Introduction to Embryology Flashcards
describe the three trimesters of gestation and their key events
> Trimester 1: 0-12 weeks - organogenesis: organ creation - most critical stage of development - teratogenic effects (nutrition, drugs, radiation, microbes, stress) > Trimester 2: 13-24 weeks - organs complete most of development - foetus appears human - can survive if born with critical care > trimester 3: 25 weeks till birth - rapid foetal growth - most systems fully functional
describe the stages of the baby
- weeks 1-2: pre-embryonic stage
- weeks 3-8: embryonic stage
- weeks 9 onwards: foetal stage
What are the three main steps in embryogenesis?
> multiplication:
- cells divide and proliferate to increase their numbers
migration:
- cells move along prescribed paths to new destinations
differentiation
- cells become specialised to perform specific functions
what are the weekly events leading up to week 5-8 of embryonic development?
> week 1: - multiplication - blastocyst formation > week 2: - implantation of the blastocyst - formation of bilaminer embryonic disc > week 3: - formation of three germ layers > week 4 - folding of embryo into 3D - some structures develop > week 5-8 - organogenesis
describe days 0 - 5 in conception
day 0 = fertilisation: - union of male and female haploid gamete to form diploid zygote - zygote is membrane bound day 1 - 3 = zygote cleavage - blastomeres - mitotic cell divisions Day 4 = morula - 16 cells day 5 = enters uterus - around 100 cells and around day 5 enters the uterus
describe days 5-6
> in uterus now
adheres to uterine lining
blastocyst formation:
- migration and differentiation of cells of morula to form blastocyst:
- trophoblasts: form shell, become placenta
- embryoblasts= group at edge of blastocyst - become embryo
hatching:
- ovum membrane dissolves and blastocyst hatches:
- can access uterine secretions
- migrates towards uterine wall
- adheres to uterine lining
what is a summary of week 1 events?
1) fertilisation - zygote formed
2) cleavage - morula formed
3) blastocyst formation and hatching
4) alignment and adhesion to uterine lining
what is implantation
> trophoblast cells group into endometrium:
- digest endometrial cells
- endometrium engulfs blastocyst
describe the formation of the bilaminer disc
> embryoblast cells of the blastocyst differentiate into two distinct layers
- epiblast cell layer (columnar and more lateral) - become the amniotic cavity - migrate to form roof of cavity
- hypoblast cell layer (cuboidal and medial) - migrate around inner blastocyst wall - become yolk sac
> the trophoblast cells expand for this new disc
- has a connecting stalk between the embryo and the trophpblasts = the umbilical cord
> mesenchyme connective tissue layer develops and surrounds the bilaminer disk and inside the trophoblast shell
- forms chorionic cavity
- for fluids and secretions for protection of the embryo
what are the different types of twins formed in terms of the chorionic and amniotic layers?
> if one fertilised egg splits = monozygotic twins
if 2 eggs fertilised by sperm = dizygotic twins
- di-chorionic, di-amniotic
if splits at days 1-4 (morula stage)
- di-chorionic, di-amniotic
if splits days 5-8 (blastocyst already formed, then splits)
- mono - chorionic, di-amniotic
if splits at days 8-12 (bilaminer disc already formed)
- mono-chorionic and amniontic
- high risk for baby
what is gastrulation?
> formation of three germ layers
bilaminer embryonic disc transformed into trilaminar embryonic disc
three primary germ layers:
- ectoderm - epiblasts
- mesoderm - epiblasts as well (check this)
- endoderm - hypoblasts
caused by migration of epiblast cells
describe the process of gastrulation (primitive streak, invagination, notochord, neural tube, internal epithelia, mesoderm)
Primitive streak formation:
- multiplication and migration of epiblast cells down the median plane on the dorsal aspect. It starts caudally and elongates cranially, cranial end forms primitive node
Invagination:
- epiblast cells invaginate through the primative streak:
- differentiate into new cells -> hypoblast = endoderm, mesoderm and epiblast = ectoderm
- two locations where ectoderm and endoderm in direct contact = oropharyngeal membrane -> future oral cavity, cloacal membrane = future anus
Notochord:
- develops in mesoderm
- important role in induction of other tissue development
- vertebral column forms around
- becomes part of the intervertebral disc
Neural tube:
- notochord induces overlying ectoderm cells to form neural tube
- neural tube development produces epidermis and nervous system later on.
- development starts in the middle and progresses towards the head and tail
- if tail doesnt form correctly = spina bifida, head = no brain
Internal epithelia:
- yolk sac = ectoderm
- derivatives = GI tract - epithelia and gland, resp tract, urinary bladder, liver and pancreas
Mesoderm development:
- derivatives = muscles and connective tissue
- segments into solomites (L and R pairs), then in three parts
- myotome = skeletal muscles of neck, limbs and trunk
- dermatome = connective tissue
- sclerotomes = vertebrae and ribs
- lateral mesoderm not tucked next to neural tube:
= splanchnic mesdoderm = organs
= somatic mesoderm = limb bones
describe the hierachy of stem cells and what they differentiate into
1) Totipotent = blastocyst
2) Pluripotent
- endoderm -> hepatocytes and GIT mucosal epithelium (multipotent),
- mesoderm -> mesenchyme cells (multipotent) -> lymphocytes, muscle cells, bone, heart muscle, connective tissue
- ectoderm -> neuroepithelial cells -> neural cells, glial cells
- > keratinocytes
Describe longitudinal and lateral folding
Longitudinal:
- rapid longitudinal growth of neural tube -> posterior, produces C shape
- heart and oropharyngeal membrane to ventral surface
- entire surface covered by ectoderm
Lateral folding:
- around the yolk sac
- from sides: form ventral surface
- encloses longitudinal tube -> primitive gut = GI tract
- neural tube posterior -> mesoderm for somatic structures postero-lateral, mesoderm for visceral structures anterior
What happens during organogensis?
week 4: - upper limb and lower limb buds, heart prominence - otic and lens placodes in week three week 5 and 6: - rapid brain growth - large head week 7 and 8: - limbs, digits, human characteristics
