Review of Embryology Flashcards
What is the first 8 weeks of human development called?
Embryonic period
What is the fetal period?
After the embryonic period
What happens during the fetal period?
Mainly growth and maturation of the developed systems
Where does fertilisation take place?
In the ampulla of the fallopian tube
What is the ampulla?
The curved end of the fallopian tube
What is the morula?
A cluster of cells that cannot grow/multiply any further due to restrictions imposed by the zona pellucida
When does the morula form?
At around 96 hours after fertilisation
How many cells form the morula?
16
What is the blastocyst?
The organisation of cells forming the morula forms the blastocyst by engendering
Describe the maturation of the blastocyst
The morular cells in contact with the zona pellucida join together, compact and flatten to become an epithelial layer = trophoblasts
The few inner cells develop into the inner cell mass = embryoblast
What does the trophoblast form?
Placenta and nutrients
What does the embryoblast form?
Bilaminar cell layer
What is blastulation?
Process that happens when blastocyst has fully formed and hatches from the zona pellucida
Described the bilaminar disc
There are two layers:
1) Hypoblast (below) - during implantation this forms the umbilical vessel next to the yolk sac
2) Epiblast (above) - during implantation this forms the amniotic cavity
This forms the bilaminar disc
What is the name of the point where the hypoblast and epiblast meet?
Embryonic plate
What is implantation?
The embryo comes into contact with the uterine mucosa after hatching
When does implantation take place?
Adhesion can only occur if the endometrium is in the secretory phase of the menstrual cycle.
What is the importance of the placenta?
It serves as the vascular connection between the embryo and the mother
What is gastrulation?
The stage of development following implantation
Which event starts the process of gastrulation?
The formation of the primitive streak following the proliferation of epiblast cells
What is the primitive streak?
Site where the epiblast cells start to migrate
What structure helps to give the primitive streak orientation?
Prechordal plate
What is the oropharyngeal membrane?
Membrane that gives rise to the opening of the mouth
Formed next to the prechordal plate
What is the cloacal membrane?
Membrane that gives rise to the opening of the anus
What is the primitive streak?
Linear structure that lies along the rostral-caudal axis formed by epiblast cell movement towards the midline
What are two important structures formed by the primitive streak?
The primitive node found in the cranial end of the primitive treak
The primitive groove found in the center of the primitive streak
What is the function of the primitive node?
Becomes the primitive pit following epiblast cell migration
Allows an entry point for the epiblast cells to migrate into the hypoblast layer
Describe the process that happens when the bilaminar embryonic disc form the trilaminar layers of the germline
- The hypoblast layer becomes the endoderm layer as the epiblast cells replace the hypoblast cells
- After the hypoblast cells become replaced by epiblast cells and form the endoderm, more epiblast cells move down, out and forward forming the mesoderm following differentiation
- The epiblast is now called the ectoderm
What structure do each layer of the trilaminar disc form?
Ectoderm - forms the epidermis, CNS and neural crest cells
Mesoderm - form the muskuloskeletal, cardiovascular and urogenital systems
Endoderm - forms the lining of the GIT and respiratory tract
What is the process of neurulation?
The cells from the primitive pit form an elongated tube moving cranially towards the prechordal plate
The tube is found underneath the ectoderm but above the endoderm
This forms the notochord
What is an important feature about the notochord?
There is no mesoderm between the ectoderm and endoderm where the notochord is found
What are the two divisions of the ectoderm?
Neural ectoderm - contributes to future brain and nerve cord
Non-neural ectoderm - contributes to the skin and placodes (hair follicles, nails)
What is the role of the notochord?
To induce change in the ectoderm above it
The notochord produces growth factors that tell the ectodermal cells to proliferate, forming the neural plate
What are the divisions of the mesoderm?
Paraxial mesoderm (somite)
Intermediate mesoderm
Lateral plate mesoderm
Describe the formation of the neural tube
The neural plate involutes through the downwards pulling of the mesoderm
The edges of the neural plate move towards one another
- Edges = neural folds
- Center tube = neural groove
The ectodermal cells at the edges of the neural folds differentiate into neural crest cells
What are the 3 layers of the neural tube?
- Marginal zone - forming neurons
- Marginal zone - forming neuroblasts
- Neuroepithelial zone - forming the spinal cord
Where are the 3 areas in development where there is no mesoderm?
Prechordal plate - oropharyngeal membrane
Notochord
Cloacal plate - cloacal membrane
What day is the neural tube formed?
21
What is important for the development of neuropores?
Folate
What happens if the embryo does not receive enough folate during development?
Failure to close the anterior neuropore at cranial end forms anencephaly
Failure to close the posterior neuropore at caudal end leads to spina bifida
What 3 primary vesicles form during the vesiculation of the cranial neural tube?
Prosencephalon
Mesencephalon
Rhombencephalon
What vesicles do the primary vesicles form during their specialisation?
Prosencephalon splits into telencephalon and diencephalon
Mesencephalon stays the same
Rhombencephalon splints into Metencephalon and Myelencephalon
What structure does the telencephalon form?
Cerebrum
What structure does the diencephalon form?
Thalamus
Subthalamus
Hypothalamus
Pineal gland
What structure does the mesencephalon form?
Midbrain
What structure does the metencephalon form?
Pons
Cerebellum
What structure does the myelencephalon form?
Medulla
Describe what happens to the caudal portion of the neural tube
The cells of the neuroepithelial zone move their way outwards into the marginal zone, forming structures towards the dorsal and ventral parts of the spinal cord
The structures in the posterior area formed from the neural tube form the alar plate
The structures in the anterior area from the neural tube form the basal plate
What does the alar plate form?
The posterior grey horn with sensory neurons
What does the basal plate form?
The anterior grey horn with motor neurons
What is the remnant of the neural tube in a mature body?
Spinal canal with cerebrospinal fluid
What do the neural crest cells differentiate into?
Melanocytes
Enterochromaffin cells
Enteric nervous system
Dorsal root ganglion
Pia mater and arachnoid mater
Many skull bones
Myelin of Schwann cells
Parts of the heart
What does the mesoderm surrounding the notochord form?
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
What does the paraxial mesoderm give rise to?
Somites which divide into:
- Sclerotome - vertebrak colum
- Myotome - axial and limb musculature
- Dermatome - dermis
What does the intermediate mesoderm give rise to?
Nephrotome making the kidneys and gonads
What does the lateral plate mesoderm giver rise to?
Splanchnic hypomere near the endoderm gives rise to the peritoneum and the gonads
The Somatic hypomere near the ectoderm gives rise to the heart, vessels and mesenteries
What is the importance of somites?
Somites are the developmental basis of segmentation in vertebrates
One somite will give rise to one of the repeating structures:
- Sclerotome - develop into neural tube, notochord and vertebrae
- Myotome - forms skeletal muscles of back and limbs, diaphragm
- Dermatome - forms dermis
Somite cells also contribute to tendon formation and endothelium of the aorta and intersegmental arteries
What structure divides the abdominothoracic cavity?
The diaphragm divides the abdominothoracic cavity into the superior thoracic and inferior abdominopelvic cavity
