Chapter 3.4 Embryonic Period

Question 1

What does the embryonic period begin with?

Answer 1

The establishment of the three primary germ layers through the process of gastrulation

Question 2

Subsequent interactions and rearrangements among the cells of the three layers prepare for what?

Answer 2

Prepare for the formation of specific tissues and organs, a process called organogenesis

Question 3

What has been established by the end of the embryonic period (week 8)?

Answer 3

The main organ systems have been established, and the major features of the external body form are recognizable

Question 4

What are the events of week 3 for the embryonic development?

Answer 4

• Primitive streak appears
• Three primary germ layers form
• Notochord develops
• Neurulation begins
• Lenght 1.5 mm

Question 5

What are the events for week 4 of the embryonic development?

Answer 5

• Cephalocaudal and lateral folding produce a cylindrical embryo
• Basic human body plan is established
• Derivatives of the three germ layers begin to form
• Limb buds appear
• Crown-rump length is 4.0 mm

Question 6

What are the events for week 5-8 of the embryonic development?

Answer 6

• Head enlarges
• Eyes, ears, and nose appear
• Major organ systems are formed by the end of week 8 (although some may not be fully functional yet)
• Crown-rump length by the end of week 8 is 30mm

Question 7

Occurs during the third week of development immediately after implantation, and is one of the most critical periods in the development of the embryo

Answer 7

Gastrulation

Question 8

Process by which the cells of the epiblast migrate and form the three primary germ layers, which are the cells from which all body tissues develop

Answer 8

Gastrulation

Question 9

Name the three primary germ layers?

Answer 9

1) Ectoderm
2 )Mesoderm
3) Endoderm

Question 10

Once the three primary germ layers have formed, the developing trilaminar (three layered) structure may be called what?

Answer 10

Embryo

Question 11

Gastrulation begins with what?

Answer 11

The formation of the primitive streak, a thin depression on the surface of the epiblast

Question 12

The cephalic (head) end of the streak, known as the primitive node, consists of

Answer 12

A slightly elevated area surrounding a small primitive pit

Question 13

Cells detach from the epiblast layer and migrate through what layers?

Answer 13

The primitive streak between the epiblast and hypoblast layers

Question 14

Inward movement of cells is known as

Answer 14

Invagination

Question 15

The layer of cells that forms between the epiblast and hypoblast layers becomes the primary germ layer known as

Answer 15

Mesoderm

Question 16

Other migrating cells eventually displace the hypoblast and form what?

Answer 16

The endoderm

Question 17

Cells remaining in the epiblast then form what?

Answer 17

The ectoderm

Question 18

What is the source of the three primary germ layers from which all body tissues and organs eventually derive?

Answer 18

The epiblast

Question 19

The third week of development produces an embryo with what

Answer 19

Three primary germ layers: Ectoderm, mesoderm, and endoderm

Question 20

The 3-week embryo is what type of structure?

Answer 20

A flattened, disc-shaped structure, it is also referred to as an embryonic disc

Question 21

The shape transformation of the embryo occurs during which weeks

Answer 21

During the late third and fourth weeks of development

Question 22

As a result of differential growth, the embryonic disc starts to do what?

Answer 22

Fold on itself and become more cylindrical

Question 23

The two types of folding that occur are

Answer 23

1) Cephalocaudal folding

2) Transverse folding

Question 24

Where does the cephalocaudal folding occur?

Answer 24

It occurs in the cephalic (head) and caudal (tail) regions of the embryo

Question 25

The embryonic disc and amnion grow very rapidly, but what does not grow at all which eventually causes the head and tail regions to fold on themselves

Answer 25

The yolk sac

Question 26

Transverse folding (or lateral folding) occurs when

Answer 26

The left and right sides of the embryo curve and migrate toward midline. As the sides come together they restrict and start to pinch off the yolk sac. Eventually the sides of the embryonic disc fuse in the midline and create a cylindrical embryo. Thus, the ectoderm is now solely along the entire exterior of the embryo, while the endoderm in confined in the internal region of the embryo. As this midline fusion occurs, the yolk sac pinches off from most of the endoderm (with the exception of one small region of communication called the vitelline duct)

Question 27

The cephalocaudal folding helps to create the future what

Answer 27

Head and buttocks region of the embryo

Question 28

The transverse folding creates what?

Answer 28

Cylindrical trunk or torso region of the embryo

Question 29

After the embryo undergoes cephalocaudal and transverse folding where is the ectoderm located

Answer 29

On the external surface of the now-cylindrical embryo

Question 30

The ectoderm is responsible for forming what?

Answer 30

The nervous system tissue as well as many externally placed structures, including the epidermis of the skin and epidermal derivatives such as hair and nails

Question 31

Neurulation is

Answer 31

The process of nervous system formation from the ectoderm

Question 32

A cylindrical structure of mesoderm, called the notochord, forms immediately where?

Answer 32

Internal and parallel to the primitive streak

Question 33

The notochord influences what?

Answer 33

Some of the overlying ectoderm to begin to form nervous tissue via a process called induction, in which one structure influences or induces another structure to change form

Question 34

The inductive action that transforms a flat layer of ectodermal cells into a hollow nervous system tube

Answer 34

Neurulation

Question 35

In the third week of development, much of the ectoderm forms a thickened layer of cells called

Answer 35

The neural plate

Question 36

By the end of the third week, the lateral edges of the neural plate elevate to form

Answer 36

Neural folds, and the depression between the folds forms the neural groove

Question 37

The neural folds approach each other gradually in the midline and fuse. Fusion of these folds produces a

Answer 37

Cylindrical neural tube

Question 38

The fusion of the neural folds begins in the middle of the neural folds and proceeds in both

Answer 38

Cephalic and caudal directions

Question 39

This eventually forms the brain and spinal cord

Answer 39

The cylindrical neural tube

Question 40

Neurulation is complete by

Answer 40

The end of the fourth week of development

Question 41

As the neural folds migrate toward each other and fuse, some cells along the lateral border of folds begin to

Answer 41

Dissociate from adjacent cells

Question 42

Neural crest cells migrate throughout the body and give rise to

Answer 42

A vast, heterogeneous array of structures

Question 43

Among the structures neural crest cells give rise to are what?

Answer 43

* Melanocytes (pigment of the skin) * Adrenal medulla (inner portion of the adrenal gland) * Some skeletal and muscular components of the head * Spinal ganglia (specific nervous system structures) * A portion of the developing heart

Question 44

Not all ectodermal cells form the neural plate. The ectodermal cells covering the embryo after neurulation form

Answer 44

The epidermis, the external layer of the skin

Question 45

Ectoderm also forms

Answer 45

Most exocrine * Glands * Hair * Nails * Tooth enamel * Sensory organs

Question 46

In general, ectoderm gives rise to those organs and structures that

Answer 46

Maintain contact with the outside world

Question 47

The mesoderm subdivides into 5 categories

Answer 47

1) Chordamesoderm 2) Paraxial mesoderm to somites 3) Intermediate mesoderm 4) Lateral plate mesoderm 5) Head mesenchyme

Question 48

Tightly packed midline group of mesodermal cells forms the notochord.

Answer 48

Chordamesoderm

Question 49

The notochord serves a s the basis for

Answer 49

The central body axis and the axial skeleton, and induces the formation of the neural tube

Question 50

Where is paraxial mesoderm found and what does it form?

Answer 50

Found on both sides of the neural tube. It then forms somites, most muscle (including the limb musculature), and most of the cartilage, dermis, and connective tissues of the body

Question 51

Define somites

Answer 51

Blocklike masses responsible for the formation of the axial skeleton

Question 52

Lateral to the paraxial mesoderm are cords of intermediate mesoderm which forms

Answer 52

Most of the urinary system and the reproductive system

Question 53

The most lateral layers of mesoderm on both sides of the neural tube remain thin and are called

Answer 53

Lateral plate mesoderm

Question 54

These give rise to most of the components of the cardiovascular system, the lining of the body cavities, and the thoracic and abdominal body walls, and all the connective tissue components of the limbs

Answer 54

Lateral plate mesoderm

Question 55

Forms connective tissues and musculature of the face

Answer 55

Head mesenchyme

Question 56

When does endoderm become the innermost tissue?

Answer 56

When the embryo undergoes transverse folding

Question 57

Among the structures formed by the embryonic endoderm are?

Answer 57

``` The linings of the 1) Digestive 2) Respiratory 3) Urinary Tracts Also forms the 1) Thyroid gland 2) Parathyroid glands 3) Thymus 4) Portions of the palatine tonsils 5) Most of the liver, gallbladder, and pancreas ```

Question 58

Once the 3 primary germ layers have formed and the embryo has undergone cephalocaudal and transverse folding what process can begin?

Answer 58

Organogenesis (organ development)

Question 59

The upper and lower limbs attain their adult shapes, and the rudimentary forms of most organ systems have developed by what week of development?

Answer 59

By week 8

Question 60

When does the embryo have an appearance of a human?

Answer 60

By the end of the embryonic period, the embryo is slightly longer than 2.5 centimeters (1 inch)

Question 61

During the embryonic period the embryo is slightly sensitive to what?

Answer 61

Teratogens, substances that cause birth defects or the death of the embryo

Question 62

Teratogens include what?

Answer 62

Alcohol, tobacco smoke, drugs, some viruses, and even some medications such as aspirin

Question 63

Because the embryonic period includes organogenesis, exposure to teratogens at this time can result in?

Answer 63

The malformation of some or all organ systems

Question 64

Although rudimentary versions of most organ systems have formed during the embryonic period, different organ systems undergo what?

Answer 64

Peak development periods at different times

Question 65

When do teratogens cause the most harm to an organ system?

Answer 65

During its peak development period

Question 66

Give examples of ectoderm

Answer 66

1) Epidermis of skin and epidermal derivatives (hair, nails, sweat glands, mammary glands) 2) Nervous tissue and sense organs 3) Pituitary gland 4) Adrenal medulla 5) Enamel of teeth 6) Lens of eye

Question 67

Give examples of Mesoderm

Answer 67

1) Dermis of skin 2) Epithelial lining of blood vessels, lymph vessels, body cavities, joint cavities 3) Muscle tissue 4) Connective tissue (including connective tissue proper, bone, cartilage, blood) 5) Adrenal cortex 6) Heart 7) Kidneys and ureters 8) Internal reproductive organs 9) Spleen

Question 68

Give an example of Endoderm

Answer 68

1) Epithelial lining of the respiratory tract, GI tract, tympanic cavity, auditory tube, urinary bladder, and urethra 2) Liver (most of) 3) Gallbladder 4) Pancreas 5) Thymus 6) Thyroid gland 7) Parathyroid gland 8) Palatine tonsils (portion of)

Question 69

The fetal period extends from

Answer 69

The beginning of the third month of development (week 9) to birth

Question 70

What is the fetal period characterized as?

Answer 70

By the maturation of tissues and organs and rapid growth of the body

Question 71

The length of the fetus is usually measured in

Answer 71

Centimeters, either as the crown-rump length (CRL) or the crown heel length (CHL)

Question 72

The fetal length increases dramatically in which months

Answer 72

3 to 5

Question 73

The 2.5-centimeter embryo will grow in the fetal period to an average length of

Answer 73

53 centimeters (21 inches)

Question 74

Fetal weight increases steadily as well, although the weight increase is most striking when?

Answer 74

In the last 2 months of pregnancy

Question 75

What is the average weight of the full-term fetus

Answer 75

It ranges from 2.5 to 4.5 kilograms

Question 76

* Primary ossification centers appear in most bones * Reproductive organs begin to develop * Coordination between nerves and muscles for movement of limbs occurs * Brain enlarges * Body elongates * Epidermis and dermis of the skin become more fully developed * Permanent kidney develops * Palate (roof of mouth) develops * Average crown-rump length at 12 weeks: 9 cm * Average weight: 28g

Answer 76

Weeks 9-12

Question 77

* Body grows rapidly * Ossification in the skeleton continues * Limbs become more proportionate to length to body * Brain and skull continue to enlarge * Average crown-rump length at 16 weeks: 14 cm * Average weight: 170 g

Answer 77

Weeks 13-16

Question 78

* Muscle movements become stronger and more frequent * Lanugo covers skin * Vernix caseosa covers skin * Brain and skull continue to enlarge * Average crown-rump length at 20 weeks: 19 cm * Average weight: 454 g

Answer 78

Weeks 17-20

Question 79

* Body gains major amount of weight * Subcutaneous fat is deposited * Eyebrows and eyelashes appear * Eyelids open * Testes descend into scrotum (month 9) * Blood cells form in marrow only * Average crown-rump length at 38 weeks: 36 cm * Average total length at 38 weeks: 53 cm * Average weight: 2.5-4.5 kg

Answer 79

Weeks 21-38

Question 80

Termination of pregnancy by premature removal of the embryo or fetus from the uterus; may be spontaneous or induced

Answer 80

Abortion

Question 81

A malformation or deformity present at birth

Answer 81

Congenital anomaly

Question 82

A pregnancy in which the embryo implants outside the uterus; commonly occurs in the uterine tube, in which case it is called a tubal pregnancy

Answer 82

Ectopic pregnancy

Question 83

Period of intrauterine development

Answer 83

Gestation