Chapter 47 Animal Development

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Fertilization

Embryonic development/fertilization• Preformation: until 18th century; miniature infant in sperm or egg• At fertilization/conception:• Acrosomal reaction: hydrolytic enzyme action on egg jelly coat….• Fast block to polyspermy: membrane depolarization prevents multiple fertilizations….• Cortical reaction: release of calcium causes hardening of egg outer layer and creates a...• Slow block to polyspermy and...• Egg activation~ increases metabolic activity; protein synthesis

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Cleavage• Repeated mitotic divisions of zygote

– 1st step to becoming multicellular– unequal divisions establishes body plan

• different cells receive different portions of egg cytoplasm & therefore different regulatory signals

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Cleavage• zygote morula blastula

– establishes future development

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Gastrulation • Establish 3 cell layers

– ectoderm• outer layers

– skin, nails, teeth, nerves

– mesoderm• blood, bone & muscle

– endoderm• inner lining

– digestive system

ectoderm

mesoderm

endoderm

• Gastrulation in a sea urchin– Produces an embryo with a primitive gut and

three germ layers

Figure 47.11

Digestive tube (endoderm)

Key

Future ectodermFuture mesoderm

Future endoderm

Blastocoel

Mesenchymecells

Vegetalplate

Animalpole

Vegetalpole

Filopodiapullingarchenterontip

Archenteron

Blastocoel

Blastopore

50 µm

Blastopore

Archenteron

Blastocoel

Mouth

Ectoderm

Mesenchyme:(mesodermforms future skeleton) Anus (from blastopore)

Mesenchymecells

The blastula consists of a single layer of ciliated cells surrounding the blastocoel. Gastrulation begins with the migration of mesenchyme cells from the vegetal pole into the blastocoel.

1

2 The vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel.2

Endoderm cells form the archenteron (future digestive tube). New mesenchyme cells at the tip of the tube begin to send out thin extensions (filopodia) toward the ectoderm cells of the blastocoel wall (inset, LM).

3

Contraction of these filopodia then drags the archenteron across the blastocoel.4

Fusion of the archenteron with the blastocoel wall completes formation of the digestive tube with a mouth and an anus. The gastrula has three germ layers and is covered with cilia, which function in swimming and feeding.

5

• The mechanics of gastrulation in a frog – Are more complicated than in a sea urchin

Figure 47.12

SURFACE VIEW CROSS SECTION

Animal poleBlastocoel

Dorsal lipof blastopore

Dorsal lipof blastopore

Vegetal pole Blastula

Blastocoelshrinking

Archenteron

Blastocoelremnant

Ectoderm

Mesoderm

Endoderm

GastrulaYolk plugYolk plug

Key

Future ectoderm

Future mesoderm

Future endoderm

Gastrulation begins when a small indented crease, the dorsal lip of the blastopore, appears on one side of the blastula. The crease is formed by cellschanging shape and pushing inward from the surface (invagination). Additional cells then rollinward over the dorsal lip (involution) and move intothe interior, where they will form endoderm andmesoderm. Meanwhile, cells of the animal pole, the future ectoderm, change shape and begin spreading over the outer surface.

The blastopore lip grows on both sides of the embryo, as more cells invaginate. When the sides of the lip meet, the blastopore forms a circle thatbecomes smaller as ectoderm spreads downward over the surface. Internally, continued involutionexpands the endoderm and mesoderm, and the archenteron begins to form; as a result, the blastocoel becomes smaller.

1

2

3 Late in gastrulation, the endoderm-lined archenteron has completely replaced the blastocoel and the three germ layers are in place. The circular blastopore surrounds a plug of yolk-filled cells.

• Gastrulation in the chick– Is affected by the large amounts of yolk in the egg

Figure 47.13

Epiblast

Futureectoderm

Migratingcells(mesoderm)

Endoderm

Hypoblast

YOLK

Primitivestreak

Organogenesis

• Various regions of the three embryonic germ layers– Develop into the rudiments of organs during the

process of organogenesis

• Early in vertebrate organogenesis– The notochord forms from mesoderm and the

neural plate forms from ectoderm

Figure 47.14a

Neural plate formation. By the timeshown here, the notochord has developed from dorsal mesoderm, and the dorsal ectoderm hasthickened, forming the neural plate, in response to signals from thenotochord. The neural folds arethe two ridges that form the lateral edges of the neural plate. These are visible in the light micrographof a whole embryo.

Neural folds

1 mm

Neuralfold

Neuralplate

NotochordEctoderm

MesodermEndoderm

Archenteron

(a)

LM

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Neurulation• 1st organ to form is notochord &

nerve chord – develop into nervous system

Neural groove

Notochord

Neural tube

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OrganogenesisUmbilical blood vessels

Chorion

Amnion

Yolksac

AllantoisFetal blood vessels

Maternal blood vessels

Bird embryo

Mammalian embryo

Placenta

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Placenta• Materials exchange across membranes

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Human fetal development• Day 1: 1st cleavage

– 1 cell becomes 2 (2-cell stage)• Day 2: 2nd cleavage

– 4-cell stage• Day 3: 6-12 cell stage

– can test at this stage for genetic diseases if done by IVF

• Day 4: 16-32 cell stage – solid ball of cells = morula

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Ovulation to implantation

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Human fetal development• Day 5:

– solid morula develops into hollow, fluid-filled blastula– embryo will develop from the inner cell mass, or embryonic disc

• Day 6 -7: – blastocyst attaches to the endometrium (uterine lining) &

burrows in: implantation– blastocyst starts to secrete HCG = human chorionic gonatotropin

• stimulates estrogen & progesterone to prevent menstrual flow• causes "morning sickness" in some women...• pregnancy test measures the amount of this hormone!

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Human fetal developmental• Days 10 - 14:

– pregnancy becomes established– fluid filled amniotic cavity starts to form– yolk sac starts to form

• will make blood cells, germ cells– embryo starts to form from embryonic disc– chorion (placenta) starts to form

• At the end of this stage, a woman will have just missed her period!

Day 14

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Human fetal developmental• Days 15 - 21:

– emergence of the vertebrate body plan– primitive streak starts to form

• the site of gastrulation (formation of 3 tissue layers = ecto, endo, and mesoderm)

– neural groove begins to form • future spinal cord & brain

– somites begin to form • bands of tissue that will become muscles & bones

– pharyngeal arches begin to form • future face, neck, mouth, nose

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Vertebrate body plan

Day 19

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Human embryonic developmental

Day 24

• Week 3 - Week 8 = embryo– development of all organ systems

• Day 22: the heart begins to beat

Day 28Week 4

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Human embryonic development

• embryo showing tail & limb buds

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Human embryonic development

• beginning of the eye can be seen, as well the bulging heart & the umbilical cord

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Human embryonic development• lens of the eye can be seen forming, the mass of

the heart bulging from the chest, & the beginnings of the finger rays

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Human embryonic development

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Human embryonic development• beginning of the ear is clearly seen

note the bend of the elbow joint has begun, the fingers are forming and toes are beginning to bud off the foot

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Human embryonic development• Note the formation of the nose, eyelids, ear flap

& well defined toes & fingers

50–60 days (8 weeks) Both knee & elbow are visible. Embryo has formed most of basic organ systems & will spend remainder of development in “fetal” period. Organs grow, mature, & begin to learn their respective functions

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Human fetal development• Day 52:

– first brain waves can be detected• weeks 4-8 is when all major organ systems of body are

formed & when most teratogens have greatest effect

Week 8

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Human fetal development

Week 10

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Human fetal development• At 15 weeks the embryo begins to take on more

of a typical baby's form & the attachment of umbilical cord can be clearly seen

Week 15

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Human fetal development• Week 9 - week 40 = fetus

– after 12 weeks or so, the baby's development is largely "finished"

• some exceptions: brain & lung development

Week 16

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Human fetal development• The fetus just spends much of the 2nd & 3rd

trimesters just growing …and doing various flip-turns & kicks inside amniotic fluid

Week 20

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Human fetal development

• 24 weeks (6 months)

fetus is covered with fine, downy hair called lanugo. It’s skin is protected by a waxy substance called vernix

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Human fetal development

30 weeks

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Human fetal development

• 32 weeks

The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

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The end of the journey!