Embryology Flashcards
Prenatal period?
- 38 weeks from conception to birth (average) “fetal” age
- gynecologic timing has been from LMP so refers to 40 weeks gestational age (LMP is on avg 2 weeks b/f ovulation)
Embryonic period?
- first 8 weeks
- all major organs are formed
Fetal period?
- remaining 30 weeks
- organs grow large and become more complex
Process of ovulation and fertilization?
- ovulation: egg released into peritoneal cavity
- fertilization: occurs as egg travels down fallopian tube
- zygote: the cell that forms when the ova and the sperm fuse
- cell division: begins immediately as zygote travels down fallopian tube and into uterus
Zygote cleavage - leads to blastomeres?
- zygote divides repeatedly moving down tube toward uterus (cleavage)
- daughter cells are called blastomeres
- morula: solid cluster of 12-16 blastomeres at about 72 hrs
- day 4: 60 cell morula enters uterus, taking up fluid becoming a blastocyst
2 distinct types of cells of blastocyst? floats around for how long?
- inner cell mass: forms embryo
- trophoblast: layer of cells surrounding the cavity which helps form the placenta
- floats for about 3 days
When does implantation occur?
- how are monozygotic twins formed?
- day 6 post conception
- trophoblast erodes into uterine wall
- takes 1 week to complete
- if inner cell mass of single blastocyst divides: monozygotic twins
What occurs during week 2 post conception?
- inner cell mass divides: epiblast and hypoblast
- 2 fluid filled sacs: amniotic sac from epiblast and yolk sac from hypoblast
- bilaminar embryonic disc - gives rise to whole body
formation of 3 primary germ layers?
- bilaminar disc - forms trilaminar disc in week 3
- ectoderm
- endoderm
- mesoderm
description of 3 germ tissues?
early specialization of cells:
- ectoderm and endoderm are epithelial tissue (form sheets of tissue)
- mesoderm is a mesenchyme tissue
- mesenchyme cells are star shaped and don’t attach to one another, they migrate freely
Formation of 3 germ layers?
- primitive streak - on dorsal surface of epiblast
- grastulation: invagination of epiblast cells
- day 14-15: they replace hypoblast becoming endoderm
- day 16: mesoderm (new third layer)
- epiblast cells remaining on surface are ectoderm
Notochord formation?
- primitive node epiblast cells invaginate
- migrate anteriorly with some endoderm cells
- rod defining the body axis is formed
- future site of the vertebral column
Process of neuralation?
- notochord signals overlying ectoderm
- formulation begins of spinal cord and brain
- neural plate to neural groove to neural tube: pinched off into body
Mesoderm differentiation in week 3?
- lateral to notochord, week 3
- extends cranially and caudally (from head to tail or crown to rump)
- division of mesoderm into 3 regions: somites - 40 pairs of body segments (repeating units) by end of week 4
- intermediate mesoderm: just lateral to somites
lateral plate: spilts to form coelom
When does neural tube close? What is impt for this step?
- formation of neural crest?
- begins at end of week 3, complete by end of week 4
- folic acid impt for this step
- extends cranially (eventually brain) and caudally (spinal cord)
- neural crest: lateral ectodermal cells, pulled along and form sensory nerve cells and other structures
Divisions of the mesodermal lateral plate?
- somatic mesoderm: apposed to ectoderm
- splanchnic mesoderm: apposed to endoderm
- coelom in b/t will become the serous cavities of the ventral body cavity:
peritoneal
pericardial
pleural
When does embryo take rudimentary shape of adult?
- week 4
- look at cross sections - basically similar
Major derivatives of ectoderm?
- epidermis, hair, nails, glands of skin
- brain and spinal cord
- neural crest: sensory nerve cells and some nervouse structures; pigment cells; portions of skeleton, blood vessels in head and neck
Major derivatives of mesoderm?
- somite: sclerotome: vertebrae and ribs
- dermatome: dermis
- myotome: trunk and limb musculature
- intermediate: kidneys, and gonads
- lateral plate mesoderm:
somatic - CT of limbs
splanchnic: wall of digestive and respiratory tracts, visceral serosa, heart, blood vessels
Major derivatives of endoderm?
- epithelial lining of glands of digestive and respiratory tract
When does heart start pumping?
- 29 days
-
Importance of early abstinence of drugs in pregnancy?
- before 2 month mark
- by 8 weeks all major organs are at least a rudimentary form, many drugs taken during this time can cause birth defects
Weeks 3-8 organogenesis significance?
- stem cell populations est for each organ primordia
- this is period when most gross structural birth defects are induced
- 3rd and 4th weeks are particularly vulnerable and mother may not even know she is pregnant (somites which produce majority of head mesenchyme starts in cervical region at day 20 and spreads caudally at rate of 3 pairs per day
- SIG is that mother MUST avoid environmental, dietary and drug exposure during this period!
