CHAPTER 47: ANIMAL

DEVELOPMENTAP Biology 2013

ZYGOTE TO ADULT• Preformation - 18th century theory that

the egg or sperm contained an embryo

• The embryo was thought to be a preformed miniature infant (homunculus) that becomes larger during development

• We now know:

• An organism’s development is determined by the genome of the zygote and by differences that arise between early embryonic cells

• Cell differentiation - specialization of cells in their structure and function

• Morphogenesis - process by which an animal takes shape

Fig. 47.2EMBRYONIC DEVELOPMENT

Sperm

Adult frog

Egg

Metamorphosis

Larval stages

Zygote

Blastula

Gastrula

Tail-bud embryo

FERT

ILIZ

ATIO

N

CLEAVAGE

GASTRULATION

ORG

ANO-

GENESIS

DEVELOPMENTAL EVENTS• Fertilization - main function is to bring the haploid nuclei of sperm and

egg together to form a diploid zygote

• Contact of the sperm on the egg’s surface initiates metabolic reactions within the egg that trigger embryonic development

• Acrosomal reaction - when sperm meets egg, hydrolytic enzymes that digest material surrounding the egg are released

• Gamete contact blocks polyspermy

Fig. 47.3

Basal body (centriole)

Sperm plasma membrane

Sperm nucleus

Sperm head

Acrosome Jelly coat

Sperm-binding receptors

Fertilization envelope

Cortical granule Fused

plasma membranes

Hydrolytic enzymes Vitelline layer

Egg plasma membrane

Perivitelline space

EGG CYTOPLASM

Actin filament

Acrosomal process

1

2

3

DEVELOPMENTAL EVENTS• Fertilization:

• Fusion of egg and sperm also initiates the cortical reaction which causes a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg

• These changes cause the formation of a fertilization envelope that also acts as a block to polyspermy Fig. 47.4

10 sec after fertilization

25 sec 35 sec 1 min 500 µm

500 µm 30 sec 20 sec 10 sec after

fertilization 1 sec before fertilization

Point of sperm nucleus entry

Spreading wave of Ca2+

Fertilization envelope

EXPERIMENT

RESULTS

CONCLUSION

DEVELOPMENTAL EVENTS

• Activation of the Egg

• Because of the rise in Ca2+ in the egg’s cytosol, the rate of cellular respiration and protein synthesis increases substantially

• In mammals, the cortical reaction modifies the zona pellucida as a slow block to polyspermy

Fig. 47.5

Zona pellucida

Follicle cell

Sperm basal body

Sperm nucleus

Cortical granules

DEVELOPMENTAL EVENTS• Cleavage - period of rapid cell division without growth

• Many animals (not mammals) have defined polarity (distribution of yolk with vegetal pole having the most and the animal pole having the least)

Figs. 47.6

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

50 µm

4

5

6

DEVELOPMENTAL EVENTS• Cleavage planes follow

a specific pattern relative to the animal and vegetal poles

• Meroblastic cleavage - incomplete division of the egg (yolk-rich eggs like reptiles and birds)

• Holoblastic cleavage - complete division of the egg (little or moderate amounts of yolk like sea urchins and frogs)

Fig. 47.7

Zygote

2-cell stage forming

4-cell stage forming

8-cell stage

Vegetal pole

Blastula (cross section)

Gray crescent

Animal pole

Blastocoel

0.25 mm

0.25 mm

8-cell stage (viewed from the animal pole)

Blastula (at least 128 cells)

DEVELOPMENTAL EVENTS

• Morphogenesis - cells occupy their appropriate locations

• Gastrulation - rearranges the cells of the blastula into a three-layered embryo called a gastrula that has a primitive gut

• Three embryonic germ layers:

• Ectoderm - outer layer of gastrula

• Endoderm - lines the embryonic digestive tract

• Mesoderm - partially fills the space between the ectoderm and endoderm

Figs. 47.8-47.9

Key

Animal pole Blastocoel

Mesenchyme cells

Vegetal plate

Vegetal pole

Archenteron

Filopodia

Archenteron

Blastocoel

Blastopore Mouth

Mesenchyme (mesoderm forms future skeleton)

Anus (from blastopore)

Digestive tube (endoderm)

Ectoderm

Future ectoderm

Future mesoderm

Future endoderm

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

GASTRULATION FROG VS. CHICK

Key

Future ectoderm Future mesoderm Future endoderm

SURFACE VIEW CROSS SECTION Animal pole

Vegetal pole Early gastrula

Blastocoel

Dorsal lip of blasto- pore

Blastopore Dorsal lip of blastopore

Blastocoel shrinking

Archenteron

Archenteron

Blastocoel remnant

Ectoderm Mesoderm Endoderm

Blastopore Yolk plug Blastopore

Late gastrula

3

2

1 Fig. 47.10

Future ectoderm

Migrating cells (mesoderm)

Blastocoel

Epiblast

YOLK

Endoderm Hypoblast

Primitive streak

Fertilized egg Primitive streak

Embryo

Yolk

Fig. 47.11

7

8

9

GASTRULATION IN HUMANS• Human eggs have very

little yolk

• Blastocyst - human equivalent of blastula

• Inner cell mass - cluster of cells at one end of the blastocyst

• Trophoblast - outer epithelial layer that does not contribute to embryo but instead initiates implantation

• Gastrulation

Blastocyst reaches uterus. 1

2

3

4

Blastocyst implants (7 days after fertilization).

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

Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

Uterus

Maternal blood vessel

Endometrial epithelium (uterine lining) Inner cell mass

Trophoblast

Blastocoel

Expanding region of trophoblast

Epiblast Hypoblast Trophoblast

Expanding region of trophoblast Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast)

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

Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm

Allantois

Fig. 47.12

ORGANOGENESIS• Regions of the three germ layers develop into the rudiments of organs during

organogenesis

• Vertebrates form a notochord from the mesoderm and a neural plate from the ectoderm

• Neural plate curves inward forming the neural tube Fig. 47.13

Mesoderm also gives rise to the

somites (later form vertebrae and

muscle) and the coelom

Neural folds

1 mm

Neural fold

Neural plate

Notochord Ectoderm Mesoderm

Endoderm

Archenteron

(a) Neural plate formation

(b) Neural tube formation

(c) Somites

Neural fold

Neural plate

Neural crest cells

Outer layer of ectoderm

Neural crest cells

Neural tube

Eye Somites Tail bud

SEM

Neural tube

Notochord

Coelom

Neural crest cells

Somite

Archenteron (digestive cavity)

1 mm

MORPHOGENESIS• Involves changes in shape,

position, and adhesion

• Changes in shape involve reorganization of the cytoskeleton

• Formation of the neural tube involves microtubules and microfilaments

• Also impacts cell migration (movement of cells from one place to another) ex. convergent extension

• Tissue invagination is caused by changes in both cells shape and migration during gastrulation

Ectoderm

Neural plate

Microtubules

Actin filaments

Neural tube

Extension Convergence

Figs. 47.15 & 47.16

10

11

12

MORPHOGENESIS

• Apoptosis - programmed cell death

• At various times during development, individual cells, sets of cells, or whole tissues stop developing and are engulfed by neighboring cells

DEVELOPMENTAL FATE• Determination - cell or group

of cells becomes committed to a particular fate

• Differentiation

• Embryonic cells must become different from on another

• Interactions with other embryonic cells influence the fate of cells by causing changes in gene expression

• Fate maps - territorial diagrams of embryonic development Fig. 47.17

Epidermis Epidermis Central nervous system Notochord

Mesoderm

Endoderm

Blastula Neural tube stage (transverse section)

(a) Fate map of a frog embryo

64-cell embryos

Blastomeres injected with dye

Larvae

(b) Cell lineage analysis in a tunicate

P GRANULES IN C. ELEGANS

Newly fertilized egg

Zygote prior to first division

Two-cell embryo

Four-cell embryo

20 µm

2

1

3

4 Fig. 47.20

13

14

15

AXES OF EMBRYOS

• Nonamniotic vertebrates - body axes are determined during oogenesis or fertilization

• Amniotes - environmental differences play a role in establishing differences between cells and body axes

• Uneven cytoplasmic determinants are important in establishing body axes

Figs. 47.21 & 47.22

Dorsal Right

Anterior Posterior

Ventral Left

(a) The three axes of the fully developed embryo

(b) Establishing the axes

Animal hemisphere

Vegetal hemisphere

Animal pole

Vegetal pole

Point of sperm nucleus entry

Gray crescent

Pigmented cortex

Future dorsal side

First cleavage

Figure 47.21

Control egg (dorsal view)

2

1a 1b

Gray crescent

Control group

Experimental group

Experimental egg (side view)

Gray crescent

Thread

Normal Normal Belly piece

EXPERIMENT

RESULTS

THE “ORGANIZER”• Initiates a chain reaction of

inductions that result in the formation of the notochord, neural tube, and other organs

• Plays a major role in pattern formation (spatial organization)

• Positional information tells a cell where it is with respect to the animal’s body axes

• Wings and legs of chicks begin as limb buds

• Limb buds respond to positional information

Fig. 47.24 & 47.25

Limb buds 50 µm

Anterior Limb bud

AER

ZPA Posterior

Apical ectodermal ridge (AER)

(a) Organizer regions (b) Wing of chick embryo

Digits

Anterior

Proximal

Dorsal Posterior

Ventral

Distal

2

3 4

Donor limb bud

Host limb bud

ZPA

Anterior

Posterior

New ZPA

4

4

3

3

2 2

EXPERIMENT

RESULTS

16

17