Development

1. Development

2. Fertilization Egg (female gamete) (n) + Sperm (male gamete) (n) fertilization Zygote (2n)

3. Embryo Development The single cell zygote must go through a vast number of changes to become a multicellular organism with differentiated cells.

4. Cleavage A series of rapid mitotic divisions occur without growth or gene expression The cells form a ball with a fluid filled center called a blastula Day 1 Day 2 This forms a solid ball of cells called a morula 16 – 32 cells Day 3 Day 4 Day 5

5. Blastula • Does not have an opening • Blastocoel – fluid filled center • Blastomeres – outer cells to become chorion • Inner cell mass will become the embryo Blastula (has no ICM) Morula blastocoel Inner Cell Mass in Blastocyst Blastomeres (prechorion)

6. Human Development Fertilization and morula formation occur in the oviduct. The blastula forms as the developing embryo enters the uterus Implantation: Blastocyst secretes enzymes allowing it to imbed in the uterine lining (day 7 - 13)

7. Implantation Human Chorionic Gonadotropin (HGC) (secreted by the embryo) maintains the corpus luteum and production of progesterone The placenta will secrete its own progesterone later. (takes over in weeks 10-12)

8. Implantation

9. Gastrulation • starts when cells on one side pulled into the embryo (invagination) • ends with the formation of a gastrula Day 16 Blastopore (opening) Gastrula: three tissue layers (ectoderm mesoderm and endoderm) and a primitive digestive tract called the archenteron invagination

10. Gastrulation All tissues and organs will develop from these three layers (called germ layers). Cell fate has been determined. Gene expression begins as differentiation starts. ectoderm endoderm mesoderm archenteron blastopore

11. Gastrulation in Humans Inner Cell Mass produces an embryonic disc Primitive streak - Site of invagination

12. Neurulation • Initiating the Nervous System • Ectoderm becomes nerve chord and brain • Induction by notochord • Cell migration forms peripheral nerve tissue • Begins development of body segmentation • Somites from mesoderm form precursors to vertebrae (~ 21 days)

13. Embryonic Germ Layers Cell differentiation begins to occur at the blastula stage. Different types of cells use (express) different parts of the DNA code

14. Extra Embryonic Tissues

15. Placenta Site of exchange of nutrients and waste Embryo part forms from chorion Maternal part forms from uterine tissues Blood of mother and embryo or fetus never mix

16. Umbilical Cord Stretches between placenta and fetus Carries oxygen and nutrients to fetus Carries carbon dioxide and wastes to placenta

17. Embryo Development Fifth Week Limb buds form, head (brain) enlarges, sensory organs noticeable

18. Embryo Development

19. Embryo Development Human Pig Chick Fish

20. Fetal Development Embryonic stages occur in the first eight weeks. All organs are initiated in the embryonic stage Fetal development occurs from week nine through week 40 Marked by a great increase in size and weight

21. 12 – 16 weeks

22. Figure 29.24 24 weeks

23. Parturition Three stages: 1. Expansion of the cervix 2. Delivery of the fetus 3. Expulsion of the placenta (“afterbirth”)