Animal Development

Chapter 43

Impacts, Issues

Mind-Boggling Births

From a single fertilized egg, all adult cells and

tissues develop – humans are learning to

manipulate the beginnings of life

43.1 Stages of

Reproduction and Development

Animals as different as sea stars and sea otters

pass through the same stages in their

developmental journey from a single, fertilized

egg to a multicelled adult

Six Processes of

Reproduction and Development

Gamete formation

• Egg and sperm production

Fertilization

• Egg and sperm join to form a zygote

Cleavage (blastula formation)

• Repeated mitotic divisions increase the number

of cells (blastomeres), not the volume

Six Processes of

Reproduction and Development

Gastrulation

• Gastrula (early embryo) forms with two or three

germ layers (forerunners of tissues and organs)

Organ formation

• Tissues become arranged into organs

Growth and tissue specialization

• Continues into adulthood

Six Processes of

Reproduction and Development

Fig. 43-2, p. 760

a Eggs form and mature in

female reproductive organs.

Sperm form and mature in

male reproductive organs.

Gamete Formation

b A sperm penetrates an egg.

Their nuclei fuse. A zygote

has formed.

Fertilization

c Mitotic cell divisions form a

ball of cells, a blastula. Each

cell gets regionally different

parts of the egg cytoplasm.Cleavage

d A gastrula, an early embryo

that has primary tissue layers,

forms by cell divisions, cell

migrations, and

rearrangements.

Gastrulation

e Details of the body plan fill in

as different cell types interact

and form tissues and organs

in predictable patterns.

Organ Formation

f Organs grow in size,

take on mature form,

and gradually assume

specialized functions.

Growth, Tissue Specialization

Growth, Tissue Specialization

f Organs grow in size,

take on mature form,

and gradually assume

specialized functions.

Organ Formation

e Details of the body plan fill in

as different cell types interact

and form tissues and organs

in predictable patterns.

d A gastrula, an early embryo

that has primary tissue layers,

forms by cell divisions, cell

migrations, and

rearrangements.

Gastrulation

Fig. 43-2, p. 760

c Mitotic cell divisions form a

ball of cells, a blastula. Each

cell gets regionally different

parts of the egg cytoplasm.Cleavage

Stepped Art

b A sperm penetrates an egg.

Their nuclei fuse. A zygote

has formed.

Fertilization

a Eggs form and mature in

female reproductive organs.

Sperm form and mature in

male reproductive organs.

Gamete Formation

Life Cycle: Leopard Frog

Fig. 43-3a, p. 760

transformation to

adult nearly completeadult, three

years old

Sexual reproduction

(gamete formation,

external fertilization)

organ

formationtadpole cleavage eggs

and

sperm

larva (tadpole) zygote

adult, three

years old

Fig. 43-3a, p. 760

transformation to

adult nearly complete

tadpole

larva (tadpole)

Sexual reproduction

(gamete formation,

external fertilization)

eggs

and

sperm

organ

formation cleavage

zygoteStepped Art

Life Cycle: Leopard Frog

Fig. 43-3b, p. 761

gray crescent

B Here we show the first three divisions of

cleavage, a process that carves up the zygote’s

cytoplasm. In this species, cleavage results in a

blastula, a ball of cells with a fluid-filled cavity.

blastocoel

C Cleavage is over

when the blastula

forms.

blastula

Fig. 43-3b, p. 761

Fig. 43-3c, p. 761

Fig. 43-3c, p. 761

blastocoel

C Cleavage is over when

the blastula forms.

blastula

Fig. 43-3d, p. 761

Fig. 43-3d, p. 761

ectodermyolk

plug

neural

plate

ectoderm

dorsal lip mesoderm

D The blastula becomes a three-layered gastrula—a process

called gastrulation. At the dorsal lip, a fold of ectoderm above

the first opening that appears in the blastula, cells migrate

inward and start rearranging themselves.

future gut

cavityendoderm

Fig. 43-3e, p. 761

Fig. 43-3e, p. 761

neural

tube

notochord

gut cavity

E Organs begin to form as a

primitive gut cavity opens

up. A neural tube, then a

notochord and other organs

form from the primary tissue

layers.

Fig. 43-3f, p. 761

Fig. 43-3f, p. 761

Tadpole, a swimming larva

with segmented muscles

and a notochord extending

into a tail.

Limbs grow and the tail

is absorbed during

metamorphosis to the

adult form.

Sexually mature, four-

legged adult leopard

frog.

F The frog’s body form changes as it grows and its tissues specialize. The

embryo becomes a tadpole, which metamorphoses into an adult.

Animation: Leopard frog life cycle

43.2 Early Marching Orders

The location of materials in an egg and

distribution of those materials to descendant cells

affects early development

Cytoplasmic localization

• In an unfertilized egg, many enzymes, mRNAs,

yolk, and other materials are localized in specific

parts of the cytoplasm

Cytoplasmic Localization

Fig. 43-4a, p. 762

Fig. 43-4a, p. 762

animal pole

pigmented cortex

yolk-rich cytoplasm

vegetal pole

sperm

penetrating

egg

gray

crescent

egg after

fertilization

Fig. 43-4b, p. 762

Fig. 43-4b, p. 762

gray crescent of

salamander zygote

First cleavage

plane; gray

crescent split

equally. The

blastomeres are

separated

experimentally.

Two normal larvae

develop from the

two blastomeres.

B Experiment 1

Fig. 43-4c, p. 762

Fig. 43-4c, p. 762

gray crescent of

salamander zygote

First cleavage

plane; gray

crescent missed

entirely. The

blastomeres are

separated

experimentally.

A ball of

undifferentiated

cells forms.

Only one

normal larva

develops.

C Experiment 2

Fig. 43-4, p. 762

Stepped Art

sperm

penetrating

egg

egg after

fertilization

gray

crescent

pigmented

cortexyolk-rich

cytoplasm

animal pole

vegetal pole

Two normal larvae

develop from the

two blastomeres.

B Experiment 1

First cleavage

plane; gray crescent

split equally. The

blastomeres are

separated

experimentally.

gray crescent

of salamander

zygote

A ball of

undifferentiated

cells forms.

Only one

normal larva

develops.

C Experiment 2

First cleavage

plane; gray

crescent missed

entirely. The

blastomeres are

separated

experimentally.

gray crescent of

salamander

zygote

A

Animation: Cytoplasmic localization

Cleavage Divides Up

the Maternal Cytoplasm

Cleavage divides a fertilized egg into a number

of small cells but does not increase its original

volume

The cells (blastomeres) inherit different parcels

of cytoplasm that will make them behave

differently later in development

Two Main Animal Lineages

Differ in Cleavage Patterns

Protostomes

• Bilateral invertebrates

• Undergo spiral cleavage

Deuterostomes

• Echinoderms and vertebrates

• Most undergo radial cleavage

• Mammals undergo rotational cleavage

Variations in Cleavage Patterns

Fig. 43-5, p. 763

a Early protostome embryo.

Its four cells are undergoing

spiral cleavage, oblique to

the anterior–posterior axis:

b Early deuterostome

embryo. Its four cells

are undergoing radial

cleavage, parallel with

and perpendicular to the

anterior–posterior axis:

Effects of Yolk Size

on Cleavage Patterns

Fig. 43-6, p. 763

a Sea urchin egg, with

little yolk. Cleavage is

complete. First cells

formed are equally

sized.

b Frog egg, with

moderate amount

of yolk. Yolk slows

cleavage so lower

cells are larger.

c Fish egg, with a

large amount of

yolk. Cleavage is

restricted to the

layer of cytoplasm

on top of the yolk.

Two cells formed

by first cleavage

mass of yolk

Structure of the Blastula

Blastula

• Cells produced by cleavage

• Structure varies with species’ cleavage pattern

Blastocyst (mammalian blastula)

• Outer cells secrete fluid into the cavity

• Inner cells, clustered against the cavity wall,

develop into the embryo

43.3 From Blastula to Gastrula

Gastrulation

• Developmental process during which cells

rearrange themselves into primary tissue layers

Most animals have three primary tissue layers

• Outermost layer (ectoderm)

• Middle layer (mesoderm)

• Inner layer (endoderm)

Gastrulation in a Fruit Fly

Initiation of Gastrulation

Gastrulation occurs when certain cells of the

blastula make and release short-range signals

that cause nearby cells to move about, either

singly or as a cohesive group

Embryonic induction

• The fate of one group of embryonic cells is

affected by its proximity to another group of cells

Experiment: Embryonic Induction

Transplanted cells of the dorsal lip of the

blastula (descended from the zygote’s gray

crescent) induced gastrulation in salamanders

Fig. 43-8, p. 764

C The embryo

develops into a

―double‖ larva, with

two heads, two tails,

and two bodies

joined at the belly.

A Dorsal lip excised

from donor embryo,

grafted to novel site

in another embryo.

B Graft induces a

second site of inward

migration.

Animation: Embryonic induction

43.4 Specialized

Tissues and Organs Form

Cell differentiation

• Process by which cell lineages become

specialized

• Lays the groundwork for formation of specialized

tissues and organs

• Based on selective gene expression

Signaling molecules contribute to differentiation

Morphogens

Morphogens

• Signaling molecules encoded by master genes

• Diffuse from a source and form a concentration

gradient throughout the embryo

• Have different effects depending on their

concentration in each region

Morphogenesis

Morphogenesis

• Process by which tissues and organs form

• Some cells migrate to new locations

• Sheets of cells change shapes to form organs

• Apoptosis shapes body parts such as fingers

Apoptosis

• Cells die on cue; signals from cells cause other

cells to self-destruct

Morphogenesis: Neural Tube Formation

Fig. 43-9, p. 765

A Gastrulation

produces a sheet of

ectodermal cells.

B As microtubules

constrict or lengthen

in different cells, the

cells change shape,

and the sheet forms

a neural groove.

neural groove

C Edges of the

groove meet and

detach from the main

sheet, forming the

neural tube.

ectoderm

neural tube

Animation: Neural tube formation

Pattern Formation

Pattern formation

• Process by which body parts form in a specific

place

Example: Limb bud formation in chicks

• AER at the tips of limb buds induces the

mesoderm beneath to form a limb

Limb Bud Formation in Chicks

Fig. 43-10, p. 765

mesoderm of

chick embryo

forelimb

A Experiment 1:

Remove wing

bud’s AER AER

removed no limb

forms

B Experiment 2:

Graft a bit of leg

mesoderm under

the AER of a wing

mesoderm

from leg

wing

AER (region of

signal-sending

ectoderm)

leg forms

Animation: AER transplant

43.5 An Evolutionary

View of Development

Similarities in developmental pathways among

animals are evidence of common ancestry

Cytoplasmic localization in the egg induces

expression of localized master genes

Concentration gradients of master gene

products cause embryonic cells to form tissues

and organs at certain locations

Homeotic Genes

Positional information established by

concentration gradients of master gene products

affects expression of homeotic genes, which

regulate development of specific body parts

Developmental Constraints

and Modifications

Physical constraints

• Surface-to-volume ratio

Architectural constraints

• Existing body frameworks, such as four limbs

Phyletic constraints

• Master genes determine basic body form

Developmental Constraints

and Modifications

Mutations that alter the effects of master genes

are often lethal

Example: Development of somites

• Mesoderm on either side of the neural tube

divides into blocks of cells that will develop into

bones and muscles

Lethal Mutation Affecting Somites

43.1-43.5 Key Concepts

Principles of Animal Embryology

Animals develop through cleavage, gastrulation,

organ formation, and then growth and tissue

specialization

Cleavage parcels out material stored in different

parts of the egg cytoplasm into different cells,

thus starting the process of cell specialization

43.6 Overview of Human Development

Humans begin life as a single cell and go

through a series of developmental stages

• Second week: Blastocyst is embedded in the

mother’s uterus, where it develops

• Embryonic period (first 8 weeks): All organs form

• Fetal period (9 weeks to birth): Organs of the

fetus grow and specialize

• Postnatal growth (after birth): Organ growth and

maturation continues until adulthood

Stages of Human Development

Prenatal and Postnatal Changes

Fig. 43-12, p. 767

8-week

embryo

12-week

embryo

newborn 2 years 5 years 13 years

(puberty)

22 years

43.7 Early Human Development

Cleavage of a zygote produces a cluster of 16

cells (morula) by the time it reaches the uterus

By the fifth day, a blastocyst forms, consisting

of an outer layer, a fluid-filled cavity (blastocoel)

and an inner cell mass

• Inner cell mass will form the embryo

• Outer cells will form supportive tissues

Implantation

Implantation

• The blastocyst ruptures the zona pellucida and

burrows into the lining (endometrium) of the

mother’s uterus

• In ectopic pregnancy, the blastocyst implants

outside the uterus

Extraembryonic Membranes

The outer layer of the blastocyst gives rise to

four external membranes

• Amnion encloses and protects the embryo in a

fluid-filled cavity

• Yolk sac gives rise to blood and germ cells

• Chorion extends into maternal tissues and

becomes part of the placenta

• Allantois gives rise to blood vessels of placenta

The Placenta

Placenta

• An organ that functions in exchange of materials

between the bloodstreams of a mother and her

developing child

• Forms from projections of chorion that extend into

blood-filled maternal tissues, and blood vessels of

allantois

Human Extraembryonic Membranes

Early Hormone Production

Human chorionic gonadotropin (HCG)

• Released by blastula after implantation

• Causes corpus luteum to keep secreting

progesterone and estrogens to maintain the

uterine lining

The placenta takes over secretion of HCG after

about three months

Fertilization to Implantation

endometrium

fertilization

in oviduct

implantation

in the uterus

Fig. 43-13a, p. 768

inner cell mass

Fig. 43-13a, p. 768

inner cell mass

endometrial

epithelium

cavity inside

the uterus

surface layer

cells of the

blastocyst

blastocoel

Fertilization to Implantation

DAYS 10–11. The yolk sac,

embryonic disk, and amniotic

cavity have started to form

from parts of the blastocyst.

start of

amniotic

cavity

start of

embryonic

disk

start of

yolk sac

actual

size

Fig. 43-13b, p. 769

blood-filled spaces

actual

size

DAY 12. Blood-filled

spaces form in maternal

tissue. The chorionic

cavity starts to form.

start of

chorionic cavity

Fig. 43-13b, p. 769

chorion

actual

size

DAY 14. A connecting stalkhas formed between the embryonicdisk and chorion. Chorionic villi,which will be features of aplacenta, start to form.

yolk sac

chorionic

villi

connective

tissue

amniotic

cavity

chorionic

cavity

Fig. 43-13b, p. 769

Animation: Cleavage and implantation

Animation: First two weeks of

development

43.8 Emergence of

the Vertebrate Body Plan

Two weeks after fertilization, the inner cell mass

of a blastocyst is a two layered embryonic disc

Gastrulation occurs in the third week, forming an

embryo with three germ layers: ectoderm,

mesoderm, and endoderm

• Primitive streak, neural tube and notochord form

• Somites appear on either side of the neural tube

Derivatives of Human Germ Layers

Features of the Embryonic Period

Fig. 43-14, p. 770

paired neural folds future brain pharyngeal

arches

yolk sac

embryonic disk

primitive

streakamniotic cavity

chorionic cavity

neural groove (below,

notochord is forming) somites

A DAY 15. A faint band appears around a depression along the axis of the embryonic disk. This band is the primitive streak, and it marks the onset of gastrulation in vertebrate embryos.

B DAYS 18–23. Organs start to form through cell divisions, cell migrations, tissue folding, and other events of morphogenesis. Neural folds will merge to form the neural tube. Somites (bumps of mesoderm) appear near the embryo’s dorsal surface. They will give rise to most of the skeleton’s axial portion, skeletal muscles, and much of the dermis.

C DAYS 24–25. By now, some embryonic cells have given rise to pharyngeal arches. These will contribute to the formation of the face, neck, mouth, nasal cavities, larynx, and pharynx.

43.6-43.8 Key Concepts

Human Development Begins

A pregnancy starts with fertilization and

implantation of a blastocyst in the uterus

After implantation, a three-layered embryo forms

and organ formation begins

All organs have formed by the end of the eighth

week

43.9 The Function of the Placenta

Maternal and embryonic blood do not mix

• Vessels of the embryo’s circulatory system

extend through the umbilical cord to the placenta,

where they run through pools of maternal blood

• Substances diffuse across membranes between

maternal and embryonic bloodstreams

Placental hormones maintain the uterine lining

The Placenta

Fig. 43-15a, p. 771

Fig. 43-15a, p. 771

4 weeks

8 weeks

12 weeks

Fig. 43-15b, p. 771

Fig. 43-15b, p. 771

appearance of the placenta at full term

umbilical

cord

uterine tissue amniotic fluid

around fetus

fetal

blood vessels

maternal

blood

vessels

movement

of solutes

to and from

maternal

blood

vessels

(red and blue

arrows)

umbilical cord

blood-filled

space

between villi

chorionic villus

tissues

of uterus

fused amniotic

and chorionic

membranes

43.9 Key Concepts

Function of the Placenta

The placenta allows substances to diffuse

between bloodstreams of a mother and her

developing child

It also produces hormones that help sustain the

pregnancy

43.10 Emergence of

Distinctly Human Features

Embryonic features disappear and the fetus

takes on human appearance about 8th week

Heartbeat and movements are detected in the

second trimester

In the third trimester, the brain is formed and

functioning

Development of the Human Embryo

yolk sacWEEK 4

embryoconnecting stalk

WEEKS 5-6

Fig. 43-16a, p. 772

Fig. 43-16a, p. 772

forebrain

future lens

pharyngeal

arches

developing heart

upper limb bud

somites

neural tube

forming

lower limb

bud

tail

Fig. 43-16a, p. 772

Fig. 43-16a, p. 772

retinal pigment

head growth exceedsgrowth of other regions

foot plate

umbilical cord formationbetween weeks 4 and 8(amnion expands, formstube that encloses theconnecting stalk and aduct for blood vessels)

upper limb differentiation(hand plates develop, thendigital rays of future fingers;wrist, elbow start forming)

future external ear

Fig. 43-16a, p. 772

Development of the Human Embryo

Fig. 43-16b, p. 773

Fig. 43-16b, p. 773

placenta

Fig. 43-16b, p. 773

WEEK 8

final week of embryonic

period; embryo looks

distinctly human

compared to other

vertebrate embryos

upper and lower limbs well

formed; fingers and then

toes have separated

primordial tissues of

all internal, external

structures now developed

tail has become stubby

Fig. 43-16b, p. 773

Fig. 43-16b, p. 773

During fetal period, length

measurement extends

from crown to heel (for

embryos, it is the longest

measurable dimension, as

from crown to rump).

WEEK 16

Length: 16 centimeters

(6.4 inches)

Weight: 200 grams

(7 ounces)

WEEK 29

Length: 27.5 centimeters

(11 inches)

Weight: 1,300 grams

(46 ounces)

WEEK 38 (full term)

Length: 50 centimeters

(20 inches)

Weight: 3,400 grams

(7.5 pounds)

Fig. 43-16b, p. 773

43.11 Mother as Provider and Protector

A developing human depends on its mother to

supply the nutrients it requires to grow and develop

• Proteins, carbohydrates, and lipids

• Vitamins and minerals

Dietary deficiencies affect many developing organs

Teratogens

The embryo/fetus is also subjected to any toxins

or pathogens to which the mother is exposed

Teratogens

• Toxic or infectious agents that interfere with

development

• Effects vary with the timing of exposure

Teratogens

Infectious agents

• Viral diseases (such as rubella), toxoplasmosis

Alcohol and caffeine

• Fetal alcohol syndrome, miscarriage

Smoking

• Affects growth and development

Prescription drugs

• Some medications cause severe birth defects

Fetal Alcohol Syndrome (FAS)

Teratogen Sensitivity

Fig. 43-17, p. 774

defects in physiology; physical abnormalities minor

major morphological abnormalities

weeks:1 2 3 4 5 6 7 8 9 16 20–36 38

cleavage, implantation

future heart

future eye

future ear

palate forming

limb buds teeth external genitals

central nervous system

heart

upper limbs

eyes

lower limbs

teeth

palate

external genitalsinsensitivity to

teratogens ear

future brain

43.10-43.11 Key Concepts

Later Human Development

By the time the fetal period begins, the

developing individual appears distinctly human

Harmful substances that get into a mother’s

blood can cross the placenta and cause birth

defects in the developing embryo or fetus

43.12 Birth and Lactation

Labor is the process of giving birth

• Amnion ruptures, cervix dilates

• Contractions force the fetus, and later the

placenta (afterbirth), through the birth canal

Oxytocin stimulates muscle contractions in a

positive feedback loop during birth

• Secreted by the posterior pituitary

Birth and Afterbirth

Fig. 43-19a, p. 776

Fig. 43-19a, p. 776

placenta

wall of uterus

umbilical cord

dilating cervix

Fig. 43-19b, p. 776

Fig. 43-19c, p. 776

Fig. 43-19c, p. 776

placenta detaching

from wall of uterus

umbilical cord

Animation: Birth

Nourishing the Newborn

Newborn humans are nourished with milk

secreted by the mother’s mammary glands

Hormonal control of lactation (milk production)

• Prolactin, secreted by the anterior pituitary,

triggers milk synthesis

• Declines in progesterone and estrogen

production after birth increase milk production

• Oxytocin stimulates release of milk into milk ducts

Lactation and Mammary Glands

Fig. 43-20, p. 776

milk-producing

mammary glandnipple

adipose

tissue milk duct

43.12 Key Concepts

Birth and Lactation

Positive feedback control plays a role in the

process of labor, or childbirth

After birth, the newborn is nourished by milk

secreted by mammary glands

Animation: Anatomy of the breast

Animation: Blastomere separation I

Animation: Blastomere separation II

Animation: Fetal development

Animation: Formation of gray crescent

Animation: Proportional changes during

development

Animation: Sensitivity to teratogens

Animation: Stages of development

Animation: Structure of the placenta

Animation: Three variations in

gastrulation

Animation: Weeks 3 to 4 of development

ABC video: Mermaid Baby

ABC video: Bonus for a Baby

Video: Mind-boggling births