SEM_03_Cleavage and cell differentiation_PARTE1.docx

Cleavage and cell differentiation A quote “Life can only be understood backwards; but it must be lived forwards.” ― Søren Kierkegaard. Learning objectives Consider the meaning of cleavage. Understand the relationship between the amount and distribution of the yolk and the type of cleavage. Und...

Cleavage and cell differentiation A quote “Life can only be understood backwards; but it must be lived forwards.” ― Søren Kierkegaard. Learning objectives Consider the meaning of cleavage. Understand the relationship between the amount and distribution of the yolk and the type of cleavage. Understand the significance of the morula and the blastocyst. Distinguish between the fate of the trophoblast and the fate of the inner cell mass. Understand the concept of differentiation and its role in cell specialisation. Understand the concept of "induction" and its role as the main mechanism of cellular differentiation. Consider the underlying causes for fusion (chimaeras) and duplication (twins) of embryos. Cleavage or segmentation refers to the initial series of rapid mitotic divisions by which the large zygote is divided into numerous “normal size” cells called blastomeres. Blastomeres can show synchronous division in the early stages but later this synchrony is lost. Cleavage begins with a zygote, progresses to the morula stage, and terminates with the formation of the blastocyst (blastula). Mammals’ cleavage has a slow rate of division, which is between 12 and 24 hours, compared with other animals (a frog egg, for example, can divide into 37,000 cells in just 43 hours). During early cleavage divisions, the zygotic genome is not transcribed. Instead, the rate of cell division and the arrangement of the blastomeres is completely under the control of maternal proteins and mRNAs stored in the oocyte by the mother. This maternal control of cleavage is responsible for this early pattern of cleavage divisions. Before the blastula stage, there is a switch from the maternal control to the expression of the zygotic genes. At that time, the rate of cleavage decreases, and nuclear genes begin to be transcribed. The early embryonic cleavages are rapid mitotic divisions, which differ from other forms of cell division in that they increase the number of cells without increasing the cytoplasmic mass. This is accomplished by abolishing the growth period between cell divisions (gap phases G1 and G2 are not required in these initial cycles,). Nuclei in the early embryo, therefore, alternate between synthesis (S-phase) and mitosis. This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division (nucleus/cytoplasm ratio increases). Consequently, each cell division amplifies the total number of cells, but the resulting cells become increasingly smaller. Thus, the gigantic zygote (150 microns in diameter) becomes a collection of many average size cells (about 7 microns in diameter). In mammals, the first blastomeres are undifferentiated; every one of them is capable of giving rise to any type of cell or a complete embryo (they are totipotent); at a certain stage of cleavage, some cells have to cater for the embryo, and therefore, they have to differentiate in specialised cells (they lose their potency). Thereafter, a cascade of cell differentiation gives rise to all the types of cells that form the tissues and organs of the embryo. Patterns of embryonic cleavage The amount and distribution of yolk determine where cleavage can occur and the relative size of the blastomeres. Thus, the pattern and speed of cleavage are determined by the cytoplasmic yolk rather than the nucleus. The yolk tends to suppress the cleavage. - Patterns of cleavage. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic. The holoblastic type of cleavage is commonly seen in eggs containing moderate (amphibians and others) to a sparse (mammals and others) amount of yolk. The meroblastic cleavage is characterised by incomplete cleavage as a result of the presence of a mass of yolk material. At one extreme are the eggs of some invertebrates and mammals. These eggs have sparse yolk (oligolecithal), equally distributed (isolecithal). In these species, cleavage is holoblastic (Greek holos, “complete”) and equal meaning that the cleavage furrows extend through the entire egg and produce blastomeres of approximately equal size. These embryos must have some other way of obtaining food. Most invertebrates will generate a voracious larval form, while mammals get their nutrition from the placenta. https://sway.office.com/D0aWfKx0uDWJmKGC#content=yn20GyU3lb4HhU - Equal holoblastic cleavage. An equal holoblastic cleavage produces blastomeres of approximately equal size. Cleavage in many amphibians is holoblastic and unequal because the large volume of yolk (mesolecithal) interferes with cleavage. One pole of the egg, the animal pole, is relatively yolk-free. The zygote nucleus is frequently displaced toward the animal pole, where the cellular divisions occur at a faster rate than at the opposite yolk-rich pole (vegetal pole). Yolk inhibits cleavage at the vegetal pole where cleavage proceeds 50-100 times slower than at the animal pole. This unequal holoblastic cleavage gives rise to a more rapidly dividing animal pole made up of smaller micromeres and a slower dividing vegetal pole made up of macromeres. https://sway.office.com/D0aWfKx0uDWJmKGC#content=EXLFT4kJV3UltS - Unequal holoblastic cleavage An unequal holoblastic cleavage produces blastomeres of unequal size. At the other extreme are the eggs polylecithal and telolecithal which undergo meroblastic cleavage. In the eggs of birds and reptiles, division planes do not extend into the vegetal pole where the yolk accumulates. Therefore, cell divisions only occur in the small disc of cytoplasm located at the animal pole. This arrangement gives rise to the discoidal cleavage. https://sway.office.com/D0aWfKx0uDWJmKGC#content=jB6LxvUHQNuet7 - Discoidal meroblastic cleavage Cleavage in which a disk of cells is produced at the animal pole of the zygote. The eggs of insects have their yolk in the centre (i.e., they are centrolecithal), and the divisions of the cytoplasm occur only in the rim of cytoplasm around the periphery of the cell (i.e., superficial cleavage). https://sway.office.com/D0aWfKx0uDWJmKGC#content=u5IqIwtqATZ0rp - Superficial meroblastic cleavage Superficial cleavage is a meroblastic cleavage in which a layer of cells is produced about a central mass of yolk (as in many arthropod eggs). Morula The morula (from Latin= small mulberry) is a solid collection of cells that are formed by the division of the zygote. The duration of this period varies between species. In domestic animals, the morula comes to have 16-64 blastomeres, while in the human embryogenesis it does not reach beyond the 16 blastomeres. Independently of the number of blastomeres, this phase ends when a cavity appears inside of the morula; then it becomes a hollow structure that is called blastocyst or blastula. As cell divisions go on, blastomeres become compacted. The compaction of the morula changes the interrelationship between blastomeres. This brings deep metabolic and structural changes so that, at the end of the morula stage cells on surface differentiate from those insides of the morula: Outer blastomeres become flattened and form tight junctions; they develop the capacity to pump sodium from the outside, which brings water and nutrients into a centrally forming cavity. Inner blastomeres keep connected by gap junctions to maximise the exchange of ions and small metabolites. https://sway.office.com/D0aWfKx0uDWJmKGC#content=WBI35rHBCLFFJV - Morula stage The morula stage is an early stage in post-fertilization development when cells have rapidly mitotically divided to produce a solid mass of cells (12-15 cells) with a "mulberry" appearance. Blastocyst The blastocyst or blastula consists of a large number of blastomeres arranged in a hollow, fluid-filled, spherical structure. The compaction of the morula makes the superficial cells to flatten forming an epithelium called the trophoblast. The trophoblast causes the surrounding fluid to penetrate inside the blastocyst to fill a cavity called the blastocoel. Then, the inner cells become grouped in the so- called inner cell mass (ICM). In summary, the blastocyst consists of three different parts: The trophoblast or trophectoderm. They form the superficial cells of the blastocyst. They will become part of the placenta (chorion and amnion). The inner cell mass (also known as embryoblast). They are a collection of cells localised inside of the blastula. They are destined to form the embryo itself plus two foetal membranes of the placenta (yolk sac and allantois). The blastocoel (also spelt blastocoele). It is a fluid-filled cavity inside of the blastocyst. https://sway.office.com/D0aWfKx0uDWJmKGC#content=ThtCkEWjOfhDU0 - Blastocyst stage The blastocyst or blastula is the term used to describe the hollow cellular mass that forms in early development. Cleavage in mammals The mammalian oocyte is released from the ovary and swept by the fimbriae into the oviduct (also called the uterine tube or fallopian tube). Fertilisation takes place in the ampulla of the oviduct, the first region close to the ovary. Meanwhile, the cilia in the oviduct push the fertilised zygote toward the uterus, the first cleavage division takes place along this journey through the uterine tube. Mammalian blastomeres do not all divide at the same time. Thus, mammalian embryos are not always found at 2-, 4-, 8-, 16-, or 32-cell stages, but frequently contain odd numbers of cells. Broadly speaking, the embryo leaves the oviduct and reaches the uterus at the end of the first week or beginning of the second week (around 4 to 7 days after fertilisation, depending on species). In most domestic species, when the embryo leaves the oviduct and enters the uterine horns, it is at about the 16- 64 cell stage. While the blastocyst is migrating along the oviduct, the zona pellucida continues to encase the early developing embryo to prevent it from adhering to the wall of the oviduct, which could result in an ectopic pregnancy. An ectopic pregnancy happens when a developing blastocyst implants in the fallopian tubes. Once the growth of the embryo starts to expand, this can be accompanied by extreme cramping and life-threatening bleeding. After reaching the uterus, rhythmic expansions and contractions of the blastocyst result in the rupture of the zona pellucida and the blastocyst emerges from the rigid envelop. This process is also known as “hatching” of the blastocyst and the blastocyst freed of the pellucid zone may also be called “hatched blastocyst”. In cattle, 32-cell stage unhatched embryos can be easily collected from the uterus at 5-7 days post ovulation. This moment is ideal for embryo retrieval in cattle. After hatching, animal blastocysts begin a phase of rapid growth before implantation with a period of exponential growth and species-specific changes in the size and shape of the conceptus. https://sway.office.com/D0aWfKx0uDWJmKGC#content=rJbtXiDLVbdiJ0 - Timing of cleavage in mammals Animation about the main events of cleavage in cows. Cleavage in birds Fertilisation of the chicken egg occurs at the beginning of the oviduct before the albumen and the shell are secreted upon it. The yolky egg of the birds is telolecithal, with a small disc of cytoplasm (2– 3 mm in diameter) at the animal pole of the egg, where the nucleus is located. After fertilisation, the egg undergoes a discoidal meroblastic cleavage; because cellular divisions are impeded in the vegetal pole by the yolk, cleavage occurs only in the animal pole. As a result, blastomeres are located in a disc-like structure called blastodisc, rather than forming a spherical or elliptical blastocyst. The first cleavage furrow appears centrally in the blastodisc; then other cleavages follow to create a single-layered blastodisc. Thereafter, equatorial and vertical cleavages divide the blastodisc into a five to six cell layers thick disc. At this stage, the deep cells in the centre of the blastoderm disappear leaving a space between the blastodisc and the underlying yolk called the subgerminal cavity. Then, the thin central region of the blastodisc is called the pellucid area (in Latins pellucida means translucent). This part of the blastoderm will form most of the actual embryo. The peripheral ring of blastomeres that keep in close contact with the yolk constitutes the opaque area (not transparent). By the time the fertilised egg is laid, the blastodisc contains some 20,000 cells and is transforming into a two-layered blastula. At this time, most of the cells of the pellucid area remain at the surface, forming the epiblast, while others migrate individually into the subgerminal cavity to form an inner thin layer called the hypoblast. The space between these two layers (epiblast and hypoblast) is called the blastocoel. https://sway.office.com/D0aWfKx0uDWJmKGC#content=JmPLcSSFyzIWm7 - Cleavage in birds Cleavage in reptiles and birds is partial (meroblastic), and, at its conclusion, the embryo consists of a disk-shaped group of cells lying on top of a mass of yolk. This cell group often splits into an upper layer, the epiblast, and a lower layer, the hypoblast. In birds, like mammals, the fertilization takes place at the beginning of the oviduct where only the cytoplasmic membrane and the initial vitelline membrane are covering the egg. Cleavage happens meanwhile the tertiary membranes are added surrounding the egg along it passes down the oviduct. By the time the egg is laid the embryo is at the bilayered blastodisc stage. https://sway.office.com/D0aWfKx0uDWJmKGC#content=0iuK9fyC5rw8SP - Timing of cleavage in birds Animation about the main events that happen in the oviduct from ovulation the ovulation of an egg until it is laid. Control of the embryonic development One of the most fascinating aspects of the embryonic development is the fact that out of a simple undifferentiated cell (zygote), an organism arises that consists of billions of differentiated cells. Basically, three major processes can be considered during embryonic development: cell division (growth), cell specialisation (differentiation) and morphogenesis (development of the form of the organs) Growth depends on cell divisions through mitosis. To control excessive/restricted growth during embryo development, certain mechanisms are needed which are able to promote/stop cell divisions at the right moment. The study of those mechanisms, although important, is beyond the scope of this course. On the other hand, the increasing complexity of the developing structures is related to the process of cell differentiation and morphogenesis. Cell differentiation The adult mammal body is composed of more than 230 different cell types, all originating from a single cell, the fertilised egg or zygote. The process by which more specialised cells develop from less specialised cell types is known as cell differentiation. During normal development, the fate of cells can be discovered by labelling these cells and observing what structures they become a part of. By doing so to all the cells at an early stage, we can build a fate map, which is a kind of future time machine for cells at an early stage of development that indicates the fate of each cell or region at a later stage of development. The potency, or developmental potential of an embryonic cell, describes the range of different cell types that it can become. The zygote and its first descendants are totipotent stem cells; these cells have the potential to develop into a complete organism, including the placenta. Totipotency is uncommon in animals after the morula stage. As development proceeds, the developmental potential of individual cells decreases. Pluripotent stem cells can give rise to all cell types of the body but not the placenta; this is the case of the cells in the inner cell mass. Multipotent stem cells can differentiate into a variety of tissues but not all; cells of the germinal layers (ectoderm, mesoderm and endoderm) are good examples of multipotent cells. When embryonic cells are completely differentiated, they have become the specialised cells of any tissue. Cell potency and cell differentiation are inversely related. Cell differentiation involves progressive restrictions in their developmental potentials. When a cell “chooses” a particular fate, it is said to be determined, although it still "looks" like its undetermined neighbours. Cell determination implies a stable change in cell organisation; the fate of determined cells will not change; it is irreversible. Cell determination and cell specification can be considered quite similar, but there are some nuances. Before one cell is determined, there is a reversible state of being committed also called cell specification. Committed cells are not yet determined but have a bias toward a certain fate that can be reversed or transformed to another fate. The reversible phase of cell specification is followed by an irreversible stage of cell determination. Once cells are determined, their fate is fixed, and they will irrevocably differentiate. Cell differentiation implies actual changes in the biochemistry, structure, and function of the cell which result in a specific cell type. In general, this means that cells determined to become neurons cannot end up as skin cells, even no observable change is apparent at this stage. Cell determination is followed by cell differentiation when specific proteins are synthesised, and the morphology and function of these cells change. - Cell differentiation and developmental potential The developmental potential, or potency, of a cell, describes the range of different cell types it CAN become. The zygote and its first descendants are totipotent - these cells have the potential to develop into a complete organism. Totipotency is uncommon in animals after the 2-4 cell stage. As development proceeds, the developmental potential of individual cells decreases (multipotent cells) until their fate is determined (differentiated cells). Mechanisms of cell differentiation How do cells become different from their parent cells? How do two identical daughter cells become different from one another? How might one daughter cell become a neuron, while the other daughter cell becomes a skin cell? Basically, it depends on two factors: genetic control and cellular environmental influences that are temporally and locationally adjusted to one another (consider the "right place/right time” metaphor). Genetic control Cell differentiation is ultimately regulated through differential gene expression. Cell differentiation is the result of cells expressing some genes and suppressing others within a common genome. Cells differ because they produce different proteins/peptides. Ultimately, proteins and peptides form all the embryo structures: cytoskeleton and extracellular structures; enzymes controlling cell metabolism; secretory products (e.g., hormones; digestive enzymes; etc.); channels and pumps (passage of molecules across membranes); receptors (communication, etc.) and so on. Differentiation proceeds by gradual specialisation of the protein contents of a cell. Each type of cell in a mature organism has a unique collection of proteins. The blueprints for making these proteins are found in the nucleus of each cell in the form of deoxyribonucleic acid (DNA). Therefore, the starting place for understanding the process of differentiation lies in the nucleus of the original zygote, which contains all the genetic instructions (DNA) to make all the cell type repertoire of the mature organism. All cells in a mature organism (muscle cells, brain cells, etc.) have the same set of genes, but only a subset of those genes are turned "on" in any specific cell type. Therefore, the process of differentiation involves the activation (turning on) of some genes and the inactivation (turning off) of other genes in order to get the specific collection of proteins that characterises that cell type. The difference between cell determination and cell differentiation can be understood keeping in mind how proteins are produced. Cell determination happens when certain genes are activated or inactivated, and differentiation completes when the cell synthesises all the tissue-specific proteins that the activated genes encode. For example, when some particular cells in a mammalian embryo activate the gene for the protein MyoD, they are determined to be muscle cells. As it turns out, the MyoD protein is a transcription factor that controls the expression of several other genes responsible for producing specific muscle proteins. When specific muscle proteins are produced the undifferentiated stem cells turn into differentiated muscular fibre cells. - The role of proteins synthesis in cell differentiation. All of the cells within a complex multicellular organism such as a human being contain the same DNA; however, the body of such an organism is clearly composed of many different types of cells. What, then, makes a liver cell different from a skin or muscle cell? The answer lies in the way each cell deploys its genome. In other words, the particular combination of genes that are turned on (expressed) or turned off (repressed) dictates cellular morphology (shape) and function. This process of gene expression is regulated by cues from both within and outside cells, and the interplay between these cues and the genome affects essentially all processes that occur during embryonic development and adult life. The role of genes in development The formation of all aspects of cells, tissues and organs is under the gene control of different types of genes, including maternal genes, which will gradually activate segmentation genes (divide the body into segments), then activate homeotic genes (determine the fate of each segment). These types of genes are so-called master or switching genes. Each of these gene groups has other subclassifications or categories of their own depending on the type of living being in formation, forming a coordinated cascade of different gene systems. - The basic three-dimensional layout of an organism is established early in embryonic development. Even an early embryo body has dorsal and ventral axes (top and bottom) as well as cranial and caudal axes (front and back). Interestingly, while different types of organisms have dramatically different morphological features, a similar family of genes controls gene expression during the early stages of pattern formation. The Hox genes (also called homeotic genes) are found in many different organisms, and they are important in controlling the anatomical identity of different parts of a body along its anterior /posterior axis. These genes include a nearly identical DNA sequence, called the homeobox region. They encode proteins that function as transcription factors. For example, homeotic genes specify the types of appendages that develop on each body segment. The importance of the Hox genes is vividly evident when one of these genes is mutated; in such mutations, the development of an entire part of the body takes place in the wrong segment of the embryo. For example, in fruit flies, the mutation in the Antennapedia gene causes fruit flies to develop legs in place of antennae on the head segment. Gene control regulation Gene regulation is the process of controlling which genes in a cell’s DNA are expressed. Various methods of gene regulation are described: the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (pretranscriptional level); the RNA is transcribed (transcriptional level); the RNA is processed and exported to the cytoplasm after it is transcribed (post- transcriptional level); the RNA is translated into protein (translational level); or after the protein has been made (post-translational level). In prokaryotic cells, the control of gene expression is mostly at the transcriptional level. The transcription regulation is the primary method to control what type and how much of each protein is expressed. In this regard, transcription factors that act by activating / repressing genes play a fundamental role in this whole process. The role of epigenetic control in development Traditionally, genetic mechanisms and processes have been thought to provide the primary control for cell differentiation and embryo development. Today it is known that a system of marks or modifications in the DNA is added to the information of the genes that will contribute to the same gene being expressed or ceased to express itself in different cells or tissues of the organism. Epigenomics would be the part of genomics that deals with the study of epigenetic marks in a specific cell type. Epigenetics (from the Greek epi, in or on, and genetics) refers to the study of factors determined by the cellular environment instead of by inheritance, which also intervene in the heritable regulation of gene expression, without change in the sequence of nucleotides. It can be said that it is the set of chemical reactions that modify the activity of DNA but without altering its sequence. These molecular changes can be stable and pass through mitotic and meiotic cell divisions, that is, they can be inherited. The most studied epigenetic modification is DNA methylation, this is the binding to the cytosine base of DNA of a molecule called the methyl group. This union causes the genes to not be expressed. Another form of epigenetic control is through histone control. Specific combinations in histone modification serve as a kind of code that determines whether the gene is to be silenced or expressed, and this is another way of how gene regulation can occur. A third epigenetic mechanism is the microRNAs (miRNAs) are small RNAs that regulate the expression of their target genes through the decrease in the expression of the proteins encoded by these genes, through repression of the translation and / or degradation of their mRNA. MiRNAs play a fundamental role in virtually all cellular processes including development, function, and survival. In the embryo, epigenetics performs an essential function since it has to establish the genes to be expressed and at what stage of development. All cells of the embryo have the same genetic information inherited from their parents. The epigenetic modifications will be the signals to determine which genes are expressed in each cell so that a correct embryonic development takes place. Cellular environment Cells develop in the context of their environment, including their intracellular components and the substances localised in their immediate cellular neighbourhood. During the early development, differences in the cellular environment may mainly arise because of two broad mechanisms: Cytoplasmic determinants, which are molecules localised in the cytoplasm of the egg that becomes unequally distributed among the daughter cells and then affect the activity of genes. Induction, which is a cell-cell interaction at close range between two or more cells or neighbouring tissues.