Embryonic Development in General

Question 1

What happens before implantation?

Answer 1

• Day 1 - sperm penetration –> then 2 nuclei fuse w/in 24 hrs
• Many divisions w/o inc size
  • Blastomere up until 8 cells (totipotent)
  • Morula - differentiated cells
  • Blastocyst - embryoblasts, trophoblasts and inner blastocele cavity

-Must also hatch from zona pellucida

Question 2

What facilitates implantation?

Answer 2

• Epidermal growth factor (EGF) on blastocyst binding heparin sulfate on endometrium wall
• Endometrial glands release growth factors and cytokines –> stromal cel COX2 –> prostaglandin spike
• Hormones - estrogen and progesterone

Question 3

5 Main Events of Week 2

Answer 3

• 2 trophoblast layers (around whole border)
  
  • Cytotrophoblasts - inner layer
  • Syncytiotrophoblasts
• 2 embryoblast germ layers
  
  • Hypoblast layer - faces into blastocele cavity
  • Cytoblast - faces outer trophoblasts
• 2 cavities
  
  • Amniotic Cavity - b/n cytoblast cells of embryoblast and outer layer of trophoblasts
  • Chorionic Cavity - b/n 2 layers of extraembryonic mesoderm
• 2 extraembryonic mesoderm layers (extraembryonic mesoderm is area b/n hypoblast layer and outer trophoblast layer) - splits in 2 to form chorionic cavity
• 2 remodelings of blastocele cavity (one lined by hypoblasts)
  
  • Primary yolk sac pinched into 2 cavities - definitive yolk sac and remnant of primary yolk sac

Question 4

Process of Gastrulation

Answer 4

• Primitive streak w/ primitive node (gathering of cells at end of streak), primitive pit (goes down from node into layers), primitive ridges (raised cells on ea side of streak) and primitive grooves
• Cells become migratory and move thru pitt to form 3 layers (ALL 3 LAYERS COME FROM EPIBLAST)
• 1- Original epiblast layer becomes ectoderm
• 2- Middle layer of cells that migrated below becomes mesoderm
• 3- Bottom layer of cells that migrated below and pushed hypoblast away becomes endoderm

Question 5

Ectoderm Derivatives

Answer 5

• Epidermis + nails, hair, sweat glands + tooth enamel
• Forward part of oral / nasal cavity; lens of eye; lining of internal and external ear, nose, sinuses, mouth and anus
• Pituitary gland (ant and post)
• CNS and PNS
• Mammary glands

Question 6

Mesoderm Derivatives

Answer 6

• Skeletal muscles, skeletone, dermis
• Kidneys + reproductive system
• Heart, blood vessels, blood cells, spleen, lymph tissue

Question 7

Endoderm Derivatives

Answer 7

• Digestive tract, tongue, lungs, liver, pancreas, tonsils
• Branchial arches –> thyroid, parathyroid, thymus
• Cloaca/allantois –> bladder, urethra

Question 8

Neuralation

Answer 8

WEEKS 3 and 4

• 50% of cells in ectoderm become neural plate; notochord induces thickening of the ectoderm cranial to the primitive node
• Neural plate becomes wider cranially –> brain
• Neural plate stay narrow caudally –> SC
• Neural plate thickens –> neural folds/ridges –> closes to form neural tube (closes in cervical area first then cont closing both cranially and caudally) –> eventually closed off at both ends
• Failure at cranial end - anencephaly
• Failure at caudal end - spina bifida

Question 9

Where do neural crest cells originate and what do they become?

Answer 9

Neural crest cells (at highest pt of neural ridges)

• become migratory –> peripheral ganglion, melanocytes, facial cartilage, dentin of teeth, calcitonin cells of thyroid, adrenal gland, Schwann cells, outflow septum of heart

Question 10

3 Components of Mesoderm

Answer 10

1 - Somites
- Lateral swellings on both sides of midline in symmetrical pairs –> axial skeleton (inc vertebrae), muscles, dermis, vessel endothelium

2- Intermediate Mesoderm - urogenital

3 - Lateral Plate Mesoderm –> outer parietal and inner visceral layers which line body cavities
***B/n these 2 layers is the intraembryonic coelmic cavity which eventually becomes pleural, pericardial, abdominal and peritoneal cavities

Question 11

When is the embryo most vulnerable?

Answer 11

Weeks 3-8 most vulnerable to to congenital anomalies

Except CNS - sensitive in wks 8-15 too

Anything in first 2 wks can be repaired by embryo or results in spontaneous miscarriage