section 9.1

Question 1

zygote

Answer 1

a single cell formed by the amalgamation of an ovum and a sperm. The zygote divides to form two daughter cells. These two divide to form four, then the four divide to form eight, etc. until a mature organism is produced.

Question 2

Three things other than cell multiplication must occur:

Answer 2

• Cells must differentiate, some must become muscle cells, some multipolar neurons, etc.
• Cells must make their way to appropriate sites and align themselves with the cells around them to form particular structures.
• Cells must establish appropriate functional relations with other cells.

Question 3

Developing neurons accomplish these three things in five phases:

Answer 3

(1) induction of the neural plate, (2) neural proliferation, (3) migration and aggregation, (4) axon growth and synapse formation, and (5) neuron death and synapse rearrangement.

Question 4

totipotent

Answer 4

includes fertilized eggs; the cell has the ability to develop into any class of cell in the body (e.g. bone, skin, neuron, or heart cells).

Question 5

After about 4 days of embryological development

Answer 5

newly created cells lose their totipotency and begin to specialize. At this stage, developing cells have the ability to develop into many, but not all, classes of body cells and are said to be pluripotent.

Question 6

As the embryo develops, new cells become

Answer 6

more and more specialized. Eventually, new cells and develop into different cells of only one class (e.g. different kinds of blood cells). These new cells are said to be multipotent.

Question 7

unipotent

Answer 7

what most developing cells become; cells that can only develop into one type of cell (e.g. bipolar neurons).

Question 8

Three weeks after conception

Answer 8

the tissue that is destined to develop into the human nervous system becomes recognizable as the neural plate.

Question 9

neural plate

Answer 9

a small patch of ectodermal tissue on the dorsal surface of the developing embryo.

Question 10

three layers of embryonic cells

Answer 10

ectoderm, mesoderm, and endoderm.

Question 11

The development of the neural plate is induced by

Answer 11

chemical signals from an area of the underlying mesoderm layer – an area consequently referred to as an organizer.

Question 12

Tissue taken from the dorsal mesoderm of one embryo (the donor) and implanted beneath the ventral ectoderm of another embryo (the host) induces

Answer 12

the development of an extra neural plate on the ventral surface of the host.

Question 13

stem cells

Answer 13

what cells of the neural plate are often referred to as; cells that meet two specific criteria:

• They have an almost unlimited capacity for self-renewal if maintained in an appropriate cell culture.
• They are pluripotent.

Question 14

Stem cells have an almost unlimited capacity for self-renewal because when

Answer 14

a stem cell divides, two different daughter cells are created: one that eventually develops into some type of body cell and one that develops into another stem cell. In theory, when stem cells are maintained in a cell culture, they can keep dividing forever, but eventually errors accumulate, which disrupt the process.

Question 15

The growing neural plate folds to form the

Answer 15

neural groove, and then the lips of the neural groove fuse to form the neural tube. The inside of the neural tube eventually becomes the cerebral ventricles and spinal canal.

Question 16

By 40 days after conception, three swellings are visible at the

Answer 16

anterior end of the human neural tube; these swellings ultimately develop into the forebrain, midbrain, and hindbrain.

Question 17

neural proliferation

Answer 17

begins when the lips of the neural groove have fused to create the neural tube. Does not occur simultaneously or equally in all parts of the tube.

Question 18

ventricular zone

Answer 18

where most cell division in the neural tube occurs; the region adjacent to the ventricle (the fluid-filled center of the tube).

Question 19

In each species, the cells in different parts of the neural tube proliferate in a particular sequence that is responsible

Answer 19

for the pattern of swelling and folding that gives the brain of each members of that species its characteristic shape.

Question 20

The complex pattern of proliferation is in part controlled by

Answer 20

chemical signals from two organizer areas in the neural tube: the floor plate, which runs along the midline of the ventral surface of the tube, and the roof plate, which runs along the midline of the dorsal surface of the tube.

Question 21

Once cells have been created through cell division in the ventricular zone of the neural tube

Answer 21

they migrate to the appropriate target location. During this migration, the cells are still in an immature form, lacking the processes (i.e. axons and dendrites) that characterize mature neurons.

Question 22

Two major factors govern the migration in the developing neural tube:

Answer 22

time and location. In a given region of the tube, subtypes of the neurons arise on a precise and predictable schedule and the migrate together to their prescribe destinations.

Question 23

Cell migration in the developing neural tube is considered to be of two kinds:

Answer 23

• Radial migration – proceeds from the ventricular zone in a straight line outward toward the outer wall of the tube.
• Tangential migration – occurs at a right angle to radial migration – this is, parallel to the tube’s walls.

Most cells engage in both radial and tangential migration to get from their point of origin in the ventricular zone to their target destination.

Question 24

There are two methods by which developing cells migrate:

Answer 24

• Somal translocation – an extension grows fro the developing cell in the direction of the migration; the extension seems to explore the immediate environment for attractive and repulsive cues as it grows. Then, the cell body itself moves into and along the extending process, and trailing processes are retracted.
• Glial-mediated migration – once the period of neural proliferation is underway and the walls of the neural tube are thickening, a temporary network of glial cells, called radial glial cells, appears in the developing neural tube. At this point, most cells engaging in radial migration do so by moving along the radial glial network.

Question 25

The only function of radial glial cells was previously assumed to only

Answer 25

provide a matrix for radial migration. Now it is clear that many develop into neurons. Some are pluripotent, but many are committed to specific neural fates.

Question 26

inside out pattern

Answer 26

radial pattern of cortical development; each wave of cortical cells migrates through the already formed lower layers of cortex before reaching its destination.

Question 27

Many cortical cells engage in

Answer 27

long tangential migrations to reach their final destinations, and the patterns of proliferation and migration are different for different areas of the cortex.

Question 28

neural crest

Answer 28

a structure situated just dorsal to the neural tube. Formed from cells that break off from the neural tube as t is being formed.

Question 29

Neural crest cells develop into the

Answer 29

neurons and glial cells of the peripheral nervous system, so many of them must migrate over considerable distances.

Question 30

aggregation

Answer 30

after developed neurons have migrated, they must align themselves with other developing neurons that have migrated to the same area to form the structures of the nervous system.

Question 31

cell-adhesion molecules (CAMs)

Answer 31

mediates both migration and aggregation; located on the surfaces of neurons and other cells. Have the ability to recognize molecules on other cells and adhere to them.

Question 32

Gap junctions between adjacent cells have been found to be

Answer 32

particularly prevalent during brain development.

Question 33

gap junctions

Answer 33

points of communication between adjacent cells (neurons and glia). The gaps are bridged by narrow tubes called connexins, through which cells can exchange cytoplasm.

Question 34

Once neurons have migrated to their appropriate positions and aggregated into neural structures

Answer 34

axons and dendrites begin to grow from them. For the nervous system to function, these projections must grow to appropriate targets.

Question 35

growth cone

Answer 35

amoebalike structure; at each growing tip of an axon or dendrite. Extends and retracts fingerlike cytoplasmic extensions called filopodia.

Question 36

Sperry

Answer 36

cut the optic nerves of frogs, rotated their eyeballs 180 degrees, and waited for the axons of the retinal ganglion cells, which compose the optic nerve, to regenerate. When he dangled a lure behind the frogs, they struck forward, indicating that their visual world, like their eyes, had been rotated 180 degrees. Frogs whose eyes had been rotated, but whose optic nerves had not been cut, responded in exactly the same way. Strong evidence that each retinal ganglion cell had grown back to the same point of the optic tectum (superior colliculus in mammals) to which It had originally been connected.

Question 37

Chemoaffinity hypothesis of axonal development

Answer 37

proposed by Sperry; hypothesized that each postsynaptic surface in the nervous system releases a specific chemical label and that each growing axon is attracted by the label to its postsynaptic target during both neural development and regeneration.

Question 38

revised hypothesis

Answer 38

a growing axon is not attracted to its target by a single specific attractant released by the target, as Sperry thought. Instead, growth cones seem to be influenced by a series of chemical signals along the route.

Question 39

pioneer growth cones

Answer 39

the first growth cones to travel along a particular route in a developing nervous system; are presumed to follow the correct trail by interacting with guidance molecules along the route. Then, subsequent growth cones embarking on the same journey follow the routes blazed by the pioneers.

Question 40

fasciculation

Answer 40

the tendency of developing axons to grow along the paths established by preceding axons.

Question 41

Much of the axonal development in complex nervous systems involves

Answer 41

growth from one topographic array of neurons to another. The neurons on one array project to another, maintaining the same topographic relation they had on the first array.

Question 42

In most species, the synaptic connections between retina and optic tectum are

Answer 42

established long before either reaches full size. Then, as the retinas and the optic tectum grow at different rates, the initial synaptic connections shift to other tectal neurons so that each retina is precisely mapped onto the tectum, regardless of their relative sizes.

Question 43

When the optic nerves were cut in mature frogs or fish

Answer 43

the axons did not grow out to their original points of connection (as the chemoaffinity hypothesis predicted they would). Instead, they grew out to fill in the available space in an orderly fashion.

Question 44

topographic gradient hypothesis

Answer 44

axons growing from one topographic surface to another are guided by specific targets that are arranged on the terminal surface in the same way as the axons’ cell bodies are arranged on the original surface. The growing axons are guided to their destinations by two intersecting signal gradients (e.g., an anterior-posterior gradient and a medial-lateral gradient).

Question 45

A single neuron can grow an axon on its own, but it takes

Answer 45

coordinated activity in at least two neurons to create a synapse between them.

Question 46

synaptogenesis

Answer 46

the formation of new synapses; depends on the presence of glial cells, particularly astrocytes.

Question 47

Developing neurons need high levels of cholesterol during synapse formation,

Answer 47

and the extra cholesterol is supplied by astrocytes.

Question 48

Astrocytes may also play a more extensive role in

Answer 48

synaptogenesis by processing, transferring, and storing information supplied by neurons.

Question 49

Studies suggest that any type of neuron will form synapses with any other type.

Answer 49

However, once established, synapses that do not function appropriately tend to be eliminated.

Question 50

Many more neurons, about 50% more, are produced than required.

Answer 50

Large scale neuron death occurs in waves in various parts of the brain throughout development.

Question 51

necrosis

Answer 51

passive cell death. Necrotic cells break apart and spill their contents into extracellular fluid, and the consequence is potentially harmful inflammation.

Question 52

apoptosis

Answer 52

active cell death; safer than necrosis. In apoptotic cell death, DNA and other internal structures are cleaved apart and packaged in membranes before the cell breaks apart. These membranes contain molecules that attract scavenger mircoglia and other molecules that prevent inflammation.

Question 53

Although, genetic programs for apoptotic cell death are blocked

Answer 53

the consequence can be cancer; if the programs are inappropriately activated, the consequence can be neurodengeratitve disease.

Question 54

Two kinds of processes trigger the genetic programs that cause apoptosis in developing neurons:

Answer 54

• Some developing neurons appear to be genetically programmed for an early death – once they have fulfilled their functions, groups of neurons die together, in the absence of any obvious external stimuli.
• Some developing neurons seem to die because they fail to obtain the life-preserving chemicals that are supplied by their targets.

Question 55

Evidence that life-preserving chemicals are supplied to developing neurons by their postsynaptic targets:

Answer 55

• Grafting an extra target structure (e.g., an extra limb) to an embryo before the period of synaptogenesis reduces the death of neurons growing into the area.
• Destroying some of the neurons growing into an area before the period of cell death increases the survival rate of the reaming neurons.

Question 56

nerve growth factors (NGF)

Answer 56

first neurotrophin to be isolated.

Question 57

neurotrophins

Answer 57

promote the growth and survival of neurons, function as axon guidance molecules, and stimulate synaptogenesis.

Question 58

Neurons that have established incorrect connections are particularly likely to

Answer 58

die. As they die, the space they leave vacant on postsynaptic membranes is filled by the sprouting axon terminal of surviving neurons. Thus, cell death results in a massive rearrangement of synaptic connections. This phase of synapse rearrangement tends to focus the output of each neuron on a smaller number of postsynaptic cells, this increasing the selectivity of transmission.