Download Fertilization. Embryonic development

Fertilization. Embryonic
development
Maria Kazakova, PhD
Fertilization
• Union of a spermatozoa and
an ovum
• Initiates human embryonic
development
• Determines the sex of the
offspring – in human
Fertilization - steps
Spermatozoa are not fully capable of fertilization
1. Capacitation - a process that strips the coat of glycoprotein
molecules of the spermatozoa
2. Acrosomal reaction – the cap of the acrosome breaks down
and releases hydrolytic enzymes – hyaluronidase, acrosin
3. The corona radiata has been penetrated
4. Contact with zona pellucida – species specific
5. Fusion of membranes
The completion of meiosis II
6. Fusion of pronuclei – 30 min., after the first step of fertilization
1. Capacitation - a process that strips the coat of
glycoprotein molecules of the spermatozoa
Sperms cannot fertilize oocytes when they are newly
ejaculated.
The process of capacitation takes 5-7 hours in humans
Pro-Acrosin (inactive) is converted to acrosin (active)
Capacitation occurs in the uterus and oviducts and is
facilitated by substances of the female genital tract.
o Capacitation alters two crucial aspects of sperm behavior:
- It greatly increases the motility of the flagellum
- It makes the sperm capable of undergoing the acrosome
reaction
2. Acrosomal reaction – the cap of the acrosome breaks down
and releases hydrolytic enzymes – hyaluronidase, acrosin
- Occurs when sperms come into contact with the corona radiata
of the oocyte
Acrosome:
- is a lysosomal-like compartment derived from the Golgi. It has
a low pH and contains soluble hydrolases (serine protease
acrosin)
3. The corona radiata has been penetrated
4. Contact with zona pellucida – species specific
Contact with zona pellucida – species
specific
Zona pellucida is composed of three
glycoproteins - ZP1, ZP2 and ZP3
Scanning electron micrograph of a human
sperm contacting a hamster egg.
The zona pellucida of the egg has been
removed. The ability of an individual’s
sperm to penetrate hamster eggs is used
as an assay of male fertility.
Penetration of more than 10-25% of the
eggs is considered to be normal.
Molecular Biology of the Cell (© Garland Science 2008)
5. Fusion of membranes
The completion of meiosis II
• The secondary oocyte – arrested in metaphase of the
2nd meiotic division
forms the mature egg (ovum) and second polar body.
Polyspermy barriers
1. A change in the egg plasma membrane
Sea urchin by
rapid depolarization
In mammalian eggs,
the mechanism is not known
2. Cortical reaction – releases various enzymes that change the
structure of the zona pellucida so that the sperm cannot bind to or
penetrate it.
6. Fusion of pronuclei – 30 min., after the first step of
fertilization - sea urchin
the chromosomes in each pronucleus condense
the pronuclear envelopes break down without
fusing together
the male and female chromosomes intermix in the
cytoplasm
form the metaphase of the first mitotic spindle
Release contents of the cortical
granules
Inactivation of ZP3
Cleave ZP2
Harding the zona pellucida
Molecular Biology of the Cell (© Garland Science 2008)
The coming together of the sperm and egg pronuclei
after mammalian fertilization
Molecular Biology of the Cell (© Garland Science 2008)
Embryonic development
Life cycles and the evolution of developmental patterns
Descriptive embryology - the idea of a generalizable life
cycle. Each animal, whether an earthworm, an
eagle, or a beagle, passes through similar stages of
development.
When does embryonic development begin?
The Stages of Animal Development
Four essential processes by which a multicellular organism is made:
1. Cell proliferation – producing many cells from one
2. Cell specialization - creating cells with different characteristics at different positions
3. Cell interaction - coordinating the behavior of one cell with that of its neighbors
4. Cell movement – rearranging the cells to form structured tissues and organs
The stages of development between fertilization and hatching
are collectively called embryogenesis.
1. Initiated by the fusion of genetic material from the two
gametes the sperm and the egg.
2. Fertilization, stimulates the egg to begin development.
3. Cleavage
Cleavage is a series of extremely rapid mitotic divisions
where in the enormous volume of zygote cytoplasm is
divided into numerous smaller cells. These cells are called
blastomeres, and by the end of cleavage, they generally form
a sphere known as a blastula.
A. The pattern of cleavage is influenced by the amount of yolk in
the egg. In eggs with less yolk, cleavages are equal, and the
resulting blastomeres are of similar size.
B. If the yolk is localized, such as in frog eggs, then cleavages are
unequal - the cells derived from the yolky region (the vegetal
pole) are larger than those derived from the region without yolk
(the animal pole).
4. Gastrulation – the series of extensive cell
rearrangements. The embryo contains three germ
layers: the ectoderm, the endoderm, and the
mesoderm.
5. Organogenesis - the cells interact with one another and
rearrange themselves to produce tissues and organs.
Principles of experimental embryology
I. Environmental Developmental Biology
1. Environmental sex determination
In the marine worm Bonellia viridis - is thus determined by
external, environmental factors (the presence or absence of
bonellin).
Larvae become:
males if they make physical contact with a female
and females if they end up on the bare sea floor
2. Adaptation of embryos and larvae to their environments
Phenotypic variations caused by environmental differences are
often called morphs.
European map butterfly, Araschnia levana, which has two
seasonal
phenotypes
The spring morph is bright orange with black spots
The summer form is mostly black with a white band
The change from spring to summer morph is controlled by
changes in both day length and temperature during
the larval period.
3. The developmental mechanics of cell specification
3.1. Specification - cells are capable to differentiate
autonomously when placed in a neutral environment such
as a petri dish or test tube.
3.2. Determination - cells are capable to autonomously
even when placed into another region of the embryo.
Three basic modes of commitment:
Autonomous
Conditional
Syncytial
Autonomous specification
Characteristic of most invertebrates.
Specification by differential acquisition of certain
cytoplasmic molecules present in the egg.
Blastomere fates are generally invariant.
Cell type specification precedes any large-scale
embryonic cell migration.
Produces "mosaic" ("determinative") development:
cells cannot change fate if a blastomere is lost.
Conditional specification
Characteristic of all vertebrates and few
invertebrates.
Specification by interactions between cells. Relative
positions are important.
Variable cleavages produce no invariant fate
assignments to cells.
Massive cell rearrangements and migrations precede
or accompany specification.
Capacity for "regulative" development: allows cells to
acquire different functions
Syncytial specification
Syncytium - a cytoplasm that contains many nuclei.
Characteristic of most insect classes.
Specification of body regions by interactions
between cytoplasmic regions prior to cellularization
of the blastoderm.
Variable cleavage produces no rigid cell fates for
particular nuclei.
After cellularization, conditional specification is most
often seen.
4. Morphogenesis and Cell Adhesion
There are two major types of cell arrangements in the embryo:
- epithelial cells, which are tightly connected to one another in
sheets or tubes
- mesenchymal cells, which are unconnected to one another and
which operate as independent units.
Morphogenesis is brought about through a limited repertoire of
variations in cellular processes:
1. the direction and number of cell divisions;
2. cell shape changes;
3. cell movement;
4. cell growth;
5. cell death;
6. changes in the composition of the cell membrane or secreted
products.
Morphogenesis and Cell Adhesion
Cadherins:
the major cell adhesion molecules
critical for establishing and maintaining intercellular
connections
crucial to the spatial segregation of cell types
the organization of animal form
Cadherins + other cadherins on adjacent cells =
Catenins
In vertebrate embryos, several major cadherin classes
have been identified: E-cadherin, P-cadherin, Ncadherin, EP-cadherin, Protocadherins