Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints, is a structural component of the rib cage, the ear, the nose, the bronchial tubes, the intervertebral discs, many other body components. It is not as hard and rigid as bone; the matrix of cartilage is made up of glycosaminoglycans, collagen fibers and, elastin. Because of its rigidity, cartilage serves the purpose of holding tubes open in the body. Examples include the rings such as the cricoid cartilage and carina. Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance, rich in proteoglycan and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in relative amounts of collagen and proteoglycan. Cartilage does not contain blood nerves. Nutrition is supplied to the chondrocytes by diffusion.
The compression of the articular cartilage or flexion of the elastic cartilage generates fluid flow, which assists diffusion of nutrients to the chondrocytes. Compared to other connective tissues, cartilage has a slow turnover of its extracellular matrix and does not repair. In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix. Following the initial chondrification that occurs during embryogenesis, cartilage growth consists of the maturing of immature cartilage to a more mature state; the division of cells within cartilage occurs slowly, thus growth in cartilage is not based on an increase in size or mass of the cartilage itself. The articular cartilage function is dependent on the molecular composition of the extracellular matrix; the ECM consists of proteoglycan and collagens.
The main proteoglycan in cartilage is aggrecan, which, as its name suggests, forms large aggregates with hyaluronan. These aggregates hold water in the tissue; the collagen collagen type II, constrains the proteoglycans. The ECM responds to compressive forces that are experienced by the cartilage. Cartilage growth thus refers to the matrix deposition, but can refer to both the growth and remodeling of the extracellular matrix. Due to the great stress on the patellofemoral joint during resisted knee extension, the articular cartilage of the patella is among the thickest in the human body; the mechanical properties of articular cartilage in load-bearing joints such as the knee and hip have been studied extensively at macro and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive and tensile loading. Cartilage displays viscoelastic properties. Lubricin, a glycoprotein abundant in cartilage and synovial fluid, plays a major role in bio-lubrication and wear protection of cartilage.
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage is difficult to heal; because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is replaced by fibrocartilage scar tissue. Over the last years and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement. Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage. Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage; some common diseases that affect the cartilage are listed below. Osteoarthritis: Osteoarthritis is a disease of the whole joint, however one of the most affected tissues is the articular cartilage.
The cartilage covering bones is thinned completely wearing away, resulting in a "bone against bone" within the joint, leading to reduced motion, pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease, it is treated by arthroplasty, the replacement of the joint by a synthetic joint made of a stainless steel alloy and ultra high molecular weight polyethylene. Chondroitin sulfate or glucosamine sulfate supplements, have been claimed to reduce the symptoms of osteoarthritis but there is little good evidence to support this claim. Traumatic rupture or detachment: The cartilage in the knee is damaged but can be repaired through knee cartilage replacement therapy; when athletes talk of damaged "cartilage" in their knee, they are referring to a damaged meniscus and not the articular cartilage. Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
Costochondritis: Inflammation of cartilage in the ribs, causing chest pain. Spinal disc herniation: Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content; the hernia compresses the adjacent nerves and causes back pain. Relapsing polychondritis: a destruction aut
The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water released from the body into the atmosphere through transepidermal water loss; the epidermis is composed of multiple layers of flattened cells that overlie a base layer composed of columnar cells arranged perpendicularly. The rows of cells develop from stem cells in the basal layer. Cellular mechanisms for regulating water and sodium levels are found in all layers of the epidermis; the word epidermis is derived through Latin from Ancient Greek epidermis, itself from Ancient Greek epi, meaning'over, upon' and from Ancient Greek dermis, meaning'skin'. Something related to or part of the epidermis is termed epidermal; the human epidermis is a familiar example of epithelium a stratified squamous epithelium The epidermis consists of keratinocytes, which comprise 90% of its cells, but contains melanocytes, Langerhans cells, Merkel cells, inflammatory cells.
Epidermal thickenings called. Blood capillaries are found beneath the epidermis, are linked to an arteriole and a venule; the epidermis itself has no blood supply and is nourished exclusively by diffused oxygen from the surrounding air. Epidermal cells are interconnected to serve as a tight barrier against the exterior environment; the junctions between the epidermal cells are of the adherens junction type, formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells, although the actin filaments are located inside the cell and run parallel to the cell membrane; because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells. The epidermis is composed depending on the region of skin being considered; those layers in descending order are: cornified layer Composed of 10 to 30 layers of polyhedral, anucleated corneocytes, with the palms and soles having the most layers.
Corneocytes contain a protein envelope underneath the plasma membrane, are filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids. Most of the barrier functions of the epidermis localize to this layer.clear/translucent layer This narrow layer is found only on the palms and soles. The epidermis of these two areas is known as "thick skin" because with this extra layer, the skin has 5 epidermal layers instead of 4.granular layer Keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier; those polar lipids are converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids.spinous layer Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols and catabolic enzymes.
Langerhans cells, immunologically active cells, are located in the middle of this layer.basal/germinal layer. Composed of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips, they are associated with cutaneous nerves and seem to be involved in light touch sensation. The Malpighian layer is both stratum spinosum; the epidermis is separated from its underlying tissue, by a basement membrane. As a stratified squamous epithelium, the epidermis is maintained by cell division within the stratum basale. Differentiating cells delaminate from the basement membrane and are displaced outward through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are shed from the surface.
Differentiated keratinocytes secrete keratin proteins, which contribute to the formation of an extracellular matrix, an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss, taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, an additional four weeks to cross the stratum corneum; the entire epidermis is replaced by new cell growth over a period of about 48 days. Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, decreasing in the stratum corneum. Calcium concentration in the stratum corneum is low in part because those dry cells are not able to dissolve the ions; this calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers. El
Anatomical terminology is a form of scientific terminology used by anatomists and health professionals such as doctors. Anatomical terminology uses many unique terms and prefixes deriving from Ancient Greek and Latin; these terms can be confusing to those unfamiliar with them, but can be more precise, reducing ambiguity and errors. Since these anatomical terms are not used in everyday conversation, their meanings are less to change, less to be misinterpreted. To illustrate how inexact day-to-day language can be: a scar "above the wrist" could be located on the forearm two or three inches away from the hand or at the base of the hand. By using precise anatomical terminology such ambiguity is eliminated. An international standard for anatomical terminology, Terminologia Anatomica has been created. Anatomical terminology has quite regular morphology, the same prefixes and suffixes are used to add meanings to different roots; the root of a term refers to an organ or tissue. For example, the Latin names of structures such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for "two-headed", brachii as in the brachial region of the arm.
The first word describes what is being spoken about, the second describes it, the third points to location. When describing the position of anatomical structures, structures may be described according to the anatomical landmark they are near; these landmarks may include structures, such as the umbilicus or sternum, or anatomical lines, such as the midclavicular line from the centre of the clavicle. The cephalon or cephalic region refers to the head; this area is further differentiated into the cranium, frons, auris, nasus and mentum. The neck area is called cervical region. Examples of structures named according to this include the frontalis muscle, submental lymph nodes, buccal membrane and orbicularis oculi muscle. Sometimes, unique terminology is used to reduce confusion in different parts of the body. For example, different terms are used when it comes to the skull in compliance with its embryonic origin and its tilted position compared to in other animals. Here, Rostral refers to proximity to the front of the nose, is used when describing the skull.
Different terminology is used in the arms, in part to reduce ambiguity as to what the "front", "back", "inner" and "outer" surfaces are. For this reason, the terms below are used: Radial referring to the radius bone, seen laterally in the standard anatomical position. Ulnar referring to the ulna bone, medially positioned when in the standard anatomical position. Other terms are used to describe the movement and actions of the hands and feet, other structures such as the eye. International morphological terminology is used by the colleges of medicine and dentistry and other areas of the health sciences, it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research and medical care. The international morphological terminology refers to morphological sciences as a biological sciences' branch. In this field, the form and structure are examined as well as the changes or developments in the organism, it is functional.
It covers the gross anatomy and the microscopic of living beings. It involves the anatomy of the adult, it includes comparative anatomy between different species. The vocabulary is extensive and complex, requires a systematic presentation. Within the international field, a group of experts reviews and discusses the morphological terms of the structures of the human body, forming today's Terminology Committee from the International Federation of Associations of Anatomists, it deals with the anatomical and embryologic terminology. In the Latin American field, there are meetings called Iberian Latin American Symposium Terminology, where a group of experts of the Pan American Association of Anatomy that speak Spanish and Portuguese and studies the international morphological terminology; the current international standard for human anatomical terminology is based on the Terminologia Anatomica. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998.
It supersedes Nomina Anatomica. Terminologia Anatomica contains terminology for about 7500 human gross anatomical structures. For microanatomy, known as histology, a similar standard exists in Terminologia Histologica, for embryology, the study of development, a standard exists in Terminologia Embryologica; these standards specify accepted names that can be used to refer to histological and embryological structures in journal articles and other areas. As of September 2016, two sections of the Terminologia Anatomica, including central nervous system and peripheral nervous system, were merged to form the Terminologia Neuroanatomica; the Terminologia Anatomica has been perceived with a considerable criticism regarding its content including coverage and spelling mistakes and errors. Anatomical terminology is chosen to highlight the relative location of body structures. For instance, an anatomist might describe one band of tissue as "inferior to" another or a physician might describe a tumor as "superficial to" a deeper body structure.
Anatomical terms used to describe location
Estonia the Republic of Estonia, is a country in North East Europe. It is bordered to the north by the Gulf of Finland with Finland on the other side, to the west by the Baltic Sea with Sweden on the other side, to the south by Latvia, to the east by Lake Peipus and Russia; the territory of Estonia consists of a mainland and 2,222 islands in the Baltic Sea, covering a total area of 45,227 km2, water 2,839 km2, land area 42,388 km2, is influenced by a humid continental climate. The official language of the country, Estonian, is the third most spoken Finno-Ugric language; the territory of Estonia has been inhabited since at least 9,000 B. C. Ancient Estonians were some of the last European pagans to be Christianized, following the Livonian Crusade in the 13th century. After centuries of successive rule by Germans, Swedes and Russians, a distinct Estonian national identity began to emerge in the 19th and early 20th centuries; this culminated in independence from Russia in 1920 after a brief War of Independence at the end of World War I.
Democratic, after the Great Depression Estonia was governed by authoritarian rule since 1934 during the Era of Silence. During World War II, Estonia was contested and occupied by the Soviet Union and Germany being incorporated into the former as the Estonian Soviet Socialist Republic. After the loss of its de facto independence, Estonia's de jure state continuity was preserved by diplomatic representatives and the government-in-exile. In 1987 the peaceful Singing Revolution began against Soviet rule, resulting in the restoration of de facto independence on 20 August 1991; the sovereign state of Estonia is a democratic unitary parliamentary republic divided into fifteen counties. Its capital and largest city is Tallinn. With a population of 1.3 million, it is one of the least-populous member states of the European Union since joining in 2004, the economic monetary Eurozone, Organisation for Economic Co-operation and Development, Schengen Area, of the Western military alliance of the North Atlantic Treaty Organization.
It is a developed country with an advanced, high-income economy, among the fastest-growing in the EU. Estonia ranks high in the Human Development Index, performs favourably in measurements of economic freedom, civil liberties and press freedom. Estonian citizens are provided with universal health care, free education, the longest-paid maternity leave in the OECD. One of the world's most digitally advanced societies, in 2005 Estonia became the first state to hold elections over the Internet, in 2014 the first state to provide e-residency. In the Estonian language the oldest known endonym of the Estonians was maarahvas, meaning "country people" or "people of the soil"; the land inhabited by Estonians was called Maavald meaning "Country Realm" or "Land Realm". One hypothesis regarding the modern name of Estonia derives it from the Aesti, a people described by the Roman historian Tacitus in his Germania; the historic Aesti were Baltic people, whereas the modern Estonians are Finno-Ugric. The geographical areas of the Aesti and of Estonia do not match, with the Aesti living farther south.
Ancient Scandinavian sagas refer to an area called Eistland, as the country is still called in Icelandic, with close parallels to the Danish, Dutch and Norwegian terms Estland for the country. Early Latin and other ancient versions of the name include Hestia. Esthonia was a common alternative English spelling before 1921. Human settlement in Estonia became possible 13,000 to 11,000 years ago, when the ice from the last glacial era melted; the oldest known settlement in Estonia is the Pulli settlement, on the banks of the river Pärnu, near the town of Sindi, in south-western Estonia. According to radiocarbon dating it was settled around 11,000 years ago; the earliest human inhabitation during the Mesolithic period is connected to the Kunda culture, named after the town of Kunda in northern Estonia. At that time the country was covered with forests, people lived in semi-nomadic communities near bodies of water. Subsistence activities consisted of hunting and fishing. Around 4900 BC appear ceramics of the neolithic period, known as Narva culture.
Starting from around 3200 BC the Corded Ware culture appeared. The Bronze Age started around 1800 BC, saw the establishment of the first hill fort settlements. A transition from hunting-fishing-gathering subsistence to single-farm-based settlement started around 1000 BC, was complete by the beginning of the Iron Age around 500 BC; the large amount of bronze objects indicate the existence of active communication with Scandinavian and Germanic tribes. A more troubled and war-ridden middle Iron Age followed, with external threats appearing from different directions. Several Scandinavian sagas referred to major confrontations with Estonians, notably when Estonians defeated and killed the Swedish king Ingvar. Similar threats appeared in the east. In 1030 Yaroslav the Wise established a fort in modern-day Tartu. Around the 11th century, the Scandinavian Viking era around the Baltic Sea was succeeded by the Baltic Viking era, with seaborne
The surface ectoderm forms the following structures: Skin Epithelium of the mouth and nasal cavity and glands of the mouth and nasal cavity Tooth enamel (as a side note and dental pulp are formed from ectomesenchyme, derived from ectoderm Epithelium of anterior pituitary Lens, lacrimal gland, tarsal glands and the conjunctiva of the eye Apical ectodermal ridge inducing development of the limb buds of the embryo. Sensory receptors in epidermis List of human cell types derived from the germ layers This article incorporates text in the public domain from the 20th edition of Gray's Anatomy https://web.archive.org/web/20071213145329/http://cwx.prenhall.com/bookbind/pubbooks/martini10/chapter18/custom3/deluxe-content.html Thomas, Jane Coad with Melvyn Dunstall. Anatomy and physiology for midwives. Edinburgh. ISBN 0723429790
The Baltic Germans are ethnic German inhabitants of the eastern shores of the Baltic Sea, in what today are Estonia and Latvia. Since their expulsion from Estonia and Latvia and resettlement during the upheavals and aftermath of the Second World War, Baltic Germans have markedly declined as a geographically determined ethnic group; the largest groups of present-day descendants of the Baltic Germans are found in Canada. It is estimated that several thousand still reside in Estonia. For centuries Baltic Germans and the Baltic nobility constituted a ruling class over native non-German serfs; the emerging Baltic-German middle class was urban and professional. In the 12th and 13th centuries Catholic Germans, both traders and crusaders, began settling in the eastern Baltic territories. After the Livonian Crusade, they assumed control of government, economics and culture of these lands, ruling for more than 700 years until 1918 — in alliance with Polish, Swedish or Russian overlords. With the decline of Latin, German became the language of all official documents, commerce and government.
At first the majority of German settlers lived in military castles. Their elite formed the Baltic nobility, acquiring large rural estates and comprising the social, commercial and cultural elite of Latvia and Estonia for several centuries. After 1710 many of these men took high positions in the military and civilian life of the Russian Empire in Saint Petersburg. Baltic Germans held citizenship in the Russian Empire until the Revolution of 1918, they held Estonian or Latvian citizenship until the occupation and annexation of these areas by the Soviet Union in 1939–1940. The Baltic German population never surpassed more than 10% of the total population. In 1881 there were 180,000 Baltic Germans in Russia's Baltic provinces, but by 1914 this number had declined to 162,000. In 1881 there were 46,700 Germans in Estonia. According to the Russian Empire Census of 1897, there were 120,191 Germans in Latvia, or 6.2% of the population. Baltic German history and presence in the Baltics came to an end in late 1939, following the signing of the Molotov–Ribbentrop Pact and the subsequent Nazi–Soviet population transfers.
All the Baltic Germans were resettled by Nazi Germany under the Heim ins Reich program into the newly formed Reichsgaue of Wartheland and Danzig-West Prussia. In 1945, most ethnic Germans were expelled from these lands by the Soviet army. Resettlement was planned for the territory remaining to Germany under terms of the border changes promulgated at the Potsdam Conference, i.e. west of the Oder–Neisse line, or elsewhere in the world. Ethnic Germans from East Prussia and Lithuania are sometimes incorrectly considered Baltic Germans for reasons of cultural and historical affinities, but the Germans of East Prussia held Prussian, after 1871, German citizenship, because the territory they lived in was part of the Kingdom of Prussia. Baltic Germans were not a purely German ethnic group; the early crusaders and craftsmen married local women, as there were no German women available. Some noble families, such as the Lievens, claimed descent through such women from native chieftains. Many of the German Livonian Order soldiers died during the Livonian War.
New German arrivals came to the area. During this time the Low German of the original settlers was replaced by the High German of the new settlers. In the course of their 700-year history, Baltic German families had ethnic German roots, but had extensive intermarriage with Estonians and Latvians, as well as with other Northern or Central European people, such as Danes, Irish, Scots, Poles and Dutch. In cases where intermarriage occurred, members of the other ethnic groups assimilated into German culture, adopting language and German family names, they were considered Germans, leading to the ethnogenesis of the Baltic Germans. Barclay de Tolly and George Armitstead, who emigrated from the British Isles, married into and became part of the Baltic-German community. Baltic German settlements in the Baltic area consisted of the following territories: Estland the northern half of present-day Estonia. Livland the southern half of present-day Estonia and the northern and eastern part of today's Latvia.
Kurland the western half of present-day Latvia. Ösel belonging to present-day Estonia. Small numbers of Ethnic Germans began to settle in the area in the late 12th century when traders and Christian missionaries began to visit the coastal lands inhabited by tribes who spoke Finnic and Baltic languages. Systematic conquest and settlement of these lands was completed during the Northern Crusades of the 12th and 13th centuries which resulted in creation of the Terra Mariana confederation, under the protection of Roman Popes and Holy Roman Empire. After the heavy defeat in the 1236 Battle of Saule the Livonian Brothers of the Sword became a part of
Embryonic development embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe stages. Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to a single diploid cell; the zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo. Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals; the egg cell is asymmetric, having an "animal pole" and a "vegetal pole". It is covered with different layers; the first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.
Fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation; the fertilized egg cell is known as the zygote. To prevent more than one sperm fertilizing the egg, fast block and slow block to polyspermy are used. Fast block, the membrane potential depolarizing and returning to normal, happens after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Cell division with no significant growth, producing a cluster of cells, the same size as the original zygote, is called cleavage.
At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta or milk, such as might be secreted from a marsupium. On the other hand, meroblastic cleavage occurs in animals; because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms: The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription.
In amniotes, the cells of the morula are at first aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, an inner cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, thus the morula is converted into a vesicle, called the blastodermic vesicle; the inner cell mass remains in contact, with the trophoblast at one pole of the ovum. After the 7th cleavage has produced 128 cells, the embryo is called a blastula; the blastula is a spherical layer of cells surrounding a fluid-filled or yolk-filled cavity Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass, distinct from the surrounding blastula. The blastocyst must not be confused with the blastula. In the mouse, primordial germ cells arise from a layer of cells in the inner cell mass of the blastocyst as a result of extensive genome-wide reprogramming.
Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, results in cellular totipotency. Before gastrulation, the cells of the trophoblast become differentiated into two strata: The outer stratum forms a syncytium, termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast or "Layer of Langhans", consists of well-defined cells; as stated, the cells of the trophoblast do not contribute to the formation of the embryo proper. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and assumes the form of a small sac, called the yolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic c