Embryology

Question 1

Define Embryology

Answer 1

The process by which a multicellular organism arises from a single cell.

Question 2

Describe what occurs in these types of basic cell movement:

• Invagination
• Evagination
• Involution
• Ingression
• Delamination
• Epibody
• Convergence
• Extension
• Cavitation

Answer 2

• Folding of a tissue
• Folding outwards
• Spreading of cells along the inner surface of an outer layer of cells
• Small cell clusters invading into the embryo
• Cells leave epithelial level and split layer into 2
• Cells spread on outside of embryo opposite of involution
• Cells move to central point
• Convergence cells form elongated structure
• Cavities form

Question 3

Define these different cell stages:

1) Oocyte
2) Ootid
3) Zygote
4) Morula
5) Blastomere
6) Blastocyst

Answer 3

1) An immature ovum
2) After fertilisation when the cell is not quite fused
3) The cell as a result of fertilisation
4) The embryo as a cluster of cells
5) The cleavage zygote/totipotent embryonic stem cells (2,4,8)
6) A thin-walled hollow structure that contains a cluster of cells called the inner cell mass from which the embryo arises.

Question 4

What are the 3 parts of the blastocyst?

Answer 4

• The inner cell mass that forms the embryo
• The trophoblast which forms the placenta and fatal membranes
• The blastocoele which is the fluid filled cavity

Question 5

What is gastrulation?

Answer 5

The process whereby the 3 primary germ layers are established in the embryo:

• Endoderm
• Mesoderm
• Ectoderm
• This occurs between implantation and organogenesis.
• The basic body plan and the primary body axes are established.

Question 6

What do the epiblast and hypoblast become in the trilaminar embryonic disk?

Answer 6

The cells of the inner cell mass differentiate into the hypoblast (small cuboidal) epiblast (high columnar):

• Epiblast- Embryo
• Hypoblast- Extra embryonic tissues such as the yolk sac membrane

Question 7

What are the 3 germ layers of the epiblast derive into?

Answer 7

Ectoderm- skin epidermis and nervous system
Mesoderm- muscle, bones, blood, blood vessels, kidneys, gonads, skin dermis, connective tissue, skeleton, muscles
Ectoderm- Gut, liver, lungs, pancreas

Question 8

How is the amniotic cavity created?

Answer 8

Epiblast cell death

Question 9

How are each of the 3 germ layers created during the formation of the primitive streak?

Answer 9

Some displace hypoblast - endoderm
Some lie between epiblast and endoderm - mesoderm
Cells remaining in epiblast - ectoderm

Question 10

Define neurulation

Answer 10

It is the formation of neural tube preceded by formation of notochord:
- Extended column of cells ventral to central nervous system which induces the formation of the neural tube.

Question 11

How is the notochord formed?

Answer 11

Prenotochordal cells invaginate & move cranially to form the notochord.

Question 12

Describe the process of neurulation

Answer 12

Firstly, the notochord induces overlying ectoderm to thicken & form neural plate. Then there’s an approach of the neural fold which hinges at the neural groove and fuse together (beginning of the spinal cord to brain) It zips up to form a tube which then seals at the anterior and posterior ends.

Question 13

What dietary deficiency has been associated with abnormal neural tube development?

Answer 13

Folic acid.

Question 14

What are the 2 types of defects related to failure of neural tube closure?

Answer 14

1) Anecephaly: failure in cranial region

2) Spina bifida: failure anywhere else - usually lumbosacral region

Question 15

What are Somites?

Answer 15

• Derived from paraxial mesoderm
• Lateral to neural tube & notochord
• Transient structures formed cranio-caudally
• Essential for segmentation
• Will form skeletal muscle, cartilage, tendons, dermis of back, meninges of spinal cord, intervertebral blood vessels

Question 16

Describe the differentiation of somites.

Answer 16

Somites contain multipoint cells, location within the somite determines fate, via factors from surrounding tissues. Chemical signals act on somites and then the somites differentiate into 3 separate layers:

• Dermatome
• Myotome
• Sclerotome

Question 17

Describe the formation of vertebrae.

Answer 17

Sclerotome cells attracted to notochord & neural tube region form vertebral bodies.
Each vertebral body develops from the caudal part of one sclerotome & cranial portion of the next.
Sclerotome cells which cover the neural tube form neural arches, each arch fuses with corresponding vertebral body.
- Cells passing ventro-laterally develop into costal processes & ribs.
- Cells surround notochord.
-Notochord regresses in the region of the vertebral bodies, but persists in the region of the intervertebral discs, forming the nucleus pulpous.
- Vertebrae formed by chondrification of sclerotomal mesenchyme.

Question 18

Describe how limb buds form.

Answer 18

It begins with proliferation of skeletal precursors from lateral plate & muscle precursors from somites.
Cells accumulate under the ectoderm creating a bulge = the limb bud.

Question 19

How do limb buds grow?

Answer 19

Thickening of the ectoderm at tip of limb bud = apical ectodermal ridge (AER) :
- induced by mesenchyme cells.
- is a major signalling centre.
- is essential for sustained outgrowth & limb development.
Progress zone:
- distal part of mesenchyme, extends the limb bud by proliferation.

As bud grows, cells differentiate & cartilaginous structures appear, proximal structures develop first.

Question 20

What specifies individual digits & gives them their identities?

Answer 20

Organiser = small block of mesoderm: “Zone of polarising activity “
Signalling molecule = Sonic hedgehog

Question 21

What is endochondral ossification?

Answer 21

• Formation of cartilage tissue from aggregated mesenchymal cells & subsequent replacement by bone.
• Responsible for the majority of fetal skeletal growth.
• Characteristic of vertebrae, ribs & limbs.

Question 22

Describe the mechanism of formation of synovial joints in a developing limb.

Answer 22

The synovial joints will form between the adjacent cartilage models, in an area called the joint interzone. Cells at the center of this interzone region undergo cell death to form the joint cavity, while surrounding mesenchyme cells will form the articular capsule and supporting ligaments. The process of endochondral ossification, which converts the cartilage models into bone, begins by the twelfth week of embryonic development.