Early embryology Flashcards
how does one cell become a multicellular body
- growth
- morphogenesis development of form and structure
- differentation specialisation for function
when is the pre-embryonic period
first two weeks of development
when is the embryonic period
weeks 3-8
when is the fetal period
weeks 9-38
what stages occur in pre-embryonic period
week 1
- cleavage
- compaction
- hatching
- implantation begins
week 2
- differentiation
- implantation complete
how does fertilisation occur (day 1)
- oocyte released from ovary
- travels along fallopian (uterine) tube
- fertilised by sperm in the ampulla
- fertilised oocyte is called zygote
when does cleavage occur
- days 2-4
- when zygote is transported to the uterus from ampulla end of fallopian tube to isthmus (junction with uterus) by specialised cilia in the oviduct
what happens in cleavage
- zygote undergoes series of mitotic divisions
- results in blastomeres which get smaller with each division
- divisions occur without an increase in cell size due to the glycoprotein shell called the zona pellucida
what is a morula
- blastomeres form a compact ball of cells held together by tight junctions
- each cell is totipotent - can become any cell type
assisted reproductive techniques
- oocytes are fertilised in vitro and alowed to divide to 4 or 8 cell stage
- morula transferred to uterus
what is PGD (pre-implantation genetic diagnosis)
a cell can be safely removed from the morula and tested for serious heritable disorders prior to the transfer of the embryo into the mother
what happens in compaction (day 4)
spaces between cells of the morula merge to form one large central cavity called the blastocoele converting the morula into a hollow sphere of cells called the blastocyst
what is a blastocyst
- result of compaction
- cells are pluripotent
- inner cell mass - embryoblast becomes embryo
- outer cell mass - trophoblast becomes placenta and fetal membranes to support and sustain developing fetus
what happens in hatching (day 5)
- blastocyst hatches from zona pellucida
- no longer constrained so free to enlarge
- can interact with uterine surface to implant
what happens in days 6-7
implantation begins
- conceptus has around 100 cells
- 8 will make the embryo and remainder begin development of fetal membranes
what occurs during week 2
- formation of bilaminar disc (epiblast and hypoblast) from embryoblast
- formation of syncytiotrophoblast and cytotrophoblast layers from the trophoblast
- formation of amniotic cavity, yolk sacs, chorionic cavity
- conceptus has implanted
how does the bilaminar embryonic disc form (day 8)
- inner cell mass (embryoblast) shows signs of cell differentiation and forms two layers
- epiblast and hypoblast
- the bilaminar disc has dorsal (ectodermal) and ventral (endodermal) surfaces but no head to tail orientation
how does implantation occur (days 9-10)
- trophoblast becomes cytotrophoblast and syncytiotrophoblast which is a special layer that adheres to and invades the endometrium
- uterine epithelium is breached and conceptus implants within uterine stroma
- establishes maternal blood flow within placenta to support embryo from maternal circulation
- establishes basic structural unit of materno-fetal exchange called the chorionic villus
two layers of trophoblast
- cytotrophoblast inner layer of trophoblast that continually differentiates into syncytiotrophoblast
- syncytiotrophoblast placental barrier between maternal and fetal blood that allows exchanges in nutrients and gases
what happens in each day of week 2
day 8
- bilaminar disc - epiblast and hypoblast
- cytotrophoblast and syncytiotrophoblast
day 9
- rapid development of syncytiotrophoblast
- primitive yolk sac formed
- yolk sac membrane in contact with cytotrophoblast
day 9-10
- implantation
day 11
- primitive yolk sac membrane pushed away from cytotrophoblast layer by reticulum
- reticulum converted to extraembryonic mesoderm by cell migration
day 12
- maternal sinusoids invaded by syncytiotrophoblast
- uteroplacental circulation begins
- uterine stroma prepares for support of embryo
day 13
- formation of secondary yolk sac which pinches off from primitive yolk sac
day 14
- spaces within extraembryonic mesoderm merge to form chorionic cavity
- embryo and cavities suspended by connecting stalk (future umbilical cord)
- implantation bleeding can occur
how does the amniotic cavity form
formed from spaces within the epiblast
what is the blastocoele
first cavity, formed as a result of compaction
how does the primitive yolk sac form
(aka exocoloemic cavity) formed by hypoblast lining blastocoele
how does the secondary yolk sac form
(aka definitive yolk sac) formed within primitive yolk sac
how does the chorionic cavity form
(aka extraembryonic coelom) formed from spaces within extraembryonic reticulum and mesoderm
early pregnancy loss
- approximately 50% of zygotes are lost in the first 2-3 weeks
- 15% diagnosed pregnancies miscarry
- 1% suffer from recurrent miscarriages
what are two implantation defects
- ectopic pregnancy
- placenta praevia
what is ectopic pregnancy
- implantation at a site other than the uterine body
- most commonly in the ampulla of the fallopian tube
- can be peritoneal or ovarian
- can very quickly become life-threatening emergency due to rupture
what is placenta praevia
- implantation in the lower uterine segment
- can cause haemmorhage in pregnancy
- requires C-section delivery
where does implantation usually occur
upper posterior uterine wall
what happens during the embryonic period
- weeks 3-8
- period of greatest change
- all major structures and systems are formed
- greatest risk of major congenital malformations due to environmental exposure or drug therapy
what stages occur in weeks 3 + 4
- gastrulation
- neurulation
- segmentation
- folding
when does gastrulation occur
3rd week, marking the start of the embryonic period
what is a germ layer
a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues
what is gastrulation
when the bilaminar disc is converted to a trilaminar disc with three germ layers: ectoderm, mesoderm and endoderm
why does gastrulation occur
to ensure the correct placement of precursor tissues to allow subsequent morphogenesis to take place
how does gastrulation occur
- primitive streak appears on dorsal surface of epiblast
- causes migration and invagination of epiblast cells
- hypoblast displaced and epiblast cells differentiate to create 3 layers
- ectoderm, mesoderm and endoderm (trilaminar disc)
what does the primitive streak and node do
make sure cells of right type get to right place
derivatives of each germ layer of the trilaminar disc
ectoderm
- organs and structures that maintain contact with outside world
- nervous system, epidermis
mesoderm
- supporting tissues
- muscles, cartilage, bone, vascular system
endoderm
- internal structures
- epithelial lining of GI tract and respiratory tract, parenchyma of glands
what is the notochord
- solid rod of cells running in the midline with an important signalling role
- it drives neurulation
- mesoderm cells differentiate into notochord
how does neurulation occur
- notochord signals overlying ectoderm to thicken and differentiate into neuroectoderm
- forms the neural plate (origin of nervous system)
- edges of the neural plate elevate out of the plane and curl towards each other, creating the neural tube
differentiation of mesoderm
- notochord
- paraxial mesoderm is either side of axis (notochord and neural tube) becomes organised into somites
- intermediate mesoderm is lateral to the somites
- lateral plate mesoderm is a sheet that splits into two layers: somatic mesoderm and splanchnic mesoderm with the intraembryonic coelem in between them
differentiation of mesoderm
- notochord
- paraxial mesoderm is either side of axis (notochord and neural tube) becomes organised into somites
- intermediate mesoderm is lateral to the somites
- lateral plate mesoderm is a sheet that splits into two layers: somatic mesoderm and splanchnic mesoderm with the intraembryonic coelem in between them
derivatives of the 6 regions of mesoderm
- notochord forms the vertebral column
- somites form vertebral column, ribs and extensor muscles of spine and muscle of body wall
- intermediate mesoderm forms the kidneys
- somatic mesoderm forms diaphragm and limb muscles
- splanchnic mesoderm forms muscle of heart and viscera
- intraembryonic coelem becomes the body cavities
how does segmentation occur
organisation of the paraxial mesoderm into segments called somites in a regular, predictable sequence to give rise to repeating structures such as vertebrae, ribs, intercostal muscles and to guide innervation
when do somites form (segmentation)
- 1st pair appear at day 20 in occipital region
- more appear at 3 pairs/day until 42-44 pairs are present by end of week 5
- some disappear, leaving 31 somites in total
what are somites
- appear as regular block of mesoderm cells arranged around a small cavity
- followed by further differentiation into subgroups of cells
- underlie pattern of outgrowth of spinal nerve roots from spinal cord, formation of vertebral column and innervation of muscles and skin
how do somites differentiate
each somite separates into a dermatome which gives rise to an area of skin, a myotome which gives rise to a block of skeletal muscle and a sclerotome which gives rise to a vertebra and rib
how does differentiation of somites lead to innervation of skin and muscles
- spinal nerve roots growing from neural tube enter and innervate dermatomes (skin) and myotomes (muscle)
- sclerotomes rearranged to lie between nerve roots and become vertebrae
why does folding occur
draws together the margins of the disk to present ectoderm to outside and endoderm inside
- creates a ventral body wall
- pulls amniotic membrane around the disk so embryo suspended within amniotic sac
- pulls connecting stalk ventrally
puts the heart and gut in right place
creates a new cavity within the embryo
types of folding that occurs
cephalocaudal folding (head and tail)
lateral folding
cephalocaudal (head+tail) folding
driven by growth of neural tube
- amniotic sac becomes wrapped around embryo
- little bit of yolk sac inside embryonic body and rest becomes umbilical cord
lateral folding
driven by pressure from growth of somites
- lateral plate mesoderm opens up to form somatic and splanchnic layers and intraembryonic coelem
- yolk sac creates tube inside embryonic body
