Animal Development

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ANIMAL DEVELOPMENTCH 47 FERTILIZATION THROUGH ORGANOGENESIS

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STAGES OF HUMAN DEVELOPMENT1. Fertilization

1. Zona pellucida2. First cell division

2. Cleavage1. Blastomere2. Holoblastic cleavage3. Meroblastic cleavage4. regulation

3. Morophogenesis1. Gastrulation2. Organogenesis

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FERTILIZATIONFemale secretions increase sperm motility and change structure to cause fertilization potential (capacitation)Moist environment necessary for spermFirst six hours

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FERTILIZATIONZona pellucida contain receptor cite and acrosomal reaction which binds sperm to eggChanges cause slow polyspermy to prevent additional sperm from entering eggNo fast polyspermy reactions in mammals

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FIGURE 47.5

Zona pellucida

Follicle cell

Spermbasal body

Spermnucleus

Corticalgranules

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FERTILIZATIONFirst Cell DivisionMitosis forms true nuclei in daughter cells12-36 hours after sperm bondingEach cell is now a blastomere

http://www.hhmi.org/biointeractive/human-embryonic-HHMI embryonic development

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CLEAVAGERapid cell division with almost continuous S and M phases of cell cycleLittle or no protein synthesis (G1 or G2)

Blastula- Hollow ball of cells form with blastocoel cavity

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FIGURE 47.6

(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula

50 m

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CLEAVAGEIn frogs and mammals is holoblastic “holo” means completeHumans have 3 divisions in first three days with little yolk formingBirds and reptiles cleavage is meroblastic (incomplete) to get extensive yolk formationThe “ends” of the blastula are called the animal pole and vegetal poleGray crescent is the area on the opposite side from sperm binding

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FIGURE 47.7

Zygote

2-cellstageforming

4-cellstageforming

8-cellstage

Vegetal pole

Blastula(crosssection)

Gray crescent

Animalpole

Blastocoel

0.25 mm

0.25 mm

8-cell stage (viewedfrom the animal pole)

Blastula (at least 128 cells)

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REGULATION OF CLEAVAGEThe total mass of the structure does not change from zygote to blastula, the cells just get smallerCells divide until the ratio of material in each nucleus to cytoplasm is sufficiently largeSmall cells balance the amount of DNA to mRNA for protein synthesis (think surface are to volume ratio)

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MORPHOGENESISTransformation of embryo orientation and shapeImportant is the cell shape, position and survivalTwo important phases:1. Gastrulation- establishment of cell layers2. Organogenesis- formation of organs

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MORPHOGENESIS:GASTRULATIONDuring gastrulation there is a mass movement of cells which results in the blastula becoming a gastrulaThree germ layers develop

ectoderm- outside layermesoderm- middle layerendoderm- inside layer

Some organisms (cniderians) do not have a mesoderm HHMI Differentiation and Cell Fatehttp://www.hhmi.org/biointeractive/differentiation-and-fate-cells

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FIGURE 47.8

ECTODERM (outer layer of embryo)

MESODERM (middle layer of embryo)

ENDODERM (inner layer of embryo)

• Epidermis of skin and its derivatives (including sweat glands, hair follicles)

• Epithelial lining of digestive tract and associated organs (liver, pancreas)• Epithelial lining of respiratory, excretory, and reproductive tracts and ducts

• Germ cells• Jaws and teeth• Pituitary gland, adrenal medulla• Nervous and sensory systems

• Skeletal and muscular systems• Circulatory and lymphatic systems• Excretory and reproductive systems (except germ cells)• Dermis of skin• Adrenal cortex

• Thymus, thyroid, and parathyroid glands

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FIGURE 47.9Animalpole

BlastocoelMesenchymecells

Vegetal plate Vegetalpole

Blastocoel

Filopodia

Mesenchymecells

Blastopore

Archenteron

50 m

Ectoderm

Mouth

Mesenchyme(mesoderm formsfuture skeleton)

Blastopore

BlastocoelArchenteron

Digestive tube (endoderm)Anus (from blastopore)

Key

Future ectodermFuture mesodermFuture endoderm

Gastrulation in Sea Urchin

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FIGURE 47.10

Key

Future ectodermFuture mesodermFuture endoderm

SURFACE VIEW CROSS SECTIONAnimal pole

Vegetal poleEarlygastrula

Blastocoel

Dorsal lip ofblasto-pore

BlastoporeDorsal lip ofblastopore

Blastocoelshrinking Archenteron

Archenteron

Blastocoelremnant

EctodermMesodermEndoderm

BlastoporeYolk plugBlastopore

Lategastrula

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2

1

Gastrulation in Frog

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FIGURE 47.11

Future ectoderm

Migratingcells(mesoderm)

Blastocoel

Epiblast

YOLK

EndodermHypoblast

Primitive streak

Fertilized eggPrimitivestreak

Embryo

Yolk

Gastrulation in Chick

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FIGURE 47.12Blastocyst reaches uterus.1

2

3

4

Blastocyst implants(7 days after fertilization).

Extraembryonic membranesstart to form (10–11 days),and gastrulation begins(13 days).

Gastrulation has produced athree-layered embryo withfour extraembryonicmembranes.

Uterus

Maternal bloodvessel

Endometrial epithelium(uterine lining)Inner cell massTrophoblast

Blastocoel

Expanding region oftrophoblast

EpiblastHypoblastTrophoblast

Expanding region oftrophoblastAmniotic cavityEpiblastHypoblastYolk sac (from hypoblast)

Extraembryonic mesoderm cells(from epiblast)Chorion (from trophoblast)

AmnionChorionEctodermMesodermEndodermYolk sacExtraembryonic mesoderm

Allantois

Gastrulation in Human

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MORPHOGENESIS:GASTRULATION IN HUMANS1. Blastocyst first 6 days Fertilization occurs in the oviductInner cell mass becomes the embryo which is the source for stem cellsLittle yolk (stored nutrients)

Blastocyst reaches uterus.

Inner cell mass

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MORPHOGENESIS:GASTRULATION IN HUMANS2. Trophoblast 7 days after fertilizationOuter epithelium secretes enzymes for implantation which allows for blood to surround trophoblastEpiblast-upper layer becomes the embryoHypoblast- lower layer

Maternal bloodvessel

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MORPHOGENESIS:GASTRULATION IN HUMANS3. Extraembryonic membranes 10-11 daysFormed by embryoEnclose special structures outside the embryoGastrulation begins day 13 when implantation is completeCell migration occurs as cells move inward from epiblast through primitive streak to form mesoderm and endoderm

Chick gastrulation

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FIGURE 47.12C

Extraembryonic membranesstart to form (10–11 days),and gastrulation begins(13 days).

Expanding region oftrophoblastAmniotic cavityEpiblastHypoblastYolk sac (from hypoblast)

Extraembryonic mesoderm cells (from epiblast)Chorion (from trophoblast)

3

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MORPHOGENESIS:GASTRULATION IN HUMANS4. End of gastrulationThree germ layers are formedExtraembryonic layers from placentaThese layers are an evolutionary necessity in land (dry) environments

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FIGURE 47.12D

Gastrulation has produced athree-layered embryo withfour extraembryonicmembranes.

AmnionChorionEctodermMesodermEndodermYolk sacExtraembryonic mesoderm

Allantois4

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ORGANOGENESISMore localized changes

Researchbrain intestineSpinal cord liverBones (or specific) Muscles (or specific)skin eyesheart ears

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