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Gametogenesis Reproduction in vertebrates is by sexual means involving haploid (1N) germ Gametogenesis Reproduction in vertebrates is by sexual means involving haploid (1N) germ

Gametogenesis Reproduction in vertebrates is by sexual means involving haploid (1N) germ - PowerPoint Presentation

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Gametogenesis Reproduction in vertebrates is by sexual means involving haploid (1N) germ - PPT Presentation

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 ID: 921369

egg sperm membrane occurs sperm egg occurs membrane extraembryonic membranes fertilization cells fusion embryo division female chorion mammals zygote

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Slide1

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

Slide2

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

Slide3

Stages in Gametogenesis

Pairing and doubling of chromosomes in ____

gonia

, followed by growth as primary ____

ocyte

(2 X 2N)

1

st

meiotic division produces two 2 X 1N cells (= secondary ____

ocytes

)

2

nd

meiotic division produces four haploid cells (spermatids, ova)

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

Slide4

Fig 14.22

- Oogenesis

Slide5

Fig 14.30

- Spermatogenesis

Slide6

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

Slide7

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

Slide8

Ovarian follicle

Zona pellucida

Primary follicle

Ovum

Spermatozoa

Slide9

Fertilization

Several obstacles must be overcome for successful fertilization:

Sperm and egg must come into proximity

Cell to cell contact must occur

Sperm must penetrate egg cell

Slide10

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)

Slide11

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

Slide12

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)

Slide13

Mechanisms for Egg Barrier Penetration

Acrosome Reaction

involves …

Release of sperm

lysins

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

Slide14

Acrosomal Reaction in Hemichordates

sperm

lysins

rupture

Binding sites exposed that bind to receptors on egg plasma membrane

Slide15

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)

Slide16

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

Slide17

Slide18

Post-fertilization Responses in Zygote

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

Egg Activation

Upon fusion (within 3 sec) get membrane depolarization or hyperpolarization (species-dependent)

 blocks entrance of > 1 sperm (=

Fast block to

polyspermy

)

Slide19

Post-fertilization Responses in Zygote

Next, get Ca

2+

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)

Slide20

Fast Block to

Polyspermy

Slow Block to

Polyspermy

Slide21

Post-fertilization Responses in Zygote

Rearrangement of internal constituents within egg

Sets up gradients of certain substances and plane of bilateral symmetry within zygote for some animals

Fusion of Haploid Nuclei

In most vertebrates, meiosis within egg arrested after 1

st

meiotic division. Sperm entry stimulates 2

nd

meiotic division to produce female

pronucleus

(and 2

nd

polar body)

Once this 2

nd

division occurs, female

pronucleus

is ready for union with male pronucleus

Slide22

Post-fertilization Responses in Zygote

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

Slide23

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

Slide24

Parthenogenesis

Parthenogenetic

individuals are expected to be haploid (and many are), but these embryos are often diploid.

Doubling of chromosomes accomplished in 3 ways:

Suppression of 2

nd

meiotic division – occurs only in eggs completing this division after fertilization

Refusion

with second polar body

Suppression of 1

st

mitotic division (= 1

st

cleavage division)

Slide25

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

Slide26

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.

Slide27

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

Slide28

“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

Slide29

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

Slide30

Fig 5.32

Slide31

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

Slide32

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 somatopleureFrom this structure develop Amnion and Chorion

Slide33

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)

Slide34

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 embryosAllantois also serves waste storage function

Slide35

Fig 5.29

Extraembryonic

membrane formation in a bird

Slide36

Fig 5.30

Fig 5.30

Extraembryonic

membrane formation in a bird

Slide37

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

Slide38

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

Slide39

Fig 5.33

– Fetal

extraembryonic

membranes in various Amniotes

Slide40

Mammalian Placenta Types

Primitive Condition

= apposition without fusion (non-deciduous)

Advanced Condition

= fusion of maternal and fetal tissues (deciduous)

Four Types occur:

Epitheliochorial

= most primitive

Occurs in pig and some other mammals

Maternal and fetal blood separated by 6

layers: endothelium

, CT, epithelium, epithelium, CT,

endothelium

Slide41

Mammalian Placenta Types

Syndesmochorial

= no uterine epithelium

Occurs in ruminant mammals (cattle, sheep, etc.)

Endotheliochorial

= no maternal epithelium or CT

Occurs in carnivores

Hemochorial

= advanced condition

Chorionic epithelium bathed in maternal blood

Occurs in primates and many rodents

Slide42