early fetal development Flashcards
What is fertilisation age (conceptual age)? How is it measured?
From time of fertilization (+1 day from last ovulation). Hard to measure time of fertilization except when IVF so not used practically
What is gestational age? How is it measured?
From time of last menstrual period. LMP calculated by: Fertilization date + 14days if known or obstetric ultrasound comparing embryo to embryo size charts
What is the carnegie stage? How many stages? What window does it cover?
Based on embryo features not time (prescence/absence) of features. Can compare development rates between species. 23 stages in humans. Covers window 0-60 days fertilization age
What happens during the embryogenic stage? What time frame?
From fertilisation to 14-16 days after. Early embryo formed from fertilised oocyte & 2 populations of cells formed - pluripotent embryonic cells (contribute to fetus) and extra-embryonic cells (contribute to support structures like placenta)
What happens during embryonic stage? What time frame?
16-50 days post-fertilization. Germ layers established and differentiation of tissue types + establishment of body plan
What happens during fetal stage? What time frame?
Day 50-270 post fertilisation (2nd/3rd trimesters). Major organ systems present. Migration of some organ systems to final location. Growth & acquisition of fetal viability.
What happens after an egg is fertilised? What covers the oocyte in these stages?
Egg fertilized and forms zygote (1 cell). Series of mitotic cleavage divisions give embryos with 2,4,8 cells (cleavage stage embryos) - become morula (16+ cells) - become blastocyst (200-300 cells). Mitotic divisions happening as oocyte migrates down fallopian tubes to uterus for implantation. Zona pellucida covers oocyte in all these stages
Until 4-8 cell stage what is the embryo dependent on to go through first divisions? When are these made and stored? hat happens with failure of these to be made/stored/interpreted?
Embryo dependent on maternal mRNAs and proteins to go through first divisions. These are made and stored during egg development (pre-ovulation). Failure to make/store/interpret these can impair embryonic development.
What is zygotic genome activation? What happens?
At 3-8 cell stage the embryonic genes take over role of maternal mRNA and proteins and start to make more proteins and maturation of organelles like mitochondria & golgi
What happens during compaction? When does it happen?
8 cell stage or later. Cells divide by mitosis and outer cells get pressed against zona pellucida causing change of cells from spherical to wedge shaped. Tight junctions & desmosomes form in outer cells making barrier to diffusion, and they get polarised. Creates 2 distinct cell types: inner cells at one side of blastocyst and outer cells covering it.
What is the inner cell mass and what will it contribute to?
Inner cell mass are pluripotent embryonic cells that will contribute to final organism.
What is the zona pellucida and its role?
Hard protein shell that inhibits polyspermy and protects early embryo
What are the outer cells called and what will they contribute to?
Outer cells are the trophoectoderm (trophoblasts) which are extra-embryonic cells for extraembryonic support
What is the blastocoel and how is it formed?
Fluid filled cavity formed osmotically by trophoblast pumping Na+ into cavity
When does hatching happen and what does it involve?
Day 5-6. for blastocyt to implant it must escape zona-pellucida. 1. enzymatic digestion - blastocyst secretes enzymes 2. cell contraction of embryo weakens point of zona pellucida causing hatching.
When do peri-implantation events happen?
Day 7-9
What happens during peri-implantation events once the embryo has initially connected with the endometrium?
Trophoblasts fuse to form synchitiotrophoblast which invades endometrium destroying maternal cells & breaking down capillaries allowing them to be bathed in maternal blood. Cytototrophoblasts are the cells that remain individual to provide source of syncitriotrophoblast cells. The inner cell mass separates into epiblast and hypoblast
What do the epiblast and hypoblast cells produce respectively?
Epiblast forms fetal tissues and hypoblast forms yolk sac
When does bi-laminar embryonic disc formation occur?
Day 12+
How is the bilaminar embryonic disc formed?
Some epiblast cells become separated from other epiblast cells by formation of amniotic cavity (which give rise to amnion). Left with bilaminar disc of epiblast and hypoblast ready for gastrulation
What do synchitiotrophoblasts start doing at stage of bilaminar embryonic disc formation? Why is this useful?
Syncitiotrophoblasts start secreting hCG which is the basis for pregnancy tests
What happens during gastrulation? What is the end result?
Thickened structure (primitive streak) forms along midline in epiblast near caudal end of disc. At cranial end, primitive streak expands to form primitive node with primitive pit (depression) which continues along midline to caudal end forming primitive groove. Epiblast cells migrate inwards towards streak, detach from epiblast and slip beneath it into interior of embryo. First cells to invaginate through groove invade hypoblast and replace its cells forming definitive endoderm. Remaining epiblast cells are ectoderm. Some of invaginated epiblast cells remain in space in between forming mesoderm. Gastrulation from cranial to caudal end forming 3 distinct primary germ layers.
What do the 3 primary germ layers form from?
epiblast
What organs does the endoderm form?
GI tract, liver, pancreas, lung, thyroid
What organs does the mesoderm form?
? Blood (endothelial cells, RBC, WBC), muscle (smooth, skeletal, cardiac), gonads, kidneys, adrenal cortex, bone, cartilage
What organs does the ectoderm form?
CNS & neural crest, skin epithelium, tooth enamel
When is the notochord formed?
day 13+
What is the notochord? What does it form from? What is its role?
Notochord is rod-like structure formed of cartilage-like cells. It forms from primitive streak and grows towards cranial end at midline under ectoderm. Acts as key organising centre for embryonic development, releases GF signals important for neuralation. Signals go from notochord up to direct neural plate to form neural tube
What is the neural plate?
Neural plate lays on top of embryo, on top of notochord
What is neurulation? When does it happen?
Formation of neural tube (CNS formation). Happens day 14/15
What happens during neuralation?
Notochord sends signals to neural plate for part of plate to invaginate and move down to notochord forming neural groove and for 2 areas of plate to move up forming neural folds (with neural crest cells). The 2 neural folds move towards eachother and fuse forming neural tube below.
What is the area over top of neural tube covered by?
epidermis (ectoderm derived)
When does closure of head end and tail end of tube happen and why?
Closure of head end at day 23 and tail end day 27. vital for subsequent structures to develop
What happens if failure of tail end of neural tube to close?
Spina bifida - open neural tube at birth usually lower spine
What happens if failure of head end of neural tube to close?
Anencephaly - absence of most of the skull and brain
What do neural crest cells do? What are they derived from? How do we classify them?
Neural crest cells (ectoderm derived) are plastic and migrate during development to give many cell types. Classified by where they end up in embryo
What are types of neural crest cells and what do they give rise to?
Cranial (neurones, glia, lower jaw, ossicles, facial cartilage), cardiac (aortic arch/pulmonary artery septum, large artery walls, musculoconnective tissue), trunk (dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes), vagal & sacral (pns ganglia & enteric nervous system ganglia)
What do defects in migration/specification of neural crest cells cause?
Birth defects including pigmentation disorders, deafness, cardiac & facial defects & failure to innervate gut
What is somatogenesis?
somite formation
What happens during somatogenesis?
Somites arise from paired blocks of paraxial mesoderm at either side of neural tube. Blocks of paraxial mesoderm bud off synchronised forming pairs of somites (starts at head end down to tail end)
How many pairs of somites in embryo? Rate?
44 pairs. After first pair formed, one new one every 90 minutes
What do somites give rise to? What do these tissues give rise to?
Somites give rise to mesodermal tissues. Form 2 types of tissue 1. sclerotome (gives rise to vertebrae and rib carrialges) & 2. dermomyotome (divides to form dermatome –> dermis, fat, connective tissue of neck & trunk, and myotome –> muscles of embryo)
When does formation of the gut tube happen?
day 16+
What is the yolk sac and its role?
Embryonic structure involved in haematopoiesis derived from hypoblast
How is the primitive gut formed and what are its parts?
Ventral folding in embryo (head and tail folds in) followed by lateral folding (sides of embryo fold) to pinch off part of yolk sack, forming primitive gut (foregut, midgut, hindgut)
Foregut structures?
Oesophagus, stomach, upper duodenum, liver, gallbladder
Midgut structures?
Lower duodenum, remainder of small intestine, ascending colon + 2/3 of transverse colon
Hindgut structures?
1/3 of transverse colon, descending colon, rectum & upper anal canal
What is the heart formed by? When is the first detectable heartbeat?
Mesoderm. Begins at tube around day 19 beating and pumping blood. First detectable heartbeat at 6 weeks gestational age
What are lungs derived from? How does it form?
Endoderm. Arises from lung bud which splits into 2 and progressively branches to form bronchi etc.
What are gonads derived from? How do gonads form?
Mesoderm. Initially bipotential - 2 thickenings of mesoderm towards lower back half of embryo
What happens in XY embryo?
Prescence of SRY gene (on Y chromosome only) switched on so gonadal cells triggered to become sertoli cells triggering testis formation, leydig cells & testosterone production
What happens in XX embryo?
Absence of SRY gene so these cells adopt granulosa cell fate and ovary development, reinforced by FPXL2
