Early Development

Question 1

Stage after gastrulation

Pharyngeal stage
characteristics

Answer 1

Embryo has
* Pharynx
* Central neural tube
* Notochord
* Somites
* Head region

Question 2

Fertilization

Answer 2

fusion of mature gametes

Question 3

Cleavage

Answer 3

Series of rapid cell divisions during which the cytoplasm is split between smaller daughter cells (blastomeres)

Results in the blastula

Question 4

Gastrulation

Answer 4

Slower cell division and dramatic cellular rearrangements

At the end of gastrulation, the gastrula has all 3 germ layers

Ectoderm, mesoderm, endoderm

Question 5

Ectoderm

Answer 5

outer layer = skin, brain, neural crest

Question 6

Mesoderm

Answer 6

middle layer = blood, heart, kidney, gonads, bones, muscles, and connective tissue

Question 7

Endoderm

Answer 7

inner layer = digestive tube and its associated organs including the lungs

Question 8

Germ layers

Answer 8

Three distinct regions of the embryo that give rise to differentiated cell types and specific organ systems

Ectoderm, mesoderm, endoderm

Question 9

Organogenesis

Answer 9

Formation of tissues and organs

Many organs actually contain cells from multiple germ layers

Cells in the outer layer of skin (epidermis) are ectodermal while inner layers are mesodermal

Question 10

Notochord

Answer 10

Rod of mesodermal cells
Signals overlying ectoderm to become the nervous system

Begins developing at 17 days and gone by 7-10 weeks

Example of an embryonic tissue that undergoes programmed cell death

Question 11

Metamorphosis

Answer 11

Process of changing from immature to sexually mature organism

Question 12

Gametogenesis

Germ cells are gamete precursors

Answer 12

Process of producing gametes for reproduction

Set aside normally during very early development

Different than somatic cells which are all other cells of the body

Question 13

Gametogenesis requires meiosis

Meiosis

Answer 13

Chromosomes replicate prior to cell division so each gene is represented 4 times
Replicated chromosomes (called chromatids) are held together by the kinetochore and all 4 chromosomes pair together
* Recombination occurs
chromatid becomes a single chromosome

Result = 4 germ cells with a haploid nucleus

1st division separates chromatid pairs
2nd division splits the kinetochore so each chromatid
becomes a single chromosome

Question 14

von Baer’s laws

Answer 14

4 generalizations of vertebrate development

Question 15

von Baer’s first law

Answer 15

The general features of a large group of animals
appears earlier in development than do the specialized features of a smaller group

Developing vertebrates look similar after gastrulation & diversity later

All vertebrate embryos have gill arches, a notochord, a spinal cord, and primitive kidneys

Question 16

von Baer’s second law

Answer 16

Less general characters develop from the more
general, until finally the most specialized appear

Early on, all vertebrates have a similar skin. Specializations such as scales, feathers, hair etc develop later

Question 17

von Baer’s third law

Answer 17

The embryo of a given species, instead of passing
through the adult stages of lower (simpler
anatomically) animals, departs more and more
from them

For example, all embryonic vertebrates have gill arches. These are not the same as adult fish gills. Rather, fish elaborate and develop these structures into gills while in mammals these structures develop into the eustachian tubes (ear-mouth connection).

Question 18

von Baer’s fourth law

Answer 18

Therefore, the early embryo of a higher animal is
never like a lower animal but only like its early
embryo.

Human embryos never pass through a stage where they look like an adult fish or bird. Rather, human embryos, fish embryos, and bird embryos initially share common characteristics and look similar

Question 19

How does the blastula become a
gastrula which becomes an adult?

Answer 19

A complicated series of cell movements and shape
changes allow the formation of the axis

Question 20

Fate map

Cells that will give rise to certain tissues have their fate determined during gastrulation

Answer 20

Identification of groups of cells in the gastrula that will become a particular tissue in the adult

Question 21

Fate mapping method to label individual cells and track them over time

Fluorescent dye labeling

Answer 21

Step 1: Inject cells with fluorescent tracking dye (green)
Step 2: See where they go

Question 22

Fate mapping method to label individual cells and track them over time

Chimeric organisms

i.e. quail chick chimeras

Chimeric organisms revealed the diversity of tissues derived from
neural crest

Answer 22

Step 1: Transplant cells from quail embryo into chick
Step 2: See where they go!
- How do we identify quail cells in a chick?
1. Differences in nuclear DNA condensation
2. Quail-specific antibodies
- Immunohistochemistry

Neural crest arises from the ectoderm
Adjacent to developing neural tube
Delaminates and migrates away

Question 23

What do the neural crest become?

Defects in neural crest migration cause cleft palates

Answer 23

The enteric nervous system and peripheral nervous system come from the neural crest.

Cell transplantation showed us a lot

Transplanting the neural crest from a pigmented chicken embryo into an albino results in black feathers

Question 24

Induction

The ability of one group of cells to change the behavior of another

Answer 24

For the nervous system, it is the ability of existing embryonic tissues to reprogram surrounding pluripotent cells to become the nervous system

Induction gives us the neural plate

Question 25

What induces the nervous system?

Answer 25

Hensen’s node and notochord

Essential for first steps of nervous system formation

In mammals it forms from a primitive structure called Koller’s sickle at the anterior edge of the primitive streak. It can induce and pattern an embryonic axis (Spemann-Mangold organizer in amphibians)

Question 26

How do we know that Hensen’s
node is an organizing center?

Answer 26

Hensen’s node is at anterior edge of primitive streak.
Cells migrate into primitive streak during gastrulation
to form the germ layers.

Transplant node to a new animal

Induces second neural tube – primitive nervous system

Question 27

All vertebrates have an organizer of some sort

Answer 27

• Zebrafish = Shield
• Frog = Spemann Mangold organizer
• Chicken and Human = Hensen’s node

Question 28

Discovery of the Spemann-Mangold
organizer’s function

Answer 28

Transplanting cells from the blastopore lip (Hensen’s node in mammals) to another embryo resulted in the
formation of a second axis/nervous system in newts.

Question 29

How does Hensen’s node/SMO induce the nervous system?

Node

Answer 29

Induces notochord and anterior nervous system

Secretes Chordin and Dickkopf

Question 30

How does Hensen’s node/SMO induce the nervous system?

Notochord

Answer 30

Once formed, participates in process

Secretes Noggin

Notochord essential to pattern posterior spinal cord

Question 31

How does Hensen’s node/SMO induce the nervous system?

Chordin and Noggin

Answer 31

Paired action of Chordin and Noggin suppress BMP signaling to promote anterior neural fate

BMP = bone morphogenetic protein
Dickkopf suppresses Wnt signaling

Knockout of either Chordin OR Noggin has no effect
Knockout both – you lose facial structures and forebrain

Question 32

Morphogens and patterning the anterior-posterior nervous system

Anterior structures require what?

Answer 32

Low BMP
Low Wnt

Node antagonizes BMPs and Wnt to promote anterior neural fate

Question 33

Morphogens and patterning the anterior-posterior nervous system

Posterior structures require what?

Answer 33

High Wnt
High BMP
High FGF

Node antagonizes BMPs and Wnt to promote anterior neural fate

Question 34

Neurulation

Answer 34

The process by which the neural plate becomes the neural tube