4: Fertilization Flashcards
process in which gametes (an egg and sperm) fuse to form a zygote
Fertilization
the egg and sperm are _ , which means they each contain one set of chromosomes; upon fertilization, they will combine their genetic material to
form a zygote that is _ , having two sets of chromosomes
haploid
diploid
a zygote that has more than two sets of chromosomes will not be
viable; therefore, to ensure that the offspring has only two sets of
chromosomes, only one sperm must fuse with one egg
fertilization accomplishes two separate ends:
a.) sex (the combining of genes derived from two parents); and
b.) reproduction (the generation of a new organism)
Function of Fertilization:
- to transmit genes from parents
to offspring - to initiate in the egg cytoplasm those
reactions that permit development to proceed
▪ prior to fertilization, the sperm and egg must travel toward each other, and _ can attract the sperm
▪ sperm-egg recognition occurs when proteins on the sperm cell membrane meet proteins on the _ of the egg
▪ in preparation for this meeting, the sperm cell membrane is altered significantly by _ events activated by the egg
▪ once inside the egg, the sperm activates development by causing the release of _ from within the egg
* _ these ions stimulate the enzymes needed for DNA synthesis, RNA synthesis, protein synthesis, and cell division
chemicals from the egg
extracellular coating
exocytotic
calcium ions (Ca2+)
the sperm and egg pronuclei travel toward one another, and the genetic material of the gametes combines to form the _ chromosome content carrying the genetic information for the development of a new organism
diploid
Angiosperm plants, several nuclei are involved in fertilization:
1. builds the _ that burrows to the ovary;
2. fuses with the haploid nucleus of the egg to make the _ ;
3. fuses with a diploid nucleus of a somatic cell to create the _ that nourishes the embryo
pollen tube
plant embryo
endosperm
fertilization of four major events (Barresi and Gilbert, 2020):
- Contact and recognition between sperm and egg.
- Regulation of sperm entry into the egg.
- Fusion of the genetic material of sperm and egg.
- Activation of egg metabolism to start development
this ensures
that the sperm and egg are of the same species
Contact and recognition between sperm and egg.
Only one sperm nucleus can
ultimately unite with the egg nucleus. This is usually accomplished by
allowing only one sperm to enter the egg and actively inhibiting any others
from entering
Regulation of sperm entry into the egg
Significance of Fertilization
1. Secondary oocyte to undergo 2nd maturation division to release 2nd polar body and to form haploid ovum
2. -
3. -
4. Increases metabolic acts as there are more mitochondria
5. -
6. -
7. Sex chromosome of sperm helps in sex determination
8. -
- Secondary oocyte to undergo 2nd maturation division to release 2nd polar body and to form haploid ovum
- restores diploidy in the zygote
- fertilization membrane prevents polyspermy
- Increases metabolic acts as there are more mitochondria
- Combines parents’ traits and induce variations
- Centrioles of sperm from the spindle to initiate the cleavage of zygote
- Sex chromosome of sperm helps in sex determination
- Copulation path sets the axis of division
COPULATION PATH is the intracytoplasmic course followed by the male pronucleus in approaching the female pronucleus during fertilization and often delineating the direction of the first cleavage furrow
Sequential Events of Fertilization
(Carlson, 2014)
PAABPMDCD
- Penetration of the corona radiata
- Attachment to the zona pellucida
- Acrosomal reaction and penetration of the zona pellucida
- Binding and fusion of sperm and egg
- Prevention of polyspermy
- Metabolic activation of the egg
- Decondensation of the sperm nucleus
- Completion of meiosis in the egg
- Development and fusion of male and female pronuclei
Penetration of the corona radiata
when the spermatozoa first encounter the ovulated egg in the _ of the uterine tube, they are confronted by the corona radiata and some remnants of the _ , which represents the outer layer of the egg complex
ampullary part
cumulus oophorus
corona radiata is a highly cellular layer with an intercellular matrix consisting
of proteins and a high concentration of carbohydrates (especially _ )
hyaluronic acid
hyaluronidase emanating from the sperm head plays a major role in penetration of the corona radiata, but the active swimming movements of the spermatozoa are also important
Attachment to the zona pellucida
- after they have penetrated the corona radiata, spermatozoa bind tightly to the zona pellucida by means of the plasma membrane of the sperm head
- the zona pellucida, which is 13 µm in humans, consists principally of _ – ZP1 to ZP4. _ combine to form basic units that polymerize into long filaments
four glycoproteins
ZP2 and ZP3
Attachment to the zona pellucida
o spermatozoa bind specifically to a _ , which is the terminal part of a sequence of four sugars at the end of O-linked oligosaccharides that are attached to the polypeptide core of the ZP3 molecule
o molecules on the surface of the sperm head are specific binding sites for the _ on the zona pellucida
sialic acid molecule
ZP3 sperm receptors
o more than 24 molecules have been proposed, but the identity of the zona-binding molecules remains unknown
Acrosomal reaction and penetration of the zona pellucida
on binding to the zona pellucida, mammalian spermatozoa undergo the _
acrosomal reaction
Acrosomal reaction and penetration of the zona pellucida
o in this acrosome of sperm undergoes acrosomal reaction and releases certain sperm _ which dissolve the egg envelopes locally and make the path for the penetration of sperm
lysins
Acrosomal reaction and penetration of the zona pellucida
these sperm lysins contain a lysing enzyme hyaluronidase
which dissolves the hyaluronic acid polymers in the
intercellular spaces which holds the _ of corona
radiata together
granulosa cells
Acrosomal reaction and penetration of the zona pellucida
corona penetrating enzyme (that dissolves the
_ )
corona radiata
Acrosomal reaction and penetration of the zona pellucida
_ (which dissolves the
zona pellucida; most important enzymee), then it dissolves the
zona pellucida
acrosin or Zona lysin
Acrosomal reaction and penetration of the zona pellucida
when the sperm has made its way through the zona and into the _ (the space between the egg’s plasma membrane and the zona pellucida), it can make direct contact with the plasma membrane of the egg
perivitelline
space
Binding and fusion of sperm and egg
after a brief transit period through the _ , the spermatozoon
makes contact with the egg
perivitelline space
Binding and fusion of sperm and egg
binding between the spermatozoon and egg occurs when the _ contacts the microvilli surrounding the egg
equatorial region
of the sperm head
Binding and fusion of sperm and egg
molecules on the plasma membrane of the sperm head, principally sperm proteins called _ , bind to α6 integrin and CD9 protein molecules on the surface of the egg
fertilins and cyritestin
Binding and fusion of sperm and egg
the acrosomal reaction causes a _ because, if the acrosomal reaction has not occurred, the
spermatozoon is unable to fuse with the egg
change in the membrane properties of the
spermatozoon
Binding and fusion of sperm and egg
actual fusion between spermatozoon and egg, mediated by _ on the
membrane of the oocyte, brings their plasma membranes into continuity
integrin
Binding and fusion of sperm and egg
after initial fusion, the contents of the spermatozoon (the head, the midpiece, and usually the tail) sink into the egg , whereas the sperm’s _ , which is anti-genically distinct from that of the egg, becomes incorporated into the egg’s plasma membrane and remains recognizable at
least until the start of cleavage
plasma membrane
Prevention of polyspermy
when a spermatozoon has fused with an egg, the entry of other spermatozoa
into the egg (polyspermy) must be _ , or abnormal development is
likely to result
prevented
two blocks to polyspermy, fast and slow, are typically present in vertebrate fertilization
Prevention of polyspermy
the fast block to polyspermy, which has been best studied in sea urchins, consists of a _ of the plasma membrane of the egg
rapid electrical depolarization
Prevention of polyspermy
the resting membrane potential of the egg changes from about _ within 2 to 3 seconds after fusion of the spermatozoon with the egg
−70 to +10 mV
this change in membrane potential prevents other spermatozoa from adhering to the egg’s plasma membrane
Prevention of polyspermy
▪ the fast block in mammals is -, lasting only several minutes, and
may not be as heavily based on membrane depolarization as that in sea urchins
▪ this time is sufficient for the egg to mount a _
▪ however, the exact nature of the fast block in the human egg is still not well defined
short-lived
permanent slow block
Prevention of polyspermy
very soon after sperm entry, successive waves of _ pass through the cytoplasm of the egg
Ca++
Prevention of polyspermy
the first set of waves, spreading from the site of sperm-egg fusion, is involved in stimulating completion of the _ of the egg
second meiotic division
Prevention of polyspermy
later waves of Ca++ initiate recruitment of _ in the egg and act on the _ as they pass by them
maternal RNAs
cortical granules
Prevention of polyspermy
exposure to Ca++ causes the cortical granules to fuse with the _ and to release their contents (hydrolytic enzymes and polysaccharides) into the perivitelline space
plasma membrane
Prevention of polyspermy
the polysaccharides released into the perivitelline space become
hydrated and swell, thus causing the zona pellucida to _ from
the surface of the egg
rise
Prevention of polyspermy
the secretory products of the cortical granules diffuse into the porous zona pellucida and _ the sperm receptor molecules (ZP3 in the mouse) in
the zona
hydrolyze
this reaction is called zona reaction,
Prevention of polyspermy
essentially eliminates the
ability of spermatozoa to adhere to and penetrate the zona
zona reaction,
Prevention of polyspermy
the zona reaction has been observed in human eggs that have
undergone _ fertilization
in vitro
Prevention of polyspermy
In addition to changes in the zona pellucida, alterations in sperm receptor molecules on the plasma membrane of the human egg cause the egg itself to become
_ to penetration by other spermatozoa
refractory
the entry of the spermatozoon into the egg initiates several significant changes
within the egg (including the aforementioned fast and slow blocks to
polyspermy)
Metabolic activation of the egg
Metabolic activation of the egg
in effect, the sperm introduces into the egg a soluble factor (currently thought to be a phospholipase [phospholipase _ ]), which stimulates a pathway
leading to the release of pulses of Ca++ within the cytoplasm of the egg
C zeta
Metabolic activation of the egg
in addition to initiating the blocks to polyspermy, the released Ca++ stimulates a rapid intensification of the egg’s respiration and metabolism through an exchange of extracellular _ for intracellular _
Na+
H+
Metabolic activation of the egg
this exchange of extracellular Na+
for intracellular H+ results in a rise in _ pH and an increase in _ metabolism
intracellular
oxidative
Decondensation of the sperm nucleus
in mature spermatozoon, the nuclear chromatin is very tightly packed, in large part because of the —SS— (_ ) cross-linking that occurs among the protamine molecules complexed with the DNA during spermatogenesis
disulfide
Decondensation of the sperm nucleus
shortly after the head of the sperm enters the cytoplasm of the egg, the permeability of its nuclear membrane begins to increase, thereby allowing cytoplasmic factors within the egg to affect the _ of the sperm
nuclear
contents
Decondensation of the sperm nucleus
after reduction of the —SS— cross-links of the protamines to
sulfhydryl (—SH) groups by reduced glutathione in the ooplasm, the protamines are rapidly _ from the chromatin of the spermatozoon, and the chromatin begins to _ within the nucleus (now called a pronucleus) as it moves closer to the nuclear material of the egg
lost
spread out
Decondensation of the sperm nucleus
it takes about _ hours for the remodeling of the sperm
6 to 8
Decondensation of the sperm nucleus
▪ after a short period during which the male chromosomes are naked, histones begin to associate with the chromosomes
o during the period of pronuclear formation, the genetic material of the male pronucleus becomes _ , whereas methylation in the
female genome is _
demethylated
maintained
Completion of meiosis in the egg
after penetration of the egg by the spermatozoon, the nucleus of the egg, which had been arrested in metaphase of the second meiotic division, completes the last division and releases a _ polar body into the
perivitelline space
second
Completion of meiosis in the egg
the nucleus of the oocyte moves toward the _ as the result of the action of myosin molecules acting on a network of actin filaments that connect one pole
of the mitotic spindle to the _
cortex
Completion of meiosis in the egg
the resulting contraction draws the entire mitotic apparatus toward the
surface of the cell; determines the _ at which both the first and second polar bodies are extruded
location
Development and fusion of male and female pronuclei
a pronuclear membrane, derived largely from the _ of the egg, forms around the female chromosomal materia
endoplasmic reticulum (ER)
Development and fusion of male and female pronuclei
cytoplasmic factors seem to control the growth of the female and the male _
pronuclei
Development and fusion of male and female pronuclei
pronuclei appear 6 to 8 hours after sperm penetration, and they persist for about _ hours
10 to 12
Development and fusion of male and female pronuclei
DNA replication occurs in the developing haploid pronuclei, and each chromosome forms _ as the pronuclei approach each other
two chromatids
Development and fusion of male and female pronuclei
when the male and female pronuclei come into contact, their membranes break
down, and the chromosomes _
intermingle
Development and fusion of male and female pronuclei
the maternal and paternal chromosomes quickly become organized around a mitotic spindle, derived from the _ of the sperm, in
preparation for an ordinary mitotic division
centrosome
After Development and fusion of male and female pronuclei , the process of fertilization can be said to be complete, and the fertilized
egg is called a _
zygote
Parthenogenesis came from Greek words
parthenos - virgin
genesis - birth
natural form of asexual reproduction in which growth and development of
embryos occur without fertilization
Parthenogenesis
when an ovum is activated in the absence of sperm
Parthenogenesis
Parthenogenesis normal development can proceed in _ invertebrates and _ vertebrates
many
some
in animals, parthenogenesis means development of an embryo from an
_
unfertilized egg cell
in plants, parthenogenesis is a component process of _
apomixis (asexual seed
formation)
asexual seed
formation
apomixis
it is one in which the egg cell is
produced through mitosis then develops directly into an embryo
without the prior fertilization
apomictic parthenogenesis
parthenogenesis naturally in some plants; some invertebrate including
- nematodes,
- water fleas,
- some scorpions,
- aphids,
- some mites,
- some bees,
- some Phasmida and
- parasitic wasps
parthenogenesis occurs in few vertebrates including
- some fish,
- amphibians,
- reptiles and
- very rarely birds
this type of reproduction has been induced artificially in a few species
including fish and amphibians
Parthenogenesis
normal egg cells form after meiosis and are _ , with half as many
chromosomes as their mother’s body cells
haploid
haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the _ chromosome number
diploid
the offspring having all of the mother’s genetic material are called _ and those having only half are called _
full clones
half clones
_ clones are usually formed without meiosis
full
if meiosis occurs, the offspring will get only a fraction of the mother’s
alleles since crossing over of DNA takes place during meiosis, creating
_
variation
parthenogenetic offspring in species that use either the XY or the X0 sex-determination system have _ that use the ZW sex-determination system, they have either two Z chromosomes
(male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female).
two X chromosomes and are female in species
