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
*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?
*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?
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?
Cells remaining in the epiblast then form what?
What is the source of the three primary germ layers from which all body tissues and organs eventually derive?
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
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
In general, ectoderm gives rise to those organs and structures that
Maintain contact with the outside world
The mesoderm subdivides into 5 categories
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
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
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
Also forms the
1) Thyroid gland
2) Parathyroid glands
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
7) Kidneys and ureters
8) Internal reproductive organs
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)
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
*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
*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
*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
*Body gains major amount of weight
*Subcutaneous fat is deposited
*Eyebrows and eyelashes appear
*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
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
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