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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


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


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


Life Cycle: Leopard Frog

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

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

Effects of Yolk Size

on Cleavage Patterns

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

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

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

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

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

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


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

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

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


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

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

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

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