Chapter 3.4 Embryonic Period Flashcards Preview

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Flashcards in Chapter 3.4 Embryonic Period Deck (83):

What does the embryonic period begin with?

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


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

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


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

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


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

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


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

*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


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

*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


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



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



Name the three primary germ layers?

1) Ectoderm
2 )Mesoderm
3) Endoderm


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



Gastrulation begins with what?

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


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

A slightly elevated area surrounding a small primitive pit


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

The primitive streak between the epiblast and hypoblast layers


Inward movement of cells is known as



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



Other migrating cells eventually displace the hypoblast and form what?

The endoderm


Cells remaining in the epiblast then form what?

The ectoderm


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

The epiblast


The third week of development produces an embryo with what

Three primary germ layers: Ectoderm, mesoderm, and endoderm


The 3-week embryo is what type of structure?

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


The shape transformation of the embryo occurs during which weeks

During the late third and fourth weeks of development


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

Fold on itself and become more cylindrical


The two types of folding that occur are

1) Cephalocaudal folding
2) Transverse folding


Where does the cephalocaudal folding occur?

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


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

The yolk sac


Transverse folding (or lateral folding) occurs when

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)


The cephalocaudal folding helps to create the future what

Head and buttocks region of the embryo


The transverse folding creates what?

Cylindrical trunk or torso region of the embryo


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

On the external surface of the now-cylindrical embryo


The ectoderm is responsible for forming what?

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


Neurulation is

The process of nervous system formation from the ectoderm


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

Internal and parallel to the primitive streak


The notochord influences what?

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


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



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

The neural plate


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

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


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

Cylindrical neural tube


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

Cephalic and caudal directions


This eventually forms the brain and spinal cord

The cylindrical neural tube


Neurulation is complete by

The end of the fourth week of development


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

Dissociate from adjacent cells


Neural crest cells migrate throughout the body and give rise to

A vast, heterogeneous array of structures


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

*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


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

The epidermis, the external layer of the skin


Ectoderm also forms

Most exocrine
*Tooth enamel
*Sensory organs


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

Maintain contact with the outside world


The mesoderm subdivides into 5 categories

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


Tightly packed midline group of mesodermal cells forms the notochord.



The notochord serves a s the basis for

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


Where is paraxial mesoderm found and what does it form?

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


Define somites

Blocklike masses responsible for the formation of the axial skeleton


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

Most of the urinary system and the reproductive system


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

Lateral plate mesoderm


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

Lateral plate mesoderm


Forms connective tissues and musculature of the face

Head mesenchyme


When does endoderm become the innermost tissue?

When the embryo undergoes transverse folding


Among the structures formed by the embryonic endoderm are?

The linings of the
1) Digestive
2) Respiratory
3) Urinary
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


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

Organogenesis (organ development)


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

By week 8


When does the embryo have an appearance of a human?

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


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

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


Teratogens include what?

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


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

The malformation of some or all organ systems


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

Peak development periods at different times


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

During its peak development period


Give examples of ectoderm

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


Give examples of Mesoderm

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


Give an example of Endoderm

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)


The fetal period extends from

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


What is the fetal period characterized as?

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


The length of the fetus is usually measured in

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


The fetal length increases dramatically in which months

3 to 5


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

53 centimeters (21 inches)


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

In the last 2 months of pregnancy


What is the average weight of the full-term fetus

It ranges from 2.5 to 4.5 kilograms


*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

Weeks 9-12


*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

Weeks 13-16


*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

Weeks 17-20


*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

Weeks 21-38


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



A malformation or deformity present at birth

Congenital anomaly


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

Ectopic pregnancy


Period of intrauterine development