Download Fertilization

Stages of Human Development
1.
2.
Fertilization
Embryo



3.
4.
5.
6.
Cleavage
Gastrulation
Neurulation
Fetus
Childhood
Puberty
Adulthood (Johnson does not discuss development past the fetus, so I will assign
some additional reading.)
Fertilization:



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Fertilization involves fusion of sperm and egg
plasma membranes.
Entry of sperm into egg triggers blocks to
polyspermy,
and in mammals, signals the egg to complete
meiosis and begin development.
See next figure.
figure 43-04.jpg
Figure
43.4
Figure 43.4
Fertilization:
Sperm and egg contribute differentially to
the zygote.
 The sperm contributes a haploid nucleus
and, in some species, a centriole.
 The egg contributes a haploid nucleus,
cytoplasm including nutrients, ribosomes,
mitochondria, and informational molecules
that control early stages of development.

Fertilization:
The cytoplasmic contents of the egg
are not distributed evenly, and are
used to set up the major axes of the
future embryo.
 See next figure.

Fertilized Egg
Figure
43.5
Figure 43.5
Fertilization:
The animal pole will become the future
embryo.
 The vegetal pole will become yolk or yolk
cells (food for the developing embryo, like
the yolk of a chicken egg).

Stages of Animal Development
1.
2.
After fertilization comes: Embryogenesis
Three subphases:

Cleavage
 Gastrulation
 Neurulation
Cleavage: Repackaging the
Cytoplasm
Cleavage follows fertilization.
 During cleavage, the cytoplasm of the
zygote is repackaged into smaller and
smaller cells.

Cleavage:
1. Cleavage is a period of rapid cell division without
cell growth or gene expression.
2. The result of cleavage is a mass of ~1000 cells called
a blastula that is ~size as the egg.
Early Development: the Embryo

Cleavage
 Zygote
to Blastula
 Increase in cell
number, not size
 Blastula
Hollow sphere of
dividing cells
 Blastocyst—in
mammals

Embryonic Development

Blastocyst (the mammalian blastula)
 One

week for egg to travel to uterus
About 100 cells in human
 Trophoblast
(gray cells)
Is an outer layer of blastocyst cells
 Becomes the placenta

 Inner
cell mass (red cells)
Clustered inside the trophoblast
 Becomes the embryo

Implantation

Into endometrium
 Inner

lining of uterus
Placenta develops
 2 Network of blood vessels
 Exchange nutrients and
waste

Amnion
 Sac

surrounding embryo
Yolk sac
 Produces
blood cells
 Normally a source for
food, but not in mammals
Embryo phase: Gastrulation

After cleavage, the blastula develops three
layers and is called the gastrula
 Different
layers give rise to various tissues
Endoderm
 Mesoderm
 Ectoderm

 Three
layers move relative to one another
table 43-01.jpg
Table
43.1
Table 43.1
Do not memorize!
Embryo phase: Gastrulation

Gastrulation is when most embryonic genes
are first expressed.

Prior, most proteins came from the mother.
 Many Gastrulas die due to errors or mutations
in the embryonic genome.
 Gastrula cells migrate (move from one place to
another).
Embryo phase: Neurulation
The Neurula follows the Gastrula.
 Cells migrate over the blastopore and
induce the overlying ectoderm to fold in on
itself to form a neural tube.
 The nervous system develops from the
neural tube.
 The next Figure shows neurulation views
from the top and inside.

Fetal Development

Follows neurulation
 Development
of organs
 1st trimester in humans
Includes embryo & early fetal development
 Vulnerable to disruption
 Most miscarriages
 Susceptible to the environment
(smoking, alcohol)

 2nd

and 3rd trimesters
Further development of existing organs &
growth
Childbirth

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Estrogen levels increase
Oxytocin
 Produced
by fetus and mother’s pituitary gland
 Stimulates uterine muscle contraction
 Positive feedback


Contractions cause release of more oxytocin
Cervix opens
 Fetus
is pushed out --birth
 Placenta follows --afterbirth

Birth:
 First stage: opening
of the cervix.
 Second stage:
expulsion of the
baby from uterine
contractions.
 Third stage:
expulsion of the placenta.
Gene Expression: Development
Embryo development depends on gene
expression
 Timing of expression is complex, yet vital
 Cell and timing specific expression
 Controlled by cascades of gene
expression that coordinate development of
specific structures

Control of Development
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Genes are turned on or off during development
Gene activation during development
 Transcription
of genes is regulated as is the
processing of transcribed RNA

Changes gene expression resulting in development
 Chemicals and hormones
 Switch genes on or off, controlling or affecting development
 Environment
 Can influence gene regulation in many animals
 Why an embryo/fetus must be protected fr/ env. insults
(alcohol, drugs, pollution, etc.)
Changes in Development

Apoptosis
 Programmed
cell death
 Remove cells from further development
Webbing between digits
 Remove unstimulated neurons

Childhood
Dev does not end w/ birth
 Growth after birth

 Continuation

of fetal growth
Humans—one quarter of life reaching full size
 Many
tissues/organs developing
Immune system
 Brain
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Puberty
Continuation of fetal growth
 Many tissues/organs still developing
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Brain till early 20s
Sexual development
 Secondary
sex characteristics
 Reproductive organs develop
Adulthood
Cessation of Growth
 Sexual maturation
 Cells still divide
 Development only ends at what time?
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Post-Reproductive Years

Females
 Reproduce

less effectively after ~40 years
Eggs decline in quality
 Menopause

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End of reproductive life
Males
 Decline

 No
in sperm
Quality and quantity
distinct end of reproductive life
Regeneration of Cells

Limited life span of cells
 Must
regenerate and be replaced
 Example: skin cells
 Two methods for regeneration
Simple cell division
 Stem cells

Stem Cells
Unspecialized cells
 Can develop into mature cells
 Unspecialized to various degrees

 Depending
on tissue and stage of life
 Embryonic stem cells
Cells haven’t undergone differentiation
 Greatest potential for developing into any tissue

Concept Quiz
Meiosis is the process of producing
gametes in both spermatogenesis and
oogenesis. Which of the two
developments results in four gamete
cells during human reproductive cell
production?
A. Oogenesis
B. Spermatogenesis
Concept Quiz
Stem cells are considered to be a great
potential source for regenerating damaged or
nonfunctional tissue. Which of the following
stem cells provide the greatest potential for
regenerating different types of tissues?
A. Embryonic stem cells
B. Bone marrow stem cells
C. Skin stem cells
Concept Quiz
Fertilization of the human egg occurs in
which location?
A. Ovary
B. Uterus
C. Oviduct
How Identical Twins Form: