Ch. 21 & 22 Development of Multicellular Organisms

what is the difference between germ cells and gametes?

germ cells → give rise to gametes (mitosis)

gametes → sperm & eggs (meiosis: diploid → haploid)

Human Embryo from fertilization to implantation

Ovary releases estrogen and progesterone

• ovulation is busting of ovary

• prepares endometrium for implantation → influx blood vessels and glands secrete sugars

Oocyte (egg) enters the fallopian tube (oviduct)

Fertilization happens at distal end of oviduct → Day 0

2-cell stage → Day 2

4-cell stage → Day 3-4

8-cell compacted: morula → Day 4

trophoectoderm (only rise to placenta) → Day 5

Zona pellucida → Day 6-7

Implantation of the blastocyst → Day 8-9

Details of Day 5 of human embryo

trophectoderm

• blastocoel

• inner cell mass → pluripotent

• early blastocyst

blastula → 1^st embryo that can implant

Clinical applications for the human embryo

• Ectopic pregnancy

• birth control pill = low, constant estrogen & progesterone

• abortin pill: mifepristone (RU486)

  • progesterone R antagonist that blocks receptors and induce menstration

Early Mammalian embryonic development

fertilized egg → 2 cells (1.5 days) → morula 8 cells (2.5 days) → 16 cells (3 days) → blastula: section of blastocyst (4 days)

• blastula:

  • inner cell mass → every tissue/germ layer

  • blastocoel

  • trophectoderm → becomes placenta

Totipotent vs. pluripotent cells

totipotent cells can become complete organism

• early embryonic cells

• 8-16 cell stage embryo

• cells fates restricted after 16-32 cell stage

pluripotent cells are the inner cell mass

what is a chimeric embryo?

organism with distinct cells with unique DNA

where are embryonic stem cells derived from?

inner cell mass

• immortal when grown in correct environment

cells of ICM are genetically altered → injection of ES cells into recipient blastocyst → cells become incorporated in inner cell mass of host blastocyst → blastocyst develops in foster mother into a healthy chimeric mouse; ES cells may contribute to any tissue

Cell Replacement therapy using Human ES cells

embryos left over after IVF/ART can be used as a replacement in humans

• early embryo destroyed → human ES cell lines are cultured in dish → signaling molecule injected to create replacement

  • examples include skeletal/cardiac myocytes

  • pancreatic islet cells

What are the problems of using human ES cells in cell replacement therapy? Alternatives?

Problems:

• embryo destruction

• tumor formation

• immune rejection

Alternatives:

• Adult stem cells (epithelial)

• SCNT-derived hES cells (cloning)

• iPS cells

Pros and Cons of Adult Stem Cells in Bone Marrow Hematopoietic Stem Cells

Pros:

• exist in many tissues

• multipotent

• no immune rejection after transplantation

• no ethical controversy

Cons

• difficult to isolate

• degree of pluripotency is controversial

What is SCNT hES cloning?

Somatic Cell Nuclear Transfer

• inject somatic cell nucleus into enucleated egg

• implant embryo into uterus for live-birthed organism

• 1997: Dolly 1^st mammal cloned from adult somatic cell

Cell replacement using iPS cells

iPS: induced pluripotent stem cells

• master gene regulator proteins: cMYC, OCT4, KLF4, SOX2 (transcription factors)

• patient → skin biopsy → patient specific iPS cells → gene targeting to repair disease causing mutation or in vitro differentiation

  • gene targeting: in vitro differentiation → healthy cells → transplantation of genetically matched healthy cells

  • in vitro differentiation: affected cell type → disease-specific drugs → treatment with drugs