C8545 Developmental Biology
Lesson 1
Introduction into the Study of Development
Jan Hejátko
Functional Genomics and Proteomics of Plants
CEITEC
and
National Centre for the Biomolecular Research,
Faculty of Science
Masaryk University, Brno
hejatko@sci.muni.cz, www.ceitec.eu
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
 Overview of development
 Asymmetrical cell division as a major force of the development
 Genetic integrity of the differentiated cells
 Gametogenesis, fertilization and lineage tracing
 Concept of stem cells
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The Course at a Glance
 Lesson 1
 Introduction into the Study of Development
 Repetitorium of Basic Terms
 Overview of Development
 Gametogenesis, Fertilization and Lineage Tracing
 Lesson 2
 Oogenesis and Early Development of Drosophila
 Lesson 3
 Early Development of Amphibians and Amniotes
 Lesson 4
 Vertebrate Organogenesis
 Development of Ectodermal Derivatives
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The Course at a Glance
 Lesson 5
 Vertebrate Organogenesis
 Development of Mesodermal and Endodermal Derivatives
 Lesson 6
 Plant Reproduction
 Lesson 7
 Plant Embryogenesis
 Lesson 8
 Postembryonic Plant Development
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The Course at a Glance
 Lesson 9
 Morphogenesis in Animals and in Plants
 Lesson 10
 Regulation of Gene Expression in the Development
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Literature
 Fred H. Wilt and Sarah Hake, Principles of Developmental Biology
(W.W. Norton & Company, New York, London, 2004)
 Scott F. Gilbert, Developmental Biology, eighth edition (Sinauer
Associates, Inc., Publishers Sunderland, Massachusetts, USA, 2006)
 Zdeněk Vacek, Embryologie (Grada Publishing, 2006)
 Dubová J., Hejátko J., Friml J. (2005) Reproduction of Plants, in
Encyclopedia of Molecular Cell Biology and Molecular Medicine (ed, R.
A. Meyers), pp. 249 – 295. Wiley-VCH, Weinheim, Germany
 Selected original papers in scientific journals
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
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http://en.wikipedia.org/wiki/Anatomical_terms_of_location
The terms describing the directions and relative positions in the body were introduced that
are independent of the body position or movement (see the figure).
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http://en.wikipedia.org/wiki/Anatomical_terms_of_location
Sagitální rovina
Frontální rovina
Transversální rovina
The terms describing the directions and relative positions in the body were introduced that
are independent of the body position or movement (see the figure).
Sagital plane goes through or is parallell to the anterio-posterior axis and divides
the body into sinister and dexter (left and right) portions.
Coronal plane is perpendicular to sagital plane and divides the body to the dorsal
and ventral portions.
Finally, transversal plane is perpendicular to both aforementioned planes and
divides the body into cranial and caudal (head and tail) portions.
In organisms that maintain a constant shape and have one dimension longer than
the other, at least two directional terms can be used. The long or longitudinal axis
is defined by points at the opposite ends of the organism. In higher organisms, this
axis might refer to e.g. section of a specimen in an anterio-posterior or apical-basal
axis. Similarly, a perpendicular transverse axis can be defined by points on
opposite sides of the organism.
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Friml et al., Trends Plant Sci (2006)
In plants, the proper terminology undergoes substantial changes recently, as there are
differences between embryonic and postembryonic development.
During embryonic development, the apical-basal axis is established that refers to the shoot
(aerial portion of the plant) and root. This polarity is specified via polar auxin transport that
is driven by polar localization of specific proteins, auxin efflux carriers.
These proteins maintain their polarity throughout the postembryonic development,
specifying thus the apical-basal directions.
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Baskin et al., Trends Plant Sci (2010)
Root/shoot junction
prýtově
kořenově
During postembryonic development, however, the anatomical term “apical” refers to two
potential directions-apex of the shoot and apex of the root. In this sense, the base is being
considered as the root/shoot junction (see the figure).
Therefore, to avoid potential misunderstanding, the novel terms shootward and rootward
were introduced that are being used for the clear determination of cell polarity, while the
terms apical and basal are suggested to be used only for anatomical purposes.
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Outline of Lesson 1
 The course at a glance
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. meiosis
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
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Cell life cycle and commented cell cycle video.
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In the course of mitosis, individual stages could be distinguished.
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 Leptotene
 Zygotene
 Pachytene
 Diplotene
 Diakinesis
Meiosis leads to reduction of the chromozome content and formation of gametes,
carrying segregated alleles.
In contrast to males, meiosis in females is asymmetric, leading to formation of
single egg cell and three polar bodies (see the next slide).
Source of the figures: http://www.le.ac.uk/ge/genie/vgec/he/cellcycle.html
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The prophase I could be further subdivided into five phases: leptotene, zygotene,
pachytene, diplotene and diakinesis (see the figure above).
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Wikipedia
Gametes formation and their independent and random combination allows novel
quality formation via distribution of individual alleles and their new
combinations.
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
 Overview of development
 Asymmetrical cell division as a major force of the development
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Hejátko et al., Mol Genet Genomics (2003)
Wikipedia and Bio-Image
The fundamental question of developmental biology is formation of multicellular,
differentiated organism from a single-cell zygote.
The answer to that question is precise regulation of development via asymmetrical
cell division. The asymmetrical cell divison is defined as a division of the mother
cell that results into two, non-equivalent daughter cells.
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Demonstration of the non-equivalent cell division.
The molecular mechanisms involved are either non-equivalent distribution of the
cell content and/or changes in the cell milieu (environment) that could be e.g.
changes in the light intensity or chemical composition produced by the surrounding
cells.
There are basically two major principles involved in the regulation of the nature of
the cell progeny-the origin of the cell line that is mostly important in animal
systems and the position of the cell, i.e. its environment that is important in both
animal and plant systems.
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Endodermis
Cortex
Dubova, Hejatko, Friml (2005) Petricka & Benfey, Curr Opinion Genet Dev (2008)
endodermis
cortex
An example of the asymmetric cell division in plants.
Early zygote division (left) or asymmetric division of the stem cell in the root of
Arabidopsis (right). The movement of the SHORTROOT (SHR) into adjacent cell
layer induces expression of SCARECROW (SCR) that is sequestering the SHR into
the nuclei of the SCR expressing cells. That allows differentiation of two cell types
(endodermis and cortex, respectively) from the common ancestor, the initial (stem)
cell. This mechanism will be discussed in more detail in later lessons.
Thus, different cellular environment leads to the nonequivalent differentiation of the
two daughter cells.
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
 Overview of development
 Asymmetrical cell division as a major force of the development
 Genetic integrity of the differentiated cells
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The experimental evidence for the genetic integrity of the differentiated cells (John
Gurdon, 40’s-50’s, Oxford).
The cells of differentiated tissue (e.g. epitelial cells in this case) have been used as
a donor of the genetic material (nuclei) for enucleated oocytes in amphibians
(Xenopus laevis). That allowed formation of complete tadpole.
However, the older (more differentiated) the donor cell is, the lower efficiency of that
process and the tadpoles often fail undergo normal morphogenesis. Similar
principles were employed for the mammals cloning (the Dolly sheep).
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Limbs regeneration in salamander
Another evidence of the genetic integrity during cell specialization, i.e. during cell
differentiation, is the opposite process, i.e. cell dedifferentiation and the ability of
regeneration in animals and particularly in plants.
In some animals, e.g. in some amphibians, there is high level of regeneration
developed. E.g. in newt that can regenerate iris cells into functional lens cells
(removed by trauma or experiment) or limbs regeneration in salamander (above).
The question remains, whether the new limb is being regenerated from the pool of
undifferentiated cells. However, there is considerable body of evidence suggesting
that at least a part of the limb is created via dedifferentiation and subsequent redifferentiation
of the cells of the connective tissue underlying the epidermis (limb
dermis). Similar principles seem to operate in plants (next slide).
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The ability of differentiated plant cells to regenerate the whole plant was
experimentally proven by Frederick Steward (50’s) at the Cornell University.
He managed to isolate phloem cells from the carrot and by the coconut milk
treatment (source of the plant hormone cytokinins) he regenerated the entire new
carrot plant.
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
 Overview of development
 Asymmetrical cell division as a major force of the development
 Genetic integrity of the differentiated cells
 Concept of stem cells
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The concept of stem cells.
Cells in the body undergo differentiation. During that process, they are losing the ability to form different cell
types and their developmental faith becomes more and more restricted (see the figure). Under normal
circumstances, this process is more less unidirectional, i.e. more differentiated cells cannot dedifferentiate.
The highest developmental potential has a zygote, that we call is totipotent, i.e. it can give rise to all cells in the
organism.
There are specialized cell types in the later development, called stem cells. These cells maintain their selfrenewal
capacity, providing thus a source of the cells at the certain differentiation level.
Embryonal stem cells are one of the most pluripotent stem cells know so far. However, recent findings suggest
a presence of stem cell populations also during later development in different tissue types.
For a long time, neural cells were believed to be non-renewable. However, recently, stem cells were identified
even in the neural tissues, e.g. in the bone marrow. In spite of there are still some unclarities in the experimental
data provided, the experiments with bone marrow cells tagged with GFP and injected into donor mice suggest
the possibility of the presence of stem cells in the neural tissue.
GFP labeled cells were injected in the mice mutants lacking the immune system (nude mutant). These cells not
only developed immune system, the GFP tagged cells were found in the brain and some of them were
producing the neuron-specific proteins suggesting that they acquired neuron identity.
There are also reports of the stem cells in the brain that have a glial appearance and might be reprogrammed to
form neurons.
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Outline of Lesson 1
 The course at a glance and literature
 Repetitorium of basic terms
 Major body directions
 Mitosis vs. Meiosis
 Overview of development
 Asymmetrical cell division as a major force of the development
 Genetic integrity of the differentiated cells
 Gametogenesis, fertilization and lineage tracing
 Concept of stem cells
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Primární
zárodečná buňka
Folikulární buňky na
povrchu vyvíjejících
se primordiálních
folikulů
Podpůrné buňky
The development starts before fertilization.
During gametogenesis, the gene set is reduced to the haploid level. Fusion of the
gametes allows formation of totipotent zygote after fertilization.
In most of the animals, there are common principles of gametogenesis, therefore,
we will discuss them in a general form at the outset. The individual differences
among different organisms will be discussed in separate lessons.
In animals, the primary (or primordial) germ cells are formed during development
and these cells are able to undergo meiosis to form female gametes, the eggs.
Ones the primordial cell reside in female gonad, they are called oogonia.
In humans, the oogonia form limited population that does not undergo mitosis, while
in frogs, the oogonia divide, allowing thus formation of many thousands of eggs
during annual breeding season. Thus, the frogs oogonia are real stem cells that is
not the case in humans. The other cell types are of supporting function.
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Zárodečný váček
Primární oocyt
Primární (primordiální) zárodečné
buňky
Primordial germ cells
Gonads
Genuine stem cells in
amphibians, but NOT in
humans
One of the hallmark of the oogenesis is an extensive growth. That allows stockpiling
of all material that will be necessary during embryogenesis. In contrast, there is no
growth (increase in the dry mass) during embryogenesis. The growth results from
both the biosynthetic activity of the oogonium and by the import of the yolk that is
produced in mother’s liver. DNA replication occurs.
Growing germ cell is now called primary oocyte, and the meiosis is started (enters
prophase I).
The ability to undergo meiosis is another important characteristics of germ cells.
This stage is characteristic by the formation of large nucleus that is called germinal
vesicle. The growth stage of the primary oocyte may take very long (4 months in
Xenopus, several weeks in humans). Often environmentally induced hormonal
signals might be necessary to continue the further development.
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Vacek, Embryologie (2006)
Section through the ovary of 2-months- (A) and 7-months-old (B) human foetus
Coelom epitel
Folicular cells at the
surface of primordial
foliculs
Oogonium
Oogonium
nucleus in
the
prophase I
Folicular
cells at the
surface of
primordial
foliculs
Nucleus of
the
primary
oocyte
Oocyte
Maturation
Inhibitor (OMI)
Stopping the cell cycle in
the leptotene of prophase
1 until the pubescence
Sections through the human foetal ovary at the age of 2 (A) and 7 month (B). The
invasion of coelom epitel in between the primary germ cells that will develop in to
follicles is apparent.
After coelom epithelial cell surround the oogonium, they will become follicular cells.
The nuclei of oogonia surrounded by the folicular cells will enter the prophase I and
further proceeding of the cell cycle is stopped at the leptotene of prophase I until
reaching the pubescence.
In some oocytes, this stage of development might take up to 30-40 years (from the
second month until climacterium) and during that stage, an intense synthesis of
RNA of all types and proteins take place.
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Sekundární oocyt
Polární tělísko
During cytokinesis, the centrosomes are formed close to the perimeter of the cell,
leading thus to assymetric cell division-formation of secondary oocyte (retains
about 95% of the cytoplasm and all the stockpiled material) and very small polar
body . The polar body might undergo second meiotic divisions, but in every case
later on degrades.
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Ootida (mateřská vaječná buňka)
Pronucleus
Second meiotic division of the secondary oocyte leads to the formation of another
polar body and the egg that might be called ootid.
The nucleus of the egg is called pronucleus. The completion of meiosis takes place
at the moment of fertilization and at a rapid pace, when the sperm cell is in the
cytoplasm of the egg.
More frequently, the primary oocyte will finish the first meiosis and enters the
prophase of the meiosis II, where it stops. Similarly to the previous case, the
hormonal signal or fertilization might induce further development.
The fertilization occurs in the stage of the arrested primary or secondary oocyte,
dependent on the species. In that case the fertilization is a “hurry-up” signal for the
finalization of meiosis. In the egg urchin, the meiosis is completed before
fertilization.
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Pigmentové granule
Samičí prvojádro
Žloutkové destičky
Kortikální granule
Kortex
Animální pól
Vegetální pól
The egg is a complete cell, although of exceptional size. The egg has a specific
intracellular structure that ensures that the cytoplasm is not mixed and progeny of
the egg mitosis after fertilization will “inherit” differentially composed cytoplasm. It is
important for the next development.
1. Pigment granules are located at the animal pole, while
2. Yolk platelets are located on the vegetative pole
3. The peripheral cytoplasm has different ultrastructure, especialy in terms of the
cytoskeleton organization and is called cortex
4. Cortex contains special abundant vesicles called cortical granules
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Ostrich’s egg…the largest cell in the world .
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vlásečníce Pojivová tkáň Bazální membrána
Leydigovy buňky
Primární spermiocyty Spermatida
Sekundární spermiocyty (Prespermatidy)
Spermiogonie
testosteron
Spermie
Semenotvorný kanálek
Seminiferous tubule
Produkce
Műllerovy
inhibiční látky
anti-Müllerian
hormone (AMH)
production
Kapacitace
Capacitation
Male germ cells in animals lye along the perimeter of the seminiferous tubules.
These are blind tubular structures, connected with other tubules into a common duct. Between tubules is
connective tissue that contains supporting cells. In mammals these are Leydig cells, producing testosteron.
The duct system is connected with secretion system of other glands, e.g. prostate that helps formation of
seminal fluid.
One of the most apparent differences in comparison to the female gametogenesis is the absence of stockpiling
of the cell material. The spermatic cells contain only few of the cytoplasm and are designed to deliver the
haploid nucleus to the egg cells.
The male primordial germ cells are real stem cells as they can undergo mitosis. Their progeny is called
sperrmatogonia and is located close to the perimeter of tubule, close to the Sertoli cells.
Sertoli cells nurture the developing sperm cells through the stages of spermatogenesis and produce several
important regulators, among them anti-Müllerian hormone (AMH) that is important during early stages of fetal
development (see later in Lesson 5). Translocation of germ cells from the base to the lumen of the seminiferous
tubules occurs by conformational changes in the lateral margins of the Sertoli cells.
Spermatogonia grow a bit, and form primary spermatocytes that undergo meiosis I.
That leads to formation of secondary spermatocytes .
Meiosis II leads to production of four spermatids.
In contrast to egg cell formation, all four spermatids survive. The spermatids are connected with cytoplasmic
bridges and are located in Sertori cells close to the lumen of the seminiferous tube, where they undergo
spermiogenesis, i.e. formation of spermatozoa (singular spermatozoon), commonly called sperm.
The process of spermiogenesis results in dramatic subcellular changes (see the slide # 40).
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Vacek, Embryologie (2006)
Spermiogonie Primární spermiocyty Sekundární spermiocyty (Prespermatidy) Spermatidy Spermie
Spermatogonium Primary spermatocyte Secondary spermatocyte Spermatid Spermatozoon
Overview of the meiotic division during human sperm development.
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Head
Hlavička spermie
Midpiece
Krček spermie
Tail
Bičík
Histones replaced
by protamines
During spermiogenesis, the most of the cytoplasm and cellular organelles are sloughed-off,
including cytoskeleton and vesicular and membraneous systems like the Golgi apparatus
and endoplasmic reticulum. Before its disappearance, Golgi apparatus participates in the
formation of acrosome, a new type of vesicle.
Spermatozoa are composed of the head, midpiece and tail.
In the head, there is nucleus (no nucleolus apparent), the histones disappear and are
replaced by the protamines, other basic proteins that facilitate dense packing of the
chromatins. At the tip, there is acrosome. In invertebrates, the chromatin is not
condensed and adjacent to the acrosome.
In the midpiece, there are located mitochondria that might fuse to form either one large
mitochondrion or few larger miotochondria. There are located also centrioles, from the
one located at the basal part of the midpiece emanate the microtubule array, the
flagellum that allows wave like motions that propel the sperm.
For the competence of the sperm to fertlize the egg it is necessary a maturation process,
called capacitation that occurs in the reproductive tracts via action of substances
produced by other glands.
Fertlization results into:
1. Formation of the diploid zygote via syngamy (fusion of haploid nuclei of gametes).
2. Activation of the mitosis. The interaction of sperm and egg leads to the
• Finalization of egg meiosis
• Activation of the mitosis after syngamy
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Fertilizační filament
Bindin
Aktivace
metabolismu spermií
The sperm is activated after its release from the male reproductive organs.
The mitochondria in the midpiece start respiration, allowing thus ATP production.
That activates movement of the sperm via sinusoid flagellum beating.
Egg might produce chemoatractive substances that direct the movement of sperms.
These substances may also further activate the spermatozoa.
A part of this activation in marine invertebrates is eversion of acrosome into a
specific membraneous structure, called acrosomal filament that contains
microfilaments (long fibrils of polymerised actin). However that is not the case in
mammals.
For the metabolic activation of the sperm, Ca2+ is necessary.
The interaction of the sperm with egg is species-specific. Bindins are specific
proteins involved in this interaction. The bindins of sperm in sea urchin interact with
specific receptors at the surface of the egg. Though bindins evolved only in
echinoderms, it is presumed that similar mechanisms are an important part of
reproductive isolation in general.
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Zona pellucida
Oolemma
Wikipedia
The interaction of the sperm acrosome with the cell membrane of unfertilized egg
leads to fast and dramatic changes in the egg membrane. These events were
studied in the sea urchin and seem to be of general validity among other animal
systems.
Among first events, release of the Ca2+ takes place (see later) and exocytosis of the
cortical granules occurs.
Vitelline membrane cosists of proteins and becames fertilization envelope upon
fertilization (see later).
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Change of the osmotic
pressure due to release of
the cortical granules
content (macromolecules)
and lifting of the vitelline
envelope.
The contents of the granules vary with the species, and are not fully understood.
The macromolecules released by the cortical granules lead to the increase of the
osmotic pressure and lifting of the vitelline envelope.
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Cortical reaction
• proteolytic enzymes
remove the sperm
receptors
• Changes in the vitelline
envelope mechanical
propeties
• Changes in the membrane
potential
Perivitelinní prostor
Avoiding of polyspermy
Macromolecules accumulate in the perivitelline space and integrate into the vitelline
envelope leading to the thickening of the membrane and forming of what is called
fertilization envelope.
Material in the cortical granules might contain proteolytical enzymes that remove the
egg receptors from the egg surface. Together with changes in the mechanical
features and composition of the vitelline envelope and finally changes in the
membrane potential (see later) these events prevent polyspermy (presence of
more sperm cells in the egg).
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karyogamie
(ve stádiu S fáze 1. mitózy)
karyogamy
(in the S stage of mitosis 1 )
Finally, the sperm cell fuses with the egg, releasing its content to the egg cytoplasm.
These events initiate finalization of meiosis, followed after karyogamy (i.e. fusion of
both male and female pronuclei) by the S-phase of the first mitotic division.
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There are numeral molecular events after fusion of the egg and sperm cells.
One of the earliest is the change in the egg cell membrane potential (from -70 mV to
approx. +20 mV, see the figure above).
How the ion channels are activated it is still unclear. Parameters of these early
changes in the mebrane potential differ among animals, but seem to be of general
importance.
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Preventing the increase of the
membrane potential
That immediate increase of the mebrane potential prevents polyspermy.
Preventing increase of the mebrane potential via decreaase of the Na+
concentration in the sea water decreases the polyspermy barrier allowing thus
increase of the percentage of polyspermic eggs (see above).
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Lectures of Michael Lehmann, Ph.D., University of Arkansas, USA
Further, one of the crucial events early after fusion with sperm and egg cells is a
transient wave of Ca2+.
Wave of calcium release (bright signal) across sea urchin eggs during fertilization.
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The Ca2+ is released and again sequestered very fast, leading to the formation of
the Ca2+ wave that spread across the surface of the egg.
Even more importantly, experimental release of Ca2+ from internal cell reservoirs
via addition of calcium ionofores leads to the activation of fertilization egg
responses, e.g. activation of respiration (graph A) or DNA synthesis (graph B).
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Activation of
respiration
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Activation of
DNA
synthesis
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trigger unknown, NO?
In spite of the molecular mechanisms leading to the activation of Ca2+ release are largely
unknown, the G-protein mediated signalling might be involved in those processes.
The release of Ca2+ might then serve as a signal for further cell processes.
Based on recent results, more signalling pathways seem to be involved in the regulation of
the fertilization egg response; phospholipase C is probably involved in Ca2+ release, but
not the changes in the membrane potential.
The identification of single receptor, responsible for the activation of the downstream
fertlization cascade is still not successful (it is not the receptor allowing the egg cell and
sperm initial interaction that is bindin in sea urchin, see previous slides).
Alternatively, the signalling molecule might be delivered via the sperm fusion directly to the
egg cell cytoplasm. One of the examples is import of NO-producing enzymes (nitric oxide
synthase) by the sperm cell. Microinjection of NO effects a fertilization reaction in the egg
cell. However, the role of NO synthesis in the Ca2+ release in unclear.
Also the role of Ca2+ in the regulation of the downstream events, e.g. activation of proton
pumps leading to extrusion of protons and increase in the intracellular pH, is unclear.
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See the video for the molecular mechanism of PLC signaling; SOCs: store-operated
channels (activated by ER-released Ca2+)
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pH-dependent initiation
of proteosynthesis from
the egg-deposited
mRNA
Fertilization activates translation of mRNAs that are included in the egg cell
cytoplasm. Fertilization also induces poly A polymerase that leads to the elongation
of the poly A tale of mRNAs. The functional meaning of the process is still not clear.
The kinetics of the poly-A end ellongation is different from the kinetics of the protein
accumulation. One of the probable explanation of the poly-A end elongation might
be an increase of the mRNA stability.
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Fertilization-initiated cleavage/rýhování vajíčka
Regular and stereotypical cell pattern in C. elegans
(558 embryonic and 9590 somatic cells)
Vital dye injection
Fertlization leads to the initiation of mitosis.
First several rounds of mitosis are not associated with growth, i.e. increase in the biomass of the egg. It is
simple several rounds of subsequent divisions of the egg into smaller cells. This process is called cleavage.
Several types of cleavage could be classified as based on the yolk type (its amount) and position in the egg
(central or lateral). If the yolk amount is large, it might lead to lateral allocation of the cytoplasm. Then, the
cleavage proceeds only in this small island of cytoplasm and is called meroblastic cleavage.
In animals, the pattern of cellular divisions during embryogenesis is more less stable. That allows, especially in
some models, e.g. Coenorhabditis, to follow the developmental fate of individual cells.
The cells in the Coenorhabditis embryo undergo completely regular and stereotype cell pattern generating 558
cells and after molting they contain exact number of somatic cells (9590 ) and large amount of germ cells.
This stereotypical cell pattern allows tracing of individual cell lines using e.g. injection of cell vital dies into
blastomere, allowing generation of the so called lineage diagram (see the figure above).
In case of Coenorhabditis, but also other organisms, including plants, the cell lineages might be manipulated by
laser ablation or isolation. Thus, as apparent from the figure, destroying the cell E that normally develops into
gut, leads into formation of gutless embryos. Similarly, isolated and cultured E cells differentiate into gut-like
cells.
In contrast, this is not true, if the cells are isolated at the early four-cell blastomere. However, presence of P2
cells in the cultivation media might overcome this defect, suggesting that the contact with P2 cells is important
for the formation of competent E cells from EMS mother cell and thus proper development of gut cell lineage.
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Key Concepts
 Asymmetric cell division and underlying molecular mechanisms are
the core problem of the developmental biology
 Genetic integrity is maintained during cell (de-)differentiation
 Gametes arise from a population of stem cells via differentiation and
reduction of the genome to the haploid level
 Formation of the oocyte places a huge demands on the organism
and stocked material allows the early development
 Fertilization restores the chromosome number and activates the
egg. The activated gametes reenter the cell cycle and trigger the
further development
 During animal development, cell lineages with predetermined
developmental fate are established in the early development,
which is not the case in the development of plants.
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Discussion