Comparative Vertebrate Anatomy

Comparative Vertebrate Anatomy Chordate Origins & Phylogeny Presented by: Geonyzl Lepiten

Comparative vertebrate anatomy - the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates: Kingdom: Animal Phylum: Chordata Subphylum: Vertebrata

Vertebrate characteristics: 1 - notochord (at least in the embryo) 2 - pharynx with pouches or slits in wall (at least in the embryo) 3 - dorsal, hollow nervous system 4 - vertebral column

Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal Fate of notochord during development: Head region - incorporated into floor of skull Trunk & tail - surrounded by cartilaginous or bony vertebrate (except in Agnathans)

Adults: Fishes & amphibians - notochord persists the length of the trunk & tail but is constricted within the centrum of each vertebra Reptiles, birds, & mammals - notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals) Protochordates - notochord remains as the chief axial skeleton Agnathans - lateral neural cartilages are located on notochord lateral to the spinal cord

Pharynx - region of alimentary canal exhibiting pharyngeal pouches in embryo; pouches may open to the exterior as slits : permanent slits - adults that live in water & breathe via gills temporary slits - adults live on land

Dorsal, hollow central nervous system - consists of brain & spinal cord & contains a central cavity (called the neurocoel)

Among the oldest & best known = ostracoderms fishes that occurred in the late Cambrian period (see The Cambrian Explosion ) through the Devonian (about 400 - 525 million years before present) had bony plates and scales (&, therefore, were easily fossilized) jawless vertebrates called 'armored fishes'

Before ostracoderms? Myllokunmingia fengjiaoa (pictured below) & Haikouichthys ercaicunensis - primitive fish that have many similarities to living hagfishes and are the oldest vertebrates (530 mybf) ever found.

Before Vertebrates? Cathaymyrus diadexus (literally the 'Chinese eel of good fortune') = is not the fossil of an eel. At just 5 cm long, but 535 m.y. old, it is the earliest known chordate. = Researchers think that Cathaymyrus is a fossil relative of modern lancelets (amphioxus).

Phylum Chordata - established in 1874 & included organisms with: 1 - notochord 2 - pharyngeal pouches or slits 3 - dorsal, hollow nervous system 4 - cells that produce the hormone thyroxine

Subphylum Urochordata = tunicates Chordate 'ancestor' of vertebrates: sessile (like adult tunicates) tail evolved as adaptation in larvae to increase mobility 'higher forms' - came about by retention of tail (neoteny) Tunicate larva - also called 'sea squirt' notochord is confined to the tail notochord is lost during metamorphosis into sessile adult possess pharyngeal slits

Tunicate anatomy Larval stage of the tunicate

Subphylum Cephalochordata = Amphioxus (or Branchiostoma) Vertebrate features : notochord dorsal, hollow nervous system pharyngeal gill slits 'circulatory' system - vertebrate pattern with 'pumping vessels' (but no heart)

Hemichordates = acorn worms Bateson added acorn worms to the phylum Chordata in 1884 because they have: 1 - a dorsal, hollow nervous system 2 - gill slits 3 - a short diverticulum of the gut called the stomochord Present consensus = the stomochord is not homologous with the notochord and Hemichordates are placed in a separate phylum

Possible invertebrate ancestors: 1 - annelid worms Evidence for: bilateral symmetry segmented central nervous system with brain & longitudinal nerve cord Evidence against: nerve cord is solid nerve cord is ventral

2 - echinodermata - chordate characteristics include: radial cleavage - blastomeres in adjacent tiers lie directly above one another (as opposed to spiral cleavage) anus forms near or at blastopore (deuterostomous) mesoderm arises as outpocketing of the gut wall indeterminate cleavage (i.e., fate of blastomeres isn't predetermined)

Phylum: Chordata Subphylum: Vertebrata Superclass: Pisces Class Agnatha Class Placodermii Class Chondricthyes Class Acanthodii Class Osteichthyes Superclass: Tetrapoda Class Amphibia Class Reptilia Class Aves Class Mammalia

Agnathans vs. Gnathostomes: semicircular canals agnathans have 1 or 2 gnathostomes have 3 jointed, paired lateral appendages agnathans have none gnathostomes do jaws agnathans have none gnathostomes do

Class Agnatha Orders: 1 - Osteostraci 2 - Anaspida 3 - Thelodonti 4 - Galeaspida 5 - Pituriaspida 6 - Petromyzontia (lampreys) 7 - Myxinoidea (hagfishes)

Ostracoderms (Osteostraci, Anaspida, Heterostraci, & Coelolepid): 1 - extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone ('bony armor') 2 - oldest known vertebrates 3 - many had flattened appearance (some may have been bottom-dwellers)

Cyclostomes (Petromyzontia & Myxinoidea): Lampreys - parasitic with horny, rasping teeth (see drawing at right) Hagfishes - primarily scavengers

Gnathostomes Acanthodians: 1 - earliest known gnathostomes ( Silurian ; about 440 mybp) 2 - probably related to modern bony fishes 3 - small (less than 20 cm long) with large eyes 4 - Acanthodians most likely died out because of the rapidly increasing number of ray-finned fishes and sharks during the Permian

Class Placodermii : 1 - Silurian (about 420 million years before present) 2 - probably off the main line of vertebrate evolution 3 - many had bony dermal shields 4 - some were probably predators (with large, sharp 'tooth plates')

Types Eggs Alecithal = Eggs with little yolk ex. Amphioxus egg b. Mesolecithal = eggs with moderate amount of yolk ex. Freshwater lampreys ganoid fishes lungfishes amphibians

C. Megalecithal = massive amount of yolk ex. Monotremes marine lampreys teleost reptiles birds

Types of distribution of yolk Isolecithal = even distribution of yolk present in alecithal eggs b. Telolecithal = the cytoplasm and yolk tends to concentrate or accumulate at the oposite poles. present in mesolecithal eggs and in megalecithal eggs

Oviparity and Viviparity vivipary : the embryo develops inside the body of the mother and living young is delivered : reared by the mother. : but the eggs of viviparous animals lack a hard outer covering or shell like the chicken egg. :Viviparous young grow in the adult female until they are able to survive on their own outside her body.

:developing fetuses of viviparous animals are connected to a placenta in the mother's body Egg-laying, or oviparous , animals obtain all nourishment as they develop from the yolk and the protein-rich albumen, or "white," in the egg itself, not from direct contact with the mother, as is the case with viviparous young. : expulsion of undeveloped eggs rather than live young.

ovoviviparity : animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch : employed by many aquatic life forms such as fish and some sharks , reptiles , and invertebrates . The young of ovoviviparous amphibians are sometimes born as larvae , and undergo metamorphosis outside the body of the mother.

In fertilization: Gametes are essential in fertilizing the eggs - sperm which came from the male - ovum from the females when the female and male gametes unite it will form into zygote.

Cleavage The fertilized egg (zygote) is transformed by cell division (cleavage) into a mutlicellular cells called Blastula During cleavage the individual cells are called blastomere The blastula is a hollow ball of cells with a cavity is called blastocoel

In microlecithal eggs like in amphioxus have total or holoblastic cleavage (the cleavage furrows the entire yolk) Divided equally The resultant blastula is a hollow ball of cells with a cavity called blastocoel

In mesolecithal egg like in frog have a total but unequal cleavage Blastomere near the vegetal pole are larger than those in the animal pole. Development is slow The blastocoel is displaced on the animal hemisphere.

Macrolecithal egg have unequal and partial or meroblastic cleavage Limited to the relatively small yolk-free region at the animal pole Yolk mass is to great to be penetrately by the cleavage furrows A cellular blastoderm is separated from the uncleaved yolk by a narrow cavity

Gastrula When the blastula developed into an embryo = at first the gastrula has two germ layer (ectoderm and endoderm) = and then later to three germ layers (ectoderm, mesoderm and endoderm) Gastrulation of a diploblast: The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

Neurulation and Neural Crest Neurulation is a process to convert the gastrula into neurula . is a part of organogenesis in vertebrate embryos Steps of neurulation include the formation of the dorsal nerve cord , and the eventual formation of the central nervous system. The process begins when the notochord induces the formation of the central nervous system (CNS) by signaling the ectoderm germ layer above it to form the thick and flat neural plate

The neural plate folds in upon itself to form the neural tube , which will later differentiate into the spinal cord and the brain , eventually forming the central nervous system.

Neurulation in vertebrates results in the formation of the neural tube , which gives rise to both the spinal cord and the brain. Neural crest cells are also created during neurulation. Neural crest cells migrate away from the neural tube and give rise to a variety of cell types, including pigment cells and neurons.

1. Neurulation begins with the formation of a neural plate , a thickening of the ectoderm caused when cuboidal epithelial cells become columnar. 2. Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube . 3. The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis . 4. These cells become the neural crest cells . Both epidermis and neural plate are capable of giving rise to neural crest cells .

Organogenesis Organogenesis is the period of animal development during which the embryo is becoming a fully functional organism capable of independent survivial. process by which specific organs and structures are formed , and involves both cell movements and cell differentiation . Organogenesis requires interactions between different tissues. These are often reciprocal interactions between epithelial sheets and mesenchymal cells .

The endoderm produces tissue within the lungs , thyroid , and pancreas . The mesoderm aids in the production of cardiac muscle , skeletal muscle , smooth muscle , tissues within the kidneys , and red blood cells . The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes .

Organogenesis from Ectoderm 1. From Somatic Ectoderm - epidermis of skin - enamel - Stomodeum (mouth) - Proctodeum (cloaca or anus ) - Gill Epithelium - Amnion and Chorion (in part)

2. Neural Plate ectoderm - Brain and Spinal Cord 3. Epidermal Placodes - Olfactory capsules - Optic capsule - Otic Capsule - Epibranchial capsule - Electroreceptors/ neuromsst organs - ganglia of some cranial nerves

4. Neural Crest - Spinal Ganglia - Splanchnocranium - Neurocranium - Dermatocranium - Dentine - Cornea - Chromatophores - Branchiomeric muscles - aortic arches - heart septa

Organogenesis From the mesoderm Epimere (dermatome) – Dermis Epimere (myotome) – Axial Muscle - Appendicular Muscle - Branchiomeric Muscle - Hypobranchial 3. Epimere (sclerotome) – Vertebrae 4. Chordamesoderm – notochord

5. Intermediate mesoderm (Mesomere) -kidney and Urogenital ducts 6. Somatic hypomere = - ribs - Parietal peritoneum - Sternum - Genital Ridge - appendicular skeleton - appendicular muscle - amnion and chorion

7. Splanchnic hypomere - Blood - heart - gut - smooth muscle - visceral peritoneum - yolk sac and allantois

Organogenesis of the Endoderm 1. Foregut – Oral Cavity - Gill Epithelium - nasal cavity - Lung epithelium - Pharynx epithelium 2. Midgut - Stomach - Liver - Bladder - Pancreas - intestines - Allantois - germ cells of gonads - yolk sac membrane 3. Hindgut – Urinary Bladder and Cloaca or anus

Comparative Vertebrate Anatomy

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