The Concepts of Language and Development

Question 1

What happens in the cleavage stage?

Answer 1

• 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

Question 2

What must occur before the development of the embryo proper?

Answer 2

The conceptus must first implant, then generate the “germ” disc. This takes ~10 days.

Question 3

What is the morula?

Answer 3

Morula is the embryo when it has 12-16 cells after mitotic division from the 2 cell zygote.

Question 4

What happens from a morula to a blastocyst?

Answer 4

• 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

Question 5

What do the trophoblasts of the blastocyst take up the behaviour of?

Answer 5

Trophoblasts take up the behaviour of the epithelium.

Question 6

Where do blastocysts implant and what process must occur in order for this to happen?

Answer 6

• 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

Question 7

Describe the process of compaction.

Answer 7

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

Question 8

At which point along the timeline is the embryo ready to implant into the uterine wall?

Answer 8

6/7 day stage. Blastocyst has formed.

Question 9

To what two cell types will the inner cell mass give rise?

Answer 9

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.

Question 10

How will the implantation site be sealed?

Answer 10

With a fibrin plug/

Question 11

What is gastrulation?

Answer 11

The formation of ectoderm, endoderm and mesoderm from the bilaminar germ disc.

Question 12

Describe the process of gastrulation?

Answer 12

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.

Question 13

Describe the first steps of neurulation as far as the ectoderm is concerned?

Answer 13

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

Question 14

To what will the neural plate give rise?

Answer 14

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.

Question 15

What is the first clear manifestation of the developing central nervous system?

Answer 15

Neural fold formation

Question 16

Does the neural fold formation occur all at once?

Answer 16

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.

Question 17

With what other shape-forming (morphogenetic) processes is the process of neurulation concomitant?

Answer 17

Others including gut formation and body folding (silk purse model).

Question 18

In which 2 planes does Folding occur?

Answer 18

Horizontal and Median. Both folding processes occur simultaneously resulting in rapid development.

Question 19

What does Folding result in?

Answer 19

1. Septum and heart move from margin to centre
2. Yolk sac, allantois and stalk make umbilical cord.
3. Prochordal and cloacal plates delimit gut tube.

Overall: formation of gut and umbilical tube.

Question 20

What does horizontal plane folding result in?

Answer 20

Formation of 2 lateral folds.

Question 21

What does median plane folding result in?

Answer 21

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.

Question 22

What does the primitive gut tube differentiate into?

Answer 22

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)

Question 23

As folding continues what happens to the yolk sac?

Answer 23

The yolk sac narrows into a stalk called the vitelline duct

Question 24

When will the cloacal membrane rupture?

Answer 24

It will rupture in the 7th week of development to form the urogenital and anal openings.

Question 25

What does organogenesis involve?

Answer 25

• Differentiation of somitic derivatives
• Development of sensory organs
• Limb formation
• Formation of face structures
• Formation of genital structures

Question 26

If mistakes in embryogenesis occur within the first 2 weeks of development what will happen?

Answer 26

Death of embryo, spontaneous abortion.

Question 27

If mistakes in the first 3-8 weeks of embryogenesis what will happen?

Answer 27

severe malformations. Often multiple organs affected at once.

Question 28

If mistakes in the first 9-38 weeks of embryogenesis what will happen?

Answer 28

This will affect function mostly, deleterious for life. All major malformations will occur when genes involving the earlier processes are affected (embryonic period) or when environmentally insults occur at these stages.