Embryology for summative Flashcards
Describe the process of fertilisation and the events of the first week
To form a zygote – the first stage of embryonic development – there needs to be a
union of a male and female gamete.
The male sex cell/gamete is the spermatozoa. The male testes produces 1500 sperm
per second / few million per day (sample shown on left)
Comparatively, females will produce only one oocyte from each month (Most of the
time a single oocyte is released during ovulation, however occasionally, ~1/100
cycles, two can be produced).
Spermatozoa have 3 main parts (sometimes the body and tail are considered as one
piece)
- Head
- Acrosome containing enzymes that break barrier
during penetration of ovum
- Condensed nucleus contains all the genetic materials.
- Middle piece/body containing the mitochondria to generate energy
for the sperm
- Tail – made up of the flagellum that propels the sperm forward.
The female ovum or oocyte has a typical cellular structure of a nucleus surrounded by
cytoplasm and cell membrane. However, in the female lifespan only ~400 (usually
less) oocytes are produced, therefore they require more protection than sperms. The
oocyte is surrounded by the transparent zona pellucidum and then corona radiata of
protective epithelial cells.
The ovum is 0.2 mm in diameter and this image on the right demonstrates the
proportionate size to the sperm.
There are 3 stages of fertilisation to produce a zygote.
1. sperm passing through the corona radiata
2. A single sperm will pass through the zona pellucida. Once this
happens the zp goes through a change/reaction to prevent other spermatozoa from
reaching the oocyte.
3. Once the spermatozoa reaches and combines with the oocyte the
plasma membranes breakdown two haploid nuclei fuse their genetic material to from
a single entity – the zygote.
The zygote stage does not last long. Cleavage occurs in the first few days resulting in
rapid mitotic cell division which produces daughter cells known as blastomeres.
Because this process occurs so rapidly is is all encased inside the ridged zona
pellucida. Therefore the blastomeres become smaller as they divide.
The process of cleavage occurs while moving along the uterine tube towards the
uterus.
This image provides an overview of this process.
During the first week the oocyte is released into the uterine tube. Fertilisation occurs
here in the uterine tube to form a zygote. Over the following week the fertilised egg
travels towards the uterus while undergoing cleavage. At around 3 days there are
around 14-16 daughter cells creating the appearance of a mulberry hence why it is
now called the morula stage.
At 4 days, spaces start to form between the cells eventually creating a cavity and
pushing the mass of cells to one side to create the inner cell mass – this is called the
blastocyst stage.
It is the blastocyst that will then implant into the endometrial lining of the uterus.
For implantation to occur effectively the blastocyst needs to separate or ‘hatch’ from
the zona pellucida.
In this image you can see the relatively transparent zona pellucida with the blastocyst
almost completely separated. You can now see some more specific groups of cells.
The outer cell mass surrounding the blastocyst is the trophoblast which will supply
the embryo with nutrients. The inner cell mass is the embryoblast which will give rise
to the embryo itself. The space shown here within the blastocyst is filled with fluid.
Describe the events of week 2
The ‘rule of twos’ can be used to describe the processes that occur in week two of
embryo development.
The first rule of two is that the outer cell mass – the trophoblast – divides into two
layers as the embryo implants into the endometrium.
- The cytotrophoblast can be seen surrounding the whole of the
trophoblast, this is the inner layer. The cytotrophoblast produces hCG as well as
differentiates into the syncytiotrophoblastic cells
- The syncytiotrophoblast is very invasive allowing it to move into the
uterine lining and pull the blastocyst into the uterine wall. As you will see shortly it is
also imported for the exchange of nutrients and waste between the mother and the
embryo.
The second rule of twos can also be observed in this image. As the blastocyst
implants into the wall of the uterus the embryoblast organizes itself into two layers of
cells.
- The epiblast which will become the ectoderm
- The hypoblast which will become the endoderm
Together these layers are named the bilaminar (meaning having two laminae or
plates) germ disk.
This new cavity forming at this point is the amnionic cavity.
In the image on the right you can see that once the blastocyst has implanted into the
uterus it is surrounded by the syncytiotrophoblast. In order to provide nutrients to
the developing embryo, fluid filled cavities form called trophoblastic lacuna.
In this image you can also see that another layer is forming around the primary yolk
sac (previously the blastocyst cavity) which is the extraembryonic mesoderm.
The third rule of twos refers to the formation of two new cavities.
- The amniotic cavity as seen in the previous slide which develops
between the epiblast and hypoblast
- The chorionic cavity which opens up within the newly formed
extraembryonic mesoderm
The embryo is now fully implanted and now has everything it needs to start
developing some of the more complex structures.
Describe the events of week 3
As we enter the third week the embryo starts to undergo a process called
gastrulation by which the bilaminar germ disk becomes a trilaminar disk.
This image shows the surrounding chorion (trophoblast and extraembryonic
mesoderm). Note that the extraembryonic mesoderm is surrounding the embryo,
amniotic cavity and yolk sac.
At this point the embryo can be orientated by the two membranes at the caudal and
cranial poles. The buccopharyngeal membrane willeventually become the mouth and
the cloacal membrane that will form the anus. On the caudal end of the bilaminar
disk a hairpin ridge forms, this is the primitive streak. At the start of this week the
streak will deepen at the cranial end to form the primitive node.
The naming of the
layers at this stage provide an indication of the future structures that they become.
Generally speaking, the ectoderm forms the covering of the body
(epidermis) and nervous system.
The endoderm (inner skin) forms the lining of the GIT and respiratory
systems.
The intra-embryonic mesoderm is formed between the two original germ layers from
the ectoderm.
As the primitive streak deepens the cells of the ectoderm layer migrate towards the
midline and then detach below the streak, spreading laterally. This new layer, the
mesoderm, will spread to fill the whole space except for in two places where the
endoderm and ectoderm remain in contact – prechordal plate (becomes the
buccopharyngeal membrane which breaks down at week 4 to establish
communication with amniotic cavity and gut tube) and cloacal plate replaced by
membrane at the caudal end.
The mesoderm gives us everything between the external and internal skin – becomes
the musculoskeletal system
Neurulation describes the formation of the brain and spinal chord.
Before we have formation of the neural tube, cells derived from the primitive node
migrate to the cranial end forming the notochord plate – which then folds into a rod
called the notochord. The notochord acts as a underlying guide for formation of the
neural tube as well as a longitudinal axis for the embryo. The notochord eventually
also creates the nuclei pulposi of the intervertebral disks of the vertebral column.
The region above notochord is the neuroectoderm and at ~19 days this area of the
ectoderm forms the neural plate. Over the next few days it will deepen to form the
neural groove which eventually closes into the neural tube. At this time the cranial
and caudal ends are still open but they will eventually close
Closing of the tube does not happen at once in the embryonic plate it starts in the
middle and then closes like a zipper moving towards the ends.
Pathological developmental defects:
Spinae bifida – occurs when there is incomplete closure of the tail end.
This is the more common
Anencephaly – occurs when there is incomplete closure of the cranial
end. This is very rare as these embryos don’t often survive
You can also see in the paraxial region, paired blocks of tissue called somites appear.
The first pair occurs ~20 days, continuing around 3 per day until 42-44 pairs are
present. They continue to differentiate as development continues but can be used as
a gauge for the age of embryo
In the 4 th week these cells differentiate to form dermomyotomes
Describe neurulation
Neurulation describes the formation of the brain and spinal chord.
Before we have formation of the neural tube, cells derived from the primitive node
migrate to the cranial end forming the notochord plate – which then folds into a rod
called the notochord. The notochord acts as a underlying guide for formation of the
neural tube as well as a longitudinal axis for the embryo. The notochord eventually
also creates the nuclei pulposi of the intervertebral disks of the vertebral column.
The region above notochord is the neuroectoderm and at ~19 days this area of the
ectoderm forms the neural plate. Over the next few days it will deepen to form the
neural groove which eventually closes into the neural tube. At this time the cranial
and caudal ends are still open but they will eventually close.
Closing of the tube does not happen at once in the embryonic plate it starts in the
middle and then closes like a zipper moving towards the ends.
Pathological developmental defects:
Spinae bifida – occurs when there is incomplete closure of the tail end.
This is the more common
Anencephaly – occurs when there is incomplete closure of the cranial
end. This is very rare as these embryos don’t often survive
You can also see in the paraxial region, paired blocks of tissue called somites appear.
The first pair occurs ~20 days, continuing around 3 per day until 42-44 pairs are
present. They continue to differentiate as development continues but can be used as
a gauge for the age of embryo
Describe gastrulation
the ectodermal layer. What wasn’t mentioned is that as the neural tube buds off from
the neural groove, the adjacent neural crest cells also break off to move around the
body to become sensory ganglia (as discussed in anatomy day 3) among other
structures.
The endoderm is innermost layer of the digestive system. When looking at the
process of embryonic folding which you were already introduced to in the lecture you
will have seen how this layer folds in to form the gut tube.
The mesoderm is most complex of these layers because forms so many distinct
tissues. At around day 17 you can already see that each side is divided into 3 parts.
- Immediately lateral to the central axis and neural tube, as you have
just seen, are the somites formed by the paraxial mesoderm. This goes on to form
much of the musculoskeletal system, maintaining that segmental arrangement of the
somites.
- The intermediate mesoderm will go on to develop into the urogenital
system which will be described later in the year.
- The lateral mesoderm is the most lateral and consists of 2 parts (this
is just an introduction to the terminology as it will be described further in relation to
embryology of the heart).
- Somatic (relating to body and is continuous with the
extraembryonic mesoderm around amnionic sac)
- Splanchnic (relating to viscera and is continuous with
the mesoderm around the yolk sac
Describe the events of week 4
Folding occurs in two directions
- longitudinal also called cephalocaudal. This type of folding occurs
around days 21-24 and is a result of rapid growth of the cranial end of neural tube to
form the brain.
- lateral also called transverse. Lateral folding occurs because of the
formation of the paraxial somites.
You can see that as the edge of disk moves closer together creating this C curve it
eventually causes the endoderm layer off to form the gut tube. Fusing of the lateral
edges occurs in opposition to that seen by the neural tube, here fusion starts at the
cephalic and caudal ends moving towards the umbilicus.
- note: chorionic villi - which increases the area of nutrient uptake from the mother’s bloodstream
Summarise the events of embryological development
- first week: fertilisation, zygote, cleavage, formation of blastocyst
- second week: formation of bilaminar germ disc, implantation, ‘rule of twos’, formation of chorionic villi
- third week: formation of trilaminar germ disc or gastrulation
- fourth week to eighth week: folding of embryo
Describe the types of tissue that arise from the three germ layers
ectoderm- covering of body (Epidermis) and nervous system
endoderm- forms lining of GIT and respiratory system
mesoderm- MSK system: bones, connective tissue, and muscle
Describe where the ectoderm and are derived
- epiblast leads to ectoderm
- hypoblast leads to endoderm
Epiblast and hypoblast constitute the bilaminar disc, which forms in week 2
