Blastula: An early staged embryo that is made up of sphere shaped cells that surround an inner fluid-filled cavity known as the blastocoel.

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1 1) Define the following terms: cleavage, blastomere, blastula, blastulation, microlecithal, mesolecithal, megalecithal, centrolecithal, isolecithal, telolecithal, holoblastic cleavage, meroblastic cleavage, superficial cleavage, discoidal cleavage, radial cleavage, spiral cleavage, bilateral cleavage, rotational cleavage, determinate cleavage, indeterminate cleavage, blastocoele, direct development, indirect development, protostomes, deuterostomes, epiboly, involution, invagination, ectoderm, mesoderm, endoderm, delamination, cell migration, archenteron, primitive gut Cleavage: period of rapid cell division of the egg cytoplasm that forms blastula. It s a series of mitotic division of the egg cytoplasm that form nucleated cells, known as the blastula. Blastomere: A cell produced from cleavage through mitosis Blastula: An early staged embryo that is made up of sphere shaped cells that surround an inner fluid-filled cavity known as the blastocoel. Blastulation: It is the stage in embryonic development in which the blastula is formed right after the morula stage and then proceeds to gastrulation. Microlecithal: very little amount of yolk in the haloblastic cells, formation of complete and separated cells. Mesolecithal: moderate amount of yolk in the haloblastic cells. Megalecithal: large amount of yolk Centroecithal: yolk in the centre of the egg in the meroblastic cells, which are incomplete separated cells. Isolecithal: uniform yolk distribution in haloblastic cells Teloecithal: yolk at one end of the meroblastic cells, which are incomplete separated cells. Holoblastic cleavage: Is the formation of complete and separated cells, this consist of very little yolk Meroblastic cleavage: is the incomplete separation of cells because it contains a lot of yolk which prevent them from going through cleavage. Superficial cleavage: Discoidal cleavage: Radial cleavage: Spiral cleavage: Bilateral cleavage: Rotational cleavage: Determinate cleavage: Indeterminate cleavage: Blastocoele: a fluid filled cavity in animal cells Direct development: Indirect development: Protostomes: develop the mouth first from the blastopore

2 Deuterostomes: develop anus first from the blastospore Epiboly: movement of epithelial sheets that spread to enclose deeper layers of the embryo, by division, change in shape, or by several layers intercalating. Involution: inward movement of expanding outer layer over internal side of remaining external cells Invagination: in-folding region of cells Ectoderm: outer layer Mesoderm: middle layer Endoderm: inner layer Delamination: splitting of one cellular sheet into two Cell migration: Archenteron: when the first invaginate 1/4 to 1/2 the way into the blastocoel forming Primitive gut: archenteron 2) Is protein synthesis essential during early cleavage? Is mrna synthesis essential during early cleavage? Proteins and mrna are essential during early cleavage. mrna encodes cyclic B, which is responsible for mitotic division in the S phase. Cyclic B is responsible for activating cyclin depended kinase, which is a protein. 3) Actinomycin D does not arrest early amphibian cleavage but blocks gastrulation. What sort of conclusions can you draw from this observation? 4) What would you consider to be two most important things that an embryo accomplishes during the process of cleavage and blastulation? (1) The amount and the distribution of the yolk in the cytoplasm determines where cleavage will occur and the formation of the blastomeres. The yolk free pole is the animal pole and the yolk rich pole is the vegetal pole. (2) The factors of the egg cytoplasm which influence the angle of the mitotic spindles. 5) How does the cell cycle which occurs during cleavage differ from the cell cycle of normal adult body cells? The cell cycle in the cleavage only has a S and M phase. Cyclin B synthesis allows the progression of M (mitosis) and then the degradation of the cyclin B allows the cells to pass into the S (synthesis) phase. Where as the cell cycle of the normal adult body cell goes through M (mitosis) which is then followed by the interphase which consists of the G 1, S (synthesis), G 2 phases. 6) Distinguish between complete (holoblastic) cleavage and incomplete (meroblastic) cleavage. Holoblastic cleavage is the formation of complete and separate cells, which contain very little yolk and meroblastic cleavage also known as incomplete is the incomplete separation of the cells as a result of too much yolk.

3 7) Distinguish between determinate and indeterminate cleavage? Indeterminate cleavage is when cells are separated from each other and have the potential to form into entire organisms on their own. Determinate cleavage is when cells are separated from each other, they will die. Neither of the cells have the potential to develop into entire organisms on its own. 8) Comment briefly on the relative importance of DNA synthesis, RNA synthesis and protein synthesis during cleavage. What do you think, what cellular structures are being most actively synthesized? 9) Why does the terrestrial environment present more difficulties for embryonic development than the fresh water environment, and why does the fresh water environment in turn present more difficulties for development than the marine environment? Terrestrial environments are harsh and the amount of yolk is substantial unless carried internally. Fresh water environments are less hospitable and the amount of yolk is moderate. Marine environment is stable/hospitable and has small amount of yolk, leading to haloblastic cells. 10) What major embryonic and reproductive adaptations are seen in terrestrial and fresh water animals for dealing with these environments and helping to ensure survival of the species? The terrestrial animal eggs are enclosed in shell or carried internally or laid in moist environments. The Fresh water animal eggs are retained by parent or get attached to the substrate. 11) What are the major developmental features which allow the metazoans to be classified into two groups, the protostomes and the deuterostomes? Bilateral symmetrical animals are divided into two groups, the protostomes and the deuterostomes. Protostomes develop the mouth first from the blastopore, if the anus is present it will develop from another region of the embryo. The deuterostomes develop anus first from the blastopore and the mouth develops later from another region of the embryo. 12) Which germ layer(s) form pseudocoelom/coelom in pseudocoelomates vs. coelomates? The pseudocoelomates has the body cavity in between the mesoderm and endoderm, which is called the pseudocoelom. The Coelomates has the body cavity entirely within the mesoderm, called the coelom. 13) What determines cell fate? Cell fate is determined by: (1) Assymetric cell division (2) Cell to cell interaction (3) Cell to cell communication (4) Position of the cell in the embryo 14) Explain the role of asymmetric cell divisions (or cell-cell communication(s) or position of the cell in an embryo) in determination of the cell fate Assymetric cell division (sister cells born different): leads to the formation of cells which have different cytoplasm. So there is an unequal number of mrna to be transcribed and protein in egg cytoplasm. Cell to cell interactions: cells in tissues are connected to each other by cell junctions. There are different kinds of cell junctions: tight junctions, anchoring junctions, communicating junctions, gap junctions, and plasmodesmata.

4 Cell to cell communication: developing cells receive signals from surrounding cells which in turn signal the cells around them. 15) Define induction and competence. Induction: the interaction of cells or tissues with different histories and properties at close range between two or more cells. Competence: the ability of the cell or tissue to respond to a specific inductive signal. 16) Define instructive and permissive effects. Instructive: a signal from the inducing cell which is necessary for initiating new gene expression in responding cell. Permissive effects: the responding cell is specified and now needs an environment that allows it to express its traits. 17) What is a morphogen? What are the types of cell-cell communications in respect to morphogen production and distribution? Morphogen: is the signaling molecule (ligand, ion, hormone, protein, or polypeptide growth factor) The types of cell-cell communication that lead to the production and distribution: Juxtacrine: cell membrane proteins on the surface of one cell are recognized by the receptors on the adjacent cells, this is known as direct contact. Paracrine: morphogen that is released from the cell that has an effect on the neighbouring cells. Endocrine: morphogens (hormones) that are released from a cell have an effect throughout the body. Autocrine: cell secrete paracrine factor = morphogens and respond to it. 18) How do morphogen gradients pattern developing tissues? (You can use diagram to aid an explanation.) A gradient of signaling molecules within tissues provide positional information, which instructs the cell to adopt a distinct fate. The cell fate is determined based on the concentration of the signaling molecules the cell is exposed to. 19) Answer briefly the following three connected questions: a) What is signal transduction? b) What is the most common outcome of signal transduction? c) What is the relationship between signal transduction and cell differentiation? (a) extracellular signals (morphogens) are received at the cell membrane (receptors) and transmitted into the cytoplasm of the cell of close proximity. External signals are transmitted to the interior of the cell. (b) 20) List the (3) modes of cell type specification and give the two most important characteristics we discussed in class. (Can you give the examples?) Autonomous Specification Conditional Specification Syncytial Specification: specification of body regions by interaction between cytoplasmic regions prior to cellularization of blastoderm. 21) Define the conditional specification of cells and give an example from the nematode [worm] C. elegans. (or the sea urchin) Conditional Specification: specification by interactions between cells. Relative positions are important, the cells fate depends on it s position in the embryo as the neighbouring cells have a great influence since they produce signals which will determine the fate of the cell, the closer the cell the higher the effect

5 22) Define autonomous specification of cells and give an example from the sea urchin (or the nematode). Autonomous Specification: differential acquisition (gaining) by certain cytoplasm molecules present in the egg. The large micromere produces larval cytoskeleton, which is an example of autonomous specification in sea urchins. Cells fate which is determined by autonomous specification are not influenced by neighbouring cells 23) Describe the mechanism by which the primary mesenchyme cells detach from the other vegetal pole cells and enter the blastocoel. The vegetal side of blastula thickens and flattens as it hatches from the fertilization envelope. At the centre of the vegetal plate a large cluster of micromere cells extend filopodia from inner surface, these cells (mesenchyme cells) dissociate from the monolayer (larval skeleton) and migrate into the blastocoels. The blastula are connected to hyaline layer and to basal lamina inside (toward blastocoels). The mesenchyme cells lose affinity to hyaline layer and migrate to the blastocoels. The filopodia is formed to provide direction and keep migrating along the extracellular matrix of the blastocoels. 24) Describe the mechanism which accounts for the invagination of the vegetal pole cells during gastrulation in the sea urchin. After primary mesenchyme migration, the rest of the vegetal cell moves inwards. First invaginate ¼ to ½ the way to blastocoels forming the archeonteron (primitive gut). The opening that is formed is called the blastopore. The archeonteron extends dramatically. It increases in length, the short gut becomes a long tube, the cells migrate over one another and flatten. The secondary mesenchyme cells extend the filopodia pulling up on the archeonteron. 25) Describe and give specific examples of how changes in cell adhesion and changes in the migratory properties of cells are important during early embryonic development.