Lecture Notes: Development

Question: How do you get that single cell from a complex, multi-cellular, multi-tissue, multi-organ, behaving organism?

I. Development proceeds in an orderly, hierarchical and sequential manner...

A. Embryonic Development involves:

1. Cell Division - increase in complexity.
2. Differentiation - Cells, non-committed vs. committed.
3. Morphogenesis - Changes in form.

B. Development is Gradual

II. Formation of the body:

A. Gametogenesis - Specialized Germ Tissue
B. Fertilization - Early Cleavage
C. Gastrulation - Formation of Tissue (Germ) Layers
D. Organogenesis - Formation of Body

III. Embryogenesis (Human with digressions)

A. Early Cleavage >> Blastula

1. Cell divisions are partitions, not growth
2. acquires/dorsal - anterior/posterior awareness
3. Blastula is a sphere of cells, with open center: blastocoel

B. Blastula >> Gastrulation

1. Gastrulation establishes three tissue types

a. ectoderm: epidermis, nervous system, lens, inner ear
b. mesoderm: dermis, muscle, skeleton, circulatory system, gonads,

kidneys, outer (body cavity) layers of digestive &
respiratory tracts.

c. endoderm: lining digestive & respiratory tracts, liver, pancreas
d. VERTEBRATE: Notochord


2. Sea Urchin

a. "Simple" Sphere blastula / blastocoel
b. at one point (blastopore) >> Invagination >> endoderm
c. cells migrate inward >>> mesoderm


CLICK FIGURE FOR LARGER IMAGE
3. Amphioxis ("Primitive" vertebrate)

a. "Simple" Sphere blastula >> invagination
b. dorsal part of invagination >> notochord and mesoderm
c. ventral part of invagination >> endoderm
d. mesoderm induces neurulation along notochord axis


CLICK FIGURE FOR LARGER IMAGE


4. Amphibian

a. Similar to Amphioxis but blastula has a ventral cell mass = yolk
b. Invagination at blastopore

surface cells move inward through blastopore
if you could mark a surface cell, you would observe the mark

move inward

c. Lip of blastopore has INDUCTIVE power - see below

CLICK FIGURE FOR LARGER IMAGE

5. Chicken

a. Blastodisc >> splits to 2 layers

epiblast (upper layer) -- produces embryo
hypoblast (lower layer, surface of yolk)

b. Epiblast and hypoblast split >> blastocoel
c. Gastrulation:

- epiblast cells migrate towards midline -- primitive streak

primitive streak grows, Hensen's node an leading end
primitive streak marks anterior-posterior axis

- cells migrate inward, into blastocoel, through primitive streak

some cells intermingle with hypoblast >> become endoderm
some cells remain in blastocoel >> mesoderm & notochord
mesodermal induction of notochord and overlying floor plate

neurulation

d. extra embryonic membranes

chorion - thin tissue inside egg shell
amnion - surrounds embryo
allantois - collects "waste"
yolk sac - contains yolk (non-cellular nutritive material

gut is open to yolk


CLICK FIGURE FOR LARGER IMAGE

6. Mammal / Human

a. like chicken
b. Blastula .. half cell mass, half blastocoel >> yolk
c. Cell mass splits >> second cavity (amniotic cavity)
d. Layer between Yolk and amniotic cavity

epiblast - hypoblast (lines yolk)

e. Gastrulation > similar to chicken
f. extraembryonic membranes

chorion - fetal part of placenta
amnion - surrounds embryo
allantois - umbilical blood vessels
yolk sac - empty yolk

g. placenta develops in humans in 4th week

(placenta takes over progersterone responsibilities)


CLICK FIGURE FOR LARGER IMAGE


C. Organogenesis: Early body form - example: vertebrates

1. Endoderm induces notochord and floor plate
2. Ectoderm overlying notochord folds up

Neural tube, neural crest form

3. Some mesoderm segments into somites along notochord

somites become back muscles and vertebrae

4. Other parts yield different parts

CLICK FIGURE FOR LARGER IMAGE




D. Morphogenesis continues >> body

E. Variations...

1. Egg cytoplasm...

proteins / mRNA from mother > encoding transcription factors
assymetrical distribution of components

2. Cleavages...

spiral vs. radial
equal vs. unequal
may be incomplete for some divisions = syncynthium
yolk, present or absent

3. Cell fate...

determined "early" or "late"

IV. Developmental guide posts: organizing principles

A. Cell Fate

1. Cytoplasmic environment

a. Asymmetry in Egg Cytoplasm >> Polarity

i. Gene expression requires time after fertilization

- Egg contains proteins and mRNA ready to be transcribed
- fertilization >> activation

ii. Unequal distribution of molecules

- early cleavages compartmentalize "different cytoplasms"

2. Cell position
3. Cell-cell interactions

a. Tissues / Cells communicate with each other

i. cell surface to cell surface
ii. chemical from one cell to receptor of another

B. Fate Maps
C. Determination
D. Morphogenic Movements - Induction

1. Gastrulation > Induction at blastopore/primitive streak

a. early experiments transplanting lip of blastopore

i. double embryos
ii. Lip seems to have some profound organizational influence

b. Lip induces cells as they migrate in. (Speman's organizer)

E. Differentiation
F. Pattern Formation

A. Development is the regulation of assymetric gene expression

1. Developmental genetics
2. Evolutionary Conservation: worms, insects and mice... Homebox genes.

VII. Contributions to Pattern Formation - Regulation of Development

CLICK FIGURE FOR LARGER IMAGE

A. Asymmetry in Egg Cytoplasm >> Polarity

1. Gene expression requires time after fertilization

a. Egg contains proteins and mRNA ready to be transcribed
b. fertilization >> activation

2. Unequal distribution of molecules

a. early cleavages compartmentalize "different cytoplasms"


B. Gastrulation > Induction at blastopore/primitive streak

1. early experiments transplanting lip of blastopore

a. double embryos
b. Lip seems to have some profound organizational influence

2. Lip induces cells as they migrate in. (Speman's organizer)


C. Tissues / Cells communicate with each other

1. cell surface to cell surface
2. chemical from one cell to receptor of another


D. Hormones - temporal coordination.

Sexual Maturation
Aging


E. Cell Commitment / Cell Fate.

F. Differentiation.

G. Metamorphosis

Direct / Indirect


H. Cell Movement / Cell Migration,

I. Growth, Apoptosis (programed cell death)

Lecture Outline: Development Chapter 40 11/1/96

I. Development:

Embryogenesis, Juvenile Growth, Reproduction Maturation
Aging, Other Physiological Change (e.g. salmon, sex reversals)

II. Embryogenesis (Human with digressions)

A. Early Cleavage >> Blastula
B. Blastula >> Gastrulation

1. Gastrulation establishes three tissue types: ectoderm; mesoderm; endoderm
2. Sea Urchin (related to star fish)
3. Amphioxis ("Primitive" vertebrate)
4. Amphibian
5. Chicken

a. Blastodisc >> 2 layers: epiblast & hypoblast
b. Epiblast and hypoblast split >> blastocoel
c. Gastrulation:
e. extra embryonic membranes

6. Mammal / Human

C. Organogenesis: Early body form - example: vertebrates / notochord & mesoderm
D. Morphogenesis continues >> body

III. Contributions to Pattern Formation - Regulation of Development

A. Asymmetry in Egg Cytoplasm >> Polarity
B. Gastrulation > Induction at blastopore/primitive streak
C. Tissues / Cells communicate with each other
D. Hormones - temporal coordination.
E. Cell Commitment / Cell Fate.
F. Differentiation.
G. Metamorphosis: Direct / Indirect Development

Vocabulary: