BIO 131 Study Guide Chapter 43 Gastrulation PDF

Study guide chapter 43 part two “Define the term ‘gastrulation’. Be able to name the 3 germ layers formed during gastrulation; also know the final main tissues or organs that each layer eventually becomes.” Gastrulation - the process by which the three germ layers of the animal embryo are formed: ec...

Study guide chapter 43 part two “Define the term ‘gastrulation’. Be able to name the 3 germ layers formed during gastrulation; also know the final main tissues or organs that each layer eventually becomes.” Gastrulation - the process by which the three germ layers of the animal embryo are formed: ectoderm, mesoderm, and endoderm. Three Germ Layers formed during Gastrulation 1) Ectoderm – Outer layer (skin/nervous system) 2) Mesoderm – Middle layer (muscles and organs) 3) Endoderm – innermost layer digestive and respiratory tract. - three model systems we used to examine gastrulation sea urchin, frog, chick “ Be familiar with the process of gastrulation in the sea urchin (know the major structures including: mesenchyme cells, blastopore, Archenteron, skeletal rods and stomodeum. Be able to distinguish these structures in a cartoon or microscope generated image.” Sea Urchin Gastrulation 1 - Blue : ectoderm precursor - Yellow: endoderm precursor - Red: Mesoderm precursor - Black: some type of micromeres??? (part of mesoderm???) [he calls them micromere descendents] - Image A: Blastomeres (pre-blastula formation) Color-coding based on different contents and have different sizes (micromeres/macromeres) but similar morphology Image B: Blastula formed Differentiation has occurred in the cells --- cell morphology has changed - no longer round and surface area has been molded such that the cells are more conducive to the attachment of cells --- Increased cell adhesion proteins (cells stick together along their membrane surface) and proteins involved in ion transport cells form cialis (hairlike structures) which are a sign that the cell has been differentiated Note color coordination Image C: The vegetal hemisphere (bottom half where mesoderm and endoderm precursors are located) gets flattened Sea Urchin Gastrulation 2 Image D: The mesoderm precursor cells begin to disassociate from the vegetal hemisphere and make their way into the blastocoel; these cells are now called the Primary mesenchyme cells see "Movement of Primary Mesenchyme cells" term [see Sea Urchin Gastrulation 2 for pics from here] Image E: The cytoskeletons of the micromere descendants (the black part) are drawn to the opposite end of the cell (the animal pole), which drives the entire vegetal pole upward in that direction The cells that eventually become the endoderm are pulled up with it and start to displace the blastocoel --- The inward push has formed a canal that is called the Archenteron, which eventually becomes the passageway used by the anus to dispel waste --- The Blastopore is the indentation at the very tip of the canal which becomes the anus in sea urchins As this indentation happens, additional mesenchyme cells are released called Secondary mesenchyme cells Image G: Mesoderm precursor cells divide and form long structures called skeletal rods that have contractile abilities which help in the movement of the sea urchin Part of the ectoderm (outer layer) begins to form an indentation called to Stomodeum that eventually forms an opening which becomes the mouth At this point we already see the beginning of the anus, gut, and mouth Image I: Distinct structures can be seen in the larval stage (called pluteus) --- anus where blastopore was --- mouth where stomodeum was --- skeletal rods circumvent the entire larva Cilia Hairlike projections that extend from the plasma membrane --- in embryonic development, the presence of cialis is a sign that differentiation has occurred. vegetal hemisphere Bottom portion of the egg which contains the yolk Primary mesenchyme cells Future Mesoderm Cells --- descendants of the micromeres During sea urchin gastrulation, these cells move from vegetal hemisphere to the blastocoel Movement of Primary Mesenchyme Cells - see "Sea Urchin Gastrulation 1", specifically image C-D, for context --- these cells are differentiated by this point --- have cilia Initially, these cells have several cell-adhesion proteins between adjacent cells proteins that bind the cells together --- causes a change in cytoskeleton dynamics so they are cube-shaped These cells are separated from the blastocoel by two layers 1) Basal Lamina, a layer of proteins that covers these cells and acts as an anchor Basal lamina acts as an anchor holding them in place 2) Extracellular matrix fibril, a long string of protein involved in the changing shape of the blastula Dissociation and Movement into Blastocoel Steps: 1) The cells suddenly lose their cell-adhesion proteins 2) Lack of cell adhesion proteins means they are no longer forced to be cube-shaped (cytoskeleton are no longer confined) and so these cells begin to expand/wiggle out from neighboring cells and pass through the basal lamina 4) Cells become more elongated until bit-by-bit they make their way up to the extracellular matrix fibril --- sometimes division can occur in the middle of this process (see pic D-E) archenteron - the primitive gastric cavity of an embryo - ---- eventually becomes the passageway used by the anus to dispel waste Blastopore - In gastrulation, it's the indentation formed at the very tip of the canal ---- becomes the anus in sea urchins secondary mesenchyme cells - are a heterogeneous population of cells with several different fates and behaviours. Taken together, a number of evidence indicates that SMCs function as multipotent stem cells skeletal rods - Part of their body part Stomodeum - primitive mouth in embryo Pluteus - larval stage of sea urchin prism stage just before pluteus (larval stage) SEM of primary mesenchyme cells inside the blastocoel [sea urchin - cross section] SEM of blastopore [sea urchin] SEM showing archenteron [sea urchin - cross section] SEM of sea urchin pluteus (larval stage) SEM of early development of sea urchin embryo, from blastula to prism stage “Know how to describe the process of gastrulation in the frog. How do the bottle cells contribute to this process? What structure contributes to the formation of the ventral lip?” Frog Blastula [recap] blastocoel is OFFSET toward ANIMAL POLE Cells are differentiated (adhesion proteins and ion channels) Different colors have different content (though have not formed the different layers yet, but indicate that they will eventually become those cells) Gray crescent that helped in blastula formation -- will be important for gastrulation Neural ectoderm is the part of the ectoderm that becomes the brain and spinal chord (CNS) new thing: bottle cells Bottle cells Cells in the frog blastula located close to the grey crescent area which are critical for the migration of cells in the blastula to their proper places as they develop into the three proper layers (direct gastrulation) --- push inward in the blastula and in the process displaces the other cells into their proper places during gastrulation --- I think they also arise from the gray crescent but idk??? As the bottle cells push inward, they creat an indentation which becomes the blastopore of a frog gastrula Frog Gastrulation 1 Blue : ectoderm precursor Yellow: endoderm precursor Red: Mesoderm precursor - [a lot of the terms in this are originally from sea urchin gastrulation, so be familiar with that as well] Step 1: Bottle cells located in the grey crescent area push inward into the blastula An indentation arises from this pushing which becomes the dorsal lip of blastopore (the formation of the dorsal lip of blastopore is a sign that gastrulation has started in frogs) [see "Frog Gastrulation 2" for pics from here] Step 2: As the bottle cell push even further into the blastula, it forms a canal called the archenteron Step 3: As the archenteron expands, the blastocoel is displaced and is eventually completely expelled from the blastula --- the blastocoel exits the cell through the ventral lip of blastopore Step 4: The movement of bottle cells displaces other cells in the blastula; which eventually leads to the ectoderm forming a layer on the outside perimeter of the blastula followed by the mesoderm forming a layer directly underneath the ectoderm --- ectoderm and mesoderm cell are pushed in the opposite direction during their formation --- endoderm cells are now closed off between the other two layers The archenteron forms a space on the inside of the blastula (in essentially the same location the blastocoel was) Step 5: Once (1) the archenteron has fully formed a space in the blastula and also (2) all the blastocoel has existed the blastula, the blastopore openings become smaller and thinner (no longer a big indention) till it is just a round, lip-shaped circle on the outside of the blastula --- top of the lip is still called the dorsal lip of blastopore and bottom of the lip is still called the ventral lip of blastopore --- center of this circular structure is filled with mesoderm cell (he says mesoderm, but the picture show endoderm cells???) called yolk plug --- formation of yolk plug is a signal of the end of gastrulation lateral tip forms brain and spinal cord Precursor to notochord - notochord: type of cell that supports the formation of the spinal chord, eventually forms vertebrate - formed from mesoderm in frog gastrulation Blastomere in frogs - As in the sea urchin, the frog blastopore becomes the anal end of the gut (deuterostome) - Two parts: dorsal lip of blastopore (top lip) and ventral lip of blastopore (bottom lip) - (also a lateral lip but idk where that comes up???_ - Dorsal lip forms when the bottle cells are pushing into the cell and creates the archenteron. - It becomes smaller once the archenteron has formed a space inside the blastula. - The ventral lip is formed underneath the dorsal lip so the blastocoel can exit the blastula. - It becomes smaller once all the blastocoel has exited the blastula. - Once both the dorsal and ventral lips have shrunk, the space formed between them is filled with mesoderm cell (he says mesoderm, but the picture show endoderm cells???) called yolk plug --- formation of yolk plug is a signal of the end of gastrulation --- yolk plug eventually becomes the anus yolk plug - --- eventually becomes the anus in frogs Frog Gastrulation Sign that gastrulation has begun: dorsal lip of blastopore Sign that gastrulation has ended:Yolk Plug Be familiar with the process of gastrulation in the in the chick embryo (know the contributions of the primitive streak, Henson’s node, primitive groove). How do the 3 main layers arise from the epiblast? What does the hypoblast contribute? Chick Blastula [recap] - Blastula forms in the oviduct, but unlike other blastula, chicks don't have a ball of cells and instead have a flat disk of cells called the blastodisc that sits on top of the yolk sac - Blastodisc undergoes multiple cleavages and forms a layer of cells called the blastoderm - Epiblast of blastoderm eventually becomes the embryo - Epiblast gives rise to a layer of cells underneath it called hypoblast that comes from the marginal zone in the posterior end; hypoblast supports epiblast during gastrulation - Space in between epiblast and hypoblast is the blastocoel - Cells in the posterior marginal zone become the Koller's sickle which will direct the formation of the primitive streak, which is an important first structure in gastrulation of the blastodisc - Membrane forms around developing a structure that eventually accumulates calcium carbonate to form a hard shell Gastrulation in Chicks 1 Primitive Streak Formation in the Chick [3-4 hours of incubation] Some cells in the blastodisc (he doesn't specify from where???) begin to move and congregate onto the posterior end of the blastodisc [location of the Koller's sickle] --- forms a dense pocket of cells [10-12 hours of incubation] The congregated cells begin to migrate towards to opposite end of the blastodisc (anterior end) in a line from one end to the other --- this line is called the primitive streak Formation of Primitive Groove (Chick Blastopore) [see "Gastrulation in Hens 2" for pic] [18-20 hours of incubation] The primitive streak has migrated as far as it can go The cells in the centermost part of the streak push inward into the developing gastrula. As they push inwards, they make an elongated indentation on the surface of the blastodisc called the Primitive Groove in the center of the primitive streak --- primitive groove is essentially the chick's blastopore (it is an inward push that results in the formation of the three germ layers) At the very anterior end of the primitive groove is the Hensen's node which is a special group of cells that direct this process, and the cells that ingress here form the head of the organism --- there is a dropoff near the Henden's node called the primitive pit As the primitive groove pushes inward into the blastodisc, the endoderm, ectoderm, and mesoderm are formed Formation of the Three Germ Layers [see "Gastrulation in Hens 3" for pic] [cross section cut across width of blastodisc] All three parts of the embryo (ectoderm, mesoderm, and endoderm) are formed from the epiblast. As the epiblast cells on the surface are pushed inwards, they start to become mesoderm and endoderm cells The remaining epiblast cells on the surface become the ectoderm Single layer of hypoblast (support gastrulation; serve as a layer between blastodisc and yolk ) remain located underneath these layers until they are displaced by the endoderm cells Primitive Streak - A thickening along the future anterior-posterior axis on the surface of an early avian or mammalian embryo, caused by a piling up of cells as they congregate at the midline before moving into the embryo. primitive ridge - portion of the primitive streak where it just starts to indent Primitive Groove - primitive groove is an elongated blastopore - A depression that forms within the primitive streak that serves as an opening through which migrating cells pass into the deep layers of the embryo in order to form the three germ layers. Hensen's node - the front [anterior] of the primitive streak in mammal and bird embryos - --- eventually becomes the head of the organism primitive pit - depression right next the Hensen's node Post-gastrulation Cross-Sections of Chick Embryo blue = epiblast that will become ectoderm orange = epiblast that will become mesoderm yellow = epiblast that will become endoderm (b) cross-section is cut along the length --- drop-off at Hensen's node is the primitive pit --- three germ layers have formed from the epiblast --- endoderm layer has displaced hypoblast (hypoblast is now off to the side) --- axial mesoderm = mesoderm close to the central axis you are looking at (c) cross-section is cut along the width --- primitive groove in the center --- primitive ridge: portion of the primitive streak where it just starts to indent --- in this case, the axial mesoderm is in the middle --- mesoderm cells that are close the axial mesoderm are paraxial mesoderm; mesoderm cells that are far off from the center are lateral mesoderm blue = epiblast that will become ectoderm orange = epiblast that will become mesoderm yellow = epiblast that will become endoderm