THE CONCEPTS AND LANGUAGE OF DEVELOPMENT Flashcards
Define Embryology
Embryology is the branch of medicine concerned with the study of embryos and their development
What is the first stage in the development of a foetus?
The process of the egg and sperm coming together.
Identify 4 key features of a fertilised egg
There are a few identifiable features of the fertilised oocyte:
Zona Pellucida (Bright ring) – this is glycoprotein material
Within the zona pellucida is the zygote with two pronuclei (with one haploid nucleus from the male and female respectively). They are yet to fuse.
The perivitelline space – a fluid filled space that surrounds the embryo
Polar bodies in the perivitelline space (either one or two bodies)
Where do polar bodies come from?
They contain a portion of female genetic material. Females generate all of their germ cells prior to birth, where they form one oocyte.
A germ cell will duplicate its nuclear DNA, they briefly enter meiosis but then arrest after prophase 1 and will remain in this state into adult life.
When a single oocyte is chosen it will then finish meiosis.
Once meiosis continues the first meiotic division occurs to produce two cells that are unequal. Half of the genetic material is extruded into a polar body which will be found in the perivitelline body (the first polar body).
The selected oocytes will arrest in metaphase 2
In order for meiosis to now resume the oocyte must be fertilised.
Second meiotic division then occurs and another polar body will be extruded
Learn the summary diagram of when events occur during embryology
On image
What is cleavage and describe the phases of it?
Cleavage stages (process of splitting into two cells without growth)
The pronuclei first fuse and the zygote will under mitosis.
Approximately thirty hours after fertilisation, the oocyte splits into two cells of equal size called blastomeres.
This will produce 2 cells and divide again to give 4 cells
All of this is contained in the zona pellucida, so the cells are getting smaller
Describe how the blastocyst will form the morula and attachment to the endometrium of the uterus
After 3 more divisions, there are 12-16 cells. At this point, the group of cells is referred to as the morula. They have divided asyncrhonly and are all loosely attached to each other.
It will then undergo compaction where the cells on the outer part of the embryo will become flatter and form an epithelial barrier (with cell junctions between).
This allows for fluid to enter the central region of the embryo which is called the blastocoel and the embryo is now called a blastocyst. It consists of two cell types:
a. Outer cell mass (trophoblast) – contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta. Inner cell mass (embryoblast) – responsible for the formation of the embryo itself.
N.B this is happening in the oviduct and anything after cleavage is called blastulation
This zona pellucida will then be dissolved by secretory enzymes from the embryo, the blastocyst will then hatch out around 6/7 days. This degrades the protein coat, the blastocyte then hatches out.
The trophoblasts have also differentiated to allow the blastocyst to implant into the uterine wall.
During the second week, the trophoblast and embryoblast divide into increasingly specialised cell types. The trophoblast divides into the syncytiotrophoblast and cytotrophoblast. The embryoblast divides into the epiblast and hypoblast, forming a two-layered structure; the bilaminar disk. The amniotic cavity forms within the epiblast.
Describe the process of implantation
After the initial rounds of cellular divisions, the embryo must implant into the endometrium of the uterus.
During this process, the syncytiotrophoblast becomes continuous with the uterus – such that maternal blood vessels (known as sinusoids) invade the spaces within the syncytiotrophoblast (known as lacunae). At this point, uteroplacental circulation has begun; and further embryonic development can occur.
What makes up the bilaminar germ disk?
The cells of the inner cell mass have also started to proliferate which have formed a bilaminar germ disk (a two layered embryonic disk). This consists of the epiblast cells and hypoblast cells.
Summarise gastrulation
During the 3rd week of embryonic development, the cells of the bilaminar disk (epiblast and hypoblast) undergo a specialised process called gastrulation. During this time, the two cell layers become three germ layers and the bodily axes observed in the mature adult are created.
It is a process of cellular rearrangement involving migration, invagination and differentiation of the epiblast, largely orchestrated by the primitive streak. This is a groove in the midline of the epiblast which appears during the third week. Within the primitive streak lies a primitive node at the cranial end, and within the primitive node lies the primitive pit.
Gastrulation – what happens to the bilaminar germ disk?
Cells of the epiblast layer break off and migrate toward the primitive pit. Here, they detach and penetrate through the epiblast layer to form three new germ cell layers:
Endoderm – formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells. (Lower)
Mesoderm – formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm. (middle)
Ectoderm – formed by the epiblast cells that remain in position. (Top)
What will the bilaminar germ disk form?
The epiblast cells will then differentiate into endoderm cells which will then replace the hypoblast layer.
Epiblast cells also forms the mesoderm. Epiblast cells will ingress and involute to form a 3 layered embryo
On the top is ectoderm (cells from the epiblast that didn’t differentiate)
Cells of the bilaminar germ disk will migrate anteriorly, posteriorly and laterally to form other tissues. These are shown by the arrows on the epiblast
What is Hensons node?
Hensons node is where gastrulation is being initiated and acts as a signalling centre by secreting growth factors that cause changes to the surrounding cells
Summarise neurulation
In the mesoderm we have a further differentiation of cells in the middle to form a chord known as notochord (beneath of the primitive streak). Its purpose is mainly in neurulation
It induces a thickening change in the ectoderm above it called the neural plate
The neural plate forms a tube and zips up it forms the neural tube, cells also invade the mesoderm from the ectoderm known as neural crest cells.
Describe what can be seen at day 16 and 20
Day 16 – the embryo will continue to divide and fold, lots of proliferation is going on but differs between tissues. This shows a sagittal section through the embryo: we have the cranial and caudal on the left and right. It is folding towards the midline from both ends.
On the transverse section we can also see lateral folding.
Folding brings the heart tissue inwards,
Endoderm will form the lining of the gut
Here we looking at a top-down view of the embryo with cranial towards the top and caudal towards the bottom.
The pink structure is a developing nervous system, this sheet of cells will role to form the neural tube and then fuse.
At day 22 the part of the neural tube has fused together to meet in the middle.
In a cross section we have somites
The grey is the notochord ventral to the neural tube
Remember the anterior and posterior neuropore are exposed due to closure of the neural tube
