O. Understanding the stages of development after fertilization to the gastrula stage. Take out a piece of paper and split some playdough with shoulder partner. Reminders: Registration forms due tomorrow. Test Wednesday.

When Harry met Sally

Stages of development Fertilized ovum 12-24hr fertilization Cleavage 30hr-day 3 mitosis Morula day 3-4 solid ball of cells Blastocyst day 5-week 2 formation of cell layers inner cell mass forms inner cell mass forms implantation implantation Gastrulation week 2-> cell layers form: endoderm endoderm mesoderm mesoderm ectoderm ectoderm

Stem cells  Totipotent stem cells – “totally man”, totally capable of becoming any cell type in the body  Pluripotent – have a limited number of cell types that they could become. Think about it… Totipotent = freshman in college Pluripotent = junior in college

Zygote forms

Fertilization  Fertilization is the joining of sperm and egg.  Meiosis II in the egg is completed at the time of fertilization, forming one egg and one polar body.  Following fertilization, chemical reactions occur preventing additional sperm from entering the egg.

Cleavage  cleavage  zygote divides mitotically a day after fertilization Totipotent or pluripotent?

Early development: ovulation to implantation Figure 3.14

Morula  morula (solid ball)  forms after several cell divisions, which hollows to form a blastocyst (hollow ball).

Blastocyst – implants in the uterine lining The developing embryo becomes a hollow ball of cells and is called a blastocyst.  Group of cells within the hollow space forms the inner cell mass (ICM).  develops into the embryo.  The cells around the ICM become the extra embryonic membranes  role in implantation  supports embryo’s growth

Outer and inner cell mass  outermost cells (trophoblast)  secrete hCG, which prevents menstruation.  hCG can be detected in a woman’s blood or urine and is an indicator of pregnancy.

Germ layers: endoderm, mesoderm and ectoderm Gastrulation: Totipotent or pluripotent?

Germ layers Ectoderm the outermost germ layer develops Skin – outer layer nervous system eye lens Mesoderm the middle germ layer develops muscle connective tissue + inner layer of skin blood vessels Kidneys Endoderm the innermost germ layer develops lining of GI tract liver pancreas thymus

The Embryo Forms  During week 2 of pregnancy the amniotic cavity forms; development of  primary germ layers: ectoderm, endoderm, and mesoderm  During week 3, the primitive streak appears, followed rapidly by development of the central nervous system, heart, notochord, neural tube, limbs, digits, and facial features.  By week 8, all organs have begun to develop and the embryo becomes a fetus.

Make your baby