1. Animal Development
Chapter 47

2. Cleavage A period of rapid cell division after fertilization
Skips the G1 and G2 phases
No growth during this period, just division
Includes the following developmental stages
Zygote
Blastomeres
Morula
Blastula (with blastocoel)
IVF Animation

3. Gastrulation
Rearrangement of blastula cells to form a gastrula with three distinct germ layers
Accomplished by change in:
Cell motility
Cell shape
Cell adhesion to other cells & extracellular matrix

4. Germ Layers Ectoderm Outer layer Endoderm
Lines embryonic digestive tract
Mesoderm
Partly fills the space between the endoderm & ectoderm

5. Organogenesis
The germ layers work together to develop into the rudimentary organs
Can be seen by folds, splits, and clustering of cells in localized tissues
By 8 weeks in humans the fetus has all the basic versions of their organs

6. First Organs in Chordates
Notochord
Formed from mesoderm
Future position of the vertebral column
Neural tube
Becomes the central nervous system
Neural crest is formed along the border which becomes the peripheral nerves, teeth, skull, and more (aka: 4th germ layer)
Somites
Lateral strips along notochords, in pairs
Forms vertebrae
Associated with axial muscle development

7. Adaptation in Amniotes
A moist environment is needed for the developing embryo
Amniotic fluid is the solution:
Shelled eggs for birds, reptiles, and monotremes
Uterus for marsupial and eutherian mammals

8. Adaptations in Mammals
Internal fertilizition
Small eggs with little amounts of stored nutrients
Reliant on a uterus to provide nutrients from the mother
Slow cleavage (first division done after ~36 hours)

9. Human Embryo Development
Blastocyst has reached the uterus
The trophoblast begins to implant into the endometrium by secreting catabolic enzymes
Trophoblast expands into maternal tissues via mitosis to gain nutrients from her blood supply
Implantation in completed and gastrulation begins
The end of gastrulation has arrived and the embryo is now surrounded by extraembryonic membranes

10. =inner cell mass becomes the embryo
-epiblast: embryo development
-Hypoblast: don’t contribute to embryo but help with primitive streak & then become part of the yolk sac

11. Extraembryonic Membrane in Humans
Chorion: surrounds all extraembryonic membranes & provides gas exchange
Amnion: provides the protective layer around the amniotic fluid
Yolk sac: found below the embryo this cavity is where blood cells are formed which travel to the embryo
Allantonis: part of the umbilical cord in the form of blood vessels to move nutrients and waste

12. The Fate of Embryonic Cells
This is dependent on the movement of cells to allow for changes in cell shape & function
Cells move by using the cytoskeleton to crawl from one location in the embryo to another
This movement is regulated by ECM and secreted glycoproteins
Fibronectin: which provides anchorage
Cell adhesion molecules: bind to the CAM’s on other cells for tissue building
Cadherins: helps with tight junctions

13. Formation of the Neural Tube

14. The Fate of Embryonic Cells
Induction: A group of embryonic cells has the ability to influence the formation of another group resulting in specialized cells (by expressing only specific genes)
Fate Mapping: Embryonic cells have a specific fate before gastrulation occurs (as cells mature they are limited in what they can become)
Cellular Asymmetry: position of the nuclei in the sperm & egg may play in role in determining developmental axes

15. The Fate of Embryonic Cells
Restriction to cell potency: in humans embryonic cells are totipotent until the 16 cell stage when they are determined to be either blastocyst or trophoblast based on location
Inductive signals: embryonic cells will be influenced by other about their final fate through gastrula organizers
Ex: dorsal lip is an organizer the controls the development of notochord, neural tube, and more
Brain Pop: Stem Cells