Revision of embryogenesis

Question 1

What are then stages leading to formation of the blastocyst?

Answer 1

1. ) Fertilization(fusion of egg and sperm)
2. )Cleavage (these cell divisions the large egg into a ball of many cells, there is no growth with these divisions)
3. )Blastocyst (composed of two cell populations, the trophoblast, which forms the placenta, and the inner cell mass, which forms the embryo proper)

Question 2

Describe the fate of a cell in mammalian embryos

Answer 2

• The blastocyst is composed of trophoblast(outer layer) and the inner cell mass(ICM)
• The blastocoel (fluid filled cavity) fills much of the space of the trophoblast
• The trophoblast forms the chorion (the embryo’s contribution to the PLACENTA)
• The ICM divides into the epiblast& hypoblast layers
• The hypoblast differentiates into Heuser’s membrane.
• The epiblast forms the amniotic membrane and the three germ layers
• All of these tissues are extraembryonic and will be discarded at birth

Question 3

What are the 3 germ layers?

Answer 3

• endoderm (innermost)
• mesoderm
• ectoderm

Question 4

Which layer is the source of embryonic stem cells for generation of cells/tissues from embryos?

Answer 4

The inner cell mass.

Question 5

What type of potency do mammalian blastomeres have?

Answer 5

Totipotency

Question 6

What is gastrulation?

Answer 6

The process of the embryo going from two layers to form three germ layers

Question 7

Which organs/cell types does each germ layer give rise to?

Answer 7

• Ectoderm: epidermis, nervous system
• Mesoderm: muscle, skeleton,dermis,kidney,blood
• Endoderm: Gut,liver,pancreas,lungs

Question 8

What is the mesoderm subdivided into and what organs/tissues arise from each region?

Answer 8

• Paraxial mesoderm: all skeletal muscle cells
• Intermediate mesoderm: most of the urogenital system
• Lateral plate mesoderm (theres a space between them splits into…
  1. )somatic mesoderm (outermost): connective tissue of body wall and limbs
  2. ) splanchnic mesoderm: mesothelial covering of the visceral organs

Question 9

What forms the coelum?

Answer 9

The space between the inner splanchnic mesoderm and the outer somatic mesoderm

Question 10

What do the first and last somites to be born form?

Answer 10

• The first somites will form the occipital bone at the base of the skull and cervical vertebrae of the neck
• The last to be born will form the vertebrae of the tail

Question 11

What different parts does a somite form?

Answer 11

• dermatome
• sclerotome
• myotome

Question 12

What is the neural crest?

Answer 12

• The 4th tissue/germ layer
• Forms at the lateral edges of the neural plate ,at the boundary with the epidermis
• crest cells delaminate from the neural tube and migrate away
• Neural crest cells are multipotent and give rise to the PNS,part of the heart and most of the face

Question 13

State and describe the name and presentation of three defective somite development & vertebrae malformations.

Answer 13

1. ) Spondylocostal dysostosis:
   - hemivertebrae
   - rib fusions
   - kyphoscoliosis
   - short vertebral column
2. ) Klippel-Feil Anomaly:
   - cervical and thoracic vertebrae
   - including vertebrae fusions
3. ) Alagille syndrome
   - butterfly vertebrae

Question 14

What may be the consequence of defects in neural crest cell development?

Answer 14

• Bilateral cleft lip

- Pierre-Robin sequence ( underdevelopment of the lower jaw)

Question 15

What is Hirschsprung’s disease?

Answer 15

• Caused by a failure of the vagal neural crest to migrate into posterior regions of the gut
• As a consequence, the bowels contents build up proximal to the affected region,causing it to expand,

Question 16

What is somitogenesis?

Answer 16

-Generates small regular epithelial balls called somites in the paraxial mesoderm of the embryo

Question 17

What does it mean when a cell is said to be ‘committed’?

Answer 17

• It’s fate cannot be changed by external changes.

- This happens as time progresses and the cell goes from totipotent-multipotent-committed.

Question 18

What can growth factors regulate?

Answer 18

• Cell proliferation
• cell survival
• cell migration
• epithelial-mesenchymal transformations
• cell fate
• Growth factors used in the embryo are also used post-natally to repair and maintain organs e.g skin & gut
• Abmormal regulations of these growth factor pathways can result in post-natal disease e.g cancer metastasis (an epithelial-mesenchymal transformation) and calcification of the arteries.

Question 19

What is a morphogen

Answer 19

• Some growth factors e.g SHH can act as morphogens

- specifies different cell fates at different doses e.g dorso-ventral patterning of neural tube by SHH

Question 20

What is the role of transcription factors?

Answer 20

-alter gene expression and can regulate cell fate

Question 21

During which period is the embryo most susceptible to teratogens and infectious agents?

Answer 21

• During the embryonic period

- The high risk during the embryonic period coincides with when most organs are developing

Question 22

Which pathogens are embryos vulnerable to?

Answer 22

-Most vulnerable in 1st trimester
Toxoplasma parasite
Other viruses e.g parvovirus
Rubella virus
Cytomegalovirus
Herpes simplex virus
TORCH acronym
HIV & syphilis& Zika