Embryology Flashcards
phases of prenatal development
- Period of the zygote: conception through to implantation
- Period of the embryo: 3rd- 8th week: organ formation, heart beat
- Period of the fetus: 9th week – birth
period of the zygote (day2 - week2)
- From conception through to implantation
- Days counted after fertilization
- Day 2: 4 cell stage
- Day 3: Morula
- Day 4-5: Early blastocyst enters uterus
- Day 6-7: Blastocyst implantation. –> Trophoblast cells proliferate and forms
o Cytotrophoblast: inner layer of cells
o Syncytiotrophoblast: digests endometrium, multinuclear mass - Day 12: Implantation complete
- Week 2: Bilaminar embryo
week 1 after fertilisation
- Blastocyst implantation begins day 6-7
- Blastocyst nourished by uterine secretions
- Blastocyst adhesion: Trophoblast cells of Blastocyst adhere to site with proper receptors and chemical signals
week 2 after fertilisation
- Blastocyst now composed of:
o Cytotrophoblast: Inner layer of cells
o Syncytiotrophoblast: Outer layer anchored into endometrium with long protrusions, enzymes help with implantation, multinucleated (fused), large surface area for gas exchange
day 12
- Embryo completely embedded in endometrium
- Formation of embryonic disc:
o Inner cell mass divides into two layers: Epiblast & Hypoblast
o 2 cell layers = Bilaminar embryo - Two hollow cavities visible:
o Amnion
o Yolk Sac
embryonic development - week 2
- By week two the embryo consists of a bilaminar disc
- Inner cell mass divides into two layers: epiblast and hypoblast
- Subdivided inner cell mass is now called embryonic disc
- Extraembryonic membranes form during first 2–3 weeks of development and include:
o Amnion: sac filled with amniotic fluid
o Yolk sac: sac at ventral surface of embryo
o Allantois: small outpocketing at caudal end of yolk sac
o Chorion: helps form placenta
Extraembryonic membranes form during first 2–3 weeks of development and include: amnion, yolk sac, allantois, chorion
Amnion: epiblast cells form transparent sac filled with amniotic fluid that envelopes embryo -> protection
o Cushioning against blows
o Temperature regulation
o Weightless environment for movement
Yolk sac: sac that hangs from ventral surface of embryo
o Forms part of digestive tube
o Source of earliest blood cells and blood vessels
Allantois: small out-pocketing at caudal end of yolk sac
o Structural base for umbilical cord
o Becomes part of urinary bladder
Chorion: helps form placenta
o Encloses embryonic body and all other membranes
o Syncytiotrophoblast plus cytotrophoblast
period of the embryo - Developmental Milestones
week 3: Formation of the trilaminar embryo
o Changes in the Chorion -> Formation of chorionic villi
o Gastrulation begins -> Formation of 3 germ layers (ectoderm, mesoderm, endoderm)
o Neurulation -> Formation of the Neural tube
week 4: Embryonic folding, heart beat, formation of limb buds
week 4-5: Placentation
week 5: Alimentary canal is continuous tube
week 7: Feet and hands emerge, rudimentary skeleton, eyes, nose and mouth shape
weeks 7-8: Sexual development
o If male, SRY on Y chromosome triggers formation of testes, otherwise ovaries
formation of chorionic villi
Changes in the Chorion:
A) Anchoring villi:
- penetrate into decidua basalis and anchor chorion vesicle to wall of uterus
B) Absorbing villi: - branch in intervillus spaces o Primary villi o Secondary villi o Tertiary villi
3 chorionic villi
Primary villi: Syncytiotrophoblast – cytotrophoblast
Secondary villi: Syncytiotrophoblast – cytotrophoblast – extraembryonic mesoderm
Tertiary villi: Syncytiotrophoblast – cytotrophoblast – extraembryonic mesoderm – afferent and efferent capillaries
week 3 chorionic villi development
- End of week 3, embryonic blood begins to flow through the capillaries in the chorionic villi
- Diffusion between chorionic blood vessels and maternal blood in lacunae -> Oxygen and nutrients diffuse through the walls of the villi and enter the embryo’s blood
- Carbon dioxide and waste products diffuse from the blood in the embryonic capillaries through the wall of the chorionic villi into the maternal blood
changes in the embryonic disk - week 3
- Gastrulation: Conversion of bilaminar embryo into trilaminar embryo
- Migration of superficial epiblast cells, consisting of totipotent cells to interior creates third layer
- Three germ layers: Ectoderm, Mesoderm, Endoderm
gastrulation
- Formation of the trilaminar embryo
- Cells proliferate from both sides of the primitive streak
- Epiblast cells migrate to primitive streak
- Cells push through primitive groove and form mesoderm
- Mesoderm spreads between epiblastic and hypoblastic layers of bilaminar embryonic disc -> formation of trilaminar embryo
o Epiblast -> forms ectoderm
o Hypoblast -> forms endoderm
o Middle layer = mesoderm - 2 areas where there are no embryonic mesoderm cells:
o Cloacal membrane
o Oral membrane
neurulation (during 3rd week)
- Some cells migrate cranially from the primitive node to form a rod-like structure = notochord
- Notochord plays a major inductive role in the subsequent development of the nervous and musculoskeletal systems
- Notochord is gradually incorporated into the pre-cartilaginous vertebrae of the developing embryo, and degenerates, except in the region of the intervertebral disc, where it persists as the nucleus pulposus
- Neurulation = formation of the neural tube
- Neural tube becomes the central nervous system (CNS)
neural tube and somite development
- Notochord induces ectodermal cells above it to thicken and form neural plate
- Neural folds move together and fuse = formation of neural tube
- Failure of neural folds to fuse dorsally may result in severe abnormalities of the CNS such as anencephaly and spina bifida
- Embryonic mesoderm adjacent to the notochord and neural tube = paraxial mesoderm
- End of week 3: Paraxial mesoderm undergoes craniocaudal condensation to form segmental blocks of mesoderm = somites
- Somites will form axial skeleton and its musculature
- Unsegmented mesoderm gives rise to heart and vascular system, bone and muscle of limbs and body wall
3 germ layers and their derivatives
Ectoderm (outer layer) (forms from epiblast)
o Nervous system
o Skin, Epidermis
o Hair
Mesoderm (middle layer)
o Muscles
o Bones
o Circulatory system
Endoderm (inner layer) forms from hypoblast
o Gastrointestinal tract & associated glands (pancreas, liver, etc.)
o Lungs
o Urinary tract
o Mucosae
week 3 summary
- Major features of the third week of human development are:
- Gastrulation: Formation of the 3 germ layers
- Formation of the primitive streak
- Formation of the notochord
- Neurulation: Formation of the Neural tube
- Somite formation
- Paired endocardial heart tubes
week 4 - embryonic folding
- Folding of cranial and caudal ends and lateral folding of the embryo occur simultaneously
- Head fold brings the primitive heart region and the oral membrane onto the ventral surface
- Lateral folding forms the abdominal walls
- Embryonic folding occurs in two different planes
Week 4 summary
- Major developmental events include:
o Folding of the embryo
o Formation of branchial/pharyngeal arches
o Formation of limb buds
o Formation of optic pits, optic vesicles, nasal placodes
week 4 1/2 - Regional differences in placental organisation (placentation)
- Chorionic villi invaded by new blood vessels -> extend to embryo as umbilical arteries and vein
- Placenta consists of fetal and maternal tissues, with fetal and maternal blood separated by the chorionic villi
- Placenta = organ that forms from chorion and endometrium to allow embryo/fetus to exchange nutrients and waste
- Chorionic villi provide surface area for exchange
- Nutrient and gas exchange happens without actual blood exchange
- Umbilical cord - contains two umbilical arteries and one umbilical vein
placenta
- Semipermeable
o Allowing nutrients and gasses to pass through
o Blood cells are too large: blood not exchanged between foetus and mother as can have different blood types and would cause immune response - Site of all metabolic transactions sustaining the embryo
week 5
- By week 5, alimentary canal is a continuous tube from mouth to anus
- Shortly after, accessory organs bud from mucosa
Oral membrane:
- Will become mouth
Primitive gut:
- Epithelial lining formed from endoderm, with rest of GIT derived from the mesoderm
Cloacal membrane:
- Will become anus
period of the foetus - 9th week –> 12th week
- After the end of 8 weeks: Fetal development
- Weeks 9-12: External genitalia form
- Week 12: all organ systems laid out
o Most teratogens not lethal anymore – but can produce major defects
period of the foetus - 3rd month
o Movement but cannot yet be felt by mother
o Digestive system and excretory systems functioning
o Reproductive system contains immature ova or sperm cells
period of the foetus - 4th –> 6th month
o Sucking, swallowing, breathing
o Movements – felt by mother
o Heart beat can be heard with stethoscope
o Sweat glands functioning
o Vernix – protects skin from chapping
o Lanugo – fine hair helps vernix stick to skin
o Visual and auditory senses are functional
period of the foetus - 7th –> 9th months
- Age of viability: 22-28 weeks – survival outside the womb is possible
- Weight is ~2kg (at end of 7th month)
- 3rd trimester mostly for size increase and maturity
lung development and 9th month –> birth
- Lungs finish developing last in the fetus
o Baby in womb gets oxygen supply from the placenta
o Surfactant develops late (necessary to keep alveoli inflated)
o Most babies born at 35 weeks will have adequate functioning lungs -> fully developed lungs by 37 weeks
9th month – growth, activity slows, sleep increases
- Birth occurs:
o Most babies are born head first
o If feet/buttock first = breech
o Have to stop breathing through placenta’s oxygen supply- switch to breathing air -> first breath
