4- MoD; Embryogenesis Flashcards
what are the two main periods of human/ embryo development?
embryonic period - up to end of week 8
foetal period - remaining time in utero
what occurs during the embryonic period?
most of organogenesis - organs, tissues and body plan is established
what occurs during the foetal period?
growth and remodelling, refinement of initially specified organs and tissues
five key processes of embryogenesis
fertilisation
cleavage - from zygote to morula to blastocyst
gastrulation
neurulation + somtiogenesis
organogenesis
timeline for the five key processes of embryogenesis
fertilisation - within 24 hours post-ovulation
cleavage:
- day 1-3 = zygote undergoes multiple mitotic divisions, forms a morula
- day 4-5 = morula develops into blastocyst
- day 5-12 = blastocyst implants into uterine wall, completes implantation
bilaminar disc formation - day 8-9
gastrulation - day 15-18 = formation of primitive streak, epiblast cells ingress and three germ-layer disc forms
neurulation - day 19-24
somtiogenesis - day 20+ onwards
organogenesis from week 4-8 = forming major organ systems
describe the process of fertilisation
- upon fertilisation - two pronuclei and two polar bodies in zygote, perivitelline space surrounded by zona pellucida
- through cleavage - series of rapid cell cleavage divisions form 8-16 cell morula = compact collection of cells without a cavity
- compaction - morula undergoes reorganisation to form two distinct cell populations
- exterior cells
= have strong cell-cell contacts with basolateral surfaces, and apical surfaces are in contact with extrac. environment
= form trophectoderm - extra embryonic strictures
- interior cells
= only have cell-cell contacts, tightly packed = form inner cell mass for embryonic structures
- exterior/ trophectoderm cells pump fluid into centre of embryo and form a blastocoel - blastocoel enlarges and pushes inner cells to one side
- morula becomes blastocyst of 32-64 cells - implantation of blastocyst attaching to uterus lining - sealed by fibrin plug
- inner cell mass differentiates into epiblast & hypoblast after implantation = form bilayer germ disc
- epiblast cells reorganise & amniotic cavity forms
- hypoblast contributes to formation of extra-embryonic structures (with trophectoderm)
what is gastrulation
second main re-organisation of embryo from bi-layered germ disc (epiblast & hypoblast) to three-layer germ disc (ectoderm, endoderm, mesoderm)
describe the process of gastrulation
formation of primitive streak on surface of epiblast from caudal end - proliferating & condensing epiblast cells towards midline of embryo, extending from caudal to cranial end
midline epiblast cells migrate & involute & ingress towards Henson’s node and between epiblast + hypoblast layers
= undergo epithelial-to-mesenchymal transition
= migrated epiblast cells differentiate into MESODERM
cells that remain in the epiblast = ECTODERM
cells in the hypoblast = ENDODERM
what will the mesoderm, ectoderm & endoderm become?
mesoderm - become musculoskeletal (bone & muscles), connective tissue + blood vessels
ectoderm - nervous system, skin & sensory organs
endoderm - lines digestive organs & resp tracts, becomes associated organs
what is neurulation?
the formation of the specialised neural tube from the neural plate
describe the process of neurulation
- regression of primitive streak lays down notochord - a rod-like structure of mesodermal cells below the ectoderm
- notochord secretes signalling molecules (e.g. Sonic Hedgehog) - tells ectoderm to differentiate into neural tissue and become the neural plate
- neural plate forms as a thickened region of ectoderm directly above the notochord - neural plate undergoes convergence + extension & cell wedging by formation of hinge points under Wnt-PCPC pathway influence
- midline hinge point/MHP cells of neural plate form the first hinge points - centre of the neural plate sinks and forms the neural groove with raised neural folds
- cells at the tip of neural folds are called neural crest cells = migrate to form structures like peripheral nerves, melanocytes and facial cartilage
- neural folds converge towards midline, appose & fuse together - form closed neural tube
- underlying mesoderm notochord cells continue to influence ectoderm neural plate - full fusion of neural tube with anterior & posterior neuropores fusing = will form the brain and spinal cord
what processes occur at the same time as neurulation?
somtiogenesis
other morphogenic processes
formation of the gut
body folding
describe the process of body folding & gut formation according to the ‘silk purse’ model
folding in longitudinal & lateral directions simultaneously to enclose mesoderm & endoderm layers as they give rise to the internal organs, and surround them with the ectoderm
body folding occurs through ‘silk purse’ model
- positions heart & septum from margin to centre
- yolk sac & stalk make umbilical cord
embryonic folding of endoderm moving towards midline incorporates dorsal part of yolk sac to for the primitive gut tube – consists of the foregut, midgut, hindgut derived from the endoderm
foregut at cranial end of embryo = temporarily closed by oropharyngeal membrane which ruptures at the end of week 4 to form the mouth
midgut between the foregut & hindgut, connected to yolk sac until week 5
- connection to yolk sac narrows into vitelline duct (stalk) with more folding
hindgut at the caudal end of embryo temporarily closed off by cloacal membrane – ruptures at end of week 7 to form urogenital & anal openings
basic body plan & gut formation laid out by the end of week 8
describe what occurs in organogenesis
differentiation of somatic derivatives - into bones, muscles, tendons
development of sensory organs - ears, eyes, olfactory pits
limb formation - forelimbs first, then hindlimbs; establishing pattern in limbs along three axis:
- proximodistal = from shoulder to fingers
- anteroposterior = thumb to little finger
- dorsoventral = back of hand to palm
forming facial structures - jaws, nose, tongue, palate
forming genital structures from undifferentiated tissue into sex-specific structures with genetic & hormonal influences
defects during embryogenesis result in congenital malformations - examples?
neural tube defects - craniorachischises, anencephaly, spina bifida
cardiac defects, limb malformations
craniofacial abnormalities - e.g. cleft lip & palate
chromosomal abnormalities - e.g. Down’s syndrome & Turner syndrome
GI defects, urogenital defects
