Download meiosis - astone

Human Development
Meiosis
Spermatogenesis/Oogenesis
Fertilization
Cleavage
Embryonic development
Overview of Sexual Reproduction
 Sexual Reproduction is the combination
of genetic information from 2 different
organisms
 Increases diversity in a population
 Sex cells AKA germ cells or gametes
 Created By Meiosis
 Meiosis is made up of 2 nuclear divisions:
 Meiosis
I – homologous chromosomes
separate
 Meiosis II – sister chromatids separate
MEIOSIS
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II
Human Gametes
 The Human Gametes are…
 Sperm – male (23 chromosomes)
 Ovum (egg) – female (23 chromosomes)
 Gametes are made through a specialized form of
meiosis…
 Spermatogenesis – formation of sperm
 Oogenesis – formation of ova
 Gametes are made in the gonads…
 Testes in males
 Ovaries in females
Sperm: There are 4 important
1
parts to a sperm cell
1. Acrosome – holds powerful
enzymes that help sperm
enter egg cell
2. Nucleus – 23 chromosomes
3. Midpiece – holds
mitochondria (provides
energy)
4. Tail – 1 flagellum, allows
sperm to ‘swim’
2
3
4
The Process of Spermatogenesis
 Begins in puberty, continues until death
 Follicle Stimulating Hormone (FSH) causes
sperm to enter a cycle producing:
 1 spermatogonia – “starter cells” for the
whole process: diploid
 Go through rapid mitosis to produce lots of cells
before puberty
 1 primary spermatocyte – The cell that
actually goes through meiosis, each
spermatocyte creates 4 haploid sperm
SPERMATOGENESIS

Spermatogonia (2n)

Primary spermatocytes
(2n) these enter meiosis

Secondary
spermatocytes (1n)
product of meiosis I

Spermatids (1n) product of
meosis 2

Sperm end product
Female Anatomy
The Ovary
 The ovaries are composed of sac-like structures
 Their job is to produce the ovum (eggs)
The Fallopian Tubes and Uterus
 Fallopian tubes extend
from the ovaries to the
uterus
 Uterus: Receives,
retains, nourishes the
fertilized ova
 Provide the pathway for
 Inner layer of the uterus
ovum to reach the uterus
is the endometrium
 The fallopian tubes are the
(this layer is shed during
site of fertilization
menstruation)
OVUM
 Large cell
 Contains a nucleus with 23
chromosomes
 Large cytoplasm containing:
 Yolk – will be the precursor for blood
and reproductive cells, in organisms
other than mammals the yolk provides
the nutrients for the developing
embryo
 Organelles – most importantly the
mitochondria
 Cell membrane – Which contains the
rest of the cell
 Layer of jelly – surrounds the cell
membrane and protects the ovum
The Process of Oogenesis
 A female is born with a total supply of ovum
(eggs)
 Ability to release eggs begins at puberty and
ends at menopause
 Oogonia – cells that divide through mitosis
 They rapidly divide until birth producing primary
oocytes
 Primary oocytes remain inactive until puberty
 Follicle Stimulating Hormone (FSH) cause
primary oocytes to complete meiosis
The Process of Oogenesis
 Only one primary oocyte goes through meiosis at
a time, this occurs once a month
 If an egg is not fertilized it dies
 Meiosis starts inside the maturing follicle and
does not end until fertilization
 4 haploid cells are created
 Nuclear contents divide evenly (4 haploid cells)
 Cytoplasm Does NOT divide evenly
 1 cell (becomes the ovum) hogs all of the
cytoplasm
 Other 3 small cells – polar bodies - die
Notice that the
picture is divided
into:
• Before Birth
• Childhood
• Puberty to
Menopause
Meiosis I results in
ovulation –
releasing the
secondary oocyte
from the ovary into
the fallopian tube
Meiosis II does not
end until fertilization
Steps From Fertilization to Birth
 Fertilization
 Zygote
 Cleavage
 Morula
 Blastula
 Implantation
 Gestation
 Gastrula
 Embryo
 Fetus
Fertilization
 Fertilization or conception - ovum and sperm
combining
 Ovum have 48 hours from time released to be
fertilized
 If not fertilized they dissolve (menstruation occurs)
 At the same time, when a man ejaculates…
 About 300 million sperm are deposited in the vagina
 Only about 300 thousand reach the top of the uterus
 And only 300 make it to the top of the Fallopian tube
where fertilization takes place
 Sperm, have approximately 48 hours before they die
Fertilization
Fertilization
 Sperm must burrow
through layers of
protective cells when
they reach the ovum
 When a sperm breaks
through the layer it
meets and fuses with
the membrane
surrounding the ovum
The steps of fertilization
1. Sperm contacts jelly layer
2. The egg cell recognizes the proteins on sperm
3.
4.
5.
6.
7.
cell
Acrosome breaks and releases enzymes that
break the jelly layer around the ovum
Sperm forms a tube that pushes to the cell
membrane
Tube fuses with cell membrane
Sperm nucleus moves into the egg, tail stays
behind
Fertilization – officially occurs when the 2
nuclei fuse
Fertilization
FERTILIZATION
 Within ~20 seconds a reaction occurs to form
fertilization membrane – blocks additional sperm
from entering the cell
 Why would the entry of multiple sperm be bad?
 A new cell called the zygote has been formed
 Diploid
 Will enter mitosis
 Genetic material will be copied millions of times
and it will divide over and over again
Cleavage
 Zygote first divides after about
24 hours after fertilization – this
is cleavage
 Each division doubles the # of
cells
 After 3 days a solid ball of cells
form this is called the morula
 Cells in the morula divide into
hundreds of cells and form a ring
around a fluid filled cavity called
a blastocyst
Implantation
 The blastocyst embeds itself in the uterine lining
(implantation) about a week after conception
 Once the embryo embeds the extraembryonic
membranes begin to form and the next step of
development: gastrulation begins.
Make sure you
study your
stages like
me!!!
Stem Cells
 Cells with the potential to develop into many
different cell types in the body.
 Stem cells are unspecialized (they do not have a
job in the body).
 Stem cells can divide and renew for long periods of
time.
 Stem cells can make specialized cells.
Two types of Stem Cells
 Human Embryonic Stem Cells
 Adult Stem Cells
Human Embryonic Stem Cells
 Harvested from the blastocyst of an embryo
 Obtained from embryos donated from invitro
fertilization clinics
 Embryonic stem cells are pluripotent – they can
differentiate into any type of cell
 Discovered in 1998
Adult Stem Cells
 Multipotent – can only develop into some types
of cells
 Can be taken from an adult and then
reintroduced without risk of rejection
 Several types:
 Hematopoietic – forms blood cells
 Stromal cells – forms bone cartilage, fat
 Some types of brain stem cells
Stem Cells Have the Potential to
Cure:
 Parkinson’s
 Diabetes
 Alzheimer’s
 Osteoathritis
 Spinal chord injury
 Rheumatoid arthritis
 Stroke
 Muscular dystrophy
 Burns
 Vision
 Heart disease
 Hearing loss
Some Questions that Still Need
to Be Answered:
 How many adult stem cells exist and where do
they exist?
 Where do adult stem cells originate from?
 What factors stimulate stem cells to relocate to
sites of injury or damage?
 What triggers stem cells to differentiate?
Gestation
 The embryo exists from
fertilization until the 8th week
of development
 Pregnancy or gestation begins
at implantation
 After implantation cells begin
to differentiate or gradually
change into specific cell types
 The 9 month period of
pregnancy is divided into
three trimesters
Gastrulation
 Gastrulation is the period of time when cells
differentiate into three major germ layers:
 Endoderm – digestive tract, respiratory system,
liver, pancreas
 Mesoderm – muscles, tissues, bones
 Ectoderm – skin, brain, nervous system
 During gastrulation a dent forms in the
blastocyst and folds inward this creates the
gastrula.
Developmental Similarities
 One way scientists
link species
evolutionarily is by
looking at the stages
of development
 The early embryos of
many organisms are
extremely similar:
Morphing
Embryos
Clip
Important Membranes of the
Embryo
 Amnion
 forms the amniotic sac
 surrounds and protects
developing embryo
 Yolk sac – site of
formation of:
 first blood and
reproductive cells
See figure 27.16
Important Membranes of the
Embryo
 Allantois
 Manages waste
 Forms the umbilical cord
 Chorion
 Surrounds all other membranes
 Secretes HCG which prevents
menstruation
 What pregnancy tests check for
 Chorionic villi – small finger
like projections on the chorion
extend into uterine lining and
form the placenta
See figure 27.16
The Placenta
 The placenta creates an
area of exchange between
the mother and baby
 Blood of mother and baby
never mix
 Constant exchange of
nutrients, gases,
pathogens, drugs and
other substances
 Fetus is attached to the
placenta by the umbilical
chord
First Trimester
 Gastrulation – week 3
 Nervous system begins to form
 Heart beats at 21 days
 All major organs begin to form
 Head is as large as body
 The sex can be determined
 At 8 weeks developing child is
called a fetus (bones ossify)
Second Trimester
 Mother’s abdomen swells
 Skeleton forms
 Heartbeat can be heard
 Hair appears on skin
 Periods of sleep and wake
 Fetus swallows, hiccups, sucks its thumb etc.
 The brain grows rapidly
 Mother begins to feel movement
 Fetus is up to 30cm in length
Premature Birth
 A baby can survive if born 23 weeks early (during 2nd
trimester)
 Increased risk of developmental and medical problems
 1 in 10 babies are premature
 Causes:
 Rupture of amniotic sac
 Certain infections
 Weak cervix (from previous pregnancy)
 Certain chronic diseases
 Uterine abnormalities
 Previous premature delivery
 Substance abuse
 Malnutrition
 Fetus with defects
Third Trimester
 Size increases rapidly
 The immune system
develops
 Can sense light and
darkness and react to
sounds
 Fat deposits begin to form
under skin
 Labor will be triggered by
the release of a hormone
called oxytocin
Twins
 Monozygotic twins
 They are genetically
 Dizygotic twins
 Non-identical twins
identical
 Occurs when two or more
 Occurs when the fertilized
eggs are released by the
egg splits completely in
mother at a time and are
half during cleavage
fertilized by two different
sperm cells
 Happens in about 0.4% of
births
 Naturally the chance of
having dizygotic twins is
1.5% but with fertility
treatment can be up to 25%
Twins
Conjoined
twins