animal development 2nd

7/29/2019 Animal Development 2nd

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2. Animal Development

Dr. Siti Akmar Ab. Rahim

([email protected])

: 082-582965

Department of Aquatic ScienceFaculty of Resource Science & Technology

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mailto:[email protected]:[email protected]


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Learning Objectives:

Animal development

Process of fertilisation & embryogenesis (cleavage,gastrulation, organogenesis)

Animal growth

Growth phases, patterns, curves Growth under extreme conditions

Dormancy

Hibernation

Aestivation

Diapause

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Animal Development or Ontogeny

An orderly, predictable sequence of eventsbeginningwith fertilization & ending with death

Qualitative change in shape, function & degree ofspecialisation

Includes fertilisation, embryogenesis, birth, infancy,childhood, adolescence, adulthood, senescence &death

Morphogenesis: a process that an animal takes

shape & the differentiated cells end up in theappropriate locations

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Fertilisation to Embryogenesis

Fertilisation activates the egg & bring together thenuclei of sperm & egg.

Sea urchins are models for the study of the early

development ofdeuterostomes.

Sea urchins eggs are fertilised externally.

Sea urchins eggs are surrounded by a jelly coat.


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Protostomes vs. Deuterostomes

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Gametes of Sea Urchin & Human

Sperm

Egg

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Fertilisation (involves 3 steps, based on sea urchin)

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Activation of egg


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1. The Acrosomal reaction

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1. Spermcontact with egg

2. Acrosomal reaction: when exposed to jelly coat the sperms

acrosome discharges its contents by exocytosis

3. Hydrolytic enzymes enable the acrosomal process to

penetrate the eggs jelly coat.

The tip of acrosomal process adheres to the vitelline layer

just external to the eggs plasma membrane

4. The sperm & egg plasma membranes fuse

5. A single sperm nucleus enterthe eggs cytoplasm.

Na+channels in the eggs plasma membrane opens.

Na+ flows into the egg & the membrane depolarises: fast

block to polyspermy.


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2. The Cortical Reaction

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Fusion of egg & sperm plasma membranes triggers a signal-

transduction pathway.

High concentrations of Ca2+ cause cortical granules to fuse with

the plasma membrane & release their contents into the

perivitelline space.

The vitelline layer separates from the plasma membrane.

An osmotic gradient draws water into the perivitelline

space, swelling it & pushing it away from the plasma

membrane.

The vitelline layer hardens into the fertilisationenvelope: a

component of the slow block to polyspermy

The plasma membrane returns to normal & the fast block to

polyspermy no longer functions.


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3. Activation of the Egg

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High concentrations of Ca2+ in the egg stimulates an

increase in the rates of cellular respiration &

proteins synthesis

In the meantime, back at the sperm nucleus

The sperm nucleus swells & merges with the eggnucleus diploid nucleus of the zygote.

DNA synthesis begins & the first cell division

occurs


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Fertilisation in Mammals (1)

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Capacitation: a function of the female reproductive system

that enhances sperm function

A capacitated sperm

migrates through a

layer of follicle cells

before it reaches the

zona pellucida.

Binding of the sperm

cell induces an

acrosomal reactionsimilar to that seen

in the sea urchin.


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Enzymes from the acrosome enable the sperm cell to

penetrate the zona pellucida & fuse with eggs plasmamembrane.

The entire sperm enters the egg

The egg membrane depolarizes: functions as a fast

block to polyspermy

A cortical reaction occurs.

Enzymes from cortical granules catalyze

alterations to the zona pellucida: functions as a

slow block to polyspermy

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The envelopes of both egg & sperm nuclei disperse.

The chromosomes from the two gametes share a

common spindle apparatus during the first mitotic

division of the zygote

After fertilisation, embryonic development proceedsthrough cleavage, gastrulation & organogenesis


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Cleavage partitions the zygote into many

smaller cells

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Cleavage follows fertilisation. The zygote is partitioned into blastomeres.

Each blastomere contains different regions of

the undivided cytoplasm & thus different

cytoplasmic determinants


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Except for mammals, most animals have both eggs &

zygotes with a definite polarity

Thus the plane of division follow a specific patternrelative to the poles of the zygote.

Polarity is defined by the heterogeneous

distribution of substances such as mRNA, proteins& yolk.

Yolk is most concentrated at the vegetal pole

& least concentrated at the animal pole.

In some animals, the animal pole defines

the anterior end of the animal.


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In amphibians, a rearrangement of the egg

cytoplasm occurs at the time of fertilisation.

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The plasma membrane &

cortex rotate toward the

point of sperm entry.

The gray crescent isexposed & marks the

dorsal surface of the

embryo.

Cleavage occurs morerapidly in the animal pole

than in the vegetal pole.


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In sea urchins & frogs, first two cleavages are

vertical.

The third division is horizontal. The result is an 8-celled embryo with two tiers

of four cells.


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Continued cleavage produces the morula.

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A blastocoel forms within the morula blastula

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In birds, the yolk is so plentiful that it restrictscleavage to the animal pole meroblastic

cleavage.

In animals with less yolk, there is completedivision of the egg holoblastic cleavage.


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Gastrulation rearranges the blastula to

form a 3-layered embryo with a primitive

gut

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Gastrulation rearranges the embryo into a

triploblastic gastrula

The embryonic germ layers are theectoderm, mesoderm & endoderm.


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Sea urchin gastrulation

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Begins at the vegetal pole where individual cells

enter the blastocoel as mesenchyme cells.

The remaining cells flatten & buckle inwards:

invagination.

Cells rearrange to form the archenteron. The open end, the blastopore will become

the anus.

An opening at the other end of the

archenteron will form the mouth of the

digestive tube.


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Various stages of sea urchins embryonic

development

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In organogenesis, the organs of the animal

body form from 3 embryonic germ layers

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The derivatives of the ectoderm germ layer are:

o Epidermis of skin & its derivatives

o Epithelial lining of the mouth & rectum

o Cornea & lens of the eyes

o The nervous system, adrenal medulla, tooth

enamel, epithelium of pineal & pituitary

glands


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The endoderm germ layer contributes to:

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o

The epithelial lining of the digestive tract(except the mouth & rectum)

o The epithelial lining of the respiratory

system.o The pancreas, thyroid, parathyroids,

thymus

o The lining of the urethra, urinary bladder &reproductive systems.


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Derivatives of the mesoderm germ layer

are:

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o The notochord

o The skeletal & muscular systems

o The circulatory & lymphatic systems

o The excretory system

o The reproductive system (except germ cells)

o And the dermis of skin, lining of the bodycavity & adrenal cortex


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Amniote embryos develop in a fluid-filled

sac within a shell or uterus

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The amniote embryo is the solution to reproduction

in a dry environment.

Shelled eggs of reptiles & birds

Uterus of placental mammals


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Avian Development (1)

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Cleavage is meroblastic or incomplete.

Cell division is restricted to a small cap of cytoplasm at

the animal pole.

Produces a blastodisc which becomes arranged into

the epiblast & hypoblast that bound the blastocoel

(the avian version of a blastula)


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During gastrulation, some cells of the epiblast

migrate (arrows) towards the interior of the embryo

through the primitive streak

Some of these cells move lateral to form the

mesoderm, while others move downward to form the

endoderm


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In early organogenesis the archenteron is formed as

lateral folds pinch the embryo away from the yolk.

The yolk stalk (formed mostly by hypoblast cells)

will keep the embryo attached to the yolk.

The notochord, neural tube & somites form as they

do in frogs.

The three germ layers

& hypoblast cells

contribute to the

extraembryonicmembrane system.


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The 4 extraembryonic membranes are the yolk sac,

amnion, chorion & allantois.

Cells of the yolk sac digest yolk

providing nutrients to the

embryo.

The amnion encloses theembryo in a fluid-filled

amniotic sac which protects

the embryo from drying out.

The chorion cushions the

embryo against mechanical

shocks.

The allantois functions as a

disposal sac for uric acid.


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The development of

tissues is known as

histogenesis & results

from cell differentiation.

The three germ layers

will form & give rise to

all the structures of theadult via organogenesis

(organ formation) &

eventually

morphogenesis(establishment of form).
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What is growth?

Irreversible quantitative increase in parameters (size,

mass, volume, length, height) of organisms over aspecific time period

Growth phases:

1. Cell division2. Cell enlargement

3. Cell differentiation

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Growth phases

1. Cell division

Mitosis 2 new & small daughter cells

Restricted to unspecialised cells before they are modified

for a particular purpose

2. Cell enlargement

Formed daughter cells grow & become larger till certain

size divides again or undergoes differentiation3. Cell differentiation (multicellular animals)

Matured cells differentiate into specialised cells with

specific functions

Involve changes in biochemical & structural characteristics

Specialisation of cells increases the efficiency of functions

(transport, locomotion, digestion, immunity)

Create variety in shape & structure

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Growth patterns

Is determined by plotting a graph using a measurable

parameters (height, dry mass, no. of individuals or colonies)

againsttimeabsolute growth curve

Type of growth curve:

1. Sigmoid (lag, exponential, linear growth, equilibrium,

negative growth)

2. Limited most animals

3. UnlimitedObelia sp. & coral reef

4. Intermittent animals with exoskeleton which undergo

ecdysis/ moulting

Human growth curve (covered by Dr. Shamsir)

1. Allometric grow at different phase & rate

2. Isometric grow at same rate as other body parts

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Growth under extreme conditions

Weather & seasonal changes affects animalsand their surroundings

How to survive extreme conditions??

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D


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Dormancy State of reduced metabolic activity adopted by many

organisms under conditions of environmental stress (imposed

dormancy) or irrespective of external conditions (innatedormancy)

Some bats are dormant each day & active each night. Some

birds (e.g. hummingbirds) are active during the day &

dormant at night. These types of dormancy are known asdiurnal torpidity.

Some animals become dormant in the summer to protect

themselves from heat & drought. This type of dormancy is

called aestivation.

Many insects experience diapause, a period of inactivity &

lack of growth. Diapause can occur in any season. When it

occurs during the winter, it is sometimes called hibernation.

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V l f d


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Value of dormancy For many animals, dormancy is an essential part of the

life cycle, allowing an organism to pass through critical

environmental stages with a minimal impact on theorganism itself.

When lakes, ponds, or rivers dry up, aquatic organisms

that can enter a period of dormancy will survive.

Moreover, animals that can become dormant during the

extreme cold winter can extend their ranges into regions

where animals incapable of dormancy cannot live.

Dormancy also ensures that these animals will be freefrom competition during their periods of activity.

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C f d


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Causes of dormancy Factors contributing to the onset of dormancy include

changes in temperature & photoperiod and the availability of

food, water, O2 & CO2. Temperature changes affect the availability of food, water

& O2, thus providing further stimuli for dormancy.

In Arctic regions, during the winter months, food is less

abundant. In deserts, the summer months are periods of reduced

food availability, intense heat, or extreme aridity.

Lack of water in summer periods (drought) or winter periods

(freezing), as well as annual changes in the duration &intensity of light, particularly at high latitudes, are other

environmental factors that can induce dormant states.

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Hib i


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Hibernation

An inactive, sleeplike state that some animals enter during the

winter.

To protect themselves against the cold & reduce their need

for food.

Body temperature is lower than normal (endothermic

animal).

Extreme cold can freeze ectothermicanimals to death

because body temperature is controlled by environment.

Heartbeat & breathing slow down greatly.

Hibernating animal needs little energy to stay alive & canutilise the fat stored in its body tissues can survive cold

winters when food is scarce.

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Aestivation A dormant state that occurs in some animals during hot,

dry periods to protect from dryness.

Breathing, heartbeat & other body processes slow down

decreases the need for water survive hot & dry

periods.

Many amphibians & reptiles aestivate, as do some insects,

snails & fish

Fish that aestivate live in ponds & streams that evaporate

during the dry season. Some aestivators (frogs, lungfish &

salamanders) form a cocoon just before entering

aestivation to prevent water loss from the skin.

The animal awakens from aestivation after the dry season

& emerges from its cocoon.

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L fi h (L id i )


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Lungfish (Lepidosiren sp.), can

aestivate up to 4 years

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Di (1)


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Diapause (1)

Occur in many insects, during any stage of the life cycle

Characterized by a cessation of growth in the immature stages &

a cessation of sexual activity in adults.

In some insects, it is a reaction to unfavourable environmental

conditions; in others (e.g. certain moths & butterflies), diapause

is a necessary stage of the life cycle.

E.g. The 17-year larval & pupal periods of the cicada

Common among insects that live in arid desert areas, where

during the dry & hot summers, the insects usually hide

themselves in the soil at suitable depths or under any available

protective objects.

Insects may overwinter as egg, larva, nymph, pupa, or adult.

Mosquitoes & butterflies, survive in sheltered, relatively dry

places. Other insects construct nests or cocoons.

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Di (2)
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Diapause (2)

Lasts only until favourable environmental conditions return.

In other species, favourable environmental conditions & some

other stimulus, such as cold or food, is necessary. E.g. mosquito

Aedes vexans, eggs remain in diapause until they are flooded

with water to form a pool suitable for the larvae. Eggs of

another mosquito,A. canadensis, will not hatch until they have

been subjected to cold. In some, the onset of diapause needs a combination of

environmental factors operating on the regulatory mechanisms

(nervous & endocrine systems) of the insect. Photoperiod &

temperature influence the brain, which synthesizes & secretesa hormone (ecdysone) that controls other endocrine organs.

Without ecdysone, all insect growth & metamorphosis are

halted.

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Diapause (3)

Diapause can also regulate development within a population to

ensure optimal timing of emergence or temporal synchrony with

environmental resources.

E.g. Female rabbit fleas have an obligate adult diapause that

is broken only by feeding on the blood of a pregnant host

rabbit.

By the time the baby rabbits are old enough to be weaned,

the flea's offspring will be mature & ready to accompany the

rabbits when they leave the nest.

In this ecological relationship, diapause is an adaptation that

keeps the flea population from exceeding the carryingcapacity of its host.

animal development 2nd

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