Animal Characteristics & Development

CH21

From Single Cell to

Multicellular Organism

Animal Development• Although the end result of growth and

development amongst organisms of different species is often very different individuals (compare say a fly to an elephant), the mechanisms that form such diversity are remarkably similar

• By applying the tools of molecular genetics, we can better understand the processes that control development

Characteristics of Animals

• Composed of Eukaryotic cells

• Multicellular

• All animals are motile at some point during their lives

• Heterotrophs

Characteristics of Animals

• Animals are heterotrophs: they consume other organisms to obtain food and energy– All animals depend directly or indirectly on

autotrophs for energy– All autotrophs depend on the sun for energy– Ultimately, every living thing depends on the

sun for energy

• Animals digest food: after ingestion, animals digest their food

Animal Cell Adaptations

• Most animal cells are differentiated and carry out different functions

• 4 main types of tissue:– Endothelial skin– Connective bone, fat, tendons …– Nervous brain and nerves– Muscle

• Skeletal muscle• Cardiac muscle• Smooth muscle

Development of Animals

• Most animals develop from a fertilized egg called a zygote

• All the genes necessary to transform the 1 celled zygote into a fully grown adult of that species are present at conception and begin to guide development immediately following fertilization

Fertilization

• Most animals reproduce sexually

• Fertilization occurs when a sperm cell penetrates an egg cell forming a zygote– This can be an internal or external process

Differential Gene Expression in Multicellular Organisms

• 3 Processes are necessary for proper embryo development– Cell Division

(Mitosis)– Cell Differentiation– Morphogenesis

Cell Differentiation & Morphogenesis

• Both processes are due to the cells ability to differentially regulate gene expression– This initially happens through cytoplasmic

determinants, and inductive elements

• Differential gene expression is possible in an early embryo through– Regulation of Chromatin Structure– Regulation of Transcription Initiation

Cell Differentiation & Morphogenesis

Regulation of Chromatin Structure

• Histone Modifications– Acetylation

– methylation

• DNA Methylation– Often methylation patterns

are tissue specific– Accounts for genomic

imprinting

• Epigenetic Inheritance• Cytoplasmic Determinants

Regulation of Transcription Initiation

• General transcription factors (TATA box)

• Specific TF’s• Enhancers

– ACTIVATORS– REPRESSORS

• GENE SILENCING

• Induction– when a cell is induced to

undergo changes and differentiation due to signals from its environment

Important Note About Inductions

• In the developing embryo, sequential inductions drive the formation of organs

• The effect of an inducer can depend on its concentration

• Inducers produce their effects via signal transduction pathways similar to those operating in adult cells

The induced cells response is often the activation (or inactivation) of genes – transcriptional regulation- which in turn establishes the pattern of gene activity characteristic of a particular kind of differentiated cell

Cell Division

• The zygote divides by mitosis to form 2 cells in a process called cleavage

• The 2 cells that result from cleavage then divide to form 4 cells

• This continues until a blastula is formed– A blastula is a cell-covered fluid filled ball– It takes human embryos 5 days after

fertilization to form a blastula

Cell Division

• As soon as cell division (mitosis) begins, the organism is no longer a zygote, it is called an embryo

• An embryo is a living organism at the early stages of growth and development

Gastrulation

• After blastula formation, the cells on one side of the blastula move inward forming the rudimentary gut

• Gastrula: a structure made up of 2 layers of calls with an opening at one end

Gastrulation

• This forms a round structure with an inner cavity

• The outer layer is called the ectoderm – As the ectoderm continues to grow and divide

it eventually forms the skin and nervous tissue of the organism

• The inner layer is called the endoderm– The endoderm cells develop into the lining of

the digestive tract and into organs associated with digestion and the respiratory system

Formation of the Mesoderm

• The mesoderm is the 3rd layer found in a developing embryo between the endoderm and ectoderm

• Mesoderm cells develop into – Muscles– Circulatory system– Excretory system– Reproductive – Gastrulation Animation

Apoptosis

• Another outcome of cell signaling that is crucial to development is programmed cell death

• Example: the timely suicide of cells occurs exactly 131 times in C. elegans (earth worms), and at precisely the same points

Apoptosis• Occurs when a cascade of suicide proteins

become activated– Proteases, especially caspases

• Caspases: are the main proteins of apoptosis

• Humans have 15 different caspases

– Nucleases

• The cell shrinks and becomes lobed, called blebbing, the nucleus condenses and the DNA becomes fragmented until neighboring cells engulf and digest it

Apoptosis & Human Development

• Apoptosis is why our fingers and toes aren’t webbed

• Certain degenerative diseases may be the result of inappropriate activation of apoptotic pathways

• Some cancers may result from failure to initiate apoptotic pathways

Protostome & Deuterostomes

• When the opening of the gastrula develops into a mouth the organism is called a protostome– Snails, earthworms, insects…

• An animal whose mouth develops from elsewhere in the gastrula is called a deuterostome– Fish, snakes, humans…

Growth & Development• Cells in developing organisms continue to

differentiate and become specialized to perform different functions

• Pics of fetus at 10 weeks taken with new 3D ultrasound technology

REMEMBER!

• As tissues and organs of an embryo take shape, the cells become visibly different in structure and function

• These changes are actually the outcome of a cells developmental history extending back to the 1st mitotic divisions of the zygote

Cell determination (observable differentiation) is marked by the

expression of genes for tissue specific proteins

It is these proteins that give a cell its phenotype, in other words, its attributes

and characteristics

Early Human Development

• By 11-12 weeks an unborn human child has every body organ present and working

• A heart beat can be measured as early as 25 days

• Brainwaves can be

measured as early

as 43 days

Comparative Studies in Early Embryo Development

• Comparative studies help explain how the evolution of development leads to morphological diversity

• Evolutionary developmental biologists compare developmental processes of different multicellular organisms

Pattern Formation: Setting Up A Body Plan

• Cytoplasmic determinants and inductive signals both contribute to the development of a spatial organization in which the tissues and organs of an organism are all in their characteristic places

• This process is called pattern formation– Begins in early embryo when major axes of

an animal are established

3 Major Body Axes

• Before tissues and organs of a bilaterally symmetrical animal appear, the relative positions must be determined– Head & tail– Right and left– And front and back

Positional Information

• The molecular cues for these positional information are provided mainly by cytoplasmic determinants and inductive signals

• These cues tell a cell its position in relation to the body axes and to neighboring cells and determine how the cell and its progeny will respond to future molecular signals

Drosophila as a Model Organism

• In the 1940s scientists began studying mutant Drosophila (fly) development

• These studies established that genes control development and have led to an understanding of the roles of molecules in defining position and directing differentiation

Life Cycle of Drosophila

• Arthropods are constructed in an ordered series of segments including– The head– The thorax (from which wings and legs extend)– The abdomen

• Like other bilaterally symmetrical animals they have – An anterior-posterior axis (head to tail)– Dorsal ventral axis (back to belly)– And right-left axisWe’ll focus on the anterior-posterior axis for our

example

Drosophila Development

• The egg develops in the female ovary surrounded by ovarian cells called nurse cells that provide it with – Nutrients– mRNA– And other substances needed for

development

Genetic Analysis of Early Development

• By studying bizarre mutant flies with developmental defects that led to extra wings or legs in the wrong place, a genetic mzap of the genes necessary for proper body orientation was created

Legs coming out

of face! (eeewy)

Fruit Fly Development Animation

Homeotic genes

• The genes that control the pattern of body formation during early embryonic development of organisms discovered by studying mutant drosophila.

• These genes encode transcription factors that direct cells to form various parts of the body.

A homeotic protein can activate one gene but

repress another, producing effects that are

complementary and necessary for the ordered

development of an organism

Widespread Conservation of Developmental Genes

• Molecular analysis of homeotic genes in Drosophila have shown that they all include a 180-nucleotide sequence called a homeobox– Specifies a 60 amino acid domain in the protein

called a homeodomain– Most vertebrates and invertebrates have almost

identical nucleotide sequences in their homeobox, even conserving their location and arrangement within the chromosomes

Homeobox

• Homeobox- are genes responsible for the big decisions of development rather than the details of engineering.– In animals homeotic genes are called Hox genes

• Homoeotic genes are almost identical in very different species. We can study homeotic genes in the fruit fly to learn about the same genes the control development in a human or mouse.

• These groups of genes have remained relatively unchanged throughout evolutionary history.

• Homeotic genes that control the form of anterior and posterior structures of the body occur in the same linear sequence on chromosomes in Drosophila and mice

• Each colored band on the chromosomes represents a homeotic gene

Axis Establishment

• Cytoplasmic determinants initially establish axes of the drosophila body

• These determinants are encoded by genes called maternal effect genes

• Maternal effect gene: a gene that when mutant in the mother results in a mutant phenotype in the offspring, regardless of the offsprings genotype

Critical Thinking

1. In what cell of a mother would there have to be a mutation in a maternal effect gene to give rise to a mutant offspring?

2. If you don’t have something at least intelligent/plausible written down you will lose 2 points of your next quiz grade

Egg-Polarity Genes

• Because they control orientation (polarity) of the egg and consequently of the fly, maternal effect genes are also called egg-polarity genes– One group controls development of the

anterior-posterior axis– A second group establishes dorsal ventral

axis– cytoplasmic determinants and development animation

What does all this mean

• Maternal environment is crucial for proper development of an embryo– Egg-polarity genes, maternal mRNAs,

cytoplasmic determinants, and maternal induction are all crucial to the proper development of a zygote into a healthy full grown multicellular animal

– So…

When your pregnant don’t drink or smoke, take your

vitamins, exercises carefully and regularly, rest

as needed, don’t over stress,and never under any

circumstances do drugs