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VAN 504, Lecture 07
Embryology: Gametogenesis,
Fertilization, Cleavage, Gastrulation
Gametogenesis
The sequential stages in the differentiation
and maturation of primordial germ cells into
gametes in male and female animals are
referred to as gametogenesis.
Two Types of Gamete Formation
• Spermatogenesis –
the process of male
gamete production
in animals
Oogenesis – the process
of female gamete
production in animals
Spermatogenesis
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Spermatogenesis occurs inside testes
Within testes spermatogenesis occurs in
walls of seminiferous tubules
Mature sperm are released into lumen of
seminiferous tubules.
Spermatigonia & Primary spermatocytes
are 2n
Secondary spermatocytes are 1n
Spermatids and sperm are 1n
The process whereby a spermatid
undergoes metamorphosis
into a spermatozoon is termed
spermiogenesis
The time required for the production
of spermatozoa from type A
spermatogonia may range from 40 to 60
days depending on the
species.
Oogenesis
• Oogenesis begins with a diploid cell
called an oogonium.
• After growth and development, one
oogonium forms one primary oocyte.
• Meiosis I produces a secondary oocyte
and one polar body (due to unequal
division of the cytoplasm).
• Meiosis II in the secondary oocyte
produces an egg and polar body. Meiosis
II in first polar body produces 2 polar
bodies.
• The egg survives, while all the polar
bodies die.
• Only one functional egg cell comes from
this process, as the unequal division of
the cytoplasm makes the egg cell big
(needs extra nutrients).
Oogenesis
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Oogenesis occurs inside the ovary
Within ovary oogenesis occurs in a follicle
Eggs are released into oviduct
Oogonium & Primary oocytes are 2n
Secondary oocytes are 1n
Polar bodies and eggs are 1n
Release of the ovum from the follicle is referred to as
ovulation
Oogenesis
Fertilization
• Fertilization is the process of union of mature
male gamete (sperm) with mature female
gamete (ovum) to produce new cell of life
which is called (zygote) through chain of
events in the oviduct (fallopian tubes). Indeed,
interruption of any event will cause
fertilization failure.
Folliculogenesis and ovulation
• Folliculogenesis is the maturation of the ovarian
follicle, describes the progression of a number of small
primordial follicles into large mature follicles under the
influence of FSH prior to puberty.
• Ovulation By the end of the follicular phase the mature
follicle will develop and rupture, excrete the oocyte
with some granulosa cells into oviduct. The oocyte is
now competent to undergo fertilization.
• Ruptured follicle transformed into the corpus luteum,
that produce large amount of progesterone that helps to
prepare the uterus for implantation of fertilized egg.
• In absence of fertilization corpus luteum degenerates.
Image showing phases of follicle
development and ovulation
Ovum structure
Mechanisms of ovum transport
• Fimbria on terminal oviduct--acts as funnel to
receive ovum
• Fluids (abdominal cavity and that escaping
from follicle during ovulation) serve as
medium for free-floating ovum
• Cilia lining oviduct and muscular contractions
assists in moving ovum to site of fertilization
The site of fertilization on most farm animals
is (ampullary-isthmic junction) while in
human is (ampulla) region.
Fertile life of ovum
• Estimate fertile life of ovum in different
mammals is shown below
Species
Fertile life
Cow
8-12 hours
Sheep
16-24 hours
Swine
8-10 hours
Human
6-24 hours
Mating process
Is the period when female receptive and will stand for mating
with males during estrous phase of estrous cycle, the main
purpose of mating is deposition of fertile semen into female
genital tract after ejaculation, characteristics of ejaculate in
different mammals is shown below
species
Cow
Sheep
Swine
Human
Site of semen
deposition
Vagina
Vagina
Cervix and
uterus
Vagina
Volume of
ejaculate (ml)
Sperm
concentration
(billion/ml)
No. of sperm
reaching site of
fertilization
4
1
4200-27500
1
2
600-5000
125
0.200
Few
3.5
0.120
Few
• Deposition of semen into female tract must be closely
synchronized with ovulation to ensure the incidence of
fertilization
• Time of ovulation in different mammals shown in table below
species
Cow
Sheep
Goat
Swine
Human
Time of ovulation
10-12 hours after end of estrous phase
Late of estrous phase
Few hours after end of estrous phase
Mid-estrous phase
Day(14) after initiation of menstrual cycle
Mechanisms of sperm transport
• Pass into cervix by own movement
• As the sperm enter the cervix, orient themselves into current
of thin mucus during estrus
• Myometrium moves them through the uterus
• Moved through oviduct by cilia of oviduct and uterine
contractions and a thin fluid secreted by glandular cells.
Fertile life of sperm
• Estimate fertile life of sperm in different
mammals is shown in table below
Species
Fertile life
Cow
24-48 hours
Sheep
30-48 hours
Swine
24-48 hours
Human
28-48 hours
Fertilization events
1. Sperm capacitation:
Freshly ejaculated sperm are unable to fertilize
an egg. Rather, they must first undergo a series
of changes known as capacitation. Capacitation
is associated with removal of adherent seminal
plasma proteins, reorganization of plasma
membrane lipids and proteins.
Acrosome reaction
The acrosome reaction involves breakdown
and fusion of outer acrosome membrane with
the plasma membrane of the sperm. This
results in formation of vesicles and release of
enzymes needed for sperm to penetrate the
cumulus oophorus and corona radiata as well
as zona pellucida . During the penetration
process the acrosome is lost with only the
inner acrosomal membrane remaining around
the sperm head.
Sperm penetration
• Sperm cell penetrate cumulus oophorus by the enzyme (hyaluronidase)
released during acrosome reaction.
• Sperm cell penetrate corona radiata by the enzyme (corona-penetrating
enzyme) also released during acrosome reaction.
• Sperm cell penetrate zona pellucida by the acrosin (trypsinlike enzyme)
also released during acrosome reaction.
• Then membrane of sperm fuses with the
vitelline membrane of the egg, the egg
cytoplasm around the area of contact surrounds
the sperm head and incorporating it into the
egg
• The nucleus of sperm is then release into the
egg cytoplasm without the tail
Consequences of fertilization
• In most mammals after releasing the sperm
nucleus into egg cytoplasm it stimulates the
diffusion of cortical granules into the previtelline
space, the erection of a barrier to prevent
fertilization by more than one sperm will occur,
this process is called zona reaction and vitelline
block.
• Then the male and female pronuclei are formed
and unite (syngamy) to establish the diploid one
cell zygote.
• Time of formation of zygote and time required to entry the
fertile ovum into uterus in different mammals is shown in
table below
Species
Cow
Sheep
Goat
Swine
Human
Time of formation of
zygote (hours)
Entry to uterus (days)
0-24
3-4
0-38
2-4
0-30
4
0-15
2-4
0-24
3
Post fertilization events
• The first of many changes following fertilization
is to become multicellular, and the one-cell
embryo rapidly cleaves into 2, 4, 8 and more
cells.
• It then starts to do some interesting things like
develop a discrete inside and an outside.
• Finally, the embryos of many species start to
secrete hormones that ensure their survival - a
process called maternal recognition of
pregnancy.
The Zygote
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The zygotea single cell formed at
fertilisation.
• Structure.
--diploid nucleus from both parents.
--cytoplasm is maternal.
--surrounded by zona pellucida
• Cleavagemitotic division at 12hours,
2days and 3days in mouse, pig and dog.
• Rate about one division/day.
• Zygote period lasts from fertilisation to
hatching of the blastocyst.
• Nutrition/embryotroph
--mammalian zygotes is provided by uterine
secretions/
--histiotroph and the zygotes own
reserve.
--avian zygotes feed on the yolk.
Cleavage
• Cleavage is the first phase of embryonic development
Functions of cleavage:
Multicellular for differentiation
The zygote is partitioned
into blastomeres. Each
blastomere contains
different regions of the
undivided cytoplasm and
thus different cytoplasmic
determinants.
Mammalian Cleavage
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in oviduct
slow cell divisions
asynchronous cell divisions
accompanied by gene expression
produces a blastocyst
– inner cell mass - primordial embryo
– trophoblast - primordial placenta component
Cleavage
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Cleavage is a rapid series of mitotic divisions that occur
just after fertilization.
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There are two critical reasons why cleavage is so
important:
1. Generation of a large number of cells that can undergo
differentiation and gastrulation to form organs.
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2. Increase in the nucleus / cytoplasmic ratio. Eggs
need a lot of cytoplasm to support embryogenesis. It is
difficult or impossible for one nucleus to support a huge
cytoplasm, and oocytes are one of the largest cells that
exist. One small nucleus just cannot transcribe enough
RNA to meet the needs of the huge cytoplasm.
Cleavage differs from normal mitoses in 2 respects
1. Blastomeres do not grow in size between
successive cell divisions as they do in most cells.
This leads to a rapid increase in the nucleus /
cytoplasmic ratio. Cells undergoing cleavage.
2. Cleavage occurs very rapidly, and mitosis and
cytokinesis in each round of cell division are
complete within an hour. Typical somatic cells
divide much more slowly (several hours to days)
and even the fastest cancer cells divide much
slower than occurs in a zygote during cleavage.
• Cleavage differs in different types of eggs. The
presence of large amounts of yolk alters the
cleavage pattern, leading to incomplete cleavage
that characterizes birds and reptiles.
Eggs are classified by how much yolk is present
1. Isolecithal eggs (iso = equal) have a small amount of
yolk that is equally distributed in the cytoplasm (most
mammals have isolecithal eggs).
2. Mesolecithal eggs (meso = middle) have a moderate
amount of yolk, and the yolk is present mainly in the
vegetal hemisphere (amphibians have mesolecithal
eggs).
3. Telolecithal eggs (telo = end) have a large amount of
yolk that fills the cytoplasm, except for a small area near
the animal pole (fish, reptiles, and birds).
4. Centrolecithal eggs have a lot of yolk that is
concentrated within the center of the cell (insects and
arthropods).
The pattern of cleavage of the zygote depends upon
the pattern of yolk distribution
1. Holoblastic cleavage: occurs in isolecithal eggs (mammals,
sea urchins). The entire egg is cleaved during each division.
2. Meroblastic cleavage occurs when eggs have a lot of yolk. The
egg does not divide completely at each division. Two types:
a. Discoidal cleavage is limited to a small disc of cytoplasm at
the animal pole. All of the yolk filled cytoplasm fails to
cleave (characteristic of telolecithal eggs such as birds).
b. Superficial cleavage is limited to a thin surface area of
cytoplasm that covers the entire egg. The inside of the
egg at is filled with yolk fails to cleave (centrolecith al
eggs such as insects).
Mammalian eggs have rotational cleavage that is
holoblastic
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The mammalian egg is a little slow. It begins to cleave in the oviduct and
continues until it implants in the wall of the uterus (1 cleavage / 24 hr).
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Asynchronous cleavage: mammalian embryos are unusual in that they have
asynchronous cleavage. Not all blastomeres divide at the same time.
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The first cleavage is meridional, and the second cleavage is rotational. The
2 blastomeres divide in different planes (one is equatorial and one is
meridional.
Mammalian embryos undergo compaction at the 8 cell stage
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At first, the blastomeres of mammalian embryos have a loose arrangement, and
touch only at the basal surfaces.
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After compaction, blastomeres adhere tightly, maximizing the area of contact.
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During compaction, each blastomere undergoes polarization. Tight junctions
develop on the outer surface, allowing proteins to specialize. Cells take up fluids
from the uterine environment and secrete into the blastocoel.
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Gap junctions form on the outer cells to aid in intercellular communication
A blastocoel develops as cleavage proceeds to the 32-64 cell
stage
• After compaction at the 8-16 cell stage, there are 2 types of
blastomeres. Outside blastomeres are tightly joined and number
about 9-14. They surround 2-7 inside blastomeres that are loosely
joined.
• Cavitation: the outside blastomeres start to take up fluid from the
uterus and pump it into the center, creating the blastocoel. The
blastocyst is the hallmark of early embryonic development in
mammals.
Trophoblast: a structure consisting of outside
blastomeres, this contributes to forming the
placenta
Inner cell mass: this
gives rise to the
embryo, and
develops from the
inside blastomeres
• The next step in development of telolecithal eggs is formation of the
upper and lower blastoderm.
• Epiblast: (epi = upon) this is the upper layer and it forms the embryo
proper. Hypoblast: (hypo = under) this is the bottom layer that will
form the extraembryonic endoderm that surrounds the yolk.
• Blastocoel: lies between the 2 layers.
• Subgerminal space: lies between the hypoblast and yolk
Timing of cleavage divisions
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Normal eukaryotic cells divide
slowly, once every several hours or
days. The cell cycle has G1 and
G2 periods. During G1 the cell
synthesizes RNA and other
components for cell growth.
Cleavage consists of very rapid
successive mitoses. Since the egg
has stored large amounts of RNA
and other material, it does not
need G1 or G2.
However, as the number of cells
increases, the nucleus /
cytoplasmic ratio also increases.
The rate of cell division slows
because the cell now needs to
synthesize its own RNA and grow
between divisions. Thus, G1 and
G2 are restored = midblastula
transition.
Unique Features of Mammalian Cleavage
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Mammalian Cleavage
Cleavage is series of mitotic division of the zygote into progressively smaller
cellular units; blastomeres
• First cleavage synchronous.
• Differences in pattern of cleavage dependent upon amount of yolk meroblastic
/incomplete and holoblastic/complete
• Totipotent early blastomeres
• 1st cleavage is meriodal
• 2nd cleavageone meriodional and one equatorial/termed rotational cleavage
• Second cleavage not synchronised in 2 blastomeres
• Compaction occurs at 8-cell stage.
--blastomeres flatten and form intercellular connections
--E-CAD(cadherin:glycoprotein) on cell surface adhesion.
--microvilli(actin) extend on surfaces of adjacent cells and anchor cells together.
--tight junctions prevent free exchange of fluid between the inside and outside
allowing accumulation of fluid inside blastomeres.
--the gap junctions couple all the blastomeres of the compacted embryo and permit
exchange of ions and small molecules from one cell to the next.
The Morula
• Morula; 16-cell stage, embryo
enclosed in the zona pellucida.
• Late morula, first differentiation
event in mammalian
development.
--cells aggregate into internal inner
mass cells(ICM) and external
trophoblast.
--at 64-cell stage ICM and
trophoblast form distinct
populations.
• Trophoblast/trophoectoderm
forms ectoderm of chorion/
placenta.
• Inner mass cells are pluripotent;
form embryo and partly
extraembryonic membranes.
Blastulation
Formation of the blastocyst
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--A series of rapid cell divisions produce a
blastula of 64 cells with an inner cell
mass(ICM) and an outer layer of
trophoblasts.
--trophoblast secrete fluid into the morula.
Creates a cavity; the blastocoele(A)
--ICM/embryo proper lies to one side
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Transition from morula to blastula
marked by:
--rapid enlargement of blastocoele
--differentiation of blastomeres into
ICM and trophoblast cells(B).
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Trophoblast cells induce special
changes in the uterine lining at
implantation.
Trophoblast cells preferentially express
maternal genes, and inactivate paternal
genes.
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The blastula hatches from the zona
pellucida and implants in the uterus.
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Blastogenesis starts at 32-cell stage.
Formation of hypoblast
and epiblast.
• Formation of hypoblast begins
late in blastulation
1. Segregation(A)
2. Delamination of ICM
cells.Cells expand beneath
trophoblast form
hypoblast(B)/extraembryonic
endoderm
• Hypoblast tube(blastocoele)
inside tube of trophoblast(C)
• Formation of hypoblast results
in two-layered embryo(C)
• Cells on surface ICM form
epiblast(C)
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Blastula: Hatching and Implantation
Blastulation embryo arrives in uterus(4)
Blastula hatches from zona pellucida and contact uterus.
Blastocyst surrounded by ZP prevents premature implantation and ectopic
pregnancy.
• Hatching involvesA trypsin-like protease lyses of ZP
--Trophoblast cells secrete proteases which degrades the endometrial wall
and blastocyst embeds(5).
3
1
4
4
5
2
Gastrulation
• Gastrulation rearranges the cells of a blastula
into a three-layered embryo, called a gastrula,
which has a primitive gut
• The three layers produced by gastrulation are
called embryonic germ layers
– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The mesoderm partly fills the space between the
endoderm and ectoderm
Organogenesis
• During organogenesis, various regions of the
germ layers develop into rudimentary organs
• Early in vertebrate organogenesis, the
notochord forms from mesoderm, and the
neural plate forms from ectoderm
• The neural plate soon curves inward, forming
the neural tube
• The neural tube will become the central
nervous system (brain and spinal cord)
• Neural crest cells develop along the neural
tube of vertebrates and form various parts of
the embryo (nerves, parts of teeth, skull
bones, and so on)
• Mesoderm lateral to the notochord forms
blocks called somites
• Lateral to the somites, the mesoderm splits to
form the coelom
ECTODERM
MESODERM
ENDODERM
Epidermis of skin and its
derivatives (including
sweat
glands, hair follicles)
Epithelial lining of mouth
and anus
Cornea and lens of eye
Nervous system
Sensory receptors in
epidermis
Adrenal medulla
Tooth enamel
Epithelium of pineal and
pituitary glands
otochord
Skeletal system
Muscular system
Muscular layer of
stomach and intestine
Excretory system
Circulatory and lymphatic
systems
Reproductive system
(except germ cells)
Dermis of skin
Lining of body cavity
Adrenal cortex
Epithelial lining of
digestive tract
Epithelial lining of
respiratory system
Lining of urethra, urinary
bladder, and reproductive
system
Liver
Pancreas
Thymus
Thyroid and parathyroid
glands
How is the characteristic body plan for
any organism developed
• Gastrulation: the first step in the process of body formation. It transforms a
complex sphere into 3 basic germ layers from which all other tissues develop.
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Ectoderm is the outer layer = forms epidermis and nervous system.
Mesoderm is in the middle and forms a many structures (i.e., heart, muscles).
Endoderm is the inner layer and forms the ‘gut’ and related organs.
•
The first change is to generate the rudiment of the digestive tract, hence the name
gastrulation (gastric = stomach).
Gastrulation is the first step of morphogenesis
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Morphogenesis is the process whereby individual cells undergo complex
movements that generate the organ rudiments. Gastrulation generates the three
basic germ layers from which organs arise.
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How do sheets of cells (epithelia) move during gastrulation?
5 methods. Invagination:
Invagination is the local inward movement of cells from a cavity
Involution is similar, but more dramatic. It is an inward expansion of epithelial cells
around an edge such as the blastpore.
• Convergent extension is elongation of an epithelium in one direction
while it shortens in the other direction (stretching taffy). The cells can
keep their relative positions and elongate or they can interdigitate.
Epiboly is spreading movement of an epithelium to a deeper or thinner layer.
• Delamination is the splitting of one layer into
two different layers.
Different combinations of these basic movements yield a variety of changes that
characterize gastrulation
• Epithelial cells are well-differentiated. They compose skin and line
the body cavities (ie, the digestive tract). They are polarized. Their
apical surface faces out and their basal surface rests on the
basement membrane (extracellular matrix that supports cells).
Epithelial cells are closely connected with adjacent cells by
specialized attachments including tight junctions, gap junctions, and
desmosomes.
Mesenchymal cells are poorly
differentiated and have the
potential to develop into many
different tissues, including
epithelial cells. They have a
leading edge with lamellipodia,
and a trailing edge. They are not
connected to adjacent cells but
they are in contact with the
extracellular matrix.
The second phase of gastrulation is caused by
convergent extension of cells into the blastocoel
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The invaginated cells of the vegetal plate extend to form a long thin archenteron. It is unclear as to
how this occurs.
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Stretching model: the extension could result from the cells changing shape to become long and
thin.
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Cell movement model: invaginated cells could move to extend the length of the tube.
• Secondary mesenchymal cells: cells at the tip of the archenteron
guide the progress. They send filopodia (thin extensions) to find the
correct area of the roof. The roof cells send back other filopodia to
direct the archenteron where to go.
• This allows the mouth to hook up with the gut.
Gastrulation(1)
• Gastrulation transforms flat two-layered
blastula(epiblast,hypoblast) into threelayered gastrulaectoderm, endoderm,
mesoderm.
Mechanism of gastrulation consists of:
1. Formation of primitive streak(PS)
2. Involution of PS
3. Regression of PS.
4.
Formation of primitive streak,
marked by:
--expansion of epiblast cells and caudal
convergence(A).
--Primitive streak(PS) forms as
longitudinal ridge in midline(B)
--PS elongates, cranial tip widened as
primitive node(C)
Gastrulation(2)
1.Involution of primitive streak(PS)
--epiblast cells leave PS
--primitive groove in midline
--first group of cells form
intraembryonic endoderm(A)
--second group of epiblast cells
intercalate between endoderm
and ectodermform mesoderm
3. PS regresses caudally
Enlarged tip Of PSHensen’s
node/primitive knot, moves to
posterior region(C).
4. Function of PS is to form three germ
layers
Formation of the Notochord
• The Notochord
--a rod-shaped aggregate of
chordamesoderm cells
extending along entire length of
embryo.
--notochord cells formed of migrating
cells from Hensen’s node
• Functions of notochord.
1. Defines cranial-caudal axis of
embryo.
2. Serves as primary inducer at
neurulation.Induces formation of
the neural tube and somitogenesis.
3. Transient, remnants in intervertebral
disc as nucleus pulposus.