Week 3 - Embryology Flashcards
What is a conceptus
The developing offspring
What is a zygote
fertilized egg
What is the gestation period
time from the last menstrual period until birth (280 days)
What is an embryo
conceptus from week 3 through to week 8
What is a foetus
conceptus from week 9 till birth
What is pregnancy
Events that occur from fertilization until the infant is born
What is embryology
The branch of biology that studies the formation, growth and development of an embryo from a fertilized egg
What is week 1
Preimplantation period
What occurs in week 1
- While travelling through the urterine tubes the conceptus undergoes cleavage up to 16 cells
- the conceptus reaches the uterus and now becomes a more solid structure called the morula
- The cells in the morula continue to divide until 100 cells and forms a tightly bounded cell called a Blastocyst
- The blastocyst consist of outer trophoblast cells and the inner cell mass
- After about 5 days the blastocyst “hatches” from the surrounding zona pellucida in preparation for implantation
What is cleavage
the rapid mitotic cell division in the conceptus - first to 16 cells then 100 cells. During this the size of the conceptus remains constant
What is the morula
The conceptus during week 1 when the cells have now been loosely grouped and looks more compacted into a solid mass
What is a Blastocoel
The final cell structure of week 1
Structure
- Trophoblast Cells
The cells which form the outer shell and will develop into the chorionic sac and the foetal portion of the placenta
- Inner Cell Mass
The centrally located cells which is destined to become the embryonic disc. At this stage the cells are totipotent meaning that they could become any cell type in the human body
What is week 2
Implantation and Blastocyst differentiation
What occurs in week 2
- The blastocyst implants into the lining of the uterus forming the syncytiophoblast
- The inner cell mass forms a 2 layered embryonic disc (epiblast and hypoblast)
- (The blastocysts starts to organise into layers called the extraembryonic membrane which supports and protects the growing embryo including:)- might not be till gastrulation
- Amnion
- Yolk Sac
- Chorion
Explain conceptus implantation
- Implantation occurs in week 2 and is where the blastocyst comes in contact with the uterine wall and adheres to it via its trophoblast cells.
- If successful the trophoblast fuse together forming the syncytiophoblast which allows passage of blastocyst into endometrial wall.
- In response the uterine mucosa rebuilds itself and envelops the blastocyst
- The trophoblast cells secrete hCG which is a hormone that increases the production of progesterone and estrogen to suppress the menstruation cycle
In week 2 what does the inner cell mass turn into
The inner cell mass forms a 2 layered embryonic (bilaminar) disc (hypoblast and epiblast).
Also creates Yolk sac on the hypoblast side and the amniotic cavity on the epiblast side.
What are the layers of the embryonic disc (week 2)and what will they form
Epiblast - will form entire embryo and an epithelial layer lining the amniotic cavity (between bilaminar disc and trophoblast’s outer wall)
Hypoblast -nourishes the embryo and turns into the placenta (yolk sac - chorion)
What are the different components of the extraembryonic membrane
- Amnion
- Yolk Sac
- Chorion
What is the amnion
- Transparent sac formed from epiblast cells
- grows around the embryo and fills with amniotic fluid from material blood and later foetal urine
- this provides a buoyant environment that protects the embryo from trauma
- it acts as a thermal regulator
- allows freedom of movement and prevents parts from fusing together
What is the yolk sac
- a part of the extraembryonic membrane
- supplies some nutrients until the placenta takes over
- Formed from the hypoblast
- provides primitive blood circulation
- later the yolk sac forms the allantosis which is a primitive excretory duct that becomes apart of the embryo
- together the stalks of the yolk sac and allantois establishes the outer structure of the umbilical cord
What is the chorion
-apart of the extraembryonic membrane
- the membrane that surrounds all others and helps to form the placenta
- protects the conceptus from attack from the mother’s immune cells
What process occurs in week 3
Gastrulation and Notochord formation
What is gastrulation
-Formation of the 3 germ layers
Explain what occurs in week 3
- The 2 layered embryotic disc becomes a 3 layered disc (endoderm, Mesoderm, Ectoderm)
- During cell formation of the disc layers the embryo take on an oval shaped disc form and creastes a raised edge called the privative streak
- Notochord forms which runs down the longitudinal axis
Explain the formation of the 3 layered embryotic disc
- Cells differentiate at the primitive streak and migrate through it
- Cells displace the hypoblast cells and form the endoderm layer
- Cells loosely fill before the endoderm and form the mesoderm
- The remaining cells from the epiblast form the ectoderm
- The mesoderm layer are less organized and exist as a loosely connected cell community
- The endoderm and Ectoderm layers form tightly connected epithelial sheets of tissue
What will the endoderm become
epithelial linings of the digestive system, lungs, liver and pancreas
what will the mesoderm become
skeleton, muscles, connective tissue, heart, blood vessels and kidney
what will the ectoderm become
central and peripheral nervous system, sensory organns, epidermis, hair and nails
What are the layers of the 3 layered embryonic disc
- Endoderm
- Mesoderm
- Ectoderm
What is the notochord
The notochord is a midline structure present in all embryos and it forms after the mesoderm forms. It runs down from the caudal to tail.
What is the function of the notochord
Structure - serves as a primitive form of cartilage until other elements form (vertebra)
Patterning - the notochord secretes a protein called Sonic Hedgehog which it releases to surrounding tissues. Sonic Hedgehog signals the creation of new genes, but different genes will be signaled to be made depending on the concentration of the protein the tissue receives. (french flag analogy)
What main event occurs in week 3 and 4
Neurulation - the development of the nervous system
What occurs during week 3-4
- The neural plate bends at its middle axis creating lateral edges called the neural crest which eventually further folds to create a tube called the neural tube which will form the spinal cord
- When the neural tube forms the cells which created the neural crest will migrate further into the embryo body and will form structures such as the cranial nerves, bones and cartilages of the skull, pituitary glands ect.
Explain the steps in the formation of the neural tube
- The neural plate is created from the ectoderm tissue thickening
- the neural plate bends dorsally within the 2 eventually joining at the neural plate borders which are now referred to as the neural crest
- The closure of the neural crest from the epidermis, and the neural crest cells differentiate to form most of the peripheral nervous system, detaching themselves from the neural tube.
What issue may arise with the migration of neural crest cells
There may be defects in the migration and formation of neural crest cells which may affect facial and oral structures
e.g.
Mandibulofacial dysostosis - the failure of full facial development due to neural crest migration failure - causes
- affected pharyngeal arch structure
- small lower jaw
- conductive hearing loss
- downward palpebral fissure
What does the endoderm specialize into
the tube of endoderm is formed by the folding of the embryonic disc and it will become the epithelial lining of the GI track
the mucosal lining of the respiratory tract is also formed from the endoderm
What is the specialisation of the ectoderm
- neurulation
- folding and formation of the neural tube
- neural crest cells migrate to form cranial, spinal and sympathetic ganglia
- most of remaining ectoderm –> epidermis (skin)
What does parts of the neural tube form
Anterior end of the neural tube –> brain
Rest –> spinal cord
What is the specialisation of the Mesoderm
- the appearance of the notochord in the embryonic disc
- 3 mesodermal aggregates (lumps of tissue) on either side of the notochord forms (largest are the somites)
- formation of intermediate mesoderm and lateral mesoderm
What is the largest mesodermal aggregates called and their location
- Somites
- Paired mesodermal blocks that hug the notochord on either side
What do intermediate mesoderm form
kidney and gonads
What do lateral mesoderms form
dermis, lining of body cavity, most tissues of the limbs (arms and legs)
What are the segments of somites
- Sclerotome (“hard piece”) venteral portion
Dermomyotomes: - Myotome (“muscle piece”) dorsal
- Dermatome (“skin piece”) dorsla
What is the sclerotome
“hard piece”
Ventral portion of the somites which are the precursor of ribs and vertebral column
What is the myotome
“muscle piece”
dorsal portion of the somites
origin of skeletal musculature - muscles
What is the dermatome
“skin piece”
dorsal part of the somites
cells help form the dermis in dorsal part of the body - skin
What germ layer does the somites form from
mesoderm
What neurons extend through the somaties
motor neurons
sensory neurons
What are the organising centers in the head
- Prosencephalic organising center (upper 1/3 face)
- Rhombencephalic organising center (lower 2/3 face)
Hindbrain
Midbrain
Forebrain
What is the substance which is required to separate the rhombomere from the hindbrain
Homebox (Hox) gene
What are Rhombomeres
- Transerse segments which are formed from the hindbrain using the Homebox (Hox) gene. -
- This exerts influence on craniofacial formation
- Creates the pharyngeal arches
What are Pharyngeal (branchial) arches
- 5 paired swellings between the developing heart and brain
- formed from rhombomeres
- they create important head structures
What does the 1st pharyngeal arch form
- the large mandibular and smaller maxillary prominance
- trigeminal nerve (CN-V)
- Muscle of mastication (chewing)
What does the 2nd pharyngeal arch form
- Frontalis (muscle)
- tongue
- mandibula cartilage
- Muscles of facial expression (expression)
What is the dentist nerve
Trigeminal nerve (CN-V)
What is the Trigeminal nerve (CN-V)
principal sensory nerve of the head and neck (dentist nerve)
Located in both the maxillary and mandibular jaws and innervate(supply with nerves) the teeth and mucous membranes of the palate, mouth and tongue
What is the opposing protein to Sonic Hedgehog protein
BMP - Bone Morphogenic Protein
What does totipotent mean
Stem which can giving rise to any cell type
When do cells become not totipotent in embryology
Week 3 - formation of germ layer and notochord
What type of cell does the notochord arise from
mesodermal cells
