Early Embryology Flashcards
What are the number of gestational weeks?
- 38
What are the stages of fertilisation?
- Oocyte is released from the ovary
- Travels down Fallopian (uterine) tube
- Fertilised by sperm in the ampulla
- Fertilised ooctyce is called the zygote
- Ideal site for implantation is the posterior uterine wall
What are the number of pregnancy weeks?
- 40
- Different to gestational as is calculated from the last menstrual period
What weeks is the pre-embryonic period?
- 1-2
What weeks is the embryonic period?
- 3-8
What weeks is the fetal period?
- 9-38
Define zygote
- A fertilised ovum/ooctye
Define cleavage
- The dividing of cells into 2 masses known as blastomeres (are identical to each other and to zygote)
- First cleavage happens ~30 hours after fertilisation (resting period)
Define zona pellucida
- The glycoprotein shell to prevent polyspermy
Define morula
- The result of further cleavage divisions of the fertilised oocyte
- Each cell is totipotent (has the capacity to become any cell)
Define ovary
- The female reproductive organ producing oocytes/eggs for fertilisation
Define Fallopian tube
- Tube that the ooctye travels along to reach the uterus
- The egg is fertilised at the ampulla of the Fallopian tube
Define uterus
- Female reproductive organ where implantation of the zygote and the growth of the embryo occurs
Define blastocyst
- Formed from the compaction of cells that make up the morula
Define trophoblast
- Outer cell mass that will later firm support structures for the embryo ie placenta
Define embryo last
- Inner cell mass that will later become the bilaminar disc
Define implantation
- The attachment of the blastocyst to the wall of the uterus at day 6-7
Define cytotrophoblast
- Is a derivative of the trophoblast
- Forms the placental membrane around the yolk sac
Define syncytiotrophoblast
- Derivative of the trophoblast
- Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation
What stages occur during week 1?
- Compaction (day 4)
- Hatching (day 5)
- Implantation begins (day 6-7)
What happens during compaction?
- Blastomeres making up the morula compact to form the blastocyst
- First differentiation has occurred (cells are now pluripotent-can become many cells
What does the blastocyst consist of?
- Embroblast (inner cell mass)
- Trophoblast (outer cell mass)
- Blastocyst cavity
What happens during hatching?
- Blastocyst hatches from the zona pellucida
- Is no longer constrained and is now free to enlarge
- Can now interact with the uterine surface to implant
What is the structure of the conceptus at the beginning of implantation?
- Conceptus now has 107 cells
~ 8 will make the embryo
~ 99 will begin the development of the foetal membrane
What is the second week know as and what begins?
- ‘Week of twos’
- Both embryo and placenta start to form
- Placenta has priority at earliest
What happens on week 2, day 8?
- Embryoblast differentiates into the epiblast and hypoblast, which both make up then bilaminar disc
- Trophoblast differentiates in the cytotrophoblast and the syncytiotrophoblast
- The amniotic cavity forms above the bilaminar disc
- The blastocyst cavity is still present below the bilaminar disc
What has happened by day 9-10?
- Implantation is complete
Describe implantation
- Is interstitial ie is invasive, surrounded and embedded
- The uterine epithelium is breached and the conceptus implants within the uterine stroma
- Establishes maternal blood flow within the placental ie support of the embryo changes from histotrophic (dependent on simple diffusion across cell membrane) to haemotrophic (has access to maternal blood flow)
What are implantation defects? Give examples
- Implantation at inappropriate sites can cause problems
- Ectopic pregnancy: implantation at site other than the uterine body (most commonly the Fallopian tube); can quickly become a life-threatening emergency
- Placenta Praevia: implantation in lower uterine segment; placenta grows across cervical opening; requires c-section delivery
- Pre-eclampsia
- IUGR (inter-uterine growth restriction)
What happens on week 2, day 9?
- At the embryonic pole:
~ rapid development of the syncytiotrophoblast - At the abembryonic pole:
~ primitive yolk sac formed by Heuser’s membrane (hypoblast cells and extracellular matrix) spreading across the blastocyst cavity
~ yolk sac membrane is in contact with the cytotrophoblast layer
What happens on week 2, day 11?
- Primitive yolk sac is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum
- Reticulum is later converted to extraembryonic mesoderm by cell migration
What happens on week 2, day 12?
- Maternal sinusoids are invaded by syncytiotrophoblast
- Lacunae becomes continuous with the sinusoids
- Ureroplacental circulation begins
- Uterine storm prepares for support of the embryo
What happens on week 2, day 13?
- Secondary yolk sac (definitive yolk sac) is formed
- Is pinched off the primary yolk sac (primitive yolk sac)
What happens on week 2, day 14?
- Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
- The connecting stalk suspends the embryo and it’s cavities in the chorionic cavity
- The connecting stalk is a column of mesoderm and is the future umbilical cord
- Bleeding around now can be confused with menstrual bleeding
What has happened by the end of the second week?
- Conceptus has implanted
- The embryo and its two cavities (amniotic and yolk) are suspended by a connecting stalk in a supporting sac (chorionic cavity)
Define bilaminar disc
- The epiblast and the hypoblast
- Derived from the embryoblast
- Later will become the trilaminar disc with the formation of the primitive streak
Define germ layer
- Layer from which other developed tissues originated from
Define ectoderm
- The outermost layer of cells giving rise to the epidermis and nerve tissue
Define endoderm
- The innermost layer of cells giving rise to the gut lining
Define doral surface
- Outermost surface during early development
Define ventral surface
- Innermost layer during early development
Define mesoderm
- Middle germ layer giving rise to muscle, bone and more
Define primitive streak
- A narrow groove that develops of the dorsal surface of the epiblast
- Has an important role in the orientation of the embryo, determining the front and the back
Define gastrulation
- The reorganisation of the germ layers into the ectoderm, mesoderm and the endoderm, and hence the establishing the origin of all tissues
Define trilaminar disc
- The three germ layers: ectoderm, mesoderm and endoderm
What are the most important ectodermal derivatives?
- Skin
- Skin derivatives (hair, nails, lens and cornea of eye, mouth and anus lining)
- Brain and spinal cord
- Peripheral nerves
- Retina and iris of eye
What are the important mesodermal derivatives?
- Smooth, skeletal and cardiac muscle
- Connective tissue
- Bone
- Cartilage
- Blood and blood vessels
- Urinary system
What are the important endodermal derivatives?
- Lining of digestive tract
- Glands associated with digestion (eg liver, pancreas)
- Other gut derivatives (eg lungs)
Describe the embryonic period
- Weeks 3 to 8
- Period of greatest change
- All the major structures and systems are formed
- Most perilous period for the developing child
What is week 3 known as and why?
- ‘Week of threes’
- By the beginning of the week there are 3 cavities: amniotic cavity, yolk sac and chorionic cavity
- Three germ layers are established during week 3: ectoderm, mesoderm and endoderm
What is gastrulation?
- Process of establishing 3 germ layers and hence the origin of all tissues
- Trilaminar disc is formed
- Axes observed in an adult are set:
~ anterior/posterior
~ dorsal/ventral
~ right/left - Starts with the appearance of the primitive streak
- Cellular rearrangement occurs through migration and invagination
What does the primitive streak consist of?
- Primitive node
- Primitive pit
Describe the appearance of the primitive streak and what happens to it?
- The primitive streak appears on the dorsal surface of the epiblast in the third week
- The streak is a narrow groove with bulging edges
- Primitive node is located at the cranial end of the streak
- Primitive pit is located at the centre of the node
- As the three germ layers are established as gastrulation proceeds, the primitive streak regresses
- Development proceeds in a cranial/rostral to caudal direction
Describe the migration and invagination of the epiblast cells
- Cells on the epiblast migrate towards the primitive streak
- They then invaginate into the epiblast and displace the hypoblast creating a third layer the mesoderm
- Forms a trilaminar disc: ectoderm; mesoderm; endoderm
- Cells spread laterally and cephalad (towards the head) in the mesoderm
- Mesoderm spreads out leaving 2 gaps
What does the fate of the invaginating epiblast cells depend on?
- Where in the streak they invaginate
What is the notochord?
- Notochord defines the phylum Chordata
- Basis for the axial skeleton
- Drives the formation of the nervous system (neurolation)
- Prenotochordal cells of epiblast migrate through the cranial part of the primitive pit
- Forms a solid rod of cells running in the midline with an important signalling role
- Defines the midline
- Axial skeleton forms around it
- Regresses - vestigial remnant in the adult is in the nucleus pulposus of the invertebral discs
How are axes set?
- Primitive streak appearing at one end of the bilaminar disc defines the front and back (anterior/posterior)
- Molecular signals from the primitive streak ensures correct left right development
What has happened by the end of week 3?
- Embryo has been gastrulated and the bilaminar disc is converted to a trilaminar disc (3 germ layers): ectoderm; mesoderm; endoderm
- Axes have been set
What is the notochord?
- Solid rod of cells running in the midline
- Has an important signalling role
How is the notochord involved with neurolation?
- Notochord directs the conversion of overlying ectoderm to neuroectoderm
- Notochord signals overlying ectoderm to thicken (forms a keyhole-shaped neural plate)
- Edges elevate out of the plane of the disc and curl towards each other, creating the neural tube
What are somites and when do they develop?
- Segments of the paraxial mesoderm
- 1st pair appears at day 20 in the occipital region
- More appear in the craniocaudal sequence at 3 pairs per day until there are 42-44 pairs by the end of week 5
- Some pairs disappear leaving 31 in total
What does the organisation of the mesoderm into somites give rise to?
- Repeating structures eg vertebrae; ribs; intercostal muscles
- Guides innervation
How is the mesoderm structured in the 4th week after the neural tube has been formed?
- Immediately on both sides of the neural tube is the paraxial mesoderm (which develops into somites)
- Intermediate mesoderm outside the paraxial mesoderm
- Somatic mesoderm on the outside at the top
- Splanchnic mesoderm on the outside at the bottom
- Intraembryonic coelom is the space between the somatic and splanchnic mesoderm
How is the intraembryonic mesoderm formed?
- Formed when mesoderm differentiates
- During lateral folding the lateral branches of the mesoderm connect causing the intraembryonic mesoderm to join within
What structures are derived from the paraxial mesoderm?
- Axial skeleton (vertebral column and ribs)
- Dermis
- Muscles of the axial body wall
- Some limb muscles
What structures are derived from the somatic mesoderm?
- Connective tissue of limbs
- Contributes to the axial body wall
What structures are derived from the splanchnic mesoderm?
- Smooth musculature
- Connective tissue and vasculature of gut
What structures are derived from the intermediate mesoderm?
- Urogenital system eg kidneys, ureters, gonads
How do somites differentiate into dermatome, myotome and sclerotome?
- Somites appear as regular blocks of mesoderm cells arranged around as small cavity
- The ventral wall of the somite breaks down in ‘organised degeneration’, which leads to the formation of the sclerotome
- Further organisation of the dorsal portion forms the combined dermomyotome
- Myotome proliferates and migrates, and the dermatome disperses
What are the fates of the dermatome, myotome and sclerotome and what will they make up together?
- Dermatome: dermis
- Myotome: muscles
- Sclerotome: bones
- Together are the fundamental precursors of the musculoskeletal system
What do the terms dermatome and myotome mean developmentally and clinically?
- Dermatome:
~ developmentally: part of the somite that gives rise to the dermis
~ clinically: a strip of skin supplied by a single spinal nerve - Myotome:
~ developmentally: gives rise to muscles
~ clinically: a muscle/group of muscles supplied by a singalong spinal nerve
What do the nerves produced by the neural tube innovate?
- Dermomyotome
What happens to the heart during embryonic folding?
- Heart lies at the cranial end
- Heart moves inwards until it lies in the future chest
What does folding of the embryo achieve?
- Achieves 3D structure
- Creates ventral body wall
- Pulls the amniotic membrane around the disc so that the embryo becomes suspended in the amniotic sac
- Pulls the connective stalk centrally
- Creation of a new cavity within the embryo (coelom)
- Creates primordium of the gut from the yolk sac
- Puts the heart and primordium of the diaphragm in the right place
What has happened by the end of week 4?
- Neurulation: formation of the primordium of the nervous system
- Organisation of the mesoderm: derivatives of the somites; segmentation
