the free border of the falciform ligament.
The falciform ligament can become canalised. Due to the increase in venous congestion, blood is pushed down from the liver towards the anterior abdominal wall and if blood pools here, will result in dilatation of veins around the umbilicus. If these veins radiate out from the umbilicus, they can give the appearance of a head with hair of snakes - this is referred to as caput medusae; this article incorporates text in the public domain from page 1192 of the 20th edition of Gray's Anatomy Anatomy photo:37:04-0100 at the SUNY Downstate Medical Center - "Abdominal Cavity: The Falciform Ligament of the Liver" Anatomy photo:38:12-0205 at the SUNY Downstate Medical Center - "The Visceral Surface of the Liver" Anatomy image:8373 at the SUNY Downstate Medical Center liver at The Anatomy Lesson by Wesley Norman Cross section image: pembody/body8a—Plastination Laboratory at the Medical University of Vienna
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
The hindgut is the posterior part of the alimentary canal. In mammals, it includes the distal third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and rectum. In zoology, the term hindgut refers to the cecum and ascending colon. Arterial supply is by the Inferior mesenteric artery, venous drainage is to the portal venous system. Lymphatic drainage is to the chyle cistern; the hindgut is innervated via the inferior mesenteric plexus. Sympathetic innervation is from the Lumbar splanchnic nerves, parasympathetic innervation is from S2-S4. Hindgut fermentation digest-035—Embryo Images at University of North Carolina
Limb development in vertebrates is an area of active research in both developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb. Limb formation begins in the morphogenetic limb field, as mesenchymal cells from the lateral plate mesoderm proliferate to the point that they cause the ectoderm above to bulge out, forming a limb bud. Fibroblast growth factor induces the formation of an organizer at the end of the limb bud, called the apical ectodermal ridge, which guides further development and controls cell death. Programmed cell death is necessary to eliminate webbing between digits; the limb field is a region specified by expression of certain Hox genes, a subset of homeotic genes, T-box transcription factors – Tbx5 for forelimb or wing development, Tbx4 for leg or hindlimb development. Establishment of the forelimb field requires retinoic acid signaling in the developing trunk of the embryo from which the limb buds emerge. Although excess retinoic acid can alter limb patterning by ectopically activating Shh or Meis1/Meis2 expression, genetic studies in mouse that eliminate retinoic acid synthesis have shown that RA is not required for limb patterning.
The limb bud remains active throughout much of limb development as it stimulates the creation and positive feedback retention of two signaling regions: the AER and its subsequent creation of the zone of polarizing activity with the mesenchymal cells. In addition to the dorsal-ventral axis created by the ectodermal expression of competitive Wnt7a and BMP signals these AER and ZPA signaling centers are crucial to the proper formation of a limb, oriented with its corresponding axial polarity in the developing organism; because these signaling systems reciprocally sustain each other’s activity, limb development is autonomous after these signaling regions have been established. Limb formation begins in the morphogenetic limb field. Limb formation results from a series of epithelial-mesenchymal inductions between the mesenchymal cells of the lateral plate mesoderm and the overlying ectodermal cells. Cells from the lateral plate mesoderm and the myotome migrate to the limb field and proliferate to the point that they cause the ectoderm above to bulge out, forming the limb bud.
The lateral plate cells produce the cartilaginous and skeletal portions of the limb while the myotome cells produce the muscle components. The lateral plate mesodermal cells secrete fibroblast growth factors to induce the overlying ectoderm to form an organizer at the end of the limb bud, called the apical ectodermal ridge, which guides further development and controls cell death; the AER secretes further growth factors FGF8 and FGF4 which maintain the FGF10 signal and induce proliferation in the mesoderm. The position of FGF10 expression is regulated by two Wnt signaling pathways: Wnt8c in the hindlimb and Wnt2b in the forelimb; the forelimb and the hindlimb are specified by their position along the anterior/posterior axis and by two transcription factors: Tbx5 and Tbx4, respectively. The limb's skeletal elements are prefigured by tight aggregates known as cellular condensations of the pre-cartilage mesenchymal cells. Mesenchymal condensation is mediated by extracellular cell adhesion molecules.
In the process of chondrogenesis, chondrocytes differentiate from the condensations to form cartilage, giving rise to the skeletal primordia. In the development of most vertebrate limbs, the cartilage skeleton is replaced by bone in development; the limb is organized into three regions: stylopod and autopod. The zeugopod and the autopod contain a number of quasi-periodic pattern motifs; the zeugopod consists of two parallel elements along the anteroposterior axis and the autopod contains 3-5 elements along the same axis. The digits have a quasi-periodic arrangement along the proximodistal axis, consisting of tandem chains of skeletal elements; the generation of the basic limb plan during development results from the patterning of the mesenchyme by an interplay of factors that promote precartilage condensation and factors that inhibit it. The development of the basic limb plan is accompanied by the generation of local differences between the elements. For example, the radius and ulna of the forelimb, the tibia and fibula of the hindlimb of the zeugopod are distinct from one another, as are the different fingers or toes in the autopod.
These differences can be treated schematically by considering how they are reflected in each of the limb's three main axes. A general consensus is that the patterning of the limb skeleton involves one or more Turing-type reaction-diffusion mechanisms; the evolution of limbs from paired fins has been an area of much focus. A reverse study of limb reduction and limb loss in the development of the snake is another active area of research, it has been shown that there are conserved sequences involved in limb development retained in the genome of the snake. It is thought that these limb-enhancer sequences are conserved since there is overlap between those for limb development and those for phallus development; this aspect has been studied in the mouse where the usual limb-development signalling components are seen to play roles both in the development of the limbs and of the genital tubercle. The study of limb reduction and limb loss is unravelling the genetic pathways that control limb development.
The Turing system has enabled a number of possible outcomes in the evolutionary steps of patterning networks. The developing limb has to align itself in relation to three axes of symmetry; these are the c
Embryology is the branch of biology that studies the prenatal development of gametes and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Embryology has a long history. Aristotle proposed the accepted theory of epigenesis, that organisms develop from seed or egg in a sequence of steps; the alternative theory, that organisms develop from pre-existing miniature versions of themselves, held sway until the 18th century. Modern embryology developed from the work of von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end. In bilateral animals, the blastula develops in one of two ways that divide the whole animal kingdom into two halves. If in the blastula the first pore becomes the mouth of the animal, it is a protostome.
The protostomes include most invertebrate animals, such as insects and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula; the gastrula with its blastopore soon develops three distinct layers of cells from which all the bodily organs and tissues develop: The innermost layer, or endoderm, give rise to the digestive organs, the gills, lungs or swim bladder if present, kidneys or nephrites. The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, blood system; the outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, skin or carapace and hair, bristles, or scales. Embryos in many species appear similar to one another in early developmental stages; the reason for this similarity is. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.
Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done. Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein; these mRNA molecules are stored to be used in what will become the developing embryo. The male and female Drosophila gametes exhibit anisogamy; the female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA mentioned. At fertilization, the male and female gametes fuse and the nucleus of the male gamete fuses with the nucleus of the female gamete. Note that before the gametes' nuclei fuse, they are known as pronuclei. A series of nuclear divisions will occur without cytokinesis in the zygote to form a multi-nucleated cell known as a syncytium. All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome.
Before the nuclei can differentiate in transcriptional activity, the embryo must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed; the resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues in development. Outlined below is the process that leads to tissue differentiation. Maternal-effect genes - subject to Maternal inheritance Egg-polarity genes establish the Anteroposterior axis. Zygotic-effect genes - subject to Mendelian inheritance Segmentation genes establish 14 segments of the embryo using the anteroposterior axis as a guide. Gap genes establish 3 broad segments of the embryo. Pair-rule genes define 7 segments of the embryo within the confines of the second broad segment, defined by the gap genes. Segment-polarity genes define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves.
Homeotic genes use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type. Humans are deuterostomes. In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception, the developing human is called a fetus; as as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that becomes larger during development. Until the birth of modern embryology through observation of the mammalian ovum by von Baer in 1827, there was no clear scientific understanding of embryology. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available; the competing explanation of embryonic development was epigenesis proposed 2,000 years earlier by
The cloacal membrane is the membrane that covers the embryonic cloaca during the development of the urinary and reproductive organs. It is formed by endoderm coming into contact with each other; as the human embryo grows and caudal folding continues, the urorectal septum divides the cloaca into a ventral urogenital sinus and dorsal anorectal canal. Before the urorectal septum has an opportunity to fuse with the cloacal membrane, the membrane ruptures, exposing the urogenital sinus and dorsal anorectal canal to the exterior. On, an ectodermal plug, the anal membrane, forms to create the lower third of the rectum; this article incorporates text in the public domain from page 47 of the 20th edition of Gray's Anatomy Swiss embryology hdisqueembry/triderm04 genital-021—Embryo Images at University of North Carolina Diagram at unsw.edu.au Overview at ana.ed.ac.uk