Vertebrates in Relation to other Animals

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Nancy Loadman

2.1 Vertebrates in Relation to Other Animals • Vertebrates are in the subphylum Vertebrata of the phylum Chordata. • Phylum Arthropoda is the only one to compare in terms of diversity and habitat. • Phylum Mollusca is the only one that we can find animals that approach the large size of many vertebrates and have a capacity for complex learning. • Tunicates (subphylum Urochordata) and cephalochordates (subphylum Cephalochordata) are placed with vertebrates in phylum Chordata, but vertebrates and cephaochordates are more closely related to each other than to tunicates. • Chordates share the following derived characters : 1. Notochord (dorsal supportive rod) 2. Dorsal hollow nerve cord 3. A segmented, muscular postanal tail 4. Paired pharyngeal gill slits 5. Subpharyngeal gland that binds iodine (endostyle or thyroid) in the ventral pharyngeal region. • All chordates are essentially bilaterally symmetrical with some left to right asymmetry such as a heart on the left side and liver on the right. • We determine the relationship between chordates and other animals with anatomical, biochemical, and embryological characters in addition to the fossil record. • Chordates, Echinoderms, Hemichordates, and Xenoturbellids are linked as deuterostomes (meaning second stomach) based on the egg cleavage formed and evidence in their larval form. • Although extant Echinoderms lack pharyngeal slits, some extinct species appear to have had them at a point of greater diversity. They were also bilaterally symmetrical, indicating that the modern fivefold symmetry is likely a derived character of the lineage. • All animals (Metazoans) are multicellular and share common embryonic and reproductive features such as an embryo that forms a blastula, sex cells that form in special organs, and motile sperm cells. • Animals derived from sponges have a nervous system and multiple germ layers that form during gastrulation which is the process in which the blastula folds in upon itself, producing two distinct layers of cells and an inner gut with an opening to the outside at one end. o Ectoderm-outer germ layer o Endoderm-Inner germ layer o Mesoderm- Additional middle layer present in triploblastic organisms rather than diploblastic. • Mesoderm forms body's muscle that allows for motile adults; larvae are small enough for cilia to move them. • Coelom- inner body cavity that forms the split within the mesoderm. • Coelomates are split into two groups based on whether or not a second process occurs after the blastula forms a gastrula with an opening called the blastopore. 1. Protostomes- Only have the blastopore opening to the body (ex. Mollusks, Arthropods, Annelids, etc.) 2. Deuterostomes- Form a second opening during embryonic development allowing for a mouth and an anus (ex. Chordates, Hemichordates, and Echinoderms). Non-Vertebrate Chordates Urochordates and Cephalochordates are the only extant non-vertebrate chordates. Due to their soft bodies, it is hard to preserve them in fossils and thus hard to establish a clear lineage. Urochordates • Tunicates ( ~2000 extant species) • Perforated pharynx for filter feeding • Most adults are sedentary, with larvae that have a notochord, a dorsal hollow nerve cord, and a muscular postanal tail. • Larvae swimming stage can last between minutes to days. • The sessile adult Tunicates appears to be a derived form. Cephalochordates (~22 species) • Fishlike marine animals less than 5cm in length that have nearly identical front and rear shapes. • Lancelets which are also known as amphioxus are perhaps most well known. • Most adults are sedentary, but those that move use fish-like myomeres which are blocks of striated muscle fibers arranged along both sides of the body and separated by sheets of connective tissue. • Amphioxus have no gill tissue associated with pharyngeal slits because they are small enough for gas exchange to occur via diffusions. The slits are used for filter feeding. • Amphioxus has a coelom and an external body cavity called the atrium which is also present in Tunicates and Hemichordates indicating that it is likely a shared primitive deuterostome character. • Atrium is formed by outgrowths of the body wall (metaplural folds), which enclose the body ventrally. • Atrium has an opening to outside called the atriopore, which is an opening in front of the anus. • The atrium, beating cilia on gill bars, and the wheel organ in the head control the passage of substances through the pharynx and probably functionally associated with primitive chordate feature of gill slits for filter feeding. • Derived characters shared with vertebrates, but not tunicates such as myomeres, similar circulatory systems, dorsal aorta, and a ventral heart-like structure that forces blood through the gills. They lack kidneys, but vertebrates and amphioxus share excretory cells called podocytes and amphioxus have a vertebrate-like tail fin. Cambrian Chordates • Best known early chordate-like animal- Pikaia that had myomeres and a notochord running across 2/3 of the body. Myomeres were straight rather than V shaped. • Southern China- 520 million year old Chengiang Fauna contains earliest-known true vertebrates and possible early chordates, Vetulicolians and Yunnanozoans. 2.2 Definition of a Vertebrate • Vertebrate derived from term vertebra. • Vertebrae form around notochord during development and encircle the nerve cord. Vertebrae may be made of bone or cartilage. However, not all animals included in the traditional subphylum Vertebrata have Vertebrae. • The cranium which can be bony, cartilaginous, or fibrous, however, is a unique derived character leading to some people preferring to refer to the subphylum as Craniates. • Hagfishes lack vertebral elements entirely and Lampreys have cartilaginous rudiments (arcualia) around nerve cord. • Gnathostomes have a fully formed vertebrae with a centrum surrounding the notochord. Two embryonic features may account for many of the differences between vertebrates and other chordates: 1. Hox gene complex- these genes regulate the expression of a hierarchy of other genes that control the process of development along the long axis of the body from front to back. Vertebrates are unique in that they have undergone the duplication of the entire Hox gene complex. • First duplication event at the start of vertebrate evolution; the amphioxus has a single Hox cluster, while the living jawless vertebrates have two. • Second duplication event took place before the evolution of gnathostomes, with all jawed vertebrates having at least four clusters. • Additional duplications took place within the ray-finned bony fishes; teleosts may have up to seven clusters. 2. Neural Crest- A type of embryonic tissue unique to vertebrates that forms many structures, especially in the head region. This can be considered as a fourth germ layers unique to vertebrates. These cells originate at the lateral boundary of the neural plate, the embryonic structure that makes the nerve cord. • Similar cells have been found in Tunicates, but they cannot migrate and change into different cell types. These cells in nonvertebrate chordates may represent the precursor to the vertebrate condition of neural crest. • Embryonic tissue that may be related to neural crest forms the epidermal placodes (i.e., thickenings), which give rise to the complex sensory organs of vertebrates including the nose, eyes, and inner ear. • The brains of vertebrates contain a forebrain, midbrain, and hindbrain. Amphioxus brains may be homologous to vertebrate brains with the exception of the telencephalon, the front of the brain. 2.3 Basic Vertebrate Structure Embryology • Invertebrates develop from cell lineages whose fate is predetermined, but vertebrates are much more flexible in their development and use inductive interactions between developing structures to determine the information of different cell types and tissues. • After the development of the germ layers, the coelom begins to form within the mesoderm. This cavity is divided into two cavities:  Pleuroperitoneal Cavity around the viscera  Pericardial Cavity around the heart. • These cavities are lined by thin sheets of mesoderm-- the peritoneum (=the pericardium around the heart). • The gut is suspended in the peritoneal cavity by sheet of peritoneum called mesenteries. • Neural crest forms the bones and muscles in the anterior of the head region, almost all of the peripheral nervous system, and contributes to parts of the brain. In more derived vertebrates, it forms the adrenal glands, pigment cells in the skin, secretory cells of the gut, and smooth muscle tissue lining the aorta. • Pharyngeal clefts  In fish- perforate into gill slits  Land vertebrates- the disappear • Lining of Pharyngeal pouches  Gives rise to half a dozen or more glandular structures often associated with the lymphatic system, including the thymus gland, parathyroid glands, carotid bodies and tonsils. • Dorsal Hollow Nerve Cord  Formed by the infolding and subsequent pinching off and isolation of a long ridge of ectoderm running dorsal to the developing notochord. The notochord has the instructions for development.  The neural tube, next to the nerve cord, will develop into the neural crest. These cells will then disperse throughout the organism. • Embryonic Mesoderm  An epimere, a series of thick-walled segmental buds (somites), is formed along the dorsal side from head to tail end.  The ventral part, surround the gut and containing the coelom, is thin- walled and unsegmented and called the lateral plate (the hypomere).  Small segmental buds linking the somites and the lateral plate are called nephrotomes (the mesomere).  The segmental somites will eventually form the dermis of the skin, the striated muscles of the body that are used in locomotion, and portions of the skeleton ( the vertebral column, ribs, and portions of the back of the skull).  Some of these segmental muscles later migrate ventrally from their originally dorsal (epaxial) position to form the layer of striated muscles on the underside of the body (the hypaxial muscles), and from there they form the muscles of the limbs in tetrapods.  The lateral plated forms all the internal, nonsegmented portions of the body, such as the connective tissue, the blood vascularly system, the mesenteries, the peritoneal and pericardial linings of the coelomic cavities, and the reproductive system. It forms the smooth muscle of the gut and cardiac muscle.  The nephrotomes form the kidneys, the kidney drainage ducts (archinephric ducts) and the gonads.  The axial and appendicular muscles and the skeleton are derived from the somites. The limb bones are primarily derived from the lateral plate (in addition to the tendon and ligaments of appendicular muscles).  The head mesoderm contains only somites which give rise to the striated eye muscles and branchiomeric muscles powering the pharyngeal arches (gills and jaws). Adult Tissue Types • There are five kinds of tissue in vertebrates: epithelial, connective, vascular, muscular, and nervous that work together to form organs. • Fibrous collagen is primarily a mesodermal tissue that forms the soft tissues of organs and forms the organic matrix of bone, tendons, and ligaments. Collagen is typically stiff and doesn't stretch, but may be found combined with elastin protein in certain tissues that allows stretching and recoil. • Keratin is another unique fibrous protein to vertebrates; this primarily ectodermal tissue is found in the epidermis of tetrapods, making structures such as hair, scales, feathers, claws, horns, beaks, etc., but it also forms horny tooth-like structures of the living jawless vertebrates. The Integument • The external covering of vertebrates that includes skin, glands, scales, dermal armor, and hair. • Vertebrate skin splits into:  Epidermis • Superficial skin layer that often contains secretory glands and may play a significant role in osmotic and volume regulation.  Dermis • Collagen fibers, blood vessels in which blood flow is controlled neurally and hormonally, melanocytes (cells with melanin that derive from neural crest), and smooth muscle fibers. • In tetrapods, dermis contains sensory structures and nerves associated with sensations of temperature, pressure and pain. • Hypodermis/ Subcutaneous layer lies between dermis and the fascia over muscles. This layer contains collagenous and elastic fibers in addition to subcutaneous fat (in birds or mammals) or striated muscles. Mineralized Tissues • Hydroxyapatite (complex compound of Calcium and Phosphorus), found in vertebrates, is more resistant to acid than is calcite (Calcium Carbonate) and developed because of the Vertebrate anaerobic metabolism that produces lactic acid. • Tissues that can mineralize in Vertebrates: 1. Cartilage • Not usually mineralized, but can be induced to mineralize in vitro lampreys, squid, whelks, and horse shoe crabs. • Found naturally to mineralized vivo in jawed vertebrates as the main skeleton in sharks. • Cartilage and bone of jawed vertebrates is formed from an initial proteinacecous matrix of collagen. • The cartilage of living jawless fishes is noncollagen-based. 1. Bone • Mineralized tissue of the internal skeleton of bony fishes and tetrapods. • Composed of osteocytes ("bone"-"cells") called osteoblasts ("blasto"= "bud"). Chondrocytes form cartilage. • Cells that form bone and cartilage cells are derived from the mesoderm, except in the region of the front of the head, where they are derived from the neural crest tissue. • Highly vascularized which allows bone to remodel itself with the help of osteoclasts, specialized blood cells that remove old bone, and osteoblasts which deposit new bone. • Two main types of bone found in vertebrates: 1. Dermal Bone- is formed in the skin without a cartilaginous precursor. This is the primitive type of bone first seen in fossil jawless vertebrates called ostracoderms. 2. Endochondral Bone- formed inside cartilage. This is seen in bony fishes and tetrapods. 1. Enamel • 99% mineralized • Formed of amyloblasts which are derived of ectoderm. 1. Dentine • 90% mineralized • Formed of odontoblasts which are derived from the neural crest tissue. 1. Enameloid • Enamel-like tissue of primitive vertebrates found in modern fishes. • Mesodermally derived 1. Cementum • Bone-like substance that fastens the teeth in their sockets in some vertebrates, including mammals, and that may grow to become part of the tooth structure itself. • Teeth form from a type of structure called a dermal papilla so they form only in the skin, usually over dermal bones. When the tooth is fully formed, it erupts through the gum line. Replacement teeth may start to develop to one side of the main tooth even before its eruption. The basic structure of the teeth of jawed vertebrates is like the structure of odontodes, which were the original tooth-like components of primitive vertebrate dermal armor. The Skeletomuscular System The Cranial Skeleton and Musculature • Skull 1. Chondrocranium (around the brain and derived of neural crest). 2. Splanchnocranium (forming gill supports; also known as "gill arches") • May also be referred to as pharyngeal arches, branchial arches or visceral arches. They are commonly termed pharyngeal arches in development and branchial arches/gill arches in adults. • Splanchnocranium precedes origin of vertebrates and neural crest. • In primitive vertebrate condition, the chondrocranium and splanchnocranium are formed from cartilage, but, in some adults of some bony fishes and tetrapods, they are made of endochondral bone. 3. Dermatocranium (forming in the skin as outer cover and not seen in earliest vertebrates) • Dermacranium made of dermal bone. (Refer to Figure 2-8 and 2-9 from text). • Striated muscle in head of vertebrates: 1. Extrinsic eye muscles- 6 muscles (7 in lampreys) in each eye move the eye except in Hagfishes (from secondary loss). • Innervated by somatic motor nerves 2. Branchiomeric muscles- associated with splanchnocranium and sucks water into mouth during feeding or respiration. • Innervated by nerves fro the brain that exit dorsally, rather than motor nerves exiting from ventral part of the spinal cord or brain. The Axial Skeleton and Musculature •Notochord original backbone to vertebrates, that is replaced by cartilage or bone vertebrae later. •Notochord o Large, closely spaced cells that are packed with incompressible fluid-filled vacuoles. o Wrapped in complex fibrous sheath as site of attachment for segmental muscles and connective tissues o Ends anteriorly just posterior to the pituitary gland and continues posteriorly to the tip of the fleshy portion of the tail. o Most of notocord form lost in adult tetrapods, but remains between vertebrae as intervertebral discs. •Axial Muscles o Myomeres that fold in 3 dimensions with one extending anteriorly and posteriorly over several body segments. o Sequential muscle blocks overlap and produce undulation of the body when they contract. o Myomeres have V shape in amphioxus and W shape in vertebrates. o Myomeres of jawed vertebrates are divided into epaxial (dorsal) and hypaxial (ventral) portions by a sheet of fibrous tissue called the horizontal septum. o Herringbone (muscles) in similar interlocking V shape as myomeres. It is less obvious in tetra
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