The Concepts of Language and Development Flashcards
What happens in the cleavage stage?
- The two haploid pronuclei fuse and the zygote enters mitosis
- We then get the formation of a 2 cell zygote and then a 4 cell zygote
What must occur before the development of the embryo proper?
The conceptus must first implant, then generate the “germ” disc. This takes ~10 days.
What is the morula?
Morula is the embryo when it has 12-16 cells after mitotic division from the 2 cell zygote.
What happens from a morula to a blastocyst?
- Embryo undergoes a process called compaction and cells start to become flatter and form an epithelial barrier
- This allows fluid to accumulate in the central region of the embryo and this region is known as the blastoceole and the embryo is now referred to as the blastocyst
What do the trophoblasts of the blastocyst take up the behaviour of?
Trophoblasts take up the behaviour of the epithelium.
Where do blastocysts implant and what process must occur in order for this to happen?
- Blastocyst implants on posterior aspect of uterine wall
- In order for this implantation, the zona pellucida needs to be removed by a process called embryo hatching
Describe the process of compaction.
Cells sort themselves – some remain in the exterior and some move to interior of the mass. The outside cells will acquire a strong sense of contact with neighbouring cells with one of their faces exposed to the external environment thus providing polarity: basolateral end (outer cell surface in contact with other cells) and an apical surface – not in contact with any cells. Inner cells are apolar – all in contact.
• Outer cells will become the trophoblast
• Inner cells will become the inner cell mass
At which point along the timeline is the embryo ready to implant into the uterine wall?
6/7 day stage. Blastocyst has formed.
To what two cell types will the inner cell mass give rise?
Hypoblast: will line the edge of blastocoele.
The epiblast: the rest of the cells.
These two groups of cells will be called the bilaminar germ disc.
The epiblast will arrange itself to form the amniotic cavity, which will fill with amniotic fluid.
How will the implantation site be sealed?
With a fibrin plug/
What is gastrulation?
The formation of ectoderm, endoderm and mesoderm from the bilaminar germ disc.
Describe the process of gastrulation?
On the caudal end of the bilaminar germ disc a groove forms towards the cranial end – primitive streak. At the end (halfwayish) a depression called the primitive node forms. This then travels towards the caudal end, thus creating a depression called the primitive groove. Process of invagination. Cells from the epiblast will migrate to the groove and spread down, displacing hypoblast cells. Mostly displaced by day 16. They will give rise to the definitive endoderm (yellow). After, some remaining ectoblast cells will migrate into the gap, forming the mesoderm (red) layer the original epiblast layer will form the ectoderm. (most exterior, distal layer). Epiblast cells no longer migrate towards the primitive streak.
Describe the first steps of neurulation as far as the ectoderm is concerned?
Ectoderm:
- Neural plate forms (day 15)
- Primitive streak extends along the epiblast – gastrulation occurs as this happens.
- Once the primitive streak reaches the other end, it starts regressing
- As it regresses it lays down a rod like structure just underneath the ectoderm = notochord
- Notochord cells excrete a number of extracellular molecules that will instruct the ectoderm just on top of it to become neural tissue.
- Notochord extends posteriorly as the embryo grows –> neural plate forms
To what will the neural plate give rise?
It will give rise to the neural tube – as it folds it becomes buried inside the ectoderm. Neural tube will start folding over itself, the edges will detach from surrounding ectoderm and come together.
What is the first clear manifestation of the developing central nervous system?
Neural fold formation
Does the neural fold formation occur all at once?
No, it starts at the edge between the hindbrain and the spinal cord, and then it will progress anteriorly and posteriorly until all of it is folded.
With what other shape-forming (morphogenetic) processes is the process of neurulation concomitant?
Others including gut formation and body folding (silk purse model).
In which 2 planes does Folding occur?
Horizontal and Median. Both folding processes occur simultaneously resulting in rapid development.
What does Folding result in?
- Septum and heart move from margin to centre
- Yolk sac, allantois and stalk make umbilical cord.
- Prochordal and cloacal plates delimit gut tube.
Overall: formation of gut and umbilical tube.
What does horizontal plane folding result in?
Formation of 2 lateral folds.
What does median plane folding result in?
Formation of cranial and caudal folds. As folding continues the endoderm moves towards the midline and fuses, incorporating the dorsal part of the yolk sac to form the primitive gut tube.
What does the primitive gut tube differentiate into?
This primitive gut tube differentiates into 3 parts: the foregut (cranial end), the midgut ~(remains attached to the yolk sac until the 5th week of development) and the hindgut (caudal end)
As folding continues what happens to the yolk sac?
The yolk sac narrows into a stalk called the vitelline duct
When will the cloacal membrane rupture?
It will rupture in the 7th week of development to form the urogenital and anal openings.
