SEM_03_Cleavage and cell differentiation_PARTE2.docx

Cytoplasmic determinants Cytoplasmic determinants are the substances in the maternal gamete that affect the course of early development by regulating gene expression which influences the fate of the cells. Cytoplasmic determinants are responsible for maternal control in the early stages of developm...

Cytoplasmic determinants Cytoplasmic determinants are the substances in the maternal gamete that affect the course of early development by regulating gene expression which influences the fate of the cells. Cytoplasmic determinants are responsible for maternal control in the early stages of development. These molecules of mRNA, proteins, and other substances and organelles can be distributed uniformly or unevenly across the egg. During segmentation, depending on the species, cytoplasmic determinants can be distributed among the daughter cells at different times and different proportion. Cytoplasmic determinants and type of development: Two faces of one coin If these cytoplasmic determinants become unevenly distributed into the daughter cells from the first divisions, this gives rise to what is called "mosaic development". The future embryo contains distinct cytoplasmic determinants distributed in distinct cells. As a result, in the case of the mosaic development, cell totipotence, if it exists, disappears very quickly during segmentation. Indeed, each new cell is responsible for the formation of a concrete region of the future organism. Thereby the cells of the early embryo can be considered as indispensable pieces of a mosaic. If they were separated from the embryo, they would end up as a specific part of the embryo and never as a complete embryo. Other animals show "regulative development". Their cytoplasmic determinants are equally distributed in the newly created cells. Therefore, the daughter cells resulting from the first divisions are totipotent: they can, independently, lead to a complete individual. Mammals are an example of animals that undergo regulative development. Cells in the early stages of development can be separated from an embryo or fused to another embryo. They are able to readjust their development and end up as normal and complete embryos. Mosaic development and regulative development are behind the paradoxical results found in the experiments carried out by Driesch (regulative development) and Roux (mosaic development). https://sway.office.com/D0aWfKx0uDWJmKGC#content=4yKSeSbFQJDKcz - The role of the cytoplasmatic determinants in the control of the embryo development Cytoplasmatic determinants are molecules that are localized in specific cytoplasmic regions of the unfertilized egg or zygote and affect cell fate decisions by segregating into different embryonic cells and controlling distinct gene activities in these cells. In the egg, such determinants are usually maternal mRNAs and proteins. Cytoplasmic determinants are also found in some post-embryonic cells, where they produce cytoplasmic asymmetry. In dividing cells, this leads to asymmetric cell division in which each of the daughter cells differentiates into a different cell type. Also called localized cytoplasmic determinants or morphogenetic determinants. Induction The phenomenon whereby one embryonic region interacts at a close range and stimulates the behaviour of the second embryonic region is called induction. Induction is the main cause of cellular differentiation and thereby for tissular and organs development. There are at least two components to every inductive interaction. The first component is the inducer: the inducing tissue that produces a signal (usually a specific protein) that changes the cellular behaviour of the responder: responding tissue. Nevertheless, for induction to be successful, the responding tissue must be competent. Competence is the ability to respond to an inductive signal. Target cells are only receptive to induction signals when they express suitable receptors. Target cells can express different receptors over time, so depending on the moment, the same induction signal may drive them to form different cell types. An inductor is a cellular signalling centre. Numerous fundamental inductions are mediated by paracrine signalling factors. These factors can be broadly grouped into four main families: the fibroblast growth factor (Fgf) family, the Wingless (Wnt) family, transforming growth factor (Tgf) family and the hedgehog (Hh) family. https://sway.office.com/D0aWfKx0uDWJmKGC#content=TsNkni0PXVRDt1 - Induction In induction, a substance secreted by one group of cells alters the development of another group. In early development, induction is usually instructive; that is, the tissue assumes a different state of commitment in the presence of the signal than it would in the absence of the signal. Target cells are only receptive to induction signals when they express suitable receptors. They can express different receptors over time, so depending on the moment, the same induction signal may drive them to form different cell types. Responding tissues can often become inducing tissues giving rise to a cascade of embryonic induction. Often, tissular interactions that give rise to the development of a particular organ, are the last steps in a cascade of inductions that began much earlier in embryogenesis. The first step of these cascades of inductions was discovered in amphibian embryos at the gastrula stage; the dorsal blastopore lip (a fundamental structure in amphibian gastrulation), was identified as an organiser centre which is responsible for the formation the first structures of the body: the notochord and neural tube. Since this initial discovery, the dorsal blastopore lip is called the Spemann´s organiser, or just "the organiser". Comparable organising centres have been found in other vertebrate groups. In birds and mammals, it is comparable to the primitive streak and primitive node. https://sway.office.com/D0aWfKx0uDWJmKGC#content=eSO9UYqJfw2UhW - The organizer Induction centres comprise groups of cells with the ability to instruct adjacent cells into specific states, represent a key principle in developmental biology. The concept of the organizer was first introduced by Spemann and Mangold, who showed that there is a cellular population in the newt embryo that elicits the development of a secondary embryo when it was transplanted to adjacent cells. Similar experiments in chicken and mammal embryos subsequently revealed groups of cells with similar instructive potential. In birds and mammals, organizer activity is often associated with a structure known as the primitive node, which has thus been considered a functional homologue of Spemann's organizer. Morphogenesis Morphogenesis is the development of the shape; it is the process by which differentiated cells are organised into functional groups of organs with a tridimensional design. Morphogenesis is achieved by complex interactions of actively differentiating cells. During development, a process called pattern formation drives the spatial organisation of tissues and organs into a defined body plan or final shape. For example, both dogs and humans have legs made up of bone, muscle, and skin. During development, differentiation produces muscle cells, and bone cells from an unspecialised set of embryo cells. Then the bone cells organise themselves into bone tissue and muscle cells develop into muscle tissue. However, morphogenesis is the process of pattern formation that organises those bones and muscles into the specific spatial organisation that makes a dog look like a dog and a human look like a human. The role of positional cues in pattern formation: During pattern formation, it is crucial for cells of the developing embryo to communicate with one another so that each cell "knows" its relative position within the emerging body plan. The intercellular molecular signals that ultimately drive the process of pattern formation provide positional information. These signals may be chemicals released by certain embryonic cells that diffuse through the embryo and bind to other cells. These diffusible signals are called morphogens, which are a specific type of signaling molecules that act according to a concentration gradient. Often, it is by the concentration of the morphogen that the target cells “sense” the information about their position. The release of a morphogen by an inducer creates a concentration gradient, with high concentrations of morphogen close to the source, and low concentrations farther away from the source. Depending on their location, cells are exposed to different threshold levels of the morphogen, which leads to different cell fates. https://sway.office.com/D0aWfKx0uDWJmKGC#content=KGhWlOo0ouIj8Q - The role of gradients in morphogenesis Inductive signals often take the form of concentration gradients of substances that evoke a number of different responses at different concentrations. This leads to the formation of a sequence of groups of cells, each in a different state of the cellular differentiation depending on their relative position. The development of the limbs is a classical model for skeletal pattern formation. Three morphogen gradients are involved in this system: apical ectodermal ridge (AER), the zone of polarization (ZPA), and the progress zone (PZ). These three induction centres work as a coordinate system for the differentiating cells. Chimaeras and mosaics An animal chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes, either of the same or different species. If the different cell populations have emerged from the same zygote (by mutation), the organism is called a mosaic. Chimaeras are also distinguished from hybrids, organisms containing genetically identical populations of cells originating from a cross of two different species. Twin chimaeras are produced when two zygotes do not undergo fusion but exchange cells and genetic material during development. The most widely known examples of twin chimerism are blood chimaeras. These individuals are produced when blood anastomoses (vascular connections) form between the placentas of dizygotic twins, thereby enabling the transfer of stem cells between the developing embryos. When blood chimerism involves male and female twins, female exposure to male hormones results in freemartin syndrome, in which the female is masculinised; this commonly is seen in cattle and rarely in other species. https://sway.office.com/D0aWfKx0uDWJmKGC#content=BKpmIoVZaCBSnw - Genetic chimaeras and mosaic A genetic chimerism or chimaera is a single organism composed of cells with distinct genotypes. In animals, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types, subtle variations in the form (phenotype). A genetic mosaic, or mosaicism, involves the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. TWINS Twins can be either identical twins (monozygotic) or fraternal twins (dizygotic). Identical twins: If two individuals are derived from a single fertilised egg, each member acquires the same chromosomal heritage and hence the similarity between the identical twins is so striking in physical and behavioural characters. They are monozygotic or duplicate twins. They are always of the same sex. Identical twins can result from either: separation of early blastomeres (up to the 8-cell stage); each of the separate blastomere(s) develops into an independent embryo; or separation of inner blastomeres within a single morula/blastula; each of the separate blastomere(s) develops into an independent embryo and both embryos usually share a common placenta (this is less common than the first possibility). Not conjoined identical twins usually come from the split of an early embryo and as long as each segregated part is able to complete a normal development, this results in two genetically identical twins. They may have separate placentas but in most cases, they share the same placenta (i.e., monochorionic pregnancy). https://sway.office.com/D0aWfKx0uDWJmKGC#content=OYcnuyyrbElVTA - Identical or monozygotic twins Monozygotic twins are formed when one zygote, created with one egg and one sperm, splits into two. Siamese twins or conjoined identical twins, happen when blastomeres only partially separate to form two connected individuals who will remain physically connected (most often at the chest, abdomen or pelvis). Conjoined twins may also share one or more internal organs. Many conjoined twins die in the womb (stillborn) or die shortly after birth. Some surviving conjoined twins can be surgically separated. The success of surgery depends on where the twins are joined and how many and which organs are shared, as well as the experience and skill of the surgical team. https://sway.office.com/D0aWfKx0uDWJmKGC#content=X5aUUQTplDmWzg - Conjoined identical twins or Siamese twins Conjoined identical twins or Siamese are twins monozygotic twins with varying extent of union and different degrees of residual duplication. The various types of union are named by the use of a prefix designating the region that is united and adding the suffix pagus, meaning joined. For example, craniopagus (united by the heads), thoracopagus (united in thoracic region). Fraternal twins result when two or more zygotes develop “independently” during the same pregnancy (independence can be compromised by a fusion of foetal membranes and blood supplies). This condition only makes sense in animals that usually only ovulate a single secondary oocyte each oestrous (monotocous species), but it is not used in litter-bearing animals which usually give birth to more than one offspring at a time (polytocous species). They have the same, degree of family resemblance, as between brothers and sisters of different ages. In some cases, there are adhesions between the two placentas of dizygotic twins. In cows, this circumstance is related to the presence of Freemartin syndrome. https://sway.office.com/D0aWfKx0uDWJmKGC#content=GEtWRazM8Ep2bu - Fraternal twins Because fraternal, or dizygotic, twins are two separate fertilized eggs, they usually develop two separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into two. Asymmetrical twins can be monozygotic or dizygotic. While one of the twins is normal the other one is a rudimentary embryo. The rudimentary twin is called by various different terms (i.e., amorphous, acephalus, acardius) and they are usually found sharing the same placenta. Asymmetrical or unequal identical twins are also known as parasitic twins. They occur when conjoined twins begin developing in utero and one embryo maintains dominant development at the expense of the other. The undeveloped twin is defined as parasitic, rather than conjoined because it is incompletely formed or wholly dependent on the body functions of the complete foetus that is referred to as autosite. https://sway.office.com/D0aWfKx0uDWJmKGC#content=hIqlcQ3QzPVUk9 - Conjoint asymmetrical twins or parasitic twins A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin.