Def. Morphogenesis: generation of tissue shape that form organs and bodies

Def. Cell Differentiation: generation of the diff. cell types in tissues

It begins with division of cells so that there are enough cells to work with —> cells start to specialize because of a change in expression of genes —> cells signal to one another and also move to diff. places

Multicellular development is studies in mice, frogs, Drosophila, plants and C. elegans
Advantage for using these: rapid generation time, can be closely applied to human health —> there are many conserved evolutionary processes that are common across diff. species.

1) Development continues in adults for maintaining tissues

E.g. skin, there are multiple layers of epithelia that are continuously renewed. The topmost layer skin is dead and sheds constantly, then stem cells from the bottom divide and resupply the tissue.

E.g. gut epithelia, the tip of the villi is constantly being replaced by stem cells on the bottom of the crypt. This process is crucial for maintaining the lining of the gut.


2) Development continues in adults for initiating new processes
E.g. When women become pregnant, the mammary gland epithelia develops to generate milk. The duct will have many alveoli growing bigger and bigger. The epithelial layer of the alveoli make and secrete milk proteins into the duct.



The best studied for of multicellular development is embryogenesis. It has common mechanisms of morphogenesis and cell differentiation with organogenesis and stem cell development.

Embryogenesis



Haploid cells from mother and father fuse together to make a diploid zygote —> cell divides multiple times to make a loose ball of cells —> after 3 days, cells go through compaction (a process which makes cells adhere to one another) —> becomes a blastocyst
trophectoderm: outer layer of the blastocyst
blastocoel: empty space on the inside >> becomes supportive tissue that supports the embryo
inner cell mass: what becomes of an embryo

Morphogenesis requires:

1) internalization of cells

grastrulation >> determines the 3 germ layers: ectoderm, mesoderm and endoderm

e.g sea urchin
ectoderm: found on the outside >> epidermis/ nervous system
mesoderm: found in the middle >> muscle/connective tissue/blood vessels 
endoderm: found on the inside >> gut/lung/liver, etc
Bending of the tissue makes digestive track, etc. Migration of cells make mesoderm.
A group of cells that are going to undergo this transition is transplanted from a quail's wing to a chick’s wing. As the wing develops, we can see that there these cells go on to make muscles.
          

2) elongation of the embryo




3) fine repositioning of cells