Lecture 6: Fertilization and Gastrulation

Question 1

What are the order of structures of the Human Ovum from superficial to deep?

Answer 1

1. Corona Radiata (superficial) –> follicular cells
2. Zona Pellucida
3. Perivitelline Space
4. Plasma Membrane
5. Cytoplasm
6. Nucleus

Question 2

Where does fertilization of the egg take place at?

Answer 2

Distal 2/3 of the ampulla (uterine tube)

Question 3

Fertilization Step 1: Capacitation

Answer 3

• sperm “mature” and prepares enzymes for fertilization (takes 7 hrs as sperm travels)
• uses hyaluronidase to move through corona radiata

Question 4

Fertilization Step 2: Penetration

Answer 4

• sperm uses acrosin to errode zona pellucida

- zona reaction: blocks polyspermy by freezing meshwork of zona pellucida

Question 5

Fertilization Step 3: Fusion

Answer 5

• plasma membranes of oocyte and sperm merge

- head/tail of sperm enter oocyte cytoplasm

Question 6

Fertilization Step 4:

Answer 6

• complete second mitotic division

- generation of male pronucleus and fusion w/female pronucleus

Question 7

How long does fertilization usually take?

Answer 7

24 hrs

• 46 chromosomes
• determination of sex (X or Y)
• metabolic activation of oocyte (initiation of cleavage)

Question 8

Cleavage: what is it and what is generated?

Answer 8

• increase in cell number and decrease in cell size
• 2 cell stage (blastomeres) –> 4 cell stage –> 8 cell stage –> MORULA (Day 3 after fertilization)
• takes place 30 hrs after fertilization

Question 9

What day does the Morula reach the uterus?

Answer 9

Day 4

Question 10

Blastocyst: What is happening from Day 5-Day 7?

Answer 10

Day 5: inner cell mass (embryonic stem cells), trophoblast outer layer, degenerating zona pellucida

Day 6: implantation in uterus

Day 7: zona pellucida gone (“embryo hatching”), cells get bigger

Question 11

What is the Trophoblast and what two structures does it give rise to?

Answer 11

• trophoblast secretates Early Pregnancy Factor (no rejection)
  1. Cytotrophoblast: stem cell layer (mitotically active)
  2. Syncytiotrophoblast: responsible for implantation and initial signal of pregnancy (hCG) –> see on preg. test

Question 12

Hydatidiform Mole (Complete vs Partial) and symptoms

Answer 12

• abnormal trophoblastic prolif., excessive hCG prod.
  1. Complete: fert. of empty oocyte, duplication of sperm OR fert. of empty oocyte by TWO sperm
  2. Partial: fert. of normal oocyte by TWO spem
  symptoms: vaginal bleed, pelvic pressure, morning sickness

Question 13

What happens in Week 1, Week 2, and Week 3?

Answer 13

Week 1: development of inner cell mass (1 layer)

Week 2: development of epiblast and hypoblast (2 layer)

Week 3: development of ectoderm, mesoderm, endoderm

Question 14

Week 2 structures

Answer 14

Epiblast –> ectoderm, amnion, amniotic cavity

Hypoblast –> extraembryonic endoderm

Question 15

What does the Hypoblast help develop? (3)

Answer 15

1. Prechordal plate = future mouth
2. Primary/Secondary Yolk Sac (umbilical vesicle)
   
   • primary yolk sac degrades
3. Extraembryonic mesoderm

Question 16

What is primitive blood and where does it form?

Answer 16

• extraembryonic splanchnic mesoderm

- forms in wall of yolk sac

Question 17

Placenta Previa

Answer 17

• implant close to interal os of cervix
• vaginal bleeding beyond 20 weeks of gestation
• DO NOT PALPATE –> can cause severe hemorrhage

Question 18

What structures do the ectoderm, mesoderm, and endoderm form?

Answer 18

ectoderm: skin, hair, CNS
mesoderm: muscle, bone, blood, spleen
endoderm: GI tract, bladder, liver, lungs

Question 19

What are the possible causes of Sacral-coccygeal and Oropharyngeal Teratomas?

Answer 19

1. persistence of the primitive streak

2. abnormal movement of primordial germ cells

Question 20

What is the Notochord and why is it important?

Answer 20

• midline that extends from primitive node to prechordal plate

1. template for vertebral column
2. induces neural plate (SONIC HEDGEHOG)

Question 21

What are Chordomas?

Answer 21

• rare, slow growing aggressive neoplasms of bone
• from remnants of NOTOCHORD
• in spheno-occipital region of skull, sacral region

Question 22

What is the allantois?

Answer 22

• posterior midline (ENDODERM)
• template for umbilical arteries and veins (gives position where they will form around)
• becomes urachus

Question 23

Paraxial, Intermediate, and Lateral Plate Mesoderm

Answer 23

Paraxial - somites (muscle, vertebral, dermis)

Intermediate - urogential (kidneys and gonads)

Lateral Plate - CT (blood, lymph, mesenteries, cardio)

1. somatic layer (nxt to ectoderm)
2. splanchnic layer (nxt to endoderm –> viscera)