Phyla Cnidaria and Ctenophora, Basic Animal Development ...

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Phyla Cnidaria and Ctenophora, Basic Animal Development & Introduction to Nervous Systems 5.4 Lab #5 - Biological Sciences 102 – Animal Biology In animals such as sponges and cnidarians, cleavage is irregular and seemingly disorganized; egg cytoplasm is partitioned randomly into daughter cells of highly variable size and shape with no apparent relevance to future cell fates. As the metazoa evolved cleavage began to follow precise patterns and rhythms. In virtually all animal groups above the cnidarians, cleavage is regular; the egg cytoplasm is segregated into specific cells called blastomeres (Gr. blastos, bud, + meros, part) occupying discrete positions and having specific developmental fates. Patterns of regular cleavage depend greatly on amount and distribution of yolk in the egg. In eggs having a large amount of yolk, cleavage may be either complete (= holoblastic), as in amphibians, or incomplete (= meroblastic), as in birds and reptiles. In birds and reptiles with extreme telolecithal (Gr. telos, end, + lekithos, yolk) eggs, cleavage is restricted to a small disc of cytoplasm on the animal pole; this type of cleavage is called discoidal. The eggs of most insects follow another pattern of cleavage called superficial. In these the nuclei divide mitotically into hundreds or thousands of "free" nuclei, which later migrate to the egg surface. Only then do cleavage furrows form, rapidly partitioning the cytoplasm into a superficial layer of cells. In most invertebrates, eggs have little yolk (= isolecithal ["equal-yolk"]), and cleavage is complete (holoblastic) and equal. Two major kinds of holoblastic cleavage exist: spiral and radial (see text page 156, fig 8-7). The first two cleavages are the same in both kinds of eggs: the cleavage planes are along the animal-vegetal axis, producing a quartet of cells. At the third cleavage, however, these two patterns, spiral and radial, can be distinguished from each other by the geometric positioning of the cells. In radial cleavage, the third cleavage is perpendicular to the first two, yielding two quartets of cells, with the upper quartet lying directly on top of the lower. In spiral cleavage, the third cleavage planes are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to lie between the furrows of the lower quartet of larger cells. There are other important differences between these two cleavage patterns. Spiral cleavage is typically mosaic, meaning that the embryo is constructed as a mosaic, with each cell fitting into its predetermined location in the larval body. If cells of the embryo are experimentally separated at this early stage, each cell will develop into partial or defective larvae because the developmental fate of each cell has already been determined. Spiral cleavage is found in several phyla, including annelids, many molluscs, some flatworms, and ribbon worms (nemerteans). All groups showing spiral cleavage belong to the grouping of animal phyla called the Protostomia, in which the embryonic blastopore forms the mouth. Early Embryonic Development - Cell Cleavages (mitosis) Radial cleavage is characteristically regulative because cell fate does not become fixed until after the first few cleavages. Radial cleavage is found in eggs of echinoderms and many chordates, especially protochordates, amphibians, and mammals. (As mentioned earlier, eggs of birds and reptiles, as well as many fishes, show discoidal cleavage.) All of these belong to the Deuterostomia, a group of phyla in which the mouth is formed from a secondary embryonic opening.
Phyla Cnidaria and Ctenophora, Basic Animal Development & Introduction to Nervous Systems 5.5 Lab #5 - Biological Sciences 102 – Animal Biology Introduction to Nervous Systems: Nerve Nets in Cnidarians The cnidarian nerve net is an excellent example of a simple, diffuse nervous system. The nerve cells form a plexus (a network) at the base of the epidermis and the gastrodermis. There are two interconnected nerve nets, one in each tissue layer. Axons (nerve processes) end on other nerve cells at synapses or neural junctions with sensory cells or effector organs. As seen in most animals, nerve impulses are transmitted from one cell to another by release of a neurotransmitter via synaptic vesicles in the presynaptic neuron. One way transmission between neurons in higher animals occurs is because of the relative activity of the protein channels for sodium and potassium in the axons that generate the electrical signals (action potentials). In addition, synaptic vesicles are only found in the nerve cell terminal (synaptic buton). Cnidarian nerve nets are comprised of cells that have synaptic vesicles on both sides and different ion channel properties allowing transmission across the synapse in either direction. Most nerve cells in the epidermis are multipolar (many processes) although some have bipolar neurons (two processes). In cnidarians, there can be both one-way and twoway synapses with other neurons. Cnidarian neurons lack the myelin sheaths which we will see in other animals. This myelin is a fatty acid membrane (in fact, a cell membrane) that will increase the speed of action potentials in larger, more complex animals. There is no “central nervous system” in cnidarians where a concentration and integration between many neurons occurs as seen in other animals. In other animals there is a cerebral ganglion or brain and dorsal or ventral nerve cord which controls the activity of other peripheral nerve cells. In cnidarians, there is some organization to the nerve net. The nerves are grouped in ring nerves in the medusae of hydrozoans and in the marginal sense organs of scyphozoan medusae. In schyphozoans there is a fast conducting nervous system to coordinate swimming movements and a slower conducting system that coordinates the activity of the tentacles. Sensory cells synapse with nerve cells that have junctions with epitheliomuscular cells and nematocysts. Collectively this combination of cells forms a neuromuscular system which is an important milestone in the evolution of nervous systems. In fact, versions of nerve nets (plexuses) coordinate the rhythmic contractions of the digestive systems of many other more complex invertebrate and vertebrate animals. Some types of sensory cells found scattered in the cnidarian epidermis: Chemoreceptors = detect molecules/chemicals in the environment (eg. prey) mechanoreceptors or tactile receptors = responds to touch or water movement Some types of effector cells found scattered in the cnidarian epidermis: cnidocytes = have nematocysts epitheliomuscular cells = for covering and muscular contraction Near the end of the lab, your instructor will describe the basic structure of a multipolar nerve cell (neuron). Be sure to copy and label the board diagram in your notes.
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