L26 – Fertilization & Post-implantation Development

Question 1

Describe the origin and migration of the primordial germ cells?

Answer 1

Origin: junction between embryo and yolk sac

Day 24: migrate from yolk sac/ allantois along mesentery to genital ridge medial to mesonephros&raquo_space; embed in genital cords

Question 2

Compare the pairs of chromosomes and chromatids in Prophase I, Prophase II and in gametes?

Answer 2

Prophase I = 2n, 4c

Prophase II = 1n, 2c

Gametes = 1n, 1c (haploid)

Question 3

Define the number of chromosomes and chromatids of primordial germ cells, oogonia, primary oocyte, secondary oocyte and mature oocyte?

Answer 3

Primordial germ cell = 2N,2c

Oogonia = 2N, 2c

Oogonia > Primary oocyte arrested at Prophase I = 2N, 4C

Secondary oocyte arrested at metaphase II = 1N, 2C + 1st polar body

Fertilization completes meiosis II = Mature oocyte (1N,1C) + second polar body

Question 4

Describe the outermost layer of the mature egg and how sperm penetrates?

Answer 4

Corona radiata:
- High intercellular matrix of protein and carbohydrate (hyaluronic acid)

Sperm penetration by Hyaluronidase at sperm head + swimming movement

Question 5

Describe how sperm binds to Zona Pellucida?

Answer 5

Surface molecule (galactosyl transferase) on sperm head bind to receptor:

O-linked oligosaccharides attached to the polypeptide core of ZP3 molecule in Zona pelludicda

Question 6

Structure of Zona pellucida?

Answer 6

3 glycoproteins: ZP1, ZP2, ZP3

ZP2,ZP3 polymerize into long filament, cross-link by ZP1

Question 7

List the 2 reactions after sperm binds to Zona pellucida?

Answer 7

• Acrosomal reaction in spermatozoon

- Cortical reaction throughout egg

Question 8

Describe the Acrosomal reaction in spermatozoon after binding to Zona pellucida?

Answer 8

ZP3 binding + Integrin binding:

• Massive Ca influx at sperm head
• Outer acrosomal membrane fuse with egg plasma membrane&raquo_space; perforation&raquo_space; release acrosomal enzyme- Acrosin
• Acrosin digest glycoprotein on egg membrane

Question 9

How does sperm penetrate Zona pellucida?

Answer 9

• Acrosomal reaction - release acrosin to digest egg plasma membrane
• Mechanical propulsion by swimming

Question 10

Describe how an egg prevents polyspermy?

Answer 10

Sperm head bind ZP3:

Ca2+ wave propagates from site of sperm binding throughout egg

> > cause Cortical granules to fuse with plasma membrane

> > release hydrolytic enzymes into Perivitelline space

> > diffuse into zona pellucida and hydrolyze sperm receptor ZP3

> > eliminate further ZP3-sperm interaction

Question 11

What process occurs after fusion of the spermatozoon and egg after penetrating zona pellucida?

Answer 11

Spermatozoon plasma membrane fuse with egg’s

> > trigger release of Ca2+ from internal stores in egg

> > Intensify respiration, metabolism of egg

Question 12

Describe how sperm nucleus fuses with egg’s?

Answer 12

1) Nuclear membrane of sperm disintegrate = increase permeability
2) Reduced GSH in ooplasm reduce Disulfide cross-link between sperm chromatin and Protamine
3) Histone in ooplasms associate with male chromatin&raquo_space; 2 pronuclei chromosomes mingle, organized into mitotic spindle

Question 13

5 functions of fertilization?

Answer 13

1) Stimulate secondary oocyte to finish Meiosis II
2) Restore normal diploid number in zygote
3) Genetic variation
4) Sex determination
5) metabolic activation of the oocyte and initiates cleavage

Question 14

List the sequence of development after zygote formation before implantation?

Answer 14

Cleavage by mitosis:

Day 1 = 2 cell
Day 2= 4 cell
Day 3 = 8 cell
Day 3-4 = Morula (compaction)
Day 4= Blastocyst: innercell mass + trophoblast
Day 5 = Blastocyst hatch from Zona pellucida
Day 6 = Implantation

Question 15

Describe the development of embryo in week 2? Susceptible to abortion or teratogens?

Answer 15

bilaminar embryo – epiblast, hypoblast

spontaneous abortions are common
not susceptible to teratogenesis

Question 16

Describe the development of embryo in week 3?

Answer 16

Trilaminar embryo: Gastrulation produce ectoderm, mesoderm, endoderm

Question 17

List some tissue formed by ectoderm, mesoderm and endoderm?

Answer 17

Ectoderm (external layer): skin cell of epidermis, neuron of brain, pigment cell

Mesoderm (middle layer): cardiac muscle, skeletal muscle cells, tubule cell of kidney, red blood cells, smooth muscle (in gut)

Endoderm (internal layer): lung cell (alveolar cell), thyroid cell, pancreatic cell, germ cells (sperm, egg)

Question 18

Which weeks of pregnancy = Organogenesis?

Answer 18

Week 4-8

Question 19

Describe which periods of development is most prone to congenital anomalies?

Answer 19

first 8 post fertilization weeks

= beginning of all major external and internal structures development

Question 20

Maturation of embryo in Fetal period?

Answer 20

growth and maturation/elaboration of tissues and organs

Question 21

3 phases of human embryonic development?

Answer 21

1) Morphogenesis: mass cell movements to form organ and tissue
2) Differentiation: formation of functional tissue and organ
3) Growth: increase in size, cell divisions, tissue complexity…etc