3. Ovulation Occurs in Two
Phases
THE FIRST PHASE of the
ovulation cycle is the follicular
phase. It begins the first day of
the last menstrual period (LMP)
and stops at the next ovulation.
This phase varies greatly for each
woman.
THE SECOND PHASE is the
luteal phase. It begins the day
of ovulation and lasts until the
next menstrual cycle begins.
Governed by hormone release, it
follows a more regular timeline of
between 12 to 16 days following
ovulation.
4. Where are the Stem Cells?
The blastocyst embryo with its two cell types: the inner cells that
can initiate all cells of the body, and the outer cells destined to
become the placenta, is the source of embryonic stem cells (esc).
Embryonic Stem Cells (esc) have the potential to become any type
of human body cell. Stem cells are the only cells of the body
which divide into two unique entities. Typically when cells divide
they produce an identical cell or "sister" cell. But stem cells make
a new stem cell AND a new body type cell. This property makes
them extremely unique.
Blastocysts only become available for use as stem cells when
donated by couples using in-vitro fertilization (IVF) techniques.
A woman in an IVF program who has multiple eggs fertilized in a
petri dish, can choose to donate extra blastocysts to research.
These stem cells are useful to children and adults needing
interventions beyond conventional medicine.
5. WEEK 1
Fertilization
Cleavage
Formation of the blastocyst
Differentiation into the
Cytotrophoblast / Syncytiotrophoblast
7. Embryology Review
Week 1
– Fertilized Egg
– Day 1.25: Two Cells
Cleavage
Compaction
– Day 3: Morula
Inner Cell Mass
Trophoblasts
– Day 5: Blastocyst
Cavitation
Hatching
– Day 6: Implantation
8. PHASES OF FERTILIZATION
DEFINITIONS
Sets
1.penetrate corona radiata,
2. penetrate zona pellucida,
3.penetrate oocyte
membrane
.1 passage of sperm through corona radiata
2. penetration of zona pellucida
3. fusion of plasma membranes of oocyte and sperm
4. completion of second meiotic division of oocyte, formation
of female pronucleus
5. formation of male pronucleus
6. pronuclei fuse to form zygote
9. Fertilization
Contact of sperm &
secondary oocyte
occurs in uterine
tube
– Sperm penetration
– Oocyte completes
meiosis II
– Nuclear fusion
10. Fertilization:
Sperm produces
hyaluronidase to penetrate
the follicular cell layer of the
corona radiata (see diagram
below). It will then interact
with only one of many
receptors.
At a ZP3 receptor, the head
of the sperm releases its
contents (acrosin) and
burrows through the zona
pellucida and perivitelline
space - the acrosomal
reaction.
12. Passage Into the
Fallopian Tube
Once the egg is
released from the
ovary, it travels into
the fallopian tube
where it remains
until a single sperm
penetrates it during
fertilization
13. The Laborious Journey of
the Sperm
An average ejaculate
discharges 40-150 million
sperm which eagerly swim
upstream toward the
fallopian tubes on their
mission to fertilize an egg.
Fast-swimming sperm can
reach the egg in a half an
hour, while other may take
days.
The sperm can live up to 48-72 hours. Only a few
hundred will even come close to the egg, due to
the many natural barriers and hurdles that exist
in the female reproductive tract.
14. Fertilization:
Sperm Penetrates
Egg
If a sperm cell
meets and
penetrates an
egg, it will fertilize
the egg. The
fertilization process
takes about 24
hours. it. At the moment of fertilization, the genetic makeup is
When fertilization happens, changes occur on the surface of the egg to prevent other
sperm from penetrating
complete, including the sex of the infant.
15.
16. Once a receptor has been
activated, a series of reactions to
prevent polyspermy (the entrance
of more than one sperm).will be
initiated.
– First, the cell surface will be
depolarized.
– Then, cortical granules (lysosomes)
released into the perivitelline space
will hydrolyze the other receptors.
17. Fusion of the plasma membranes of the
oocyte and sperm occurs; the sperm
nucleus is released into the cytoplasm
of the oocyte; the rest of the sperm
degenerates
Entrance of sperm into the oocyte
causes the secondary oocyte to
complete its second meiotic division (2
polar bodies at this point)
Male pronucleus forms and swells;
pronuclei membranes become porous
18. Both the male and female pronuclei are essential for
normal development. Evidence for this exists when one or
the other is absent, as in the case of a hydatiform mole.
20. CORONA RADIATA
The corona radiata surround an ovumor
unfertilized egg cell, and consist of two or
three strata (layers) of follicular cells. They
are attached to the outer protective layer of
the ovum, the zona pellucida, and their
main purpose in many animals is to supply
vital proteins to the cell.
They are formed by follicle cells adhering to the oocyte before it leaves
the ovarian follicle, and originate from the squamous granulosa cells
present at the primordial stage of follicular development.
The corona radiata is formed when the granulosa cells enlarge and become
cuboidal, which occurs during the transition from the primordial to primary
stage.
These cuboidal granulosa cells, also known as the granulosa radiata, form
more layers throughout the maturation process, and remain attached to the
zona pellucida after the ovulation of the Graafian follicle.
21. Cone of attraction and
Vitelline membrane
Where the spermatozoon is about to
pierce, the yolk (ooplasm) is drawn
out into a conical elevation, termed
the cone of attraction.
Once the spermatozoon has
entered, the peripheral portion of the
yolk changes into a membrane, the
vitelline membrane, which prevents
the passage of additional
22. Acrosome reaction
The acrosome reaction must occur to
mobilise enzymes within the head of
the spermatozoon to degrade the zona
pellucida. example: hyaluronidase.
23. Cortical reaction
Once the sperm cells find
their way past the zona
pellucida, the cortical
reactionoccurs: cortical
granules inside the secondary
oocyte fuse with the plasma
membrane of the
cell, causing enzymes inside
these granules to be expelled
by exocytosis to the zona
pellucida. This in turn causes
the glyco -proteins in the
zona pellucida to cross-link
with each other—that is, the
enzymes cause the ZP2 to
hydrolyseinto ZP2f—making
the whole matrix hard and
impermeable to sperm. This
prevents fertilization of an
egg by more than one sperm.
24. Fusion
After the sperm enters the cytoplasm of the
oocyte, the cortical reaction takes place, preventing
other sperm from fertilizing the same egg. The
oocyte now undergoes its second meiotic division
producing the haploid ovum and releasing a polar
body.
The sperm nucleus then fuses with the
ovum, enabling fusion of their genetic material.
25. capacitation
To become competent to accomplish these tasks, ejaculated
mammalian sperm must normally be modified by conditions in
the female reproductive tract, a process called, which requires
about 5–6 hours in humans.
Capacitation is triggered by bicarbonate ions (HCO3–) in the
vagina, which enter the sperm and directly activate a soluble
adenylyl cyclase enzyme in the cytosol.
The cyclase produces cyclic AMP, which helps to initiate the
changes associated with capacitation.
Capacitation alters the lipid and glycoprotein composition of the
sperm plasma membrane, increases sperm metabolism and
motility, and markedly decreases the membrane potential (that
is, the membrane potential moves to a more negative value so
that the membrane becomes hyperpolarized).
Once a capacitated sperm has penetrated the layer of follicle
cells, it binds to the zona pellucida The zona usually acts as a
barrier to fertilization across species, and removing it often
eliminates this barrier.
26. Cell membranes
The cell membranesof the secondary
oocyte and sperm fuse.
27. Transformations
In preparation for the fusion of their genetic
material both the oocyte and the sperm undergo
transformations as a reaction to the fusion of cell
membranes.
The oocyte completes its second meiotic division.
This results in a mature ovum.
The nucleus of the oocyte is called a pronucleusin
this process, to distinguish it from the nuclei that
are the result of fertilization.
The sperm's tail and mitochondria degenerate with
the formation of the male pronucleus.
This is why all mitochondria in humans are of
maternal origin.
28. Replication
The pronuclei migrate toward the
centre of the oocyte, rapidly
replicating their DNA as they do so to
prepare the embryo for its first mitotic
division.
29. Mitosis
The male and female pronuclei don't fuse, although their
genetic material do.
Instead, their membranes dissolve, leaving no barriers
between the male and female chromosomes.
During this dissolution, a mitotic spindle forms between them.
The spindle captures the chromosomes before they disperse
in the egg cytoplasm.
Upon subsequently undergoing mitosis (which includes pulling
of chromatids towards centrioles in anaphase) the cell gathers
genetic material from the male and female together.
Thus, the first mitosis of the union of sperm and oocyte is the
actual fusion of their chromosomes.
Each of the two daughter cells resulting from that mitosis has
one replica of each chromatid that was replicated in the
previous stage. Thus, they are genetically identical.
30. Spindle microtubules are
stained in green with anti-
tubulin antibodies, and DNA is
labeled in blue with a DNA
stain.
(A) A meiotic spindle in a
mature, unfertilized oocyte.
(B) This fertilized egg is
extruding its second polar body
and is shown about 5 hours
after fusion with a sperm. The
sperm head (left) has nucleated
an array of microtubules.
(C) The two pronuclei have
come together.
D) By 16 hours after fusion with a sperm, the centrosome that entered the
egg with the sperm has duplicated, and the daughter centrosomes have
organized a bipolar mitotic spindle.
The chromosomes of both pronuclei are aligned at the metaphase plate of the
spindle. As indicated by the arrows in (C) and (D), the sperm tail is associated
with one of the centrosomes.
31. Summary
Mammalian fertilization begins when the head of a sperm binds in a
species-specific mannerto the zona pellucida surrounding the egg.
This induces the acrosome reactionin the sperm, which releases the
contents of its acrosomal vesicle, exposing enzymesthat help the
spermto digest its way through the zonato the egg plasma
membranein order to fuse with it.
The fusion of the sperm with the egg induces a Ca2+ signal in the
egg.
The Ca2+ signal activates the egg to undergo the cortical reaction, in
which cortical granules release their contents, including enzymes that
alter the zona pellucida and thereby prevent the fusion of additional
sperm.
The Ca2+ signal also triggers the development of the zygote, which
begins after sperm and egg haploidpronuclei have come
together, and their chromosomes have aligned on a single mitotic
spindle, which mediates the first division of the zygote.
32. Diseases
Various disorders can arise from defects in
the fertilization process.
Polyspermy results from multiple sperm
fertilizing an egg.
However, some researchers have found that
in rare pairs of fraternal twins, their origin
might have been from the fertilization of one
egg cell from the mother and two sperm cells
from the father. This possibility has been
investigated by computer simulations of the
fertilization process.
34. cleavage
In embryology, cleavage is the division of cells in the
early embryo.
The zygotes of many species undergo rapid cell cycles with no
significant growth, producing a cluster of cells the same size as the
original zygote.
The different cells derived from cleavage are called blastomeres and
form a compact mass called the morula.
Cleavage ends with the formation of the blastula.
Depending mostly on the amount of yolk in the egg, the cleavage can
be holoblastic (total or entire cleavage) or
meroblastic (partial cleavage).
The pole of the egg with the highest concentration of yolk is referred to as the
vegetal pole while the opposite is referred to as the animal pole.
Cleavage differs from other forms of cell division in that it increases the
number of cells without increasing the mass. This means that with each
successive subdivision, the ratio of nuclear to cytoplasmic material
increases.
35. The Cells Begin to Divide
The fertilized egg
begins dividing
rapidly, growing into
many cells. It leaves
the fallopian tube
and enters the
uterus three to four
days after
fertilization.
Rarely, the fertilized
egg does not leave
the fallopian tube;
this is called a tubal
pregnancy or ectopic
pregnancy and is a
danger to the
mother.
36. Cleavage
•Following mitosis, two blastomeres
form, each one totipotent (capable of
forming its own organism)
•Continuing along the uterine tube,
the zygote divides to four, then eight
cells
•Size of individual blastomeres
decreases with progressive divisions
as zygote maintains size
•A ball of 12 or more cells(32), the
morula, enters the uterus around day
3
38. Mechanism
The rapid cell cycles are facilitated by
maintaining high levels of proteins that control
cell cycle progression such as the cyclins and
their associated cyclin-dependent kinases (cdk).
The complex CyclinB /cdc2 a.k.a. MPF
(maturation promoting factor) promotes entry
into mitosis.
The processes of karyokinesis (mitosis) and
cytokinesis work together to result in cleavage.
The mitotic apparatus is made up of a central
spindle and polar asters made up of polymers
of tubulinprotein called microtubules.
39. The asters are nucleated by centrosomes and the
centrosomes are organized by centrioles brought into the egg
by the sperm as basal bodies. Cytokinesis is mediated by the
contractile ring made up of polymers of actinprotein called
microfilaments. Karyokinesis and cytokinesis are independent
but spatially and temporally coordinated processes. While
mitosis can occur in the absence of cytokinesis, cytokinesis
requires the mitotic apparatus.
The end of cleavage coincides with the beginning of zygotic
transcription. This point is referred to as the midblastula
transition and appears to be controlled by the nuclear:
cytoplasmic ratio (about 1/6).
40. Types of cleavage
DETERMINATE
Determinate is the form of cleavage in
most protostomes.
It results in the developmental fate of
the cells being set early in the embryo
development. Each cell produced by
early embryonic cleavage does not
have the capacity to develop into a
complete embryo.
41. INDETERMINATE
A cell can only be indeterminate if it has
a complete set of undisturbed
animal/vegetal cytoarchitectural features.
It is characteristic of deuterostomes -
when the original cell in a deuterostome
embryo divides, the two resulting cells
can be separated, and each one can
individually develop into a whole
organism.
42. HOLOBLASTIC
In the absence of a large concentration of yolk, four major
cleavage types can be observed in isolecithal cells (cells
with a small even distribution of yolk) or in mesolecithal
cells (moderate amount of yolk in a gradient) –
– bilateral holoblastic,
– radial holoblastic,
– rotational holoblastic, and
– spiralholoblastic, cleavage.T
These holoblastic cleavage planes pass all the way through
isolecithal zygotes during the process of cytokinesis.
Coeloblastula is the next stage of development for eggs that
undergo these radial cleavaging.
In holoblastic eggs the first cleavage always occurs along the
vegetal-animal axis of the egg, the second cleavage is
perpendicular to the first. From here the spatial arrangement
of blastomeres can follow various patterns, due to different
planes of cleavage, in various organisms.
43. MEROBLASTIC
In the presence of a large amount of yolk in the fertilized egg
cell, the cell can undergo partial, or meroblastic, cleavage.
Two major types of meroblastic cleavage are discoidal and
superficial.
– DISCOIDAL
In discoidal cleavage, the cleavage furrows do not penetrate the yolk.
The embryo forms a disc of cells, called a blastodisc, on top of the yolk.
Discoidal cleavage is commonly found in monotremes, birds, reptiles, and
fish that have telolecithal egg cells (egg cells with the yolk concentrated
at one end). Superficial
– I SUPERFICIAL CLEAVAGE,
mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With the
yolk positioned in the center of the egg cell, the nuclei migrate to the
periphery of the egg, and the plasma membrane grows
inward, partitioning the nuclei into individual cells. Superficial cleavage
occurs in arthropods that have centrolecithalegg cells (egg cells with the
yolk located in the center of the cell).
45. BLASTOCYST
The is a structure formed in the early embryogenesis
of mammals, after the formation of the morula. It is a
specifically mammalian example of a blastula.
It possesses an inner cell mass (ICM), or
embryoblast, which subsequently forms the
embryo, and an outer layer of cells, or
trophoblast, which later forms the placenta. The
trophoblast surrounds the inner cell mass and a fluid-
filled blastocyst cavity known as the blastocoele or
the blastocystic cavity. The human blastocyst
comprises 70-100 cells.
Blastocyst formation begins at day 5 after fertilization
in humans, when the blastocoele opens up in the
morula, a process known as hatching.
46.
47. Formation of the
Blastocyst
Compaction: Blastomeres clump
together; the amount of cytoplasm is
reduced
Adhesion: E- cadherin gene is
expressed; these proteins facilitate
intercellular adhesion and
communication
Zona pellucida is shed, allowing for cell
growth and for uterine fluid to
infiltrate
A fluid-filled cavity ( blastocoele) forms
in the center, pushing cells to the
periphery
The outer cell layer, the trophoblast
("tropho" - to nourish), may have a
nutritive role as the future embryonic
part of the placenta
The inner cell mass, the future
embryo, forms as a ball of cells on one
side of the spherical blastocyst
48. Parts of the blastocyst
Formation of the blastocyst
49. Parts of the blastocyst
The blastocyst consists of two primary cell types:
the inner cell mass, also known as the
"embryoblast" (this part of the embryo is used in
stem cell research)
the trophoblast, a layer of cells surrounding the
inner cell mass and the blastocyst cavity
(blastocoele)
The former is the source of embryonic stem cells
and gives rise to all later structures of the adult
organism.
The latter combines with the maternal
endometrium to form the placenta in eutherian
mammals
50. Inner cell mass
In early embryogenesisof most eutherian mammals, the
inner cell mass (abbreviated ICMand also known as
the embryoblast or pluriblast, the latter term being
applicable to all mammals) is the mass of cells inside the
primordial embryo that will eventually give rise to the
definitive structures of the fetus.
This structure forms in the earliest steps of
development, before implantation into the endometrium
of the uterus has occurred.
The ICM lies within the blastocoele(more correctly
termed "blastocyst cavity," as it is not strictly
homologous to the blastocoele of anamniote
vertebrates) and is entirely surrounded by the single
layer of cells called trophoblast
51. Trophoblasts (from Greek
trephein:to feed, and
blastos: germinator)
are cells forming the outer layer of a
blastocyst, which provide nutrients to the
embryo and develop into a large part of
the placenta.
They are formed during the first stage of
pregnancyand are the first cells to
differentiate from the fertilized egg. This
layer of trophoblasts is also collectively
referred to as "the trophoblast",or, after
gastrulation,the trophectoderm, as it is
then contiguous with the ectoderm of the
embryo.
am. Amniotic cavity.b.c. Blood- clot.b.s. Body-stalk.ect. Embryonic ectoderm.ent.
Entoderm.mes. Mesoderm.m.v. Maternal vessels.tr. Trophoblast.u.e. Uterine
epithelium.u.g. Uterine glands.y.s. Yolk-sac.
52. A blastocoel(e) or blastocele (also
called blastocyst cavity,cleavage
cavity or segmentation cavity)
is the fluid-filled central region of a
blastocyst. A blastocoele forms during
embryogenesis when a zygote (a fertilized
ovum) divides into many cells through
mitosis.
A blastocoel can be described as the first
cell cavity formed as the embryo enlarges.
It is essential for later gastrulation.
53.
54. Formation of the
blastocyst
The morula is a solid
ball of about 16
undifferentiated, spheri
cal cells. As cell division
continues in the
morula, the
blastomeres change
their shape and tightly
align themselves
against each other. This
is called compaction
and is likely mediated Blastulation. 1 - morula,
by cell surface adhesion 2 - blastula.
glycoproteins.
55.
56. First stages of segmentation of a fertilized mammalian ovum.
Semidiagrammatic.
z.p. Zona pellucida.
p.gl. Polar bodies.
a. Two-cell stage.
b. b. Four-cell stage.
c. c. Eight-cell stage.
d. d, e. Morula stage
57. Implantation
After entering the uterus, the
fertilized egg attaches to the
uterine lining, or endometrium.
This process is called
implantation. The cells continue
to divide.
58. Differentiation into the
Cytotrophoblast/Syncytiotrophoblast
(day 7)
Near the end of the first week, the
blastocyst implants in the posterior wall
of the uterus in the presence of high
hormone levels; the blastocyst attaches
at the embryonic pole (inner cell mass)
so the embryo will eventually be attached
dorsally.
Occasionally, the blastocyst will implant
on another spot in the uterus, potentially
causing problems (placenta previa).
59. The trophoblast layer
undergoes mitosis
upon contact with
the uterine wall and
differentiates into
the cytotrophoblast
("cellular" layer) and
the
syncytiotrophoblast
("multinucleated"
layer).
60. Early Development
Implantation
– 6-7 days after
fertilization
– Trophoblast
forms placenta
– ―Inner cell mass‖
forms embryo
61. Pregnancy Hormones
Human chorionic
gonadotropin (hCG) is a
hormone present in the
blood within about a week of
conception. It is the hormone
detected in a blood or urine
pregnancy test, but it usually
takes three to four weeks for
levels of hCG to be high
enough to be detected by
pregnancy tests. It is
secreted by cells that
develop into the placenta.
Editor's Notes
The day of ovulation determines the length of a menstrual cycle. The common belief that stress can affect the timing of a period is half true. Stress can affect ovulation which then affects hormone release which begins the next menstrual period ; but, stress around the time of an expected period will not make it late - that date was decided 12-16 days earlier.OvulationEach month, a mature egg is released from one of a woman's two ovaries -- this is called ovulation. Ovulation takes place about two weeks after the first day of the last menstrual period.
When stem cells divide, one becomes a specialised cell,while the other remains a stem cell. Specialised cells areall the different cells in our body that perform a function.They are all the cells we see on us and in us,such as lung cells, skin cells, hair cells, and kidney cells."
original work, are licensed under a SIZE: 0.1 - 0. 15 mmTIME PERIOD: 1 day post-ovulation Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours.A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female reproductive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus. The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization.
Fertilization FertilizationFertilization, the process by which male and female gametes fuse, occurs in theampullary region of the uterine tube. This is the widest part of the tube and38 Part One: General Embryologyis close to the ovary (Fig. 2.4). Spermatozoa may remain viable in the femalereproductive tract for several days.Only 1% of sperm deposited in the vagina enter the cervix, where theymay survive for many hours. Movement of sperm from the cervix to the oviductis accomplished primarily by their own propulsion, although they may be assistedby movements of fluids created by uterine cilia. The trip from cervixto oviduct requires a minimum of 2 to 7 hours, and after reaching the isthmus,sperm become less motile and cease their migration. At ovulation, spermagain become motile, perhaps because of chemoattractants produced by cumuluscells surrounding the egg, and swim to the ampulla where fertilizationusually occurs. Spermatozoa are not able to fertilize the oocyte immediatelyupon arrival in the female genital tract but must undergo (a) capacitation and(b) the acrosome reaction to acquire this capability.Capacitation is a period of conditioning in the female reproductive tractthat in the human lasts approximately 7 hours. Much of this conditioning,which occurs in the uterine tube, entails epithelial interactions between thesperm and mucosal surface of the tube. During this time a glycoprotein coatand seminal plasma proteins are removed from the plasma membrane thatoverlies the acrosomal region of the spermatozoa. Only capacitated sperm canpass through the corona cells and undergo the acrosome reaction.The acrosome reaction, which occurs after binding to the zona pellucida,is induced by zona proteins. This reaction culminates in the release of enzymesneeded to penetrate the zona pellucida, including acrosin and trypsin-like substances
passage of sperm through corona radiata *penetration of zona pellucida *fusion of plasma membranes of oocyte and sperm *completion of 2nd meiotic division of oocyte and formation ofthe female pronucleus *formation ofthe male pronucleus *asthe pronuclei fuse into a single diploid aggregation,theootid becomes a zygote
Human fertilizationis the union of a humanoid egg and sperm, usually occurring in the ampulla of the uterine tube. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development. The process of fertilization involves a sperm fusing with an ovum—usually following ejaculation during copulation. It is possible, but less probable, for fertilization to occur without copulation, artificial insemination, or In vitro fertilization.Upon encountering the ovum, the acrosome of the sperm produces enzymes which allow it to burrow through the outer jelly coat of the egg. The sperm plasma then fuses with the egg's plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitroOocyte Fertilized••••• original work, are licensed under a SIZE: 0.1 - 0. 15 mmTIME PERIOD: 1 day post-ovulation Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours.A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female reproductive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus. The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization.
The phases of fertilization include phase 1, penetration of the corona radiata;phase 2, penetration of the zona pellucida; and phase 3, fusion of theoocyte and sperm cell membranes.PHASE 1: PENETRATION OF THE CORONA RADIATAOf the 200 to 300 million spermatozoa deposited in the female genital tract,only 300 to 500 reach the site of fertilization. Only one of these fertilizes theegg. It is thought that the others aid the fertilizing sperm in penetrating thebarriers protecting the female gamete. Capacitated sperm pass freely throughcorona cells (Fig. 2.5).PHASE 2: PENETRATION OF THE ZONA PELLUCIDAThe zona is a glycoprotein shell surrounding the egg that facilitates and maintainssperm binding and induces the acrosome reaction. Both binding and theacrosome reaction are mediated by the ligand ZP3, a zona protein. Releaseof acrosomal enzymes (acrosin) allows sperm to penetrate the zona, therebycoming in contact with the plasma membrane of the oocyte (Fig. 2.5). Permeabilityof the zona pellucida changes when the head of the sperm comesin contact with the oocyte surface. This contact results in release of lysosomalChapter 2: First Week of Development: Ovulation to Implantation 39enzymes from cortical granules lining the plasma membrane of the oocyte.In turn, these enzymes alter properties of the zona pellucida (zona reaction)to prevent sperm penetration and inactivate species-specific receptor sites forspermatozoa on the zona surface. Other spermatozoa have been found embeddedin the zona pellucida, but only one seems to be able to penetrate the oocytePHASE 3: FUSION OF THE OOCYTE ANDSPERM CELL MEMBRANESThe initial adhesion of sperm to the oocyte is mediated in part by the interactionof integrins on the oocyte and their ligands, disintegrins, on sperm. Afteradhesion, the plasma membranes of the sperm and egg fuse (Fig. 2.5). Becausethe plasma membrane covering the acrosomal head cap disappears during theacrosome reaction, actual fusion is accomplished between the oocyte membraneand the membrane that covers the posterior region of the sperm head(Fig. 2.5). In the human, both the head and tail of the spermatozoon enter thecytoplasm of the oocyte, but the plasma membrane is left behind on the oocytesurface. As soon as the spermatozoon has entered the oocyte, the egg respondsin three ways:1. Cortical and zona reactions. As a result of the release of cortical oocytegranules, which contain lysosomal enzymes, (a) the oocyte membranebecomes impenetrable to other spermatozoa, and (b) the zona pellucidaalters its structure and composition to prevent sperm binding andpenetration. These reactions prevent polyspermy (penetration of morethan one spermatozoon into the oocyte).2. Resumption of the second meiotic division. The oocyte finishes its secondmeiotic division immediately after entry of the spermatozoon. Oneof the daughter cells, which receives hardly any cytoplasm, is known asthe second polar body; the other daughter cell is the definitive oocyte.Its chromosomes (22+X) arrange themselves in a vesicular nucleusknown as the female pronucleus (Figs. 2.6 and 2.7).3. Metabolic activation of the egg. The activating factor is probably carriedby the spermatozoon. Postfusion activation may be considered toencompass the initial cellular and molecular events associated with earlyembryogenesis.The spermatozoon, meanwhile, moves forward until it lies close to thefemale pronucleus. Its nucleus becomes swollen and forms the male pronucleus(Fig. 2.6); the tail detaches and degenerates. Morphologically, the maleand female pronuclei are indistinguishable, and eventually, they come intoclose contact and lose their nuclear envelopes (Fig. 2.7A). During growth ofmale and female pronuclei (both haploid), each pronucleus must replicate itsDNA. If it does not, each cell of the two-cell zygote has only half of the normalamount of DNA. Immediately after DNA synthesis, chromosomes organize onthe spindle in preparation for a normal mitotic division. The 23 maternal and23 paternal (double) chromosomes split longitudinally at the centromere, andsister chromatids move to opposite poles, providing each cell of the zygotewith the normal diploid number of chromosomes and DNA (Fig. 2.6, D andE ). As sister chromatids move to opposite poles, a deep furrow appears on thesurface of the cell, gradually dividing the cytoplasm into two parts (Figs. 2.6Fand 2.7B).The main results of fertilization are as follows: Restoration of the diploid number of chromosomes, half from the fatherand half from the mother. Hence, the zygote contains a new combinationof chromosomes different from both parents. Determination of the sex of the new individual. An X-carrying spermproduces a female (XX) embryo, and a Y-carrying sperm produces a male(XY) embryo. Hence, the chromosomal sex of the embryo is determinedat fertilization. Initiation of cleavage. Without fertilization, the oocyte usually degenerates24 hours after ovulation.
Conception: From Egg to EmbryoAt the moment when a lone sperm penetrates a mature egg, conception or fertilization takes place. To better understand the incredible process of conception, take a journey with us from tiny egg to growing embryo.
Thus, if all functions properly, only one sperm can enter the ovum since only one receptor can be activated.
The corona radiata surround an ovum or unfertilized egg cell, and consist of two or three strata (layers) of follicular cells. They are attached to the outer protective layer of the ovum , the zonapellucida, and their main purpose in many animals is to supply vital proteins to the cell.They are formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originate from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage. These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle. For fertilization to occur, sperm cells rely on hyaluronidase (an enzyme found in the acrosome of spermatozoa) to disperse the corona radiata from the zona pellucida of the secondary (ovulated) follicle, thus permitting entry into the perivitelline space and allowing contact between the sperm cell and the nucleus of the oocyte.The corona radiata are layers of follicle cells, that protect the secondary oocyte as it passes through the ruptured follicular wall, on its way to the infundibulum of the uterine (AKA fallopian) tubes. In order for fertilization to occur, the sperm must break through this layer of follicular cells by secreting the enzyme hyaluronidase. It takes the secretions of dozens of sperm to weaken the layer enough for one sperm to penetrate.
OoplasmOoplasm (also: oöplasm) is the yolk of the ovum, a cell substance at its center, which contains its nucleus, named the germinal vesicle, and the nucleolus, called the germinal spot.The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm, often called the formative yolk; and the nutritive yolk or deutoplasm, made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm.Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation
The zona pellucida (plural zonaepellucidae, also egg coat) is a glycoproteinmembrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the oocyte, external but of essential importance to it. The zona pellucida first appears in multilaminar primary oocytes. It is secreted by both the oocyte and the follicular cellsThis structure binds spermatozoa, and is required to initiate the acrosome reaction.In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes to be replaced by the underlying layer of trophoblastic cells.The zona pellucida is essential for oocyte death and fertilization.In some older texts, it has also been called zona striata and stratum lucidum
The cortical reaction occurs in fertilisation when a spermcell unites with the egg's plasma membrane, (zona reaction).This reaction leads to a modification of the zona pellucidathat blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.The cortical reaction is exocytosisof the egg's cortical granules. Cortical granules are secretory vesicles that reside just below the egg's plasma membrane. When the fertilizing sperm contacts the egg plasma membrane, it causes calcium to be released from storage sites in the egg, raising the intracellular free calcium concentration. This triggers fusion of the cortical granule membranes with the egg plasma membrane, liberating the contents of the granules to the extracellular space. Fusion begins near the site of sperm contact, and then as the wave of calcium release sweeps around the egg, a wave of cortical granule fusion results. The contents of the granules vary with the species, and are not fully understood
Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "second mouth") are a superphylum of animals. They are a subtaxonof the Bilateriabranch of the subregnumEumetazoa, and are opposed to the protostomes. Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth. Deuterostomes are also known as enterocoelomatesbecause their coelom develops through enterocoely.
FunctionTrophoblasts are specialised cells of the placenta that play an important role in embryo implantation and interaction with the decidualised maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetal circulation via the umbilical cord. This core is surrounded by two layers of trophoblast; a single layer of mononuclear cytotrophoblast that fuse together to form the overlying multinucleated syncytiotrophoblast layer that covers the entire surface of the placenta. It is this syncytiotrophoblast that is in direct contact with the maternal blood that reaches the placental surface, and thus facilitates the exchange of nutrients, wastes and gases between the maternal and fetal systems.
A morula (Latin, morus: mulberry) is an embryo at an early stage of embryonic development, consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida.[1]The morula is produced by embryonic cleavage, the rapid division of the zygote. Once the zygote has divided into 32 cells, it begins to resemble a mulberry, hence the name morula (Latin, morus: mulberry).[2] Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable. This process is known as compaction.[3][4] The cells of the morula then secrete a viscous liquid[specify], causing a central cavity to be formed, forming a hollow ball of cells known as the blastocyst.[5][6]The blastocyst's outer cells will become the first embryonic epithelium (the trophectoderm). Some cells, however, will remain trapped in the interior and will become the inner cell mass(ICM), and are pluripotent. In mammals (except monotremes), the ICM will ultimately form the "embryo proper", while the trophectoderm will form the placenta and other extra-embryonic tissues.[7][8][9][10]
