Presentation on theme: "Life History and Embryology of Vertebrates"— Presentation transcript:
1Life History and Embryology of Vertebrates Hazel Anne L. Tabo, MS
2Embryology Study of development from fertilization to embryo. The nature of relationship from: ancestor to offspring can be studied.It reveals ancestry.Haeckel: “Ontogeny recapitulates phylogeny.” (development repeats evolution).De Beers: Recapitulation is not the only relationship between embryos and ancestors.
4Embryo (De Beers, 1930)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.
5GametesGametes (sex cells, haploid chromosomes) union to form diploid organism.Gametes perform meiotic division haploidySperms (males) have various head appearance: spherical, spatulate, hooked, lancet-shaped, spiraled, capped (acrosome).Sperm: Head (nucleus); Neck (mitochondria); Flagella (movement).Egg cells: Microlecithal (small yolk); Mesolecithal (moderate yolk); Macrolecithal (abundant yolk).
7OOCYTE (egg cell; ovum) Developed and released by ovarian follicle; vitelline membrane (nourishes the egg)In therian mammals: egg is enclosed in thick zona pellucida, corona radiata, cells of ovarian follicle.Other vertebrates: egg is enclosed after ovulation; Jelly layers (amphibians); albumen (birds); horny, membranous (fishes) or calcareous shells (reptiles, birds).Microlecithal: Amphioxus; Eutherian mammals (placental nourishment)Mesolecithal: Lampreys; some fishes; AmphibiansMacrolecithal: Most fishes; Reptiles; Birds; Monotremes (egg-laying mammals)
8FertilizationIn therian mammals: Penetration of sperm from corona radiata to vitelline membrane of the egg. Fertilization involves enzymatic and physical interactions between sperm acrosome and egg cortex.Union of gametes: restores the diploid # of chromosomes.Activation of egg to refract additional sperm entry.Zygote (fertilized egg) Embryo (multicellular)
10CleavageBlastula – structure from series of cell division into multicellular (embryo).Blastomeres – daughter cellsBlastocoel - coelom / cavityYolk – nutrition of developing embryo; process of cleaving & blastula is dependent on the yolk present.Morula (blastula) – multiple numbered cells ready for uterine implantation.
11Cleavage: 2-celled; 4-celled; 8-celled; 16-celled; 32-celled stages of blastula
12Vertebrate blastulaComposed of single tissue layer with hundred of cells.The cells exhibit cellular polarity related to the axis of the adult body.Polarity: 1) animal pole - embryo2) vegetal pole – yolk (gut)Blastocoel – body cavity
14Blastula: Animal and vegetal poles Animal pole – EPIBLAST or EPIMERE; developing embryo nourished by the vegetal pole (dorsal cells--ECTODERM)Vegetal pole – HYPOBLAST or HYPOMERE or developing yolk; nourishes the embryo GUT (ventral cells--ENDODERM)
15Microlecithal (Amphioxus) – “holoblastic” (total cleavage furrows penetrate the entire yolk); equal-sized blastomeresAnimal poleBlastocoel or GutVegetal pole
16Cleavage in Amphibians Mesolecithal - holoblastic; unequal-sized blastomeresVegetal pole – larger blastomeres; nourishes the embryo; slower developmentAnimal pole – smaller blastomeres; developing embryoBlastocoel – towards animal pole
18Cleavage in Birds (Aves) Macrolecithal – “meroblastic” (partial cleavage); unequal-sized blastomeresVegetal pole – large size yolk mass; too great to be penetrated by cleavage furrowAnimal pole – is relatively small (blastoderm)Narrow blastocoel
20Cleavage in MammalsMicrolecithal - holoblastic; unequal-sized blastomeresBlastula Blastocyst (inner cell mass formation)Trophoblast – superficial layer of cellsBlastocoele – towards vegetal poleEmbryoblast – animal pole; epimeric cellsEndometrium – inner lining of the uterus (mother)
21Cleavage patterns followed by holoblastic (total cleavage) and meroblastic (partial cleavage) eggs Radial (sea urchin, amphioxus)Bilateral (tunicates, amphibians)Spiral (annelids, mollusks)Rotational (mammals)Discoidal (fish, birds, reptiles)Superficial (insects)
22Gastrulation: formation of three germ layers Blastula GastrulaAnimal pole – EPIBLAST or EPIMERE; developing embryo (dorsal cells--ECTODERM)Vegetal pole – HYPOBLAST or HYPOMERE or Yolk; nourishes the embryo GUT (ventral cells--ENDODERM)Germ layers:1) Ectoderm – from epiblast (animal pole); outermost layer2) Mesoderm – middle layer; MESENCHYME3) Endoderm – from hypoblast (vegetal pole); innermost layer
25GastrulationCells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) into three (triploblastic) germ layers.The embryo during this process is called a gastrula.Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:Epiboly - expansion of one cell sheet over other cellsIngression - cells move with pseudopodsInvagination - forming the mouth, anus, and archenteronDelamination - the external cells divide, leaving the daughter cells in the cavityPolar proliferation
27In most animals a blastopore is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore (formerly the mouth), while in protostomes it develops into the mouth.
28BlastoporeBlastopore – an opening that resulted from invagination of cells towards embryo
30Holoblastic Gastrula: mesoderm formation Vegetal (yolk) pole – folds inward towards animal pole double-walled cupCup opening – blastopore continues to another endGastrocoel – body cavity (coelom)Ectoderm – uppermost layer of cellsArchenteron – primitive gut (yolk) lined by endodermMesoderm – middle layer formed from dorsolateral outpocketing of archenteronNotochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron
34Holoblastic gastrulaNotochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteronChordamesoderm – roof of the archenteron, forms the notochord in the midline and somites (series of paired mesodermal tissue blocks)No outpocketing from the gut tube - schizocoelCoelom – formed by splitting of the hypoblast in the somites (schizocoel – pouch form)
36Mammalian embryoBlastocoel – indirectly filled with maternal fluid and enlarges BlastocystDelamination of endoderm (hypoblast) from inner cell mass and cover the trophoblastBlastoderm – (or embryoblast) flattened inner cell mass forms primitive streak (notochord CNS)Mesoderm – forms extraembryonic membranes (fetal membranes) and body of embryo (musculoskeletal and circulatory system); Mesenchymal tissueCoelom – forms by schizocoel (pouch form)
38Embryoblast or Blastoderm inner cell mass + primitive streakPrimitive streak – notochord (anlagen of CNS); promotes neurulation
39Germ layersEctoderm – Central nervous system, retina and lens, cranial and sensory, ganglia and nerves, pigment cells (melanocytes), head connective tissue, epidermis of skin, hair, mammary glands.Mesoderm – musculoskeletal system, circulatory system, dermis of skin, connective tissue, urogenital system, heart, blood (lymph cells), and spleen.Endoderm – gastrointestinal system (gut); stomach, colon, liver, pancreas, urinary bladder; lining of urethra, epithelial parts of trachea, lungs, pharynx, thyroid, parathyroid, intestine.
40NeurulationOverlaps with gastrulation establishing the central nervous system (CNS)Gastrula NeurulaNeurocoel – coelomChordamesoderm – induces thickening of the ectoderm into a neural plateNeural crest cells – (Ectomesenchyme) considered as the 4th germ layer which arised from ECTODERM forms cartilage & bones of the head, pharyngeal cartilages; peripheral nerve ganglia, some glandular tissues; melanocytes
41Neural plate – ectodermal cells Neural folds – formed from neural plate; fold of cells that arches and meet at the mid-dorsal line and forms the neural tube.Neural tube – encloses the neurocoel (cavity)Neurocoel – anlagen of the dorsal hollow nerve cord (+) chordates
43Organogenesis Migration of primordial germ cells Holoblastic embryo: Establishes the head and tailMeroblastic embryo: 3 germ layers spread faced down on the uncleaved yolkEmbryo increase in length while the archenteron becomes part of the gut tubeSchizocoely Enterocoelom (blastopore becomes anus while the mouth opens anteriorly.
45Organogenesis: Endoderm Endoderm form the lining of the gut.Archenteron: Foregut, Midgut & HindgutGutMajor StructuresForegutEpithelium: oral cavity,nasal cavity, pharynx, gill, esophagus, lungMidgutVisceral organs: stomach, liver, gall bladder, pancreas, intestines, germ cells of gonads (ovary & testis), part of the yolk sac membrane, part of the allantoisHindgutUrinary bladder; cloaca / anus
50FishesYolk remains uncleaved, 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 bodyRespiration and excretion are direct contact with the environment
51Reptiles and Birds (Avians) Primitive streak extraembryonic mesoderm splits forms extraembryonic coelom SPLANCHNOPLEURESplanchnopleure + adjacent Yolk = Yolk sacAllantois (vascular) – under the shell; near the chorioallantoic membrane. Allantois (respiration, receives excretory wastes; absorbs albumen; takes some minerals from the shell)Somatopleure + head fold of amnion = Chorion (outer) and Amnion (inner)Amnion – “water bag” contains amniotic fluid that bathes the embryo; cushions the embryo (shelter for growth)
54Eutherian mammalsPlacental mammals – nourish their young in the uterus, exchange and nutrition of fetus occurs between fetal and maternal bloodstream by PLACENTAYolk sac and Allantois – vascularized (blood vessels) supplies nutrition to placentaChorion and Amnion – avascular (no blood vessels) cannot support placenta
55Allantoic circulation (birds & reptiles) – homologous to umbilical circulation (mammals) Amnion – sac filled with amniotic fluid bathes the fetus; allows the early fetus to move freely and protects the fetus from pressure of the maternal abdomenAmniotes – collective terms for reptiles, birds and mammals because the possess amnion.PLACENTAL FORMATIONMaternal contribution: Chorion (lies between allantois & uterus) incorporated into the placenta CHORIOALLANTOIC MembraneFetal contribution: Allantois (in most mammals) as fetus grows, allantois decreases in size