ORIGIN_OF_CHORDATES_and_EMBRYOLOGY(4) (1).pdf

ORIGIN OF CHORDATES  CHORDATE A large phylum of animals that includes the vertebrates together with the sea squirts and lancelets. Animals having at least at some stage of development a notochord, dorsally situated central nervous system, and gill slits. Evolv...

ORIGIN OF CHORDATES  CHORDATE A large phylum of animals that includes the vertebrates together with the sea squirts and lancelets. Animals having at least at some stage of development a notochord, dorsally situated central nervous system, and gill slits. Evolved during the Cambrian period from a deuterostome ancestor (echinoderm, hemichordates).26 ECHINODERM ORIGIN Johannes Muller (1860) Proposed that echinoderm larvae gave rise to chordates by neoteny (retention of juvenile features in the adult animal). Echinoderms are also deuterostomes and possess mesodermal skeletal elements (made from CaCO3). Calcichordate fossil HEMICHORDATE ORIGIN Romer (1959) Suggested that ancestral deuterostomes were sedentary tentacle feeders whose mucous-laden Hemichordate fossil ciliated tentacles served to trap planktons as they were waved in water (like modern hemichordates). Acorn worm UROCHORDATE ORIGIN W. Garstang (1928) and N.J. Berrill (1955), tadpole-like larva of urochordates which carries typical chordate characters. Garstang (1928) suggested Tunicate adult that chordates evolved from some sessile filter-feeding urochordate by the larval stage evolving into adult by neoteny and by losing the sedentary adult stage. Tunicate larva CEPHALOCHORDATE ORIGIN Chamberlain (1900). Primitive and advanced characters of Pikaia gracilens fossil cephalochordates possess all chordate characters in typical state. Modern Amphioxus COMBINED THEORY E.J.W. Barrington (1965) Proposed that the common ancestor of echinoderms and chordates was a sessile ciliary arm feeder that lived in the Yuknessia plankton-rich environment of the Cambrian. Later evolved in a free swimming form at a time of food scarcity. Pikaia gracilens  What makes you a chordate? ❑Dorsal hallow nerve cord ❑Notochord ❑Pharyngeal pouches ❑Tail Classification of Chordates Subphylum Urochordata Subphylum Cephalochordata Superclass Agnatha Class Cephalaspidomorphi Class Myxini Superclass Gnathostomata Class Chondrichthyes Class Osteichthyes Class Actinopterygii Class Sarcopterygii Class Amphibia Class Reptilia Class Aves Class Mammalia Identify if they are chordates or not? Subphylum Vertebrata (Unranked group) Hyperoartia (lampreys) Class †Conodonta Subclass †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Galeaspida Class †Pituriaspida Class †Osteostraci Infraphylum Gnathostomata (jawed vertebrates) Class †Placodermi (Paleozoic armoured forms) Class Chondrichthyes (cartilaginous fish) Class †Acanthodii (Paleozoic "spiny sharks") Superclass Osteichthyes (bony fish) Class Actinopterygii (ray-finned fish) Class Sarcopterygii (lobe-finned fish) Subclass Coelacanthimorpha (coelacanths) Subclass Dipnoi (lungfish) Subclass Tetrapodomorpha (ancestral to tetrapods) Superclass Tetrapoda (four-limbed vertebrates) Class Amphibia (amphibians) Series Amniota (amniotic embryo) Class Sauropsida (reptiles and birds) Class Aves (birds) Class Synapsida (mammal-like reptiles) Class Mammalia (mammals) Anvier (1981, 1997), Shu et al. (2003), and Benton (2004) EMBRYOLOGY EMBRYOLOGY the branch of biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses.  Ernst Haeckel: “Ontogeny recapitulates phylogeny”, theorizes that the stages an animal embryo undergoes during development are a chronological replay of that species' past evolutionary forms.  Gavin de Beers: Recapitulation is not the only relationship between embryos and ancestors. ✓Embryonic structures may or may not be present from their ancestors or descendants. ✓Embryonic structures - vestigial (lost); or retained to adulthood. ✓Developmental stages may be repeated in earlier stage but not in later stages. ✓Developmental sequence may or may not be altered in the descendant. GAMETES are an organism's reproductive cells or sex cells. are haploid cells, and each cell carries only one copy of each chromosome. GAMETOGENESIS – process of formation of gametes. SPERMATOGENESIS Leydig cells produces testosterone in the presence of luteinizing hormone (LH). Sertoli cell is a "nurse" cell of a seminiferous tubule and helps in the process of spermatogenesis Spermiogenesis is the final stage of spermatogenesis, which sees the maturation of spermatids into mature spermatozoa. OOGENESIS CORPUS LUTEUM – an endocrine structure that continuously release hormones responsible for the thickening of the endometrium in the uterus. FERTILIZATION Is the fusion of haploid gametes, egg and sperm, to form the diploid zygote. CLEAVAGE Also known as cellulation (the formation of the blastomeres). The process by which the zygote undergo repeated mitotic cell division. MORULA - is a mass of 16 totipotent cells in a spherical shape BLASTULA - spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).  ANIMAL POLE – region in the developing embryo that consist of small cells that divides rapidly. VEGETAL POLE – region in the developing embryo consists of large cells and yolk that divides very slowly.  INFLUENCE OF YOLK ON CLEAVAGE HOLOBLASTIC Type of cleavage furrow bisects the entire egg. MEROBLASTIC Type of cleavage furrow restricted to the active cytoplasm found either in the animal pole (macrolecithal egg).  CLASSIFICATION OF EGG BASED ON THE AMOUNT OF YOLK ALECITHAL - Egg contains no yolk (Eutherian mammals). MICROLECITHAL - Egg contain small or negligible amount of yolk (Amphioxus, Tunicates). MESOLECITHAL - Egg contains moderate amount of yolk (Dipnoi, Petromyzon). MACROLECITHAL/POLYLECITHAL - Egg contains high amount of yolk (Reptile, Birds).  DISTRIBUTION OF YOLK ISOLECITHAL/HOMOLECITHAL A very little amount of yolk present and is uniformly distributed through out the egg (Echinoderm, Amphioxus, Mammals). TELOLECITHAL Egg contains moderate or large amount of yolk, distribution of yolk is unequal (concentrated in the Vegetal pole).  Cleavage in Amphibians Mesolecithal - holoblastic; unequal-sized blastomeres Vegetal pole – larger blastomeres; nourishes the embryo; slower development Animal pole – smaller blastomeres; developing embryo Blastocoel – towards animal pole  Cleavage in Birds (Aves) Macrolecithal – “meroblastic” (partial cleavage); unequal- sized blastomeres Vegetal pole – large size yolk mass; too great to be penetrated by cleavage furrow Animal pole – is relatively small (blastoderm) Narrow blastocoel  Cleavage in Mammals Microlecithal - holoblastic; unequal-sized blastomeres. GASTRULATION Process in which the single-layered blastula is reorganized into a multilayered structure known as the gastrula. Cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism.  IMPORTANT OUTCOMES OF GASTRULATION Formation of the three (3) embryonic germ layers. Formation of the embryonic gut (archenteron). Appearance of the major body axes.  THREE (3) GERM LAYERS ❑ECTODERM – the outermost layer (forms skin, brain, the nervous system, and other external tissues). ❑MESODERM – the middle layer (forms muscle, the skeletal system, and the circulatory system) ❑ENDODERM – the innermost layer (forms the lining of the gut and other internal organs).  MESENCHYMAL STEM CELL Multipotent stromal cells that can differentiate into a variety of cell types. Unspecialized pack of tissue of a developing embryo and its cells enter into the formation of specialized tissues. DIPLOBLASTIC - having a body derived from only two embryonic cell layers (ectoderm and endoderm, but no mesoderm), as in sponges and coelenterates. TRIPLOBLASTIC - having a body derived from three embryonic cell layers (ectoderm, mesoderm, and endoderm), as in all multicellular animals except sponges and coelenterates. PROTOSTOME - Animals in which the blastopore becomes the mouth (primitive invertebrates) DEUTEROSTOME - Animals in which the blastopore becomes the anus (echinoderms and chordates) GASTRULATION IN AMPHIBIANS GASTRULATION IN CHICKEN NEURULATION Refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula. ORGANOGENESIS It is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Migration of primordial germ cells (Mesenchyme). Organogenesis: Endoderm Organogenesis: Mesoderm Organogenesis: Ectoderm EXTRAEMBRYONIC MEMBRANES membranes which assist in the development of the embryo. originate from the embryo, but are not considered part of it. typically perform roles in nutrition, gas exchange, and waste removal.  EXTRAEMBRYONIC MEMBRANES AMNION – protects the embryo in a sac filled with amniotic fluid YOLK SAC – contains yolk, source of food for a developing embryo. CHORION – the inner surface of the shell, participates in gas exchange between the embryo and the outside air. ALLANTOIS – stores metabolic wastes of the embryo, also participates in gas exchange. FISH Eggs are laid in water; yolk sac is the only fetal membrane formed. Yolk sac – formed from 3 germ layers to absorb the yolk into the body Respiration and excretion are direct contact with the environment REPTILES AND BIRDS AMNION - protects the embryo in a sac filled with amniotic fluid. YOLK SAC - contains yolk — the sole source of food until hatching (mixture or proteins and lipoproteins). CHORION - lines the inner surface of the shell (which is permeable to gases) and participates in the exchange of O2 and CO2 between the embryo and the outside air. ALLANTOIS - stores metabolic wastes (chiefly uric acid) of the embryo and participates in gas exchange. MAMMALS: MONOTREMES Mammals that produce a shelled egg like their reptilian ancestors. Spiny anteater (echidna) and Duckbill platypus MAMMALS: METAHERIAN Pouched mammals (no typical placenta), Marsupials (Kangaroo, Wombat). Embryo is poorly supplied with yolk, yolk sac provides a rudimentary connection to the mother's blood (food, oxygen, and other essentials). The young are born in a very immature state (they are able to crawl into a pouch on the mother's abdomen, attach themselves to nipples, and drink milk from her mammary glands). MAMMALS: EUTHERIAN Placental mammals. The extraembryonic membranes form a placenta and umbilical cord (connect the embryo to the mother's uterus) Placenta functions include gas exchange, metabolic transfer, hormone secretion, and fetal protection.