Week 3 of development Flashcards
What is gastrulation?
The formation of the trilaminar disc, and the establishment of axial orientation
What brings about gastrulation?
Gastrulation is brought about due to:
▪ Cell-shape changes
▪ Re-arrangement
▪ Movement
▪ Alteration in adhesive properties
What is the first sign of gastrulation?
The formation of the primitive streak on the epiblast
How does the primitive streak form?
-Proliferation and migration of epiblast cells to the median plane.
-The thickened linear band forms in the caudal direction
-
The formation of the primitive streak allows for…? (3)
- identification of a craniocaudal axis
- dorsal and ventral surfaces
- right and left sides
What is found in the cranial end of the primitive streak?
Primitive node
The depression in the primitive node is called the____________.
Primitive pit
What is a primitive groove?
It is a furrow along the length of the primitive streak, which forms as the primitive streak forms in the caudal direction.
What causes the formation of the primitive pit and the primitive groove?
The primitive groove and pit form as a result of
invagination of epiblastic cells
When and how is mesenchyme formed?
Shortly after the primitive streak appears several cells leave the deep surface and form mesenchyme – an embryonic connective tissue that consists of small spindle-shaped cells loosely arranged in an extracellular matrix.
Describe the process of formation of three germ layers.
Process of Invagination:
* Cells of epiblast migrate towards primitive streak
* At arrival, these cells detach from the epiblast
* Once detached the cells slip under the epiblast.
Once all cells have invaginated:
▪ Some displace the hypoblast – ENDODERM
▪ Some lie between epiblast and endoderm – MESODERM
▪ Some cells remain in the epiblast - ECTODERM
Cells of the epiblast replace the hypoblast and form the____________.
Endoderm
What is derived from the endoderm?
Endoderm gives rise to the lining of the future gut and gut derivatives.
- Endoderm is the source of:
- Epithelial lining of respiratory and digestive tracts
- Glands opening into the gastrointestinal tract
- Glandular cells of the associated organs of the gastrointestinal tract (e.g. liver and pancreas)
What is derived from the mesoderm?
-Skeletal muscles, most of the CVS, and visceral smooth muscular coat.
- Blood cells and lining of blood vessels
- Serosa of body cavities
- Ducts and organs of the urogenital system
Name the three types of mesoderm and where each mesoderm is derived from the primitive streak.
PARAXIAL MESODERM: Lateral aspect of the primitive node and the cranial aspect of the primitive streak.
INTERMEDIATE MESODERM: Midstreak
LATERAL PLATE MESODERM: Caudal aspect of the primitive streak.
Which mesoderm is invloved in the formation of somites, responsible for development of sclerotome
(vertebral column + ribs), dermatome and myotome?
Paraxial mesoderm
Intermediate mesoderm is involved in the formation of…?
Urogenital structures,, including the kidneys and gonads.
Name the two divisions of the lateral plate mesoderm, and what each division is responsible of forming.
Parietal (Splanchnic) Mesoderm – forms dermis of skin, bones of limbs and sternum
➢ Visceral (Somatic) Mesoderm – forms the main aspects of the gastrointestinal tract,
and forms the gut tubes together with endoderm
What happens to the epiblast cells that remain after the endoderm and mesoderm has been formed?
They become the ectoderm.
What does the ectoderm differentiate into?
It differentiates into the the neural plate and surface ectoderm.
What structures are formed from the ectoderm?
- Epidermis
- Central and peripheral nervous systems
- Eyes and internal ear
- Neural crest cells
- Connective tissues of the head
What happens to the primitive streak after its function is over?
-It diminishes in size and becomes an insignificant structure in the sacrococcygeal region.
-It then degenerates by the end of the 4th week
What is the pathology that occurs if the primitive streak does not disappear?
Sacrococcygeal teratoma, which is a tumor that forms in the sacrococcygeal region before birth
Explain how the notochordal process is formed.
- As cells mesodermal ‘slip’ through the primitive, streak some of these cells migrate cranially from the primitive pit and node.
- These cells form a median cellular cord – NOTOCHORDAL PROCESS
- The notochordal process eventually gets a lumen which forms the
notochordal canal.
Explain how the notochordal process is formed.
- As cells mesodermal ‘slip’ through the primitive, streak some of these cells migrate cranially from the primitive pit and node.
- These cells form a median cellular cord – NOTOCHORDAL PROCESS
- The notochordal process eventually gets a lumen which forms the
notochordal canal.
The notochordal process grows cranially until it reaches the____________
Prechordal plate
What is a prechordal plate?
It is a small circular area where the endoderm and the ectoderm are fused. It is located cranial to the primitive node
The prechordal plate gives rise to____________.
The oropharyngeal membrane, which is the site for future oral cavity.
What is the cloacal membrane?
It is a small circular area where the endoderm and the ectoderm are fused, caudal to the primitive streak. It is the site for the future anus
Signals from the__________________ induce notochordal precursor cells to form the notochord
primitive streak
Extension of notochord
- Extends from oropharyngeal membrane to primitive node
What happens to the notochord?
The notochord degenerates as bodies of vertebrae form; however, small portions remain as the nucleus pulposus of the intervertebral discs
How is the neural plate formed?
-Notochord development causes a thickening of the ectoderm
around the midline.
*This thickening forms the elongated NEURAL PLATE
The neural plate broadens and extends cranially to the oropharyngeal membrane and eventually extends past the notochord.
What does the neural plate form?
The neuroectoderm of the neural plate forms the central nervous system as well as other structures such as the retina
When does neurulation occur?
Around day 18
Describe the process of neurulation
-Neural plate invaginates along the central axis forming the MEDIAN LONGITUDINAL NEURAL GROOVE with NEURAL FOLDS on either side.
-These neural folds become prominent at the cranial end of the embryo and are the first signs of brain development.
When do the neural folds fuse to form the neural tube?
By the end of the third week the folds move together to fuse thereby forming the NEURAL TUBE
What are the cranial and caudal ends of the neural tube?
- Cranial and caudal ends of the neural tube are known as the anterior and posterior neuropores (respectively) until the neural tube closes.
When is neurulation completed?
- Neurulation is completed during the 4th week.
How are neural crest cells formed?
As the neural folds close, some of the neuroectodermal cells at the inner margin of these folds detach.
* These cells, termed the NEURAL CREST CELLS, form a flattened irregular mass between the neural tube and the overlying ectoderm.
-The neural crest then separates into left and right parts that move to the dorsolateral aspect of the neural tube where they form sensory ganglia of the spinal and cranial nerves.
What are the derivatives of cranial neural crest cells?
Craniofacial bone and cartilage
Cranial neurons and glia
Odontoblasts
Melanocytes
What are the derivatives of cardiac neural crest cells?
Cardiac septa
Cardiac neurons and glia
Smooth muscle cells
Melanocytes
What are the derivatives of vagal neural crest cells?
Enteric neurons and glia
Melanocytes
What are the derivatives of trunk neural crest cells?
Sensory neurons and glia
Autonomic neurons
Chromaffin cells
Melanocytes
Where are somites derived from, and when does this happen?
They are derived from the paraxial mesoderm, around the end of the 3rd week
How is the age of the embryo determined?
Determined using prominence of somites
True or false: somites develop from the caudal region to the cranial region
False
They develop cranio-caudally from the future occipital region
What do somites give rise to?
The axial skeleton and the associated muscles and dermis
____________from the spinal cord innervate the muscle cells in the somites.
Motor axons
How does the intraembryonic coelom form?
It forms from the fusion of the coelomic spaces
that appear in the lateral plate mesoderm and cardiogenic mesoderm.
This coelom then divides the lateral plate mesoderm into two layers:
Parietal (Somatic) layer
Visceral (splanchnic) layer
Describe the parietal (somatic) layer of the lateral plate mesoderm
beneath the ectodermal epithelium which is continuous with extraembryonic mesoderm covering the amnion.
Describe visceral (splanchnic) layer of the lateral plate mesoderm
Adjacent to the endoderm and continuous with the
extraembryonic mesoderm covering the umbilical vesicle.
- Forms the embryonic gut or SPLANCHNOPLEURE
During the second month of development, the intraembryonic coelom becomes divided into three body cavities.
Names these cavities
Pericardial Cavity, pleural cavity and the Peritoneal Cavity.
What do neural crest cells form
-sensory ganglia for spinal cord (dorsolaterally)
-spinal nerves
What forms the CNS and retina
Neuroectoderm
What innervates muscle cells in somites?
Motor axons from spinal cord
Intraembryonic coelom forms from…?
from fusion of coelomic spaces that appear in lateral plate mesoderm and cardiogenic mesoderm
What do somites form
Axial skeleton and skeletal muscles
