Formation of the Basic Body Plan Flashcards
Explain formation of the primitive streak and its components
Day 15
- Thickening, primitive streak, containing a midline groove forms along the midsagittal plane of the embryonic disc
- Along the length of the primitive streak, the primitive groove becomes deeper and more defined
- the cranial end of the primitive streak is expanded to form the primitive node
- the primitive then forms a depression called the primitive pit, which is continuous caudally with the primitive groove
What does the primitive streak define?
All major body axes
- primitive streak forms midline of embryonic disc, thus the cranial to caudal axes, and medial- lateral axis
left-right axis
Gastrulation definition
The process of forming endoderm, ectoderm and mesoderm from the epiblast layer
Stages of gastrulation
- epiblast cells undergo ingression, moving in through the primitive streak
- Epiblast cells undergo epithelial to mesenchymal transformation EMT
- The ingressing epiblast cells invade the hypoblast and displace the cells
- Hypoblast is eventually completely replaced by a new layer of cells called the definitive endoderm
- Epiblast cells migrate through the primitive streak and diverge into the space between the epiblast and definitive endoderm- form third layer called intraembryonic mesoderm
- Remaining epiblast cells constitute the ectoderm
- By week 4 primitive streak disappears
What is the difference between epithelial cells and mesenchymal cells?
Epithelial- sheets of regular shaped cells tightly interconnected to one another at their lateral surfaces.
Mesenchyme- more more irregular, stellate loosely connected cells
What happens during EMT?
epiblast cells become more flask shaped, elongate, extend rootlike processes called pseudopodia which allow them to migrate through the primitive streak
What happens between the ectoderm and endoderm?
Two depressions are formed, one at cranial and one at caudal end.
- oropharyngeal membrane and cloacal membrane, exclude mesoderm
- eventually bind to gut tube which form the oral cavity and opening of the anus respectively
How does the endoderm induce the formation of the mesoderm?
Mesoderm induction requires TGF beta and FDF families of secreted growth factors. Deregulate production of E cadherin, preventing the binding of epiblast cells so they can move into the primitive streak
Two important regions of the embryo explained
Nieuwkoop centre- induces the formation of the organiser-
Spemann-Mangold organiser- sends out signals to pattern the newly formed mesoderm into its medial and lateral subdivisions
-primitive node in humans
What does the ectoderm differentiate into?
Central neural plate and peripheral surface ectoderm
In which direction does the differentiation of cells take place?
Cranial to caudal
Teratogenesis definition
Abnormalities of physical development
Example of a teratogenic condition + explained
Sirenomelia
- insufficient caudal mesoderm
notochord definition
Flexible rod made out of a material similar to cartilage, which eventually forms the vertebral column
Stages of notogenesis
- Prospective notochord cells, formed of mesoderm, migrate cranially at the midline to form the notochord process
- Day 18 the notochordal process fuses with the endoderm to then become the notochord plate
- provides a passage from the amniotic sac to the yolk sac- forming the neurenteric canal
- Eventually, two edges of notochordal plate fuse to become the notochord which ends up in the mesoderm
What does the notochord eventually form?
Nucleus pulposus
What is another role of the notochord?
Secretes the patterning signal sonic hedgehog, which initiates tissue development and differentiation
Organisation of mesoderm
para-axial mesoderm slightly lateral to
notochord (eventually forms somites)- thickened plate
intermediate mesoderm
lateral plate mesoderm- eventually forms splanchnic, somatic and coleum mesoderm- remains thin
Function of intermediate mesoderm
- produces urinary and genital system
temporarily holds paraxial and lateral plate mesoderm together
Stages of differentiation of lateral plate mesoderm
Day 17
1. lateral plate mesoderm splits into two layers: ventral layer associated with endoderm and dorsal layer associated with ectoderm
- ventral layer gives rise to splanchnic mesoderm- forming mesothelial coverings of viscera
- dorsal layer- somatic mesoderm- gives rise to inner lining of body and parts of limbs
- middle layer forms the coelom, which is the mesodermally lined cavity between the outer body wall and gut
Where does mesoderm begin differentiating?
Cranial to caudal direction
Somite definition
block-like condensations of mesoderm
What occurs in the head region?
Mesoderm forms a band of cells that remain unsegmented
Explain somite pair formation sequence and days
Day 20 - first pair of somites form at the head-trunk border
remainder form in the cranial-caudal progression- finishing on day 30
Approximately 42-44 pairs form, flanking the notochord from the occipital region to the tip of the embryonic tail
Several caudal most somites disappear- forming 37 pairs in total
What do somites form?
most of the axial skeleton- form limb buds
vertebral column, occipital bones, voluntary musculature of neck
body wall
limbs and dermis of neck
Segmentation definition
Formation of serially repeated and functionally equivalent units
Stages of somitogenesis
- The presomitic paraxial mesoderm is patterned using two molecular activities known as the ‘clock’ and ;wave front’
- The segmentation clock controls the expression of cyclic genes.
- these include the notch family and Wnt pathway
- Outer shell of the somite formed by mesenchymal to epithelial transformation
- core of mesenchymal cells then contains a cavity called the somitocoel that fills with the proliferating mesoderm cells
How is the spacing of the segments achieved?
Wavefront is generated using a gradient of Fgf8, which is transcribed in the tail bud.
Cells migrate out of the tail bud into the presomitic mesoderm
High concentration in tail, low in head, thus controlling the position of the somite boundaries along the cranial/caudal axis
What two regional components does the somite form originally?
ventromedial region- sclerotome forms vertebral body and intervertebral disc
dorsolateral region- dermomyotome forms dermis and skeletal muscle
Explain structure of sclerotome
- left and right sclerotome from the same segmental level fuse and engulf notochord
- becomes segmented in the rostral and caudal directions by a von Ebner’s fissure
- one half forms single vertebral body
- other half forms intervertebral disc
Explain structure of dermomyotome
Divided into a dorsal and ventral half
dorsal- dermatome
ventral- myotome that will further vide into dorsal and ventral half
Structure and function of myotome
dorsal- epaxial
ventral- hypoaxial- muscles of the ventrolateral body wall, girdle, limb and Tonge
How is sclerotome formed?
- Notochord secretes Sonic Hedgehog, along with noggin (a Bmp inhibitor) also secreted by the notochord
- Express the factor Pax1- a paired box transcription factor which characterises the vertebral bodies and vertebral discs.
- Pax 1 induces epithelial to mesenchymal transformation
How is dermamyotome formed?
- dorsal neural tube and surface ectoderm produce various Wnts, which induce dermamyotome.
- Wnts express Pax3 which allows the dermamyotome further develop and form the myotome
- Wnt6 from ectoderm also maintains epithelial characteristics of the dermatome
- long range Shh signalling is needed for the initial specification of the epaxial part of the myotome
- Further patterned by a gradient of Bmp4, which is secreted by the lateral plate mesoderm and noggin secreted by the neural tube and notochord (which attenuates the gradient)
Proof of the importance of the notochord in sclerotome formation?
Remove notochord and add it to mesoderm of another organism.
Will form a second vertebral column
What will the dermatome eventually form?
Dermis- connective tissue and fat
Teratology definition
The study of abnormal development in embryos and the causes of congenital malformations or birth defects
What are the most sensitive periods of time in relation to teratogenesis?
First two weeks- teratogenic agents will kill the embryo, rather than cause congenital malformations
During organogenesis- between days 15 and 60, agents most likely to cause major congenital malformations- during most rapid cell deviation
Example of teratogenic agent + defect
Thalidomide- caused malformation of organs and limbs. Thalidomide is antiangiogenic thus embryos cannot receive enough oxygen. Also inhibits Tumour Necrosis Factor
alcohol- fetal alcohol syndrome and retardation
Name of mesodermal-ectoderm structures
Somatopleure
Name of mesodermal-endoderm structures
Splanchopleure
What two forms of mesoderm are formed first?
Cardiac mesoderm and extra embryonic mesoderm which helps form around the embryo
