SEM_04_Gastrulation.docx

Gastrulation A quote "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." Lewis Wolpert, 1986 Learning objectives Consider the importance of gastrulation from the point of view of evolution. Understand the emergence of an embryonic bilamin...

Gastrulation A quote "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." Lewis Wolpert, 1986 Learning objectives Consider the importance of gastrulation from the point of view of evolution. Understand the emergence of an embryonic bilaminar stage as the precursor of the three germ layers stage (trilaminar stage). Consider the importance of cellular movements in gastrulation. Understand the role of the primitive streak and the primitive node. Consider how by the end of gastrulation the embryo has begun to differentiate the main tissue-specific cell lineages. Consider gastrulation as a fundamental step in the formation of the vertebrate body plan and body axes. Gastrulation Gastrulation is a crucial phase in the development of multicellular animals during which the blastula is reorganised into a multi-layered structure known as the gastrula. The word gastrulation literally means "formation of a stomach", and this is precisely the primary outcome of this process. The process of gastrulation occurs between days 14 and 19 post-fertilisation (second week). It is a series of rapid, complicated, but coordinated movements of cells from the surface of the bilaminar embryo into the interior. Because of the complexity of this process, many embryos do not gastrulate correctly. It is estimated that improper gastrulation occurs in one-third of all human embryos. When this happens, a miscarriage usually takes place, even before the woman realises that she is pregnant. The primitive streak is an important concept in bioethics, where some experts have argued that experimentation with human embryos is permissible, but only before the primitive streak develops, generally around the fourteenth day of existence. The development of the primitive streak is taken, by such bioethicists, to signify the creation of a unique, human being. In some countries, it is illegal to develop a human embryo for more than 14 days outside a woman's body. The basic mechanism of gastrulation is a series of cell movements that rearrange the inner cell mass into a series of layers (known as germ layers) with specific commitments. The outer layer (ectoderm) is related to the protection and interaction of the embryo with its environment. The inner layer (endoderm) assumes nutritive and metabolic functions. An intermediate layer (mesoderm) is incorporated later in development to form a great variety of supporting tissues. Besides the formation of the three germ layers, other several important things are accomplished during gastrulation: Cell movements result in a massive reorganisation of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism, the gastrula. As a result, cells are brought into new positions, allowing them to interact with cells that were initially not near them. This paves the way for inductive interactions (induction), which are the hallmark of following phases of neurulation and organogenesis. The basic body plan is established, including the definition of the primary body axes. One physical landmark is the formation of the notochord (axial mesoderm) that is a defining structure in all chordate embryos. As the embryo forms, its overall body pattern is determined by the establishment of three clear axes—the anterior-posterior axis (head-tail), the dorsal-ventral (back-belly) axis, and left-right asymmetry. - Cell movements during gastrulation Outer cells from the epiblast move toward the primitive streak and through it they migrate toward the interior of the developing gastrula. Gastrulation throughout evolution Besides its original meaning related to the digestive system, gastrulation is an essential step by which the embryo acquires the three germ layers (ectoderm, mesoderm and endoderm) from which all body structures are derived. From an evolutionary perspective, the primary goal of gastrulation was to form a primitive gut or archenteron by a simple invagination in the blastocyst. As a result, the body was first delimited by two layers (diploblastic species), an outer layer (epiblast/ectoderm) and an inner layer (hypoblast/endoderm). Diploplasts (animals with only two germ layers) do not have mesodermal cells. These animals, which include jellyfish and comb jellies, have radial rather than bilateral symmetry and have far fewer tissue types than triploplasts due the lack of a mesoderm. The appearance of the third layer of filler cells (mesoderm) was a very successful achievement as it allowed the development of more complex embryos, the triploblastic animals which expanded so extensively that the current number of diploblastic species is very scarce. Although the way by which gastrulation is accomplished exhibit a range of variations throughout the animal kingdom, they are unified by a common basic mechanism: cells move from the outermost layer toward the interior resulting in a massive reorganisation of the embryo. During gastrulation, many cells on the surface of the embryo move inward to a new deeper location. In the small and yolk-deprived ancestral oligolecital eggs, gastrulation was easily achieved by bending inward the outermost layer. Over evolution, the accumulation of yolk in the mesolecithal and polilecithal eggs was an increasing handicap for the outer cells to keep moving toward the interior. The gastrulation mechanism had to evolve into different gastrulation patterns according to the increased yolk content in mesolecithal and polilecithal eggs. To understand gastrulation, let’s briefly review this process from the simplest primitive oligolecithal eggs to the more complex mesolecithal and polylecithal eggs considering mammal’s gastrulation at the end. Ancestral oligolecithal eggs (chordates) Gastrulation in sea urchins The simplest gastrulation pattern occurs in chordates (e.g., sea urchins). During early animal development, embryos develop into a blastula, which is a hollow ball of cells. Gastrulation involves a process of invagination at one end of the blastula. It is like “pushing in” a tennis ball on one side, causing that part of the blastula wall first to flatten, and then bend inwardly (invaginate). It results in the formation of a two-layered (bilaminar) structure of which the outer layer is called the epiblast and the inner is called the hypoblast. Gastrulation results in the virtual obliteration of the blastocoel and its replacement by a new cavity that is referred to as the archenteron or primitive stomach. The opening to the exterior, where invagination occurred, is called the blastopore. Eventually, some cells from the blastopore detach and migrate into the space between the epiblast and the hypoblast giving rise to a third layer called the mesoderm (trilaminar gastrula). Besides the mesoderm, the inner layer of cells enclosing the archenteron (formerly called hypoblast) constitutes the endoderm, and the outer layer of cells (formerly called epiblast) constitutes the ectoderm. https://sway.office.com/tmRWmxnynpRohJa1#content=P5fv11oP2LacuE - Gastrulation in primitive chordates The blastocyst is a ball of cells which then folds inward to form a gastrula. Think of this in-folding as if you were pressing your fist into a deflated balloon. It would form a u-shaped cavity (primitive stomach) that is open to the exterior (blastopore). Mesolecithal eggs (amphibians) Gastrulation in frogs In frogs, gastrulation is also accomplished when cells on the outer layer move to the interior. Nevertheless, the invagination of the blastula through the blastopore is hampered by the increased amount of yolk. Despite this drawback, gastrulation is still accomplished by inward movements of the outer cells (epiblast) through the blastopore to give rise to the primitive stomach or archenteron. But migrating cells have to squeeze under the surface and over the mass of yolk so that the shape of the embryo elongates. Over evolution, the elongation of the embryo was accompanied by a diversification of the mesoderm functions. Besides forming filling tissues, the mesoderm gives rise to the notochord: a rod-like structure that will be decisive for the first inductions needed for the development of the body organs. The notochord also determines the body plan by establishing the bilateral symmetry and the body axes of the embryo. https://sway.office.com/tmRWmxnynpRohJa1#content=oSmy4HUh5Vha5S - Gastrulation in amphibians Gastrulation in frogs is similar to the sea urchin, but it’s more complicated. One of the main differences is that the blastula is not hollow but is filled with yolk cells which are concentrated in the vegetal hemisphere; this is an impediment to cleavage and gastrulation. Nevertheless, superficial cells keep moving toward the blastopores and condense in its dorsal lip which is the point where the cells begin to turn and migrate inward. Convergent extension of cells elongates the embryo along the anterior-posterior axis while the mesoderm in the midline forms the notochord, a transient mesodermal “backbone” that plays an important role in distinguishing and patterning the nervous system. Polylecithal eggs (reptiles and birds) Gastrulation in birds In birds, the meroblastic cleavage produces a bilaminar blastula that lies atop of a significant amount of yolk resembling a disc (embryonic disc or blastodisc) rather than a ball. The bilayered blastula of the birds corresponds to a blastodisc that is arranged in two layers (epiblast and hypoblast), which somewhat resembles the primitive organization of diploblastic animals. One of the unique features of gastrulation in reptiles and birds is the replacement of the blastopore by the primitive streak. The ancestral blastopore is replaced by a slit in the midline of the embryonic disc, the primitive streak, through which the outer cells keep moving inwards to form the three germ layers (ectoderm, mesoderm and endoderm). Besides, the midline mesoderm gives rise to the notochord, a flexible rodlike structure of mesodermal cells that plays an organizational role in the early embryos. https://sway.office.com/tmRWmxnynpRohJa1#content=0Er5cCTR7LOCmY - Gastrulation in reptiles and birds The major structural characteristic of avian, reptilian, and mammalian gastrulation is the primitive streak. This streak is first visible as a thickening of the epiblast at the posterior region of the embryo. As soon as the primitive streak has formed, epiblast cells begin to migrate through it to form the three germ layer as well as ad the notochord. Derived oligolecithal eggs (mammals) The emergence of the placenta in mammals led to a loss of yolk content so that their eggs became oligolecithal like those of the primitive chordates. Nonetheless, the gastrulation mechanism in mammals and birds remains very alike, suggesting that mammals and birds evolved from a common ancestor with polylecithal eggs. Gastrulation in mammals Birds and mammals are both descendants of one common reptilian ancestor. Therefore, it is not surprising that cells movements that made possible gastrulation in yolk-laden eggs were retained in mammals, even when mammalian eggs lost most of the yolk. As a result, birds and mammals gastrulate in an almost identical manner. For this reason, before gastrulation, the mammalian inner cell mass must be reorganised as an embryonic disc sitting atop an imaginary ball of yolk, following instructions that seem more appropriate to a reptilian ancestor. Formation of the mammalian embryonic disc and the bilaminar blastocyst. Associated with the development of the placenta the amount of yolk was decreased and the egg became smaller. The meroblastic cleavage was then replaced by the holoblastic cleavage but the gastrulation continued taking place in a manner remarkably similar to that observed in polylecithal eggs. That's why the first step of the gastrulation in mammals is the formation of a bilayered embryonic disc which closely resembled the arrangement of the avian bilayered blastula. Just before implantation, the inner cell mass begins to transform into a disc made of two distinct epithelial layers. Cells of the inner cell mass next to the fluid-filled cavity (blastocoel) proliferate, break loose (delaminate), and migrate to form a new cell layer inside the trophoblast called hypoblast. The hypoblast forms a lining that surrounds the blastocyst cavity. Cells of the inner cell mass next to the trophoblast form the epiblast which, after degeneration of the adjacent trophoblast, emerges on the surface of the blastocyst and assumes a disc shape, conforming the embryonic disc morphology at this stage. In mice and humans, the trophoblast cells overlying the embryonic disc, instead of degenerating, contribute to the formation of the amniotic sac. In summary, the bilaminar embryonic disc is formed when the inner cell mass forms two layers of cells, the external layer is called the epiblast and the internal layer is called the hypoblast. Together, they compose the bilaminar embryonic disc that is ready to start the actual gastrulation. https://sway.office.com/tmRWmxnynpRohJa1#content=Qi1GXPEaf8pRtq - Formation of the embryonic disc in mammals Mammals—apart from the egg-laying mammals—do not have yolk in their eggs but, having passed through an evolutionary stage of animals with yolky eggs, retain, particularly in gastrulation, features common to reptiles (and birds, which also had reptilian ancestors). As a result, before gastrulation, the formative cells of the embryo—the cells that will actually build the body of the animal— have to be arranged in the form of a disk over a cavity that takes the place of the yolk of the reptilian ancestors of mammals. For that purpose, as a previous step to gastrulation, the mammalian inner cells mass is rearranged in a two-layered structure, the inner hypoblast that engulfs the blastocoel and the outer epiblast which defines the embryonic disc. Formation of the primitive streak and primitive node The primitive streak is a transient structure whose formation, about the end of the second week of gestation in most mammals or at the beginning of incubation in birds, marks the end of the blastocyst and the start of gastrulation. Gastrulation lasts as long as the primitive streak is present and therefore, the loss of this structure also marked the end of gastrulation. During gastrulation, the outer cells of the embryonic disc (epiblast) converge and condense in the midline of the embryonic disc surface forming a rod-like condensation of epiblast cells called the primitive streak. The primitive streak starts to grow at the caudal edge of the embryonic disc and as gastrulation proceeds, the primitive streak gradually elongates from the caudal margin of the embryonic disc towards the cranial territories. Thereby, the beginning and growth of the primitive streak define the caudal-cranial axis of the embryo. The primitive streak also determines the bilateral symmetry of the embryo (right-left axis). The cranial end of the primitive streak forms a rounded tip called primitive node, or Hensen's node. - Gastrulation in mammals: the beginning The beginning of gastrulation is marked by the formation of the primitive streak. Epiblast cells proliferate along primitive streak margins and migrate inwards through the primitive streak. Ingression of these cells results in the formation of the mesoderm and reinforcement of the hypoblast cells to produce the definitive endoderm. - Gastrulation in mammals: the process The cells of the primitive streak move inwardly, forming an intermediate filler layer, the mesoderm. The final result of gastrulation is the formation of the three germ layers: ectoderm, mesoderm and endoderm. When the epiblast migration is completed the primitive streak cease to exist; as this process is first completed at the cranial territories the primitive streak seems to recede gradually towards the caudal territories until its disappearance marks the end of the gastrulation. - Gastrulation in mammals: the end The end of gastrulation is marked by the regression and disappearance of the primitive streak. https://sway.office.com/tmRWmxnynpRohJa1#content=zPv7N1Qk2yImS7 - Gastrulation in mammals Epiblastic cells migrate through the primitive streak and contribute to the formation of the endoderm (migrated epiblastic cells + the former hypoblast), mesoderm (migrated epiblastic cells) and the ectoderm (non-migrated epiblastic cells that remain on the surface). Formation of the germ layers As the cells in the primitive streak migrate, the primitive streak forms a pair of ridges separated by a depression called primitive groove whereas a primitive pit is formed in the tip of the primitive pit. Epiblast migrating cells give rise to the following structures: Initial migrating cells join the hypoblast layer, forming embryonic endoderm. Previous hypoblast cells constitute the extra-embryonic endoderm: yolk sac endoderm. The space between the hypoblast and epiblast is filled by epiblast migrating cells, forming the mesoderm. Mesoderm divides into axial, paraxial, intermediate, and lateral plate mesodermal regions. The axial mesoderm constitutes the notochord: a rod-shaped aggregate of cells located in the midline between ectoderm and endoderm. The notochord is important because it is fundamental in inducing the earliest structures of the body. The paraxial mesoderm will form the somites. The intermediate mesoderm will form the urogenital system. The lateral plate mesoderm splits into two layers bordering the cavity that is referred to as the coelom. The lateral plate of the lateral mesoderm is called the parietal (somatic) mesoderm and is attached to the ectoderm; the medial plate of the lateral mesoderm is called the visceral (splanchnic) mesoderm is joined to endoderm. After gastrulation, the non-migrated epiblast cells that remain on the surface of the embryonic disc constitute the ectoderm. https://sway.office.com/tmRWmxnynpRohJa1#content=pkDU0mISWMaSS3 - Formation of the germ layers The three germ layers differ greatly in shape and extension. The ectoderm remains on the surface occupying the elongated area that define the embryonic disc. The endoderm acquires the shape of the hypoblast like a sac, which from this moment is called the yolk sac. In contrast, the endoderm migrated all around the gastrula differentiates into the lateral, intermediate, paraxial and axial mesoderm. Germ layers derivatives The three primary germ layers (the ectoderm, the mesoderm, and the endoderm) are in place at the end of gastrulation. They have been designated germ layers because the origins of all organs can be traced back to these three layers. The ectoderm is the outermost layer and gives rise to the integument including hair, nails and epidermis, and the epithelia of the nose, mouth and the sense of the eye, the nervous system and inner ear. The mesoderm is the middle layer and it forms the musculoskeletal, circulatory and most of the excretory systems. Mesoderm also develops into gonads and the muscular and connective tissue in the digestive and respiratory system. The endoderm is the inner layer, which will eventually develop into the epithelial linings of the digestive and respiratory tracts including the lungs. It also forms the pancreas, liver, bladder and distal urinary tracts. Finally, it will give rise to endocrine glands as well (thyroid, parathyroid, thymus). Establishing body axes during development As mentioned above, the three main axes of the organism are clearly established during gastrulation, when the primitive streak and later the notochord clearly define the dorso-ventral, cranio-caudal axes and the right-left asymmetry. The establishment of these body axes at the correct time is fundamental to normal embryonic development. For instance, the central nervous system develops along the dorsal surface, with the largest concentration of neuronal tissue—the brain—at the anterior end of the embryo. The limbs develop symmetrically and bilaterally, whereas the heart— although it begins as a symmetrical structure—ultimately comes to point toward the left side of the trunk. Some internal structures are paired (the kidneys, lungs, adrenal glands, testes, and ovaries), whereas many are not (the heart, gut, pancreas, spleen, liver, and uterus). The dorso-ventral axis is the first that ca be identified. The mammalian blastocyst clearly shows an axis between the area where the inner cell mass (ICM) is found and the opposite area without ICM. This is clearly the first axis of embryonic polarity that can be clearly distinguished in the mammalian embryo. This dorso-ventral axis divides the cells of the ICM into two groups, those close to the blastocele destined to form the hypoblast (ventral) and those arranged on the surface destined to form the epiblast (dorsal). One question that can be raised with regard to this polarity is whether they occur randomly with respect to morula cells or whether they are predisposed from earlier stages of development. Information about the establishment of these body axes and their role in development is far from complete. The cranio-caudal axis of the blastocyst can also be determined before gastrulation. The anterior hypoblast may play a role in primitive streak formation and in determining the head region through a complex network of signaling pathways. An important group of cells that produces molecular signals that help determine the anterior-posterior axis of the mouse embryo is the anterior visceral endoderm (AVE). The AVE expresses different genes along its length. The right-left axis of asymmetry is established during gastrulation; there is a mutation called "situs inversus" in which the individual has a mirror-mode inversion of the development side of the thoracic and abdominal viscera with little or no clinical consequences, while the mutation that causes a developmental side random arrangement results lethal. Defects of normal right-left asymmetry are related to defects or absence of cilia in the cells of the primitive node. These cells present ciliary movements to the left that increase the concentration of signaling factors, such as "nodal" or "lefty" on this side, this being the determining cause of the right-left asymmetry in the development of the organs. Therefore, coordinating the embryo's "decisions" about its body pattern is a hierarchy of genes. Overall, the Hox genes specify anterior-posterior polarity. Their normal function can be subverted by retinoic acid, which can activate Hox genes in inappropriate places. Less is known about the establishment of the dorsal-ventral axis. It may be determined in the blastocyst, or even in the oocyte; it is clearly established when the notochord develops. Genes such as Nodal and Lefty help determine left-right asymmetry. Genes that regulate body patterning in embryonic development are well conserved throughout evolution among both vertebrates and invertebrates.