Download Gametogenesis and Fertilization

Gametogenesis
• Reproduction in vertebrates is by sexual
means involving haploid (1N) germ cells
– Ovum = female component
– Spermatozoa = male component
• Both arise through meiosis = cell division
where each daughter cell receives ½ genetic
material from original cell
• Primordial germ cells derived from
extraembryonic endoderm (yolk sac) 
migrate to gonads
Gametogenesis
• Oogenesis occurs in Ovary within a follicle of
epithelial cells
• Spermatogenesis occurs in germinal epithelium
lining seminiferous tubules of testis
• Oogenesis begins with oogonium;
Spermatogenesis begins with spermatogonium
– Both are normal 2N cells
• Reduction in chromosome number
accomplished via two meiotic divisions
Stages in Gametogenesis
1. Pairing and doubling of chromosomes in ____gonia,
followed by growth as primary ____ocyte (2 X 2N)
2. 1st meiotic division produces two 2 X 1N cells (=
secondary ____ocytes)
3. 2nd meiotic division produces four haploid cells
(spermatids, ova)
4. Spermatids mature and differentiate to form
functional spermatozoa
• In spermatogenesis, all 4 sperm cells produced are
viable
• In oogenesis, only 1 of 4 cells produced is viable.
– Others become abortive as polar bodies (only small
amount of cytoplasm) that later degenerate
Fig 14.22 - Oogenesis
Fig 14.30 - Spermatogenesis
Egg Membranes and Structure
• Cytoplasm enclosed within plasma membrane
• Vitelline membrane = thin membrane closely
attached to plasma membrane
– Zona pellucida = glycoprotein layer (mammals)
• Corona radiata (mammals) = layer of follicle
cells that become sloughed off after
fertilization
Sperm Structure
• Spermatozoa from different animals have a
wide variety of forms
• All have head and tail regions
– Head region serves two functions:
• Contains nucleus (genetic function)
• Acrosomal cap = contains enzymes that allow sperm to
break down membranes around egg and fertilize egg
• Tail = flagellum that provides motility
• Midpiece between head and tail contains
mitochondria that provide ATP to fuel
swimming
Ovarian follicle
Primary follicle
Ovum
Zona pellucida
Spermatozoa
Fertilization
• Several obstacles must be overcome for
successful fertilization:
1. Sperm and egg must come into proximity
2. Cell to cell contact must occur
3. Sperm must penetrate egg cell
Mechanisms for Proximity
• Transport occurs in liquid medium
• EXTERNAL FERTILIZATION
– Eggs and sperm simultaneously shed into water
– Occurs in fishes (except Chondrichthyes) and most
anurans
• INTERNAL FERTILIZATION
– Sperm introduced directly into female tract
– Usually involves copulatory organs in males (none
present in tuatara, birds, salamanders = copulation by
cloacal “kiss”)
– Occurs in animals with shelled eggs or viviparous
habits as sperm must reach egg before shell is added
(Chondrichthyes, most Amphibians, Amniotes)
Mechanisms for Contact
• For internal fertilization, sperm travel within
female tract by passive transport (dependent
on muscular contractions and ciliary currents
provided by female tract).
– Little active swimming by sperm for transport
function
• Contact in external fertilization accomplished
by random swimming movements of sperm in
water
Mechanisms for Egg Barrier Penetration
• Once contact with egg has been established, the
next step is to penetrate the egg so that nuclear
materials can unite to form the diploid zygote.
• Barrier penetration mechanisms are chemical in
nature and involve acrosomal reaction
– Sperm Lysins = enzymes that locally dissolve egg
membranes
– Produced by acrosomal cap
– Sperm lysins differ among animal groups as
membranes surrounding eggs differ (e.g., jelly coat in
amphibians, follicle cells of corona radiata in
mammals)
Mechanisms for Egg Barrier Penetration
• Acrosome Reaction involves …
1. Release of sperm lysins
2. Fusion of egg and sperm membranes
— In some animals, acrosomal reaction involves
exposure of binding sites on plasma membrane of
sperm, via acrosomal tubule or filament, which bind
to receptors on p.m. of egg in species-specific manner
— This binding precedes fusion of sperm and egg
plasma membranes
rupture
 sperm lysins
Acrosomal
Reaction in
Hemichordates
Binding sites
exposed that
bind to
receptors on
egg plasma
membrane
Mechanisms for Egg Barrier Penetration
• In mammals, there is no development of
acrosomal filaments
• Instead, fluids of female reproductive tract induce
capacitation  primes sperm for fertilization and
includes removal of some components from
sperm surface.
• After capacitation, hyaluronidase on the sperm
head is exposed and breaks down the hyaluronic
acid cementing the follicle cells of corona radiata
(which surround the egg) together  allows
sperm passage through corona radiata to contact
zona pellucida (a glycoprotein layer surrounding
the egg)
Mechanisms for Egg Barrier Penetration
• Zona pellucida has species-specific receptors for
binding sperm
– Binding causes rupture of acrosome, which releases
contents that break down zona pellucida and allow
contact with egg plasma membrane
– Binding also exposes proteins on sperm surface that
bind with receptors on egg plasma membrane to
facilitate fusion of sperm and egg
• Fusion of plasma membranes releases sperm
genetic material into egg as sperm pronucleus
• Male and female genetic material will soon
combine forming a diploid zygote
Post-fertilization Responses in Zygote
1. Formation of Fertilization Cone = outward
bulge of egg cytoplasm that serves to engulf
sperm
– Occurs upon fusion of sperm and egg plasma
membranes
– Recession of cone brings sperm nucleus into egg
cytoplasm
2. Egg Activation
— Upon fusion (within 3 sec) get membrane
depolarization or hyperpolarization (speciesdependent)  blocks entrance of > 1 sperm (=
Fast block to polyspermy)
Post-fertilization Responses in Zygote
— Next, get Ca2+ release from internal stores within
egg triggers cortical reaction  release of cortical
granules to perivitelline space around egg
— Cortical granule release causes development of
fertilization membrane blocking further sperm
entry (= Slow block to polyspermy)
— Slow block to polyspermy occurs about 25-30 sec
post-fusion
— Seems to occur only for microlecithal eggs (e.g.,
mammals); entrance of > 1 sperm into eggs of
birds, reptiles and some amphibians common,
but only 1 sperm contributes to zygote (others
somehow inactivated)
Fast Block to Polyspermy
Slow Block to Polyspermy
Post-fertilization Responses in Zygote
3. Rearrangement of internal constituents within
egg
— Sets up gradients of certain substances and plane
of bilateral symmetry within zygote for some
animals
4. Fusion of Haploid Nuclei
— In most vertebrates, meiosis within egg arrested
after 1st meiotic division. Sperm entry stimulates
2nd meiotic division to produce female pronucleus
(and 2nd polar body)
— Once this 2nd division occurs, female pronucleus is
ready for union with male pronucleus
Post-fertilization Responses in Zygote
4. Fusion of Haploid Nuclei (cont.)
— Male and female pronuclei next approach each
other (mechanism by which this movement occurs
is not known with certainty)
— Next get fusion of pronuclei
— In some animals (including most vertebrates),
pronucleus membrane degenerates  free
chromosomes arrange themselves at spindle
(metaphase of mitosis)  completion of mitosis
 dipolid zygote
Parthenogenesis
• Definition = development of the egg in the
absence of sperm
• Occurrence suggests that:
– (1) egg activation and nuclear fusion are separate
developmental processes
– (2) the ovum contains all the capacities necessary
for embryo formation – all that is necessary is some
triggering agent
• Eggs can be activated by a number of chemical,
thermal, electrical or mechanical means
Parthenogenesis
• Parthenogenetic individuals are expected to be
haploid (and many are), but these embryos are
often diploid.
• Doubling of chromosomes accomplished in 3
ways:
1. Suppression of 2nd meiotic division – occurs only
in eggs completing this division after fertilization
2. Refusion with second polar body
3. Suppression of 1st mitotic division (= 1st cleavage
division)
Parthenogenesis
• Haploid embryos generally show premature
developmental arrest
• Parthenogenetic diploid embryos also usually
show premature developmental arrest
• However, in several invertebrates
parthenogenesis is normal (e.g., male drones of
bee colony) and there are several species of
naturally occurring parthenogenetic lizards (the
entire population is female)
• Artificial selection procedures have developed
parthenogenetic strain of turkeys
The asexual, all-female whiptail species Cnemidophorus neomexicanus (center),
which reproduces via parthenogenesis, is shown flanked by two sexual species
having males, C. inornaus (left) and C. tigris (right), which hybridized naturally to
form the C. neomexicanus species.
Methods of Bearing Young
• Oviparous = egg laying
– Primitive condition for vertebrates
– Occurs in most fishes, amphibians, reptiles, all birds,
monotremes
• Viviparous = live-bearing
– Advanced condition in vertebrates
– Some live-bearers occur in all vertebrate classes
except cyclostomes and birds
– Evolved by retention of eggs within body to increase
survival of young
“Placental Connections” in Viviparous
Vertebrates
• Anamniotes with connection between yolk sac
and maternal tissues through which exchange
of metabolites occurs (e.g., Chondrichthyes)
• Reptiles use yolk sac, chorion, allantois
(extraembryonic membranes) or some
combination for connection
• Mammals with a variety of connections
Early Development/Placentation in Mammals
• After formation of zygote  cleavage  produces
blastula
• Blastula forms before embryo reaches uterus
• Mammalian blastula consists of trophoblast and inner
cell mass (ICM becomes embryo)
• Upon reaching uterus, trophoblast overlying ICM
makes contact with uterine endometrium 
trophoblast cells rapidly multiply and insert among
epithelial cells lining endometrium and endometrial
cells degenerate  implantation
• Continued trophoblast cell division  placentation;
embryo becomes buried w/in endometrial lining
Fig 5.32
Mammalian Placenta Formation
• Structure produced by apposition and fusion of
extraembryonic membranes of embryo with
uterine endometrium of mother
• Extraembryonic membranes = tissues external
to embryo not participating in embryo
formation, but functioning in maintenance of
the embryo
• In Amniotes, four extraembryonic membranes
exist
Extraembryonic Membranes
• Yolk Sac = forms from extraembryonic hypomere
(splanchnopleure) that expands to enclose yolk
– This is the only extraembryonic membrane present in
Anamniotes, so it occurs in all vertebrates
– Functions to derive nutrients from yolk in yolky eggs to
nourish developing embryo
• In Amniotes, extraembryonic somatopleure grows
over embryo by folding back on itself producing a
double hood of somatopleure
• From this structure develop Amnion and Chorion
Extraembryonic Membranes
• Amnion = forms from inner somatopleure +
ectoderm (inside)
• Chorion = forms from outer somatopleure +
ectoderm (outside)
• Amnion serves as fluid-filled sac for embryonic
development
– Replicates aquatic developmental environment of
primitive vertebrates.
– Allows complete conquest of terrestrial habitats
• Chorion functions in protection of embryo and in
exchange of gases (and metabolites in placenta)
Extraembryonic Membranes
• Outgrowth of splanchnopleure from posterior
region of gut in Amniotes eventually expands
to fill extraembryonic coelom (= space between
amnion and chorion)
• This membrane is the Allantois = composed of
splanchnic mesoderm (outside) + endoderm
• Mesoderm fuses with mesoderm of chorion to
form Chorioallantoic Membrane = main gas
exchange organ for Amniote embryos
• Allantois also serves waste storage function
Fig 5.29 – Extraembryonic membrane formation in a bird
Fig 5.30
Fig 5.30 – Extraembryonic membrane formation in a bird
Mammalian Placenta
• From chorion (outermost extraembryonic
membrane), finger-like processes grow outward
to interlock with uterine endometrium
• Blood streams of mother and fetus never mix –
always separated by epithelial membrane, so
exchange of gases and nutrients occurs by
diffusion across this membrane
• Chorion is not in direct contact with embryo so
some means of blood supply from embryo to
placenta (and back) must occur
Mammalian Placenta
• Blood Supply to developing embryo differs
between marsupial and placental mammals
• Marsupials = mostly Choriovitelline fetal placenta
– Yolk sac associated with inner surface of chorion
– Blood vessels develop in mesoderm of yolk sac
– This situation also occurs to some extent in several
placental groups (e.g., rodents)
• Placentals = Chorioallantoic fetal placenta
– Dominant connection to chorion provided by allantois,
yolk sac usually degenerates
– Allantoic mesoderm forms blood vessels that function
in gas & nutrient exchange, waste removal
Fig 5.33 – Fetal extraembryonic
membranes in various
Amniotes
Mammalian Placenta Types
• Primitive Condition = apposition without fusion
(non-deciduous)
• Advanced Condition = fusion of maternal and
fetal tissues (deciduous)
• Four Types occur:
1. Epitheliochorial = most primitive
— Occurs in pig and some other mammals
— Maternal and fetal blood separated by 6 layers:
endothelium, CT, epithelium, epithelium, CT,
endothelium
Mammalian Placenta Types
2. Syndesmochorial = no uterine epithelium
— Occurs in ruminant mammals (cattle, sheep, etc.)
3. Endotheliochorial = no maternal epithelium or
CT
— Occurs in carnivores
4. Hemochorial = advanced condition
— Chorionic epithelium bathed in maternal blood
— Occurs in primates and many rodents