1. Embryology Development After Fertilization http://www.hhmi.org/biointeractive/ media/human_emb_dev-lg.mov

2. Stem Cells Embryonic development depends on stem cells. During development, cells divide, migrate, and specialize. Early in development, a group of cells called the inner cell mass (ICM) forms. THE ICM produce all the tissues of the body. The outer cells produce the placenta.

3. Stages of Embryo Development Segmentation – Zygote A single sperm enters an ovum (conception) (day 1) Zygote spends several days travelling down fallopian tube Zygote is dividing (2,4,8 and 16 cells) forms and inner and outer group of cells Called the Morula – 3 days after conception

4. Morula

5. Blastula At 128 cell stage – Starts ~ fourth day and continues until the embryo has become implanted in the wall of the uterus, around the sixth or seventh day

6. As a blastula the protective envelope (or pellucid area) around the ball of cells gradually decays allowing blastula to implant on the uterine wall. Blastula has two kinds of cells – Outer cells called trophoblasts, which will form the placenta – Inner cells called embryoblasts, which will form the embryo

7. Implantation

8. ~ day 7 Trophoblasts secrete an enzyme that lets the blastocyste burrow into the uterine wall. Trophoblasts proliferate, foring two distinct layers of cells: – one that surrounds the embryo – One that penetrates uterine wall

9. Grastrulation Embryoblasts form embryo

10. EMBRYO Cells multiply and begin to take on specific functions. This process is called differentiation. It leads to the various cell types that make up the organism

11. Three Germ Layers Embryonic cells made of: – Endoderm – Mesoderm – Ectoderm

12. Endoderm The inner layer of cells – become a simple tube lined with mucous membranes. – Forms: the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.

13. Mesoderm Middle layer – Forms skeletal muscle, the skeleton, the dermis of skin, the crystal lens of the eye, connective tissue, the urogenital system, the heart, blood (lymph cells), and the spleen.

14. Ectoderm Outer layer – Forms: the central nervous system, the lens of the eye, peripheral nervous sytem, pigment cells, connective tissues, the epidermis, hair, and mammary glands.

15. Organogensis Organogensis refers to that period of time during development when the organs are being formedtime