Germ layers Flashcards
Describe the stages the egg goes through btw fertilisation & implantation.
week 1:
1. Fertilisation - ovum becomes zygote - no mitotic divisions yet.
- Cleavage occurs - zygote develops into morula (solid ball of cells).
- Ingress of fluid forms a blastocyst cavity - this forces cells to one side of the morula to form a cavity.
- Blastocyst – 2 cell layers.
- Inner cell mass – embryoblast (becomes embryo)
- Outer cell mass – trophoblast (becomes placenta) - Implantation - trophoblasts release digestive enzymes which allow implantation of blastocyst into endometrium of uterus. Now known as an embryo.
NOTE: At this stage, embryo is gaining nutrition through diffusion in process called Uterine Secretion
When does the bilaminar disc form?
2 weeks
How does the bilaminar germ disc form?
Embryo further progresses into the endometrium & the inner cell mass differentiates to form 2 layers (the bilaminar disc):
- Epiblast – these cells are in contact w/ the amniotic cavity & they are the top layer of cells w/ columnar shape. (BLUE)
- Hypoblast – these cells are in contact w/ the primary yolk sac & are the bottom layer of cuboidal shaped cells. (YELLOW)
- Formation of the amniotic cavity & primary yolk sac also occurs at this point.
NOTE- view image on notes!
What structures are forming simultaneously or just after the bilaminar disc?
- Formation of extra-embryonic cavity & plate.
- Development of uteroplacental circulation - by end of 3rd week, diffusion can no longer supply embryo so primitive blood vessels form (primary villi & trophoblastic lacunae)
- Connecting stalk - primitive umbilical cord.
- Greater formation of yolk sac = formation of secondary yolk sac
When does the development of the 3 germ layers occur?
Occurs at 3 weeks
What is gastrulation?
Gastrulation- the process of forming the 3 germ layers in the trilaminar disc from the bilaminar disc.
The 3 layers are:
- Ectoderm
- Mesoderm
- Endoderm
How do the 3 germ layers form?
- Cells of the inner cell mass (epiblast & hypoblast) undergo differentiation
. - Epiblast cells migrate into the midline to form primitive streak (streak elongates from bottom to top)
- Day 14-15: Some epiblast cells migrate into primitive streak & enter hypoblast, transforming hypoblast toendoderm
- Day 14-15: The remaining epiblast cells differentiate intoectoderm
- Day 16: majority of epiblast cells occupy space between epiblast & hypoblast layers & differentiate intomesoderm
What does ectoderm layer develop into?
Formed from the epiblast layer.
Differentiates into:
- Skin - epidermis, dermis, hypodermis & associated glands, hair.
- Enamel of teeth & nails.
- Sensory epithelium
- Eyes, ears & nose
- Nervous systems
- Pituitary
What does mesoderm layer develop into?
Forms between initial epiblast & hypoblast layers during folding.
Differentiates into:
- Musculoskeletal, cardiovascular, reproductive systems & connective tissue.
- Lining of body cavity & surrounding organs, muscle, connective tissue, bone, blood & blood vessels.
Different parts of the mesoderm differentiate into different things:
- Paraxial mesoderm – musculoskeletal system of trunk.
- Intermediate mesoderm- urogenital system
- Lateral (plate) mesoderm – musculoskeletal system of limbs and girdles
What does the endoderm layer develop into?
Formed from the hypoblast layer.
Differentiates into:
- The gut - gastrointestinal tract, liver, pancreas, bladder.
- Epithelial layer of lungs.
- Thyroid & parathyroid glands
- Lining of digestive & respiratory tracts.
What occurs during organogenesis? When does it occur?
Occurs btw weeks 3-8.
- Neurulation
- Development ofsomites
- Development of intraembryonic coelom – body cavity.
- Development of primitive cardiovascular system
- Development of all major organ systems
Describe the development of the neural tube. What is the process called?
Neurulation:
- The developing notochord & surrounding mesenchyme induce the formation of the neural plate
Neural plate: made of neuroectoderm cells.
- Neural groove forms
- 19 days: neural groove deepens & folds dorsally (backwards), then fuses to form neural tube.
- This is primitive CNS.
Some neuroectoderm cells migrate along the fusion crest before fusion to form neural crest cells.
NOTE: view notes for image
What do neural crest cells form?
- Craniofacial skeleton (i.e. skull).
- Dorsal root ganglia & ganglia of autonomic nerveous system.
- Meninges – lining surrounding the skull.
- Skeletal & muscular components of the head & neck.
What structures do the somites give rise to?
The axial skeleton & associated muscle & dermis
cells are described as pluripotent mesenchymal cells- means some will develop into bone, some cartilage & some muscle.
Describe the TRANSVERSE folding of the embryo from flat disc to embryonic body.
Embryonic disc is flat to embryonic body (3D): weeks 4-8
- Fluid ingresses into the amniotic cavity.
- Causes lateral edges of amniotic cavity to bulge out & fold down.
- Folds disc into a C shape & then into a body cavity.
- Results in a constriction btw the embryo & the secondary yolk sac.
- Ventral fold(folding forwards) - the dorsal (upper surface) yolk sac becomes incorporated into the embryo as the primitive gut.
Then differentiation of trilaminar germ cells into specific tissues & organs occurs.
Trilaminar disc folding:
- Ectoderm - dorsal (backward) folding to form the neural tube – neurulation.
- Endoderm - folds ventrally (forwards) to form the inner lining of the gut tube.
- Mesoderm - sits between the 2 tubes forming most other tissues.
