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Life History and Embryology of Vertebrates Hazel Anne L. Tabo, MS.

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1 Life History and Embryology of Vertebrates Hazel Anne L. Tabo, MS

2 Embryology Study of development from fertilization to embryo. Study of development from fertilization to embryo. The nature of relationship from: ancestor to offspring can be studied. The nature of relationship from: ancestor to offspring can be studied. It reveals ancestry. It reveals ancestry. Haeckel: “Ontogeny recapitulates phylogeny.” (development repeats evolution). Haeckel: “Ontogeny recapitulates phylogeny.” (development repeats evolution). De Beers: Recapitulation is not the only relationship between embryos and ancestors. De Beers: Recapitulation is not the only relationship between embryos and ancestors.

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4 Embryo (De Beers, 1930) Embryonic structures may or may not be present from their ancestors or descendants. Embryonic structures may or may not be present from their ancestors or descendants. Embryonic structures - vestigial (lost); or retained to adulthood. Embryonic structures - vestigial (lost); or retained to adulthood. Developmental stages may be repeated in earlier stage but not in later stages. Developmental stages may be repeated in earlier stage but not in later stages. Developmental sequence may or may not be altered in the descendant. Developmental sequence may or may not be altered in the descendant.

5 Gametes Gametes (sex cells, haploid chromosomes) union to form diploid organism. Gametes (sex cells, haploid chromosomes) union to form diploid organism. Gametes perform meiotic division  haploidy Gametes perform meiotic division  haploidy Sperms (males) have various head appearance: spherical, spatulate, hooked, lancet-shaped, spiraled, capped (acrosome). Sperms (males) have various head appearance: spherical, spatulate, hooked, lancet-shaped, spiraled, capped (acrosome). Sperm: Head (nucleus); Neck (mitochondria); Flagella (movement). Sperm: Head (nucleus); Neck (mitochondria); Flagella (movement). Egg cells: Microlecithal (small yolk); Mesolecithal (moderate yolk); Macrolecithal (abundant yolk). Egg cells: Microlecithal (small yolk); Mesolecithal (moderate yolk); Macrolecithal (abundant yolk).

6 Gametogenesis – formation of gametes

7 OOCYTE (egg cell; ovum) Developed and released by ovarian follicle; vitelline membrane (nourishes the egg) 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. 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). 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) Microlecithal: Amphioxus; Eutherian mammals (placental nourishment) Mesolecithal: Lampreys; some fishes; Amphibians Mesolecithal: Lampreys; some fishes; Amphibians Macrolecithal: Most fishes; Reptiles; Birds; Monotremes (egg-laying mammals) Macrolecithal: Most fishes; Reptiles; Birds; Monotremes (egg-laying mammals)

8 Fertilization In 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. In 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. Union of gametes: restores the diploid # of chromosomes. Activation of egg to refract additional sperm entry. Activation of egg to refract additional sperm entry. Zygote (fertilized egg)  Embryo (multicellular) Zygote (fertilized egg)  Embryo (multicellular)

9 FERTILIZATION

10 Cleavage Blastula – structure from series of cell division into multicellular (embryo). Blastula – structure from series of cell division into multicellular (embryo). Blastomeres – daughter cells Blastomeres – daughter cells Blastocoel - coelom / cavity Blastocoel - coelom / cavity Yolk – nutrition of developing embryo; process of cleaving & blastula is dependent on the yolk present. Yolk – nutrition of developing embryo; process of cleaving & blastula is dependent on the yolk present. Morula (blastula) – multiple numbered cells ready for uterine implantation. Morula (blastula) – multiple numbered cells ready for uterine implantation.

11 Cleavage: 2-celled; 4-celled; 8-celled; 16-celled; 32-celled stages of blastula

12 Vertebrate blastula Composed of single tissue layer with hundred of cells. Composed of single tissue layer with hundred of cells. The cells exhibit cellular polarity related to the axis of the adult body. The cells exhibit cellular polarity related to the axis of the adult body. Polarity: 1) animal pole - embryo Polarity: 1) animal pole - embryo 2) vegetal pole – yolk (gut) 2) vegetal pole – yolk (gut) Blastocoel – body cavity Blastocoel – body cavity

13 BLASTULA (animal pole) (vegetal pole) (blastocoel)

14 Blastula: Animal and vegetal poles Animal pole – EPIBLAST or EPIMERE; developing embryo nourished by the vegetal pole (dorsal cells--ECTODERM) 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) Vegetal pole – HYPOBLAST or HYPOMERE or developing yolk; nourishes the embryo  GUT (ventral cells--ENDODERM)

15 Microlecithal (Amphioxus) – “holoblastic” (total cleavage furrows penetrate the entire yolk); equal-sized blastomeres Microlecithal (Amphioxus) – “holoblastic” (total cleavage furrows penetrate the entire yolk); equal-sized blastomeres Blastocoel or Gut Animal pole Vegetal pole

16 Cleavage in Amphibians Mesolecithal - holoblastic; unequal-sized blastomeres Mesolecithal - holoblastic; unequal-sized blastomeres Vegetal pole – larger blastomeres; nourishes the embryo; slower development Vegetal pole – larger blastomeres; nourishes the embryo; slower development Animal pole – smaller blastomeres; developing embryo Animal pole – smaller blastomeres; developing embryo Blastocoel – towards animal pole Blastocoel – towards animal pole

17 Amphibians

18 Cleavage in Birds (Aves) Macrolecithal – “meroblastic” (partial cleavage); unequal-sized blastomeres Macrolecithal – “meroblastic” (partial cleavage); unequal-sized blastomeres Vegetal pole – large size yolk mass; too great to be penetrated by cleavage furrow Vegetal pole – large size yolk mass; too great to be penetrated by cleavage furrow Animal pole – is relatively small (blastoderm) Animal pole – is relatively small (blastoderm) Narrow blastocoel Narrow blastocoel

19 Birds: Yolk > Embryo

20 Cleavage in Mammals Microlecithal - holoblastic; unequal-sized blastomeres Microlecithal - holoblastic; unequal-sized blastomeres Blastula  Blastocyst (inner cell mass formation) Blastula  Blastocyst (inner cell mass formation) Trophoblast – superficial layer of cells Trophoblast – superficial layer of cells Blastocoele – towards vegetal pole Blastocoele – towards vegetal pole Embryoblast – animal pole; epimeric cells Embryoblast – animal pole; epimeric cells Endometrium – inner lining of the uterus (mother) Endometrium – inner lining of the uterus (mother)

21 Cleavage patterns followed by holoblastic (total cleavage) and meroblastic (partial cleavage) eggs HoloblasticMeroblastic  Radial (sea urchin, amphioxus)sea urchin amphioxus  Bilateral (tunicates, amphibians)tunicates amphibians  Spiral (annelids, mollusks)annelids mollusks  Rotational (mammals)mammals  Discoidal (fish, birds, reptiles)fishbirds reptiles  Superficial (insects)insects

22 Gastrulation: formation of three germ layers Blastula  Gastrula Blastula  Gastrula Animal pole – EPIBLAST or EPIMERE; developing embryo (dorsal cells--ECTODERM) Animal pole – EPIBLAST or EPIMERE; developing embryo (dorsal cells--ECTODERM) Vegetal pole – HYPOBLAST or HYPOMERE or Yolk; nourishes the embryo  GUT (ventral cells-- ENDODERM) Vegetal pole – HYPOBLAST or HYPOMERE or Yolk; nourishes the embryo  GUT (ventral cells-- ENDODERM) Germ layers: Germ layers: –1) Ectoderm – from epiblast (animal pole); outermost layer –2) Mesoderm – middle layer; MESENCHYME –3) Endoderm – from hypoblast (vegetal pole); innermost layer

23 BLASTULA (animal pole) (vegetal pole) (blastocoel)

24 Ectoderm Endoderm Mesoderm Gastrula

25 Gastrulation Cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) into three (triploblastic) germ layers. Cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) into three (triploblastic) germ layers.diploblastictriploblastic germ layersdiploblastictriploblastic germ layers The embryo during this process is called a gastrula. The embryo during this process is called a gastrula.gastrula Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo: 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 cells Epiboly –Ingression - cells move with pseudopods –Invagination - forming the mouth, anus, and archenteron Invagination –Delamination - the external cells divide, leaving the daughter cells in the cavity –Polar proliferation

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27 In most animals a blastopore is formed at the point where cells are entering the embryo. In most animals a blastopore is formed at the point where cells are entering the embryo.blastopore 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. 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.deuterostomesprotostomesdeuterostomesprotostomes

28 Blastopore Blastopore – an opening that resulted from invagination of cells towards embryo Blastopore – an opening that resulted from invagination of cells towards embryo

29 A. ProtostomesB. Deuterostomes A. ProtostomesB. Deuterostomes

30 Holoblastic Gastrula: mesoderm formation Vegetal (yolk) pole – folds inward towards animal pole  double-walled cup Vegetal (yolk) pole – folds inward towards animal pole  double-walled cup Cup opening – blastopore continues to another end Cup opening – blastopore continues to another end Gastrocoel – body cavity (coelom) Gastrocoel – body cavity (coelom) Ectoderm – uppermost layer of cells Ectoderm – uppermost layer of cells Archenteron – primitive gut (yolk) lined by endoderm Archenteron – primitive gut (yolk) lined by endoderm Mesoderm – middle layer formed from dorsolateral outpocketing of archenteron Mesoderm – middle layer formed from dorsolateral outpocketing of archenteron Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron

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33 Ectoderm Endoderm Mesoderm Gastrula Archenteron (primitive gut)

34 Holoblastic gastrula Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron Notochord – anlagen of nervous system (primitive), formed from dorsal wall of archenteron Chordamesoderm – roof of the archenteron, forms the notochord in the midline and somites (series of paired mesodermal tissue blocks) Chordamesoderm – roof of the archenteron, forms the notochord in the midline and somites (series of paired mesodermal tissue blocks) No outpocketing from the gut tube - schizocoel No outpocketing from the gut tube - schizocoel Coelom – formed by splitting of the hypoblast in the somites (schizocoel – pouch form) Coelom – formed by splitting of the hypoblast in the somites (schizocoel – pouch form)

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36 Mammalian embryo Blastocoel – indirectly filled with maternal fluid and enlarges  Blastocyst Blastocoel – indirectly filled with maternal fluid and enlarges  Blastocyst Delamination of endoderm (hypoblast) from inner cell mass and cover the trophoblast Delamination of endoderm (hypoblast) from inner cell mass and cover the trophoblast Blastoderm – (or embryoblast) flattened inner cell mass forms primitive streak (notochord  CNS) Blastoderm – (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 tissue Mesoderm – forms extraembryonic membranes (fetal membranes) and body of embryo (musculoskeletal and circulatory system); Mesenchymal tissue Coelom – forms by schizocoel (pouch form) Coelom – forms by schizocoel (pouch form)

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38 Embryoblast or Blastoderm inner cell mass + primitive streak inner cell mass + primitive streak Primitive streak – notochord (anlagen of CNS); promotes neurulation Primitive streak – notochord (anlagen of CNS); promotes neurulation

39 Germ layers Ectoderm – Central nervous system, retina and lens, cranial and sensory, ganglia and nerves, pigment cells (melanocytes), head connective tissue, epidermis of skin, hair, mammary glands. Ectoderm – 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. 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. Endoderm – gastrointestinal system (gut); stomach, colon, liver, pancreas, urinary bladder; lining of urethra, epithelial parts of trachea, lungs, pharynx, thyroid, parathyroid, intestine.

40 Neurulation Overlaps with gastrulation establishing the central nervous system (CNS) Overlaps with gastrulation establishing the central nervous system (CNS) Gastrula  Neurula Gastrula  Neurula Neurocoel – coelom Neurocoel – coelom Chordamesoderm – induces thickening of the ectoderm into a neural plate Chordamesoderm – induces thickening of the ectoderm into a neural plate Neural crest cells – (Ectomesenchyme) considered as the 4 th germ layer which arised from ECTODERM  forms cartilage & bones of the head, pharyngeal cartilages; peripheral nerve ganglia, some glandular tissues; melanocytes Neural crest cells – (Ectomesenchyme) considered as the 4 th germ layer which arised from ECTODERM  forms cartilage & bones of the head, pharyngeal cartilages; peripheral nerve ganglia, some glandular tissues; melanocytes

41 Neural plate – ectodermal cells Neural 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 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) Neural tube – encloses the neurocoel (cavity) Neurocoel – anlagen of the dorsal hollow nerve cord  (+) chordates Neurocoel – anlagen of the dorsal hollow nerve cord  (+) chordates

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43 Organogenesis Migration of primordial germ cells Migration of primordial germ cells Holoblastic embryo: Establishes the head and tail Holoblastic embryo: Establishes the head and tail Meroblastic embryo: 3 germ layers spread faced down on the uncleaved yolk Meroblastic embryo: 3 germ layers spread faced down on the uncleaved yolk Embryo increase in length while the archenteron becomes part of the gut tube Embryo increase in length while the archenteron becomes part of the gut tube Schizocoely  Enterocoelom (blastopore becomes anus while the mouth opens anteriorly. Schizocoely  Enterocoelom (blastopore becomes anus while the mouth opens anteriorly.

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45 Organogenesis: Endoderm Endoderm form the lining of the gut. Endoderm form the lining of the gut. Archenteron: Foregut, Midgut & Hindgut Archenteron: Foregut, Midgut & Hindgut GutMajor Structures ForegutEpithelium: oral cavity, nasal cavity, pharynx, gill, esophagus, lung MidgutVisceral organs: stomach, liver, gall bladder, pancreas, intestines, germ cells of gonads (ovary & testis), part of the yolk sac membrane, part of the allantois HindgutUrinary bladder; cloaca / anus

46 Organogenesis: Mesoderm Mesoderm layerMajor structure Outer Epimere – DermatomeSkin Dermis Middle Epimere – MyotomeMuscles: axial, appendicular, branchiomeric, hypobranchial Inner Epimere – SclerotomeVertebral column ChordamesodermNotochord Intermediate mesodermKidney; Urogenital ducts Somatic hypomereBones: ribs, sternum (breast plate), appendicular skeleton, some appendicular muscle, parietal peritoneum (outer abdominal wall); genital ridge; part of amnion and chorion Splanchnic hypomereBlood, heart, gut, smooth muscle, visceral peritoneum (inner abdominal wall); yolk sac and allantois

47 Organogenesis: Ectoderm Ectoderm layerMajor structure Somatic ectodermSkin epidermis; Teeth enamel; stomodeum (mouth); proctodeum (anus/cloaca); gill epithelium; part of amnion and chorion Neural plate ectodermBrain; Spinal cord Epidermal placodesCapsules: olfactory (nasal); optic (eye); otic (ear); epibranchial capsules; neuromast (electroreceptors organs); ganglia of some cranial nerves Ectomesenchyme (Neural crest) Spinal ganglia; splanchnocranium; neurocranium (part); dermatocranium (part); teeth dentine; eye cornea; chromatophores (pigment cells); branchiomeric muscles (part); aortic arches; heart septum (divides the right and left side of heart)

48 Extraembryonic membranes (fetal membranes) and Placenta Amphibians – yolk cleaves and directly incorporated into somatic cells (body cells) Amphibians – yolk cleaves and directly incorporated into somatic cells (body cells) Yolk supply – limited Yolk supply – limited Early hatching  Larva Early hatching  Larva No fetal membranes required to sustain larva No fetal membranes required to sustain larva

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50 Fishes Yolk remains uncleaved, eggs are laid in water; yolk sac is the only fetal membrane formed. Yolk 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 body Yolk sac – formed from 3 germ layers to absorb the yolk into the body Respiration and excretion are direct contact with the environment Respiration and excretion are direct contact with the environment

51 Reptiles and Birds (Avians) Primitive streak  extraembryonic mesoderm splits  forms extraembryonic coelom  SPLANCHNOPLEURE Primitive streak  extraembryonic mesoderm splits  forms extraembryonic coelom  SPLANCHNOPLEURE Splanchnopleure + adjacent Yolk = Yolk sac Splanchnopleure + adjacent Yolk = Yolk sac Allantois (vascular) – under the shell; near the chorioallantoic membrane. Allantois (respiration, receives excretory wastes; absorbs albumen; takes some minerals from the shell) Allantois (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) 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) Amnion – “water bag” contains amniotic fluid that bathes the embryo; cushions the embryo (shelter for growth)

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54 Eutherian mammals Placental mammals – nourish their young in the uterus, exchange and nutrition of fetus occurs between fetal and maternal bloodstream by PLACENTA Placental mammals – nourish their young in the uterus, exchange and nutrition of fetus occurs between fetal and maternal bloodstream by PLACENTA Yolk sac and Allantois – vascularized (blood vessels)  supplies nutrition to placenta Yolk sac and Allantois – vascularized (blood vessels)  supplies nutrition to placenta Chorion and Amnion – avascular (no blood vessels)  cannot support placenta Chorion and Amnion – avascular (no blood vessels)  cannot support placenta

55 Allantoic circulation (birds & reptiles) – homologous to umbilical circulation (mammals) Allantoic 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 abdomen 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 abdomen Amniotes – collective terms for reptiles, birds and mammals because the possess amnion. Amniotes – collective terms for reptiles, birds and mammals because the possess amnion. PLACENTAL FORMATION Maternal contribution: Chorion (lies between allantois & uterus) incorporated into the placenta  CHORIOALLANTOIC Membrane Maternal contribution: Chorion (lies between allantois & uterus) incorporated into the placenta  CHORIOALLANTOIC Membrane Fetal contribution: Allantois (in most mammals)  as fetus grows, allantois decreases in size Fetal contribution: Allantois (in most mammals)  as fetus grows, allantois decreases in size

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57 Prototherians – egg-laying mammals; deposits egg in a pouch (oviparous) Prototherians – egg-laying mammals; deposits egg in a pouch (oviparous) Metatherians – marsupials (pouched animals) – no typical placenta Metatherians – marsupials (pouched animals) – no typical placenta Eutherians – placental mammals (viviparous) Eutherians – placental mammals (viviparous)


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