Chapter 3.4 Embryonic Period Flashcards
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
Gastrulation
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
Gastrulation
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?
Embryo
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
Invagination
The layer of cells that forms between the epiblast and hypoblast layers becomes the primary germ layer known as
Mesoderm
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