1. Chapter 47Animal Development

2. 2005-2006 Fertilization

3. Embryonic development/fertilization Preformation: until 18th century; miniature infant in sperm or egg At fertilization/conception: Acrosomal reaction: hydrolytic enzyme action on egg jelly coat…. Fast block to polyspermy: membrane depolarization prevents multiple fertilizations…. Cortical reaction: release of calcium causes hardening of egg outer layer and creates a... Slow block to polyspermy and... Egg activation~ increases metabolic activity; protein synthesis

4. 2005-2006 Cleavage Repeated mitotic divisions of zygote – 1st step to becoming multicellular – unequal divisions establishes body plan different cells receive different portions of egg cytoplasm & therefore different regulatory signals

5. 2005-2006 Cleavage zygote  morula  blastula – establishes future development

6. 2005-2006 Gastrulation Establish 3 cell layers – ectoderm outer layers – skin, nails, teeth, nerves – mesoderm blood, bone & muscle – endoderm inner lining – digestive system ectoderm mesoderm endoderm

7. Gastrulation in a sea urchin – Produces an embryo with a primitive gut and three germ layers Figure 47.11 Digestive tube (endoderm) Key Future ectoderm Future mesoderm Future endoderm Blastocoel Mesenchyme cells Vegetal plate Animal pole Vegetal pole Filopodia pulling archenteron tip Archenteron Blastocoel Blastopore 50 µm Blastopore Archenteron Blastocoel Mouth Ectoderm Mesenchyme: (mesoderm forms future skeleton) Anus (from blastopore) Mesenchyme cells The blastula consists of a single layer of ciliated cells surrounding the blastocoel. Gastrulation begins with the migration of mesenchyme cells from the vegetal pole into the blastocoel. 12 The vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel. 2 Endoderm cells form the archenteron (future digestive tube). New mesenchyme cells at the tip of the tube begin to send out thin extensions (filopodia) toward the ectoderm cells of the blastocoel wall (inset, LM). 3 Contraction of these filopodia then drags the archenteron across the blastocoel. 4 Fusion of the archenteron with the blastocoel wall completes formation of the digestive tube with a mouth and an anus. The gastrula has three germ layers and is covered with cilia, which function in swimming and feeding. 5

8. The mechanics of gastrulation in a frog – Are more complicated than in a sea urchin Figure 47.12 SURFACE VIEW CROSS SECTION Animal pole Blastocoel Dorsal lip of blastopore Dorsal lip of blastopore Vegetal pole Blastula Blastocoel shrinking Archenteron Blastocoel remnant Ectoderm Mesoderm Endoderm Gastrula Yolk plug Key Future ectoderm Future mesoderm Future endoderm Gastrulation begins when a small indented crease, the dorsal lip of the blastopore, appears on one side of the blastula. The crease is formed by cells changing shape and pushing inward from the surface (invagination). Additional cells then roll inward over the dorsal lip (involution) and move into the interior, where they will form endoderm and mesoderm. Meanwhile, cells of the animal pole, the future ectoderm, change shape and begin spreading over the outer surface. The blastopore lip grows on both sides of the embryo, as more cells invaginate. When the sides of the lip meet, the blastopore forms a circle that becomes smaller as ectoderm spreads downward over the surface. Internally, continued involution expands the endoderm and mesoderm, and the archenteron begins to form; as a result, the blastocoel becomes smaller. 1 2 3 Late in gastrulation, the endoderm-lined archenteron has completely replaced the blastocoel and the three germ layers are in place. The circular blastopore surrounds a plug of yolk-filled cells.

9. Gastrulation in the chick – Is affected by the large amounts of yolk in the egg Figure 47.13 Epiblast Future ectoderm Migrating cells (mesoderm) Endoderm Hypoblast YOLK Primitive streak

10. Organogenesis Various regions of the three embryonic germ layers – Develop into the rudiments of organs during the process of organogenesis

11. Early in vertebrate organogenesis – The notochord forms from mesoderm and the neural plate forms from ectoderm Figure 47.14a Neural plate formation. By the time shown here, the notochord has developed from dorsal mesoderm, and the dorsal ectoderm has thickened, forming the neural plate, in response to signals from the notochord. The neural folds are the two ridges that form the lateral edges of the neural plate. These are visible in the light micrograph of a whole embryo. Neural folds 1 mm Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron (a) LM

12. 2005-2006 Neurulation 1st organ to form is notochord & nerve chord – develop into nervous system Neural groove Notochord Neural tube

13. 2005-2006 Organogenesis Umbilical blood vessels Chorion Amnion Yolk sac Allantois Fetal blood vessels Maternal blood vessels Bird embryo Mammalian embryo Placenta

14. 2005-2006 Placenta Materials exchange across membranes

15. 2004-2005 Human fetal development Day 1: 1 st cleavage – 1 cell becomes 2 (2-cell stage) Day 2: 2 nd cleavage – 4-cell stage Day 3: 6-12 cell stage – can test at this stage for genetic diseases if done by IVF Day 4: 16-32 cell stage – solid ball of cells = morula

16. 2004-2005 Ovulation to implantation

17. 2004-2005 Human fetal development Day 5: – solid morula develops into hollow, fluid-filled blastula – embryo will develop from the inner cell mass, or embryonic disc Day 6 -7: – blastocyst attaches to the endometrium (uterine lining) & burrows in: implantation – blastocyst starts to secrete HCG = human chorionic gonatotropin stimulates estrogen & progesterone to prevent menstrual flow causes "morning sickness" in some women... pregnancy test measures the amount of this hormone!

18. 2004-2005 Human fetal developmental Days 10 - 14: – pregnancy becomes established – fluid filled amniotic cavity starts to form – yolk sac starts to form will make blood cells, germ cells – embryo starts to form from embryonic disc – chorion (placenta) starts to form At the end of this stage, a woman will have just missed her period! Day 14

19. 2004-2005 Human fetal developmental Days 15 - 21: – emergence of the vertebrate body plan – primitive streak starts to form the site of gastrulation (formation of 3 tissue layers = ecto, endo, and mesoderm) – neural groove begins to form future spinal cord & brain – somites begin to form bands of tissue that will become muscles & bones – pharyngeal arches begin to form future face, neck, mouth, nose

20. 2004-2005 Vertebrate body plan Day 19

21. 2004-2005 Human embryonic developmental Day 24 Week 3 - Week 8 = embryo – development of all organ systems Day 22: the heart begins to beat Day 28 Week 4

22. 2004-2005 Human embryonic development embryo showing tail & limb buds

23. 2004-2005 Human embryonic development beginning of the eye can be seen, as well the bulging heart & the umbilical cord

24. 2004-2005 Human embryonic development lens of the eye can be seen forming, the mass of the heart bulging from the chest, & the beginnings of the finger rays

25. 2004-2005 Human embryonic development

26. 2004-2005 Human embryonic development beginning of the ear is clearly seen note the bend of the elbow joint has begun, the fingers are forming and toes are beginning to bud off the foot

27. 2004-2005 Human embryonic development Note the formation of the nose, eyelids, ear flap & well defined toes & fingers 50–60 days (8 weeks) Both knee & elbow are visible. Embryo has formed most of basic organ systems & will spend remainder of development in “fetal” period. Organs grow, mature, & begin to learn their respective functions

28. 2004-2005 Human fetal development Day 52: – first brain waves can be detected weeks 4-8 is when all major organ systems of body are formed & when most teratogens have greatest effect Week 8

29. 2004-2005 Human fetal development Week 10

30. 2004-2005 Human fetal development At 15 weeks the embryo begins to take on more of a typical baby's form & the attachment of umbilical cord can be clearly seen Week 15

31. 2004-2005 Human fetal development Week 9 - week 40 = fetus – after 12 weeks or so, the baby's development is largely "finished" some exceptions: brain & lung development Week 16

32. 2004-2005 Human fetal development The fetus just spends much of the 2 nd & 3 rd trimesters just growing …and doing various flip-turns & kicks inside amniotic fluid Week 20

33. 2004-2005 Human fetal development 24 weeks (6 months) fetus is covered with fine, downy hair called lanugo. It’s skin is protected by a waxy substance called vernix

34. 2004-2005 Human fetal development 30 weeks

35. 2004-2005 Human fetal development 32 weeks The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

36. 2004-2005 The end of the journey!