Chapter 7 - Development

Question 1

Growth and Development of the Brain Are _______ Processes

Answer 1

Growth and Development of the Brain Are Orderly Processes

Question 2

Development of the Nervous System in the Human Embryo and Fetus

At 18 days

Answer 2

At 18 days the embryo has begun to implant in the uterine wall and consists of three layers of cells: endoderm, mesoderm, and ectoderm. A thickening of the ectoderm leads to development of the neural plate (insets).

Question 3

Development of the Nervous System in the Human Embryo
and Fetus

At 20 days

Answer 3

At 20 days the neural groove begins to develop.

Question 4

Development of the Nervous System in the Human Embryo and Fetus

At 22 days

Answer 4

At 22 days the neural groove has closed to form the neural tube, with the rudimentary beginning of the brain at the anterior end.

Question 5

Development of the Nervous System in the Human Embryo and Fetus

A few days later then 22 days

Answer 5

A few days later, three major divisions of the brain—forebrain (prosencephalon, consisting of the telencephalon and diencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon)—are discernible.

Question 6

zygote

Answer 6

The fertilized egg

has 46 chromosomes—23 from each parent—

Question 7

ectoderm

Answer 7

The outer cellular layer of the developing fetus, giving rise to the skin and the nervous system.

Question 8

neural groove

Answer 8

In the developing embryo, the groove between the neural folds.
Uneven rates of cell division at the head end form the neural groove, which will become the midline. The pace of events then quickens. The ridges of the groove come together to form the neural tube

Question 9

forebrain

Answer 9

Also called prosencephalon. The anterior division of the brain, containing the cerebral hemispheres, the thalamus, and the hypothalamus.

Question 10

neural tube

Answer 10

An embryonic structure with subdivisions that correspond to the future forebrain, midbrain, and hindbrain.
At the head end of the neural tube, three subdivisions become apparent
The interior of the neural tube becomes the fluid-filled cerebral ventricles of the brain, the central canal of the spinal cord, and the passages that connect them.

Question 11

midbrain

Answer 11

Also called mesencephalon. The middle division of the brain.

Question 12

hindbrain

Answer 12

Also called rhombencephalon. The rear division of the brain, which, in the mature vertebrate, contains the cerebellum, pons, and medulla.

Question 13

embryo

Answer 13

The earliest stage in a developing animal.

the developing human is called an embryo during the first 10 weeks after fertilization; thereafter it is called a fetus

Question 14

fetus

Answer 14

A developing individual after the embryo stage.

Question 15

neurogenesis

Answer 15

The mitotic division of non The process of division of somatic cells that involves duplication of DNA.neuronal cells to produce neurons.

Question 16

mitosis

Answer 16

The process of division of somatic cells that involves duplication of DNA.

Question 17

ventricular zone

Answer 17

Also called ependymal layer. A region lining the cerebral ventricles that displays mitosis, providing neurons early in development and glial cells throughout life.

Question 18

Within 12 hours after fertilization,

Answer 18

Within 12 hours after fertilization, the single cell begins dividing, so after 3 days it has become a small mass of homogeneous cells, like a cluster of grapes,

Question 19

Within a week the emerging embryo

Answer 19

shows three distinct cell layer

Question 20

shows three distinct cell layer

Answer 20

the human embryo shows the rudimentary beginnings of most body organs. The rapid development of the brain is reflected in the fact that by this time the head is half the total size of the embryo.

Question 21

Development of the Nervous System Can BeDivided into Six Distinct Stages

Answer 21

1. Neurogenesis, the mitotic division of nonneuronal cells to produce neurons
2. Cell migration, the movements of cells to establish distinct nerve cell populations (brain nuclei, layers of the cerebral cortex, and so on)
3. Differentiation of cells into distinctive types of neurons or glial cells
4. Synaptogenesis, the establishment of synaptic connections, as axons and dendrites grow
5. Neuronal cell death, the selective death of many nerve cells
6. Synapse rearrangement, the loss of some synapses and development of others, to refine synaptic connections

The six stages proceed at different rates and times in different parts of the nervous system. Some of the stages may overlap even within a region. We will take up the stages in order.

Question 22

Cell proliferation produces cells that become neurons or glial cells

Answer 22

The production of nerve cells is called neurogenesis. Nerve cells themselves do not divide, but the cells that will give rise to neurons begin as a single layer of cells along the inner surface of the neural tube. These cells divide (in a process called mitosis) and gradually form a closely packed layer of cells called the ventricular zone. All neurons and glial cells are derived from cells that originate from this ventricular mitosis. Eventually, some cells leave the ventricular zone and begin transforming into either neurons or glial cells.

Question 23

What is meant by: Each part of an animal’s brain has a species-characteristic “birth date.”

Answer 23

That is,
there is an orderly chronological program for brain development, and we know on
approximately which days of embryonic development the precursor cells of each
group of neurons stop dividing.

Question 24

A favorite animal of researchers who study the lineage of nerve cells is the

Answer 24

nematode Caenorhabditis elegans, a tiny worm with fewer than a thousand cells, precisely 302 of which are neurons. Because the body of C. elegans is almost transparent (FIG-URE 7.4A), researchers have been able to trace the origins of each neuron (Pines, 1992). By observing successive cell divisions of a C. elegans zygote, investigators can exactly predict the fate of each cell in the adult—whether it will be a sensory neuron, muscle cell, skin cell, or other type of cell—on the basis of its mitotic “ancestors”
Whereas cell fate in C. elegans is a highly predetermined and unvarying result of mitotic lineage, in vertebrates the paths that cells take to form the completed nervous system are more complex.
in vertebrates, the paths of development include more-local regulatory mechanisms

Question 25

cell-cell interactions

Answer 25

The general process during development in which one cell affects the differentiation of other, usually neighboring, cells.

The hallmark of vertebrate development is that cells sort themselves out via cell-cell interactions, taking on fates that are appropriate in the context of what neighboring cells are doing. Thus, vertebrate development is less hardwired and more susceptible to being shaped by environmental signals and, as we’ll see, experience.

Question 26

adult neurogenesis

Answer 26

The creation of new neurons in the brain of an adult.

Question 27

cell migration

Answer 27

The movement of cells from site of origin to final location.

Neurons in the developing nervous system are always on the move. At some stage the cells that form in the ventricular layer through mitotic division move away,

Cells do not move in an aimless, haphazard manner. Cells in the developing brain move along the surface of a particular type of glial cell

Question 28

radial glial cells

Answer 28

Glial cells that form early in development, spanning the width of the emerging cerebral hemispheres, and guide migrating neurons.

Cells do not move in an aimless, haphazard manner. Cells in the developing brain move along the surface of a particular type of glial cell. Like spokes (radii) of a wheel, these radial glial cells extend from the inner to the outer surfaces of the emerging nervous system (FIGURE 7.5). The radial glial cells act as a series of guides, and the newly formed cells mostly creep along them, as if “riding the glial monorail” (Hatten, 1990). Some migrating cells move in a direction perpendicular to the radial glial cells (S. A. Anderson et al., 1997), like Tarzan swinging from vine to vine; others move in a rostral stream to supply the olfactory bulbs

Question 29

cell adhesion molecule (CAM)

Answer 29

A protein found on the surface of a cell that guides cell migration and/or axonal pathfinding.

The migration of cells and the outgrowth of nerve cell extensions (dendrites and axons) involve various chemicals that promote the adhesion of developing elements of the nervous system. These cell adhesion molecules (CAMs) guide migrating cells and growing axons (Reichardt and Tomaselli, 1991). Genetic abnormalities in CAMs can disrupt cell migration, resulting in either a vastly reduced population of neurons or a disorderly arrangement and, not surprisingly, behavior disorders. CAMs may also guide axons to regenerate when they are cut in adulthood

Question 30

The postnatal increase of human brain weight is primarily due to

Answer 30

The postnatal increase of human brain weight is primarily due to growth in the
size of neurons, branching of dendrites, elaboration of synapses (as we’ll see in Figure 7.6), increase in myelin, and addition of glial cells.

Question 31

nerve cells of _________ _______ are replaced throughout life

Answer 31

nerve cells of the olfactory organ are replaced throughout life

Question 32

________ ________ also boosts neurogenesis, at least in rats—an effect that can be blocked by ________ such as _______ __________

Answer 32

Physical exercise also boosts neurogenesis, at least in rats—an effect that can be blocked by stresses such as social isolation (Stranahan et al., 2006)—so invest in exercise and a network of friends too.

Question 33

single-file migration

Answer 33

The single-file migration of nerve cell precursors (see Figure 7.5A) is followed by
the aggregation of cells into the brain nuclei we discussed in Chapter 2. For example,
cells of the cerebral cortex arrive in waves during fetal development, each successive
wave forming a new outer layer, until the six layers of the adult cortex are formed,
with the latest arrivals on the outside.

Question 34

Degeneration and Regeneration of Nervous Tissue

Answer 34

When a mature nerve cell is injured, it can regrow in several ways.

Complete replacement of injured nerve cells is rare in mammals,

Injury close to the cell body of a neuron produces a series of changes resulting in the eventual destruction of the cell; this process is called retrograde degeneration

If the injured neuron dies, the target cells formerly innervated by that neuron may show signs of trans-neuronal degeneration

Cutting through the axon also produces loss of the distal part of the axon (the part that is separated from the cell body). This process is called anterograde degeneration. The part of the axon that remains connected to the cell body may regrow.

Cell adhesion molecules (CAMs) help guide the regenerating axons.

Studying regeneration, then, may increase our understanding of the original processes of growth of the nervous system, and vice versa. From a therapeutic view-point, these studies may help scientists learn how to repair and regrow dam-aged tissue in human brains.

Question 35

retrograde degeneration

Answer 35

Destruction of the nerve cell body following injury to its axon.

Injury close to the cell body of a neuron produces a series of changes resulting in the eventual destruction of the cell; this process is called retrograde degeneration

Question 36

anterograde degeneration

Answer 36

Also called Wallerian degeneration. The loss of the distal portion of an axon resulting from injury to the axon.

Cutting through the axon also produces loss of the distal part of the axon (the part that is separated from the cell body). This process is called anterograde degeneration. The part of the axon that remains connected to the cell body may regrow.

Question 37

expression

Answer 37

The process by which a cell makes an mRNA transcript of a particular gene.
Once they reach their destinations, however, the cells begin to use, or express, particular genes. This means that the cell transcribes a particular subset of genes to make the specific proteins it needs.

Question 38

cell differentiation

Answer 38

The developmental stage in which cells acquire distinctive characteristics, such as those of neurons, as the result of expressing particular genes.

the cell transcribes a particular subset of genes to make the specific proteins it needs. This process of cell differentiation shapes the cell into the distinctive appearance and functions of neurons characteristic of that particular region

What controls differentiation is not completely under-stood, but two classes of influence are known.
First, intrinsic self-organization is an important factor;
However, the neural environment also greatly influences nerve cell differentiation. In other words, neighboring cells are a second major influence on the differentiation of neurons.

young neural cells seem to have the capacity to become many varieties of neurons, and the particular type of neuron that a cell becomes depends on where it happens to be and what its neighboring cells are.

Question 39

in vitro

Answer 39

Literally “in glass” (in Latin). Usually, in a laboratory dish; outside the body.

Question 40

cell-autonomous

Answer 40

Referring to cell processes that are directed by the cell itself rather than being under the influence of other cells.

When a cell shows characteristics that are independent of neighboring cells, we say that it is acting in a cell-autonomous manner. In cell-autonomous differentiation, presumably only the genes within that cell are directing events.

Question 41

notochord

Answer 41

A midline structure arising early in the embryonic development of vertebrates.

in the notochord, a rodlike structure that forms along the midline. The notochord
releases a protein (named Sonic hedgehog) that diffuses to the spinal cord and directs some (but not all) cells to become motoneurons

Question 42

induction

Answer 42

The process by which one set of cells influences the fate of neighboring cells, usually by secreting a chemical factor that changes gene expression in the target cells.

The influence of one set of cells on the fate of neighboring cells is known as induction

Question 43

Because each cell influences the differentiation of others, vertebrate neural development is very complex, but also very flexible.

Answer 43

Because each cell influences the differentiation of others, vertebrate neural development is very _______, but also very _________.

Question 44

regulation

Answer 44

An adaptive response to early injury, as when developing individual cells compensate for missing or injured cells.

Embryologists refer to such adaptive responses to early injury as regulation: the developing animal compensates for missing or injured cells

Question 45

stem cell

Answer 45

A cell that is undifferentiated and therefore can take on the fate of any cell that a donor organism can produce.

if cells that have not yet differentiated extensively can be obtained and placed into a particular brain region, they will differentiate in an appropriate way and be-come properly integrated. Such undifferentiated cells, called stem cells, are present throughout embryonic tissues, so they can be gathered from umbilical-cord blood, miscarried embryos, or unused embryos produced during in vitro fertilization.

Question 46

process outgrowth

Answer 46

The extensive growth of axons and dendrites.

Question 47

synaptogenesis

Answer 47

The establishment of synaptic connections as axons and dendrites

Question 48

growth cone

Answer 48

The growing tip of an axon or a dendrite.

At the tips of axons and dendrites alike, specialized swellings called growth cones are found. Very fine extensions, called filopodia (singular filopodium, from the Latin filum, “thread,” and the Greek pous, “foot”), extend from the growth cone

Question 49

filopodia

Answer 49

Very fine, tubular outgrowths from the growth cone.

Very fine extensions, called filopodia (singular filopodium, from the Latin filum, “thread,” and the Greek pous, “foot”), extend from the growth cone

Just as migrating cells are guided by CAMs, the filopodia of growth cones adhere to CAMs in the extracellular environment and then contract to pull the growth cone in a particular direction (the growing axon or dendrite trailing behind). Dendritic growth cones are found in adults, mediating the continued elongation and change in dendrites that occurs throughout life in response to experience

Question 50

The biggest change in brain cells early in life is the

Answer 50

The biggest change in brain cells early in life is the extensive growth of axons and dendrites (termed process outgrowth) and the proliferation of synapses (synaptogenesis).

Question 51

What guides axons along the paths they take?

Answer 51

The CAMs guiding growth cones are released by the target nerve cells or other tissues, such as muscles. The axon growth cone responds to the concentration gradients of these chemicals that pro-vide directional guidance.

Question 52

chemoattractants

Answer 52

Compounds that attract particular classes of growth cones.

Question 53

chemorepellents

Answer 53

Compounds that repel particular classes of growth cones.

Question 54

cell death or apoptosis

Answer 54

The developmental process during which “surplus” cells die.

As strange as it may seem, cell death is a crucial phase of brain development, especially during embryonic stages. This developmental stage is not unique to the nervous system. Naturally occurring cell death, also called apoptosis (from the Greek apo, “away from,” and ptosis, “act of falling”), is a kind of sculpting process in the emergence of other tissues in both animals and plants.

These cells are not dying because of a defect. Rather, it appears that these cells have “decided” to die and are actively committing suicide

apoptosis in vertebrates is regulated by cell-cell interactions, such as the availability of synaptic targets. Reducing the size of the synaptic target invariably reduces the number of surviving nerve cells.

Question 55

death gene

Answer 55

A gene that is expressed only when a cell becomes committed to natural cell death (apoptosis).

Your chromosomes carry death genes—genes that are expressed only when a cell undergoes apoptosis

Question 56

caspases

Answer 56

A family of proteins that regulate cell death (apoptosis).

Question 57

Diablo

Answer 57

A protein released by mitochondria, in response to high calcium levels, that activates apoptosis inhibitors of apoptosis

Question 58

inhibitors of apoptosis proteins (IAPs)

Answer 58

A family of proteins that inhibit caspases and thereby stave off apoptosis.

Question 59

Bcl-2

Answer 59

A family of proteins that regulate apoptosis.

Question 60

neurotrophic factor

Answer 60

Also called trophic factor. A target-derived chemical that acts as if it “feeds” certain neurons to help them survive.

Neurons that receive enough of the chemical survive; those that do not, die. Such target-derived chemicals are called neurotrophic factors (or simply trophic factors) because they act as if they “feed” the neurons to help them survive (in Greek, trophe means “nourishment”). The neurotrophic factor that was the first to be identified prevents the death of developing sympathetic neurons,

nerve growth factor (NGF)
brain-derived neurotrophic factor (BDNF)

The exact role of these various factors (and other neurotrophic factors yet to be discovered) is under intense scientific scrutiny. One role of neurotrophic factors seems to be guiding the rearrangement of synaptic connections,

Question 61

nerve growth factor (NGF)

Answer 61

A substance that markedly affects the growth of neurons in spinal ganglia and in the ganglia of the sympathetic nervous system.

that markedly affects the growth of neurons in spinal ganglia and in the ganglia of the sympathetic nervous system

Administered to a chick embryo, NGF resulted in many more sympathetic neurons than usual. These cells were also larger and had more extensive processes

Various target organs normally produce NGF during development. It is taken up by the axons of sympathetic neurons that innervate those organs and transported back to the cell body, where NGF prevents the sympathetic neurons from dying. The amount of NGF produced by targets during development is roughly correlated with the amount of sympathetic innervation that the targets maintain into adulthood. Thus, cell death, controlled by access to NGF, provides each target with an appropriate amount of sympathetic innervation

Question 62

brain-derived neurotrophic factor (BDNF)

Answer 62

A protein purified from the brains of animals that can keep some classes of neurons alive.

Question 63

neurotrophin

Answer 63

A chemical that prevents neurons from dying.

Question 64

synapse rearrangement

Answer 64

Also called synaptic remodeling. The loss of some synapses and the development of others; a refinement of synaptic connections that is often seen in development.

Just as not all the neurons produced by a developing individual are kept into adulthood, some of the synapses formed early in development are later retracted. Originally this process was described as synapse elimination, but later studies foundthat, although some original synapses are indeed lost, many new synapses are also formed as they compete for neurotropic factors (FIGURE 7.14). Thus, a more accurate term is synapse rearrangement, or synaptic remodeling. In most cases, synapse rearrangement takes place after the period of cell death.

Question 65

myelination

Answer 65

The process of myelin formation.

The development of sheaths around axons—the process of myelination changes the rate at which axons conduct messages.Myelination has a strong impact on behavior because it allows large networks of cells to communicate rapidly.

In humans, the earliest myelination in the peripheral nervous system is evident in cranial and spinal nerves about 24 weeks after conception. But the most intense phase of myelination occurs shortly after birth.

The first nerve tracts in the human nervous system to become myelinated are in the spinal cord. Myelination then spreads successively into the hindbrain, midbrain, and forebrain. Within the cerebral cortex, sensory zones are myelinated before motor zones; correspondingly, sensory functions mature before motor functions.

Question 66

multiple sclerosis

Answer 66

Literally, “many scars”; a disorder characterized by widespread degeneration of myelin.

Multiple sclerosis is a disorder in which myelin is de-stroyed by the person’s own immune system in random distinct patches (Manova and Kostadinova, 2000). The resultant desynchronization of activity in these loca-tions can cause devastating disruptions of sensory and motor function.

Question 67

intellectual disability

Answer 67

A disability characterized by significant limitations in intellectual functioning and adaptive behavior.

intellectual disability refers to a variety of conditions that impede mental growth.

Question 68

hypoxia

Answer 68

A transient lack of oxygen.

are at greater risk for intellectual dis-ability than are children who have a problem-free birth.

Question 69

behavioral teratology

Answer 69

The study of impairments in behavior that are produced by embryonic or fetal exposure to toxic substances.

Question 70

fetal alcohol syndrome (FAS)

Answer 70

A disorder, including intellectual disability and characteristic facial anomalies, that affects children exposed to too much alcohol (through maternal ingestion) during fetal development.

Prominent anatomical effects of fetal exposure to alcohol include distinctive changes in facial features (e.g., a sunken nasal bridge and altered shape of the nose and eyelids) and stunted growth. In some cases, the children may lack a corpus callosum (FIGURE 7.17). Few FAS children catch up in the years following birth. The most common problem associated with FAS is intellectual disability, which varies in severity.

Question 71

autism

Answer 71

A disorder arising during child-hood, characterized by social withdrawal and perseverative behavior.

Autism is a developmental disorder characterized by impaired social interactions and language and a narrow range of interests and activities.

Usually autism is discovered when apparently normal toddlers begin regressing, losing language skills, and withdrawing from family interaction. Children with autism tend to perseverate (such as by continually nod-ding the head or making stereotyped finger movements), actively avoid making eye contact with other people, and have a difficult time judging other people’s thoughts or feelings

Several structural differences between the brains of people with autism and controls have been reported, including a reduction in the size of the corpus callosum and certain cerebellar regions (Egaas et al., 1995). Even though people with autism have fewer neurons in the amygdala than control subjects have (Schumann and Amaral, 2006), they show greater activation of the amygdala when they are gazing at faces (K. M. Dalton et al., 2005). The amygdala has been associated with fear (see Chapter 15), so this finding suggests that children with autism avoid making eye contact with people because they find it aversive.

Underactivation of Mirror Cells in Autism
.

Question 72

perseverate

Answer 72

To continue to show a behavior repeatedly.

Question 73

Asperger’s syndrome

Answer 73

Sometimes called high-functioning autism. A syndrome characterized by difficulties in social cognitive processing; usually accompanied by strong language skills.

is also characterized by difficulties in understanding social interactions, yet children with Asperger’s do not lose their language capabilities, and they may indeed be quite articulate. They have difficulty interpreting other people’s emotional facial expressions, but they tend to be very good at classifying objects and noting details

Question 74

genotype

Answer 74

All the genetic information that one specific individual has inherited.

Your genotype, some 20,000 genes (Pertea and Salzberg, 2010), was determined at the moment of fertilization and remains the same throughout your life.

Question 75

phenotype

Answer 75

The sum of an individual’s physical characteristics at one particular time.

Changes constantly, as you grow up and grow old and even, in a tiny way, as you take each breath. In other words, phenotype is determined by the interaction of genotype and extrinsic factors, including experience. Thus, as we’ll see, individuals who have identical genotypes do not have identical phenotypes, because they have not received identical extrinsic influences. And since their nervous system phenotypes are somewhat different, they do not behave exactly the same.

Question 76

mutation

Answer 76

A change in the nucleotide sequence of a gene as a result of unfaithful replication.

In rare instances, an animal inherits a sudden change in genetic structure, a mutation,that results in marked anatomical or physiological change. Researchers can increase the frequency of mutations by exposing animals to radiation or chemicals that produce changes in genes.