Theme: Molecular and
genetic mechanisms of
ontogenesis. Ontogenesis
abnormalities
Lector Pryvrotska Iryna
Plan of the lecture:
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Asexual reproduction in unicellular and
multicellular organisms
Sexual reproduction in unicellular and
multicellular organisms
The periods of ontogenesis
Spermatogenesis and oogenesis
Morphogenetic specialization of sex cells:
a sperm and an ovum
Fertilization. Parthenogenesis
Biological peculiarities of human
reproduction
Reproduction is the method by which
individuals give rise to other
individuals of same type.
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There are two types of
reproduction :
asexual and sexual
Types of reproduction
asexual
Organisms single-cellular
organisms,
multi-cellular
organisms: one ore
more somatic cells of
parental organisms;
Parents
One person
Offspring's genetically identical to
its parent like a xero
copy.
Cell
formation
Mitosis
sexual
Parental organisms
forms gametes
Two persons
genetically different
from parents
Meiosis
Forms of asexual reproduction
in single-cellular organisms:
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Asexual is a
reproduction without the
fusion of sexual cells,
identical offspring grows
directly from a one or
few body cells, which
divides mitotically
binary fission – parent
cell splits in two cells by
mitosis
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Asexual reproduction
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endogony -
in
parent cell forms only
two daughter cells
by internal budding
schizogony -
formation a great
amount of daughter
cells in parent cell
Asexual reproduction
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budding -
after
karyokinesis the special
region in parent cell
rapid grows and
organized into new
organism.
sporogony -
is
reproduction by the
spores
Forms of asexual reproduction in
multi-cellular organisms
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vegetative
(regeneration) –
a group of cells
from the parent
organism
separates and
new organism
forms
Asexual reproduction
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polyembriony the production of
two or more
embryo's from the
one zygote
sporogony -
is
reproduction by the
spores
Sexual - is a reproduction by
fusion of sexual cells and
formation of zygote
In sexual reproduction, genetic
material from two individuals combines
to begin the life of a third individual
who has a new combination of
inherited traits
Sexual reproduction
Forms of sexual reproduction
in single-cellular organisms
are
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Conjugation – a cytoplasm bridge forms
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Copulation - two individuals acquire the
between two organisms, the nuclei
transfer across this bridge and after
exchange ones forms a new gene
combination but no new offspring.
gametes properties, fuse and form a
zygote – the life of a new individual
begins.
In multicellular organism sexual
reproduction may be two forms:
- with fertilization
- without fertilization
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Parthenogenesis is the development of
new organism from an egg without
fertilization.
Ontogenesis
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the development of the individual
organism.
It includes the set of morphological,
phisiological and biochenical
transformations from the moment of
germing up to death.
Types of ontogenesis in animals:
The larval type of an ontogenesis is characterized by
development of an organism by metamorphosis.
Metamorphosis – is change of shape or structure of an
organism from one developmental stage to another.
F/e mosquito: ovum- larva- pupa - imago;
louses – ovum- larva - imago;
pincers – ovum- larva- nymph - imago.
2.
The non-larval type of an ontogenesis is characterized
by formation of an organism in
an egg (birds)
3.
Intrauterine ontogenesis – is development of an
organism inside a maternal organism. (mammalian).
1.
The ontogenesis of multicellular
organisms is divided in two
periods
1 embryonic
2 postembryonic.
For higher animals
and man there are:
1) prenatal (before
birth)
2) posnatal (after birth)
periods of development
Germ cells
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The ovum
nourishes the
embryo with yolk,
which contains rich
stores of lipids.
It provides the
machinery for
protein synthesis
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An ovum is enormous in size. It
protects the developing embryo
inside jellylike protein coatings and
strong membranes (zona pellucida),
sacs of fluid, and sometimes hard or
leathery shells.
Corona radiata outside the cell
consists of the great amount of
follicular cells, which produce
follicular fluid for attracting the
sperms.
Distinguish the following
types of ovum cells:
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The isolecythal ovum contain a little yolk. It is
distributed in regular intervals on all cytoplasm of
an ovum.
(ovum of mollusca, lancelet, mammalian).
The telolecythal ovum have much yolk of grains.
They collect at a vegetative pole. On animal pole
there is cytoplasm without yolk and with
nucleus.(ovum of fishes, amphibians, reptilie).
The centrolecythal ovum has the central nucleus
and around it settles down yolk as grains.
(insects).
Unlike the egg, the sperm is
one of the smallest cells in the
body.
Each sperm consists of:
 a head region
 a body or midpiece
 a tail or flagellum.
The head has a haploid nucleus.
An acrosome – a small bump on
the front end of the head
contains enzymes that help the
cell penetrate the ovum’s outer
membrane.
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Sperm cell
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The body or midpiece has
mitochondria to provide the cell
energy and centrioles.
A tail consists of microtubules
for propulsion.
The sperm’s streamlined size
and shape effect its narrow
objective: to reach the egg,
penetrate its coating, and
deliver a haploid nucleus into
the egg’s cytoplasm
Oogenesis
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This process begins in a
haploid oogonium.
An oogonium
accumulates cytoplasm,
replicates its
chromosomes- primary
oocyte.
In meiosis I, the primary
oocyte divides to form a
small polar body and a
large, haploid secondary
oocyte.
Oogenesis
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Ovulation – dischange (going out) of a secondary
oocyte from a follicule of the ovary.
In meiosis II, the secondary oocyte divides to yield
another small polar body and a mature ovum.
Therefore, each cell undergoing meiosis in female can
potentially divide to yield a maximum of four cells, only
one of which will become the ovum
Spermatogenesis
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Sperm development begins
with spermatogonia.
A diploid spermatogonium
divides mitotically and
becomes a primary
spermatocyte as it moves
toward the lumen of the
tubule.
In meiosis I- to form two
secondary spermatocytes.
In meiosis II, each
secondary spermatocyte
divides to yield two equal –
sized spermatids.
Therefore, each cell
undergoing meiosis in male
can potentially divide to
yield a maximum of four
spermatids.
Gametogenesis
Periods
Spermatogenesis
Cells
Genetic
formula
Oogenesis
Cells
Genetic
formula
Reproduction
Cells divides
mitoticaly
♂ - from
puberty to
death
♀- strongly
at 3-7 m. in
embryogeny
completed at
3-d year
spermatog
onia
2n2c
2n4c
oogonia
2n2c
2n4c
Cells
Genetic
formula
Cells
Genetic
formula
Growth
Cell growth and
increase in size.
Duplication of
DNA.
primary
2n4c primary
spermatoc
oocyte
yte
2n4c
Cells
Genetic
formula
Cells
Genetic
formula
Maturation
Meiosis I- two
cells halves
genetically
material
secondary
spermatocyt
es
Meiosis II two
spermatids
1n2c Secondary
1n2c
1n1c mature
1n1c
Oocyte
ovum
Cells
Genetic
formula
Cells
Formation (spematogenesis)
Spiralization sperms 1n1c
of
chromosome
s, formation
of
acrosome,cel
l centre, tail
Genetic
formula
Gametogenesis
Ontogenesis abnormalities
1) multynuclear oocytes;
2) 10% of sperms can be abnormal;
3) development of germ cells with 22 or 24
chromosomes
The fusion of haploid gametes to
form a new diploid cell is called
fertilization or syngamy
Fertilization may be
external and
internal
Fertilization
During fertilization two processes
take place:
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Egg’s activation – a wave of
chemical reactions sweeps across
the surface of the newly aroused
egg, causing that surface to
harden and present a barrier to
the entry of any additional sperm.
The egg’s oxygen consumption
skyrockets, as does its rate of
protein synthesis.
Syngamy – male and female
haploid nuclei converge and fuse
to form the zygote’s single diploid
nucleus
Fertilization
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3 functions
– transmission of genes
– restoration of the
diploid number of
chromosomes reduced
during meiosis
– initiation of
development in
offspring
Fertilization
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internal fertilization
– capacitation
– sperm must penetrate cumulus and zona
pellucida
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extracellular matrix consisting of 3 types of
glycoproteins one of these, ZP3, acts as a sperm
receptor
Fertilization
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activates the egg, initiating
metabolic processes
acrosomal reaction
– sperm are activated
– acrosomal process
– sperm and egg membranes fuse
– ion channels open, allowing Na+
to flow in
– fast block to polyspermy
cortical reaction
– egg’s ER releases Ca2+ into the
cytosol at site of sperm entry
Fertilization
– slow block to
polyspermy
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Ca2+ causes cortical
granules underneath the
plasma membrane to
fuse
mucopolysaccharides
draw water into the
space, swelling it
vitelline layer becomes
the fertilization
membrane
Cleavage
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rapid divisions following fertilization
– often skip G1 and G2 phases
– blastomeres result
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most animal eggs have polarity
– substances are heterogeneously distributed in
cytoplasm
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vegetal pole
animal pole
Kinds of cleavage:
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Holoblastic (total cleavage) – the zygote is divided
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Meroblastic (incomplete cleavage) – the part of cytoplasm of
completely. There are 1) uniform and 2) irregular holoblastic
cleavage. They are haracteristic for isolecythal and
telolecythal cells
an zygote is divided where yolk is absence.
There are: 1) discoidal and 2) superficial meroblastic
cleavage.
In discoidal meroblastic cleavage the segmentation occurs
on an animal pole in telolecythal cells. Birds’ eggs contain so
much yolk that the small disc of cytoplasm on the surface is
dwarfed by compasion. No cleavage of the massive yolk is
possible, and all cell division is restricted to the small
cytoplasmic disc, or blastodisc.
In superficial meroblastic cleavage the segmentation occurs
on an peripheric zone of cytoplasm in centrolecital cells.
In the man the cleavage of zygote is
holoblastic, irregular and asynchronous
Mammalian Development
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implantation
– ICM forms flat disk with 2 layers (epiblast
and hypoblast)
– embryo develops from epiblast cells,
hypoblast forms yolk sac
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gastrulation
Gastrulation
series of cell migrations to positions where they will form the
three primary cell layers:
1) ectoderm (outside germinal layer); 2) endoderm (inside
germinal layer) and 3) mesoderm (medium germinal layer)
The germinal layers give rice to various tissues and organs of
animals. It is called as histogenesis and organogenesis
The fate of primary germinal layers is given
bellow:
Ectoderm
Mesoderm
Endoderm
Skin (epidermis),
hair, nails, the
eye lens, the
pituitary gland,
the epithelium of
the nasal cavity,
mouth, anal
canal, nervous
system, sense
organs
Connective tissue, bones,
muscles, dermis, heart,
blood vessels, gonads,
excretory organs
(kidneys) and the
notochord (the dorsally
located supportive rod
found in all chordates, at
least in embryonic
stages)
Digestive tract,
lungs, liver,
pancreas,
thyroid gland,
urinary bladder
Mammalian Development
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4 extraembryonic membranes form:
– chorion- from trophoblast, surrounds embryo
and all other membranes
– amnion- from epiblast, encloses embryo in
amniotic fluid
– yolk sac- from hypoblast, site of early blood cell
formation
– allantois- outpocketing of embryo’s gut,
incorporated into umbilical cord, forms blood
vessels of umbilical cord
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Provisional organs
are present during
embryonic period
and absent after
birth.
Provisional organs
in human embryo
are:
an youlk sac
an amnion
an allantois
a chorion.
The youlk sac – the extraembryonic membrane that connects with the midgut. In
human it is the first site of blood cell formation, but has no nutritive function.
Chorion [Gr. «membane»] – in human it is cellular, outermost
extraembryonic membrane, composed of trophoblast lined with mesoderm,
it develops villi about 2 weeks after fertilization, is vascularized by allantoic
vessels a week later.
Amnion [Gr. «bowl» ; «membrane enveloping the fetus»] – the thin but tough
extraembryonic membrane of reptilies, birds, and mammals that lines the chorion and
contains the embryo and later the fetus, with the amniotic fluid around it.
Placenta
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By the 23-rd day of human
embryonic development, two
other embryonic membranes
– the chorion and the
allantois to give rise to a
placenta
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Exchange of material takes
place in the placenta by
diffusion between the blood
of the mother and that of
the embryo
Within the placenta there is
no mixing of maternal and
fetal blood
Human Gestation
– organogenesis
– fetus- all major
structures are present
– human chorionic
gonadotropin (hCG)produced by embryo,
maintains corpus luteum
– mucous plug formation
in cervix
– negative feedbackcessation of ovulation
and menstrual cycles
Human Gestation
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2nd trimester
– increased movement
of fetus
– hCG levels decline,
leading to
deterioration of
corpus luteum
– placenta secretes
progesterone
– uterus increases to
visible size
Human Gestation
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3rd trimester
– rapid growth of fetus
– estrogens and oxytocin initiate labor
– positive feedback- oxytocin stimulates
prostaglandin secretion by placenta,
leading to increased contractions
In embrionic development of
man there are following critical
periods:
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Implantation (7 day after a fertilization) –
introduction of a zygote in a wall of an uterus
Placentation (the end of 2 week of pregnancy) – form
at embryo of a placenta
Perinatal period (from 28 week of pregnancy to 7 day
after birth) – is transferring a fetus out of aqueous into
air (on the average, about 280 days after the beginning
of the mother’s last regular menstrual period).
Intrauterine pregnancy
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Any substance that
can causes abnormal
development of the
egg in the mother's
womb is called a
teratogen.
Growth is rapid, and
each body organ has a
critical period in which
it is especially
sensitive to outside
influences. About 7%
of all congenital
defects are caused by
exposure to
teratogens.
Drugs
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Alcohol.
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Ontogenesis abnormalities
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Chances of Down syndrome rapidly
increase with parental age starting at
about age 35 in women and 55 in
men. Previously, it was thought there
might be a tendency toward
nondisjunction as a women ages
because her eggs age as she does a women is born with all the eggs
she will ever have and they remain in
suspended animation until one
matures each month.
Ontogenesis abnormalities
References:
1. Biology. – Sylvia S. Mader, Wm. C.
Brown Publishers: Dubuque, Lowa –
Melbourne, Australia – Oxford, England,
IV edition. – p.169-181, 694-705.
2. Biology. Art notebook – Sylvia S.Mader,
Wm. C. Brown Publishers: Dubuque, Lowa
– Melbourne, Australia – Oxford, England,
IV edition. – p.42-47.
3. Human biology. – McMillan, Beverly. –
p. 310-327.