Embryology Flashcards
Follicular phase
(days 1–14)
– Selection of the dominant
“ovulatory” follicle
– ↑ estrogen levels
– Endometrial thickening
Luteal phase
(days 14–21)
– Corpus luteum produces
estrogen and progesterone
– Endometrium prepared for
implantation
How would implantation occur?
Blastocyst produces human chorionic gonadotropin (hCG) thus maintaining progesterone production
Gestational Age or Menstrual Age
Time since the 1st day of the last menstrual period (LMP), which predates conception
EDC & Naegele rule
Estimated Date of Confinement (EDC)
– Naegele Rule
1st Day of LMP + 7 day − 3 months = EDC
Fertilization timeline
Takes place within hours (no more than a day) after ovulation
* Fusion of the nuclei of the maternal & paternal gametes
* Haploid chromosome sets intermingle & create the Zygote
Fertilization Steps
- Passage of sperm through the corona radiata
- Penetration of the zona pellucida
- Fusion of plasma membranes of oocyte &
sperm - Completion of 2nd meiotic division of oocyte
- Formation of male pronucleus
- Breakdown of pronuclear membranes
What happens 24 hrs after fertilization?
The diploid cell with 46 chromosomes undergoes cleavage –> forming 2 blastomeres.
* Blastomeres & polar body
are surrounded by the zona
pellucida
* Blastomeres divide & form a solid
a 16-cell morula
What happens 3 days post fertilization?
- Morula enters the uterine cavity
- Fluid forms between the cells of
the morula creating the
early blastocyst
What happens ~5 days postfertilization?
- 58-cell blastula differentiates into
the inner cell mass
– 5 embryo-producing cells
– 53 outer cells
(trophectoderm) will become
trophoblasts
What is the 107-cell blastocyst composed of?
- 8 embryo-producing cells surrounded by 99 trophoblastic cells
- Secretory-phase endometrial glands release proteases
– Releases blastocyst from the
zona pellucida
What happens ~6-7 day postfertilization?
Implantation into the uterine wall
1. Apposition—Initial contact of the blastocyst to the uterine wall
2. Adhesion— ↑contact between the blastocyst & decidua (thick modified mucous membrane, lines pregnant uterus)
3. Invasion—Penetration & invasion of
syncytiotrophoblasts & cytotrophoblasts into the functional layer of the endometrium
The placenta will develop from the
_____ encircling the blastocyst
trophoblast cell layer
What happens ~Day 8 postfertilization?
- Trophoblasts have differentiated
– Outer layer - primitive syncytiotrophoblast
– Inner layer - cytotrophoblasts
After implantation is complete, trophoblasts further differentiate & give rise to _____
villous & extravillous trophoblasts
What do Villous trophoblasts do?
generate chorionic villi (fingerlike projections)
– Transport oxygen, nutrients, & other compounds between
the fetus & mother
What do Extravillous trophoblasts do?
are further classified as interstitial
trophoblasts & endovascular trophoblasts
What do Interstitial trophoblasts do?
invade the decidua & myometrium to
form placental-bed giant cells & surround spiral arteries
What do Endovascular trophoblasts do?
invade & transform spiral arteries
during pregnancy to create low-resistance blood flow that is characteristic of the placenta
What happens ~9-10 days postfertilization?
- Blastocyst wall facing the uterine
lumen is a single layer of flattened cells - Becomes totally encased within the
endometrium
What happens ~12 days postfertilization?
- Maternal blood fills lacunar network
- Large cavities appear in the
extraembryonic mesoderm - Extraembryonic endodermal cells
form on the inside of the primitive
yolk sac
What are Chorionic Villi?
- Primary villi arise from buds of
cytotrophoblasts - Lacunae join & form a complicated
labyrinth partitioned by cytotrophoblastic
columns - Trophoblast-lined channels form the
intervillous space, & the solid cellular columns
form the primary villous stalks
What happens ~15-17 days postfertilization?
- Mesenchymal cords from extraembryonic
mesoderm invade the solid trophoblast columns
& form secondary villi - Angiogenesis* in the mesenchymal cords forms tertiary villi
- Maternal arterial blood enters the intervillous
space & fetal blood vessels become functional
Postovulatory age:
~15-17 (week 3)
* Organogenesis begins
* Gastrulation = Conversion of
bilaminar embryonic disc to a
trilaminar embryonic disc
* Morphogenesis begins
* Primitive streak forms
Largest susceptibility to
developmental defects occurs when?
Postovulatory age: ~15-17 (week 3)
Week 3
- Development of the Notochord
– Migrates ventral and cranial to the primitive streak/node
– Defines the axis of the embryo - Neurulation begins = formation of the neural tube
- Primitive tube (heart) develops (cardiogenic mesoderm)
- Mesonephric duct
Week 5
- Chorionic/gestational sac
measures ~1 cm in diameter - Embryo ~3 mm long
- Arm & leg buds developed
- Amnion begins to ensheath
the body stalk
Week 6
- Embryo ~9 mm long
– Neural tube closed - cranial end 38 days from LMP
- caudal end closes 40 days from LMP
- Cardiac motion is discernable sonographically
Week 8
End of the embryonic period
* Crown-rump length ~22 mm
* Fingers & toes are present
* Arms bend at the elbows
* Upper lip is complete
Week 9
- ~24 mm in length
- Most organ systems have
begun to develop
Week 10
- Bone growth of the front of the
skull - Placenta supplants the yolk sac
Week 11
- Neck has lengthened
- Eyes & ears are visible, but not
functional
Week 12
- Crown-rump length ~5 to 7 cm
- Most bones have centers of ossification
- Fingers & toes forming
- Patches of hair; skin & nails develop
- Male/female external genitalia show
- Spontaneous movements
Week 13
- Crown-rump length ~7 cm
- Wt ~25 gm
- Pancreas producing insulin
- Female clitoris, labia have formed
End of 1st Trimester
End of week 13
Week 14
- Forearms, wrists, hands, fingers are
differentiated - Eyes migrate toward midline; behind
the sealed eyelids
Week 15
- Jaw lengthens, chin held away from
the chest - Hands & outstretched fingers are
commonly close to the face - Canaliculi (small channels/ducts) of
the lungs begin to form
Week 16
- Crown-rump length* ~12 cm; Wt ~100 gm
- Biparietal diameter, head circumference,
abd. circumference, & femur length are
measured & used to estimate weight - Eyes can blink
- Finger/toe-prints form
- Male penis, urethra, & scrotum formed
Week 17
- ~12.7 cm; ~140 gm
– Would fit comfortably in your palm - Cranial bone replacing cartilage
- Blood vessels/supply proliferating
- Small fat deposits begin to develop
- Heart pumps 25+ oz of blood/day
- Moving joints
Week 18
- Full movement of the arms & legs
- Yawning
- Nerves developing/myelinating
- Hearing developing
- Vision developing
- Female uterus is formed
- Vaginal canalization begins
Week 19
- ~15 cm & Wt ~240 gm
- Skin wrinkled, vernix caseosa
- Umbilical cord coiled (prevents kinking)
- Teeth forming, bones ossifying
- Muscles developing
- Kidneys producing urine
- Digestive tract producing meconium
Week 20
– Midpoint of gestation
* ~15 cm & Wt ~300 gm
* Thumb sucking, stretching
* Facial movements
* “Quickening” (mom feels movement)
* Brown fat forming; skin less transparent
* Lanugo covers the body; some scalp hair present
Week 21
- Head to heal length ~26 cm long
- Wt ~355 gm
- Intestines contract & relax
- Heart rate is ~120-160 BPM
- Red blood cell production begins to
shift from liver to bone marrow
Week 22
- Hearing internal & external sound
- Sense of touch developing
- Fingers moving
- Taste buds developing
- Liver begins breaking down bilirubin
- Male testes begin descent
Week 23
- ~29 cm & Wt ~450 gm
- Eyes fully formed, no iris pigment
- Skin pigment developing
- Organs continue development
Week 24
- ~30 cm ~565 gm
- Secretory type II pneumocytes
secrete surfactant
– Terminal alveoli not yet fully formed - Neural pain system is developed
Week 25
~34 cm ~680 gm
* Spine structures begin to form
* Lungs rapidly developing
Week 26
- ~35.5 cm ~900 gm
- Active brain growth
- Male testes descended
End of 2nd Trimester
End of week 26
Week 27
- ~36.8 cm ~907 gm
- Able to stick out the tongue out
– Esp. before or after a particularly
large gulp of amniotic fluid
Week 28
- ~37.5 cm ~925 gm
- Brain activity indicates dreaming
Week 29
- ~38 cm ~1200 gm
- Fat deposition continues
- Skin smoothing
Week 30
- ~39 cm ~1400 gm
- Brain sulci & gyri becoming
prominent - Lanugo diminishing
- Bone marrow producing more RBC
- Digestive tract nearly fully formed
Week 31
- ~41 cm ~1500 gm
- May be positioned head down
- All 5 senses intact
- Eye lids blink
Week 32
- ~42 cm ~1800 kg
- Skin smooth & thickening
- Bones calcifying
Week 33
- ~43 cm ~2000 gm
- Hair on the head is growing
- Regularly practicing “breathing”: Would likely need some support breathing if born this early
Week 34
- ~44.5 cm ~2500 gm
- Fat deposition increases
Week 35
- ~46 cm ~2500 gm
- Kidney development complete
- Lung development nearly complete
Week 36
- 47 cm ~2.7 gm
- Body fat increases
- Skin is pink 2° to blood vessel
proliferation subcutaneously
Week 37
- ~48 cm ~2800 gm
- No longer considered a “preemie”
- ACOG:
– Babies born 37 – 38 weeks 6 days
– Early-Term
Week 38
- ~46 cm ~3000 gm
- Rooting reflex initiates suckling at
every opportunity (parts of the
hands, thumbs or fingers)
Week 39
- ~50 cm ~3-3500 gm
- ACOG:
– Babies born at 39 weeks 0 days
– Full Term
When do ovaries develop?
11-13 weeks gestation
When do testes develop
6 weeks gestation
– Disorders of Sex Development (DSD)
DSD are congenital conditions in which development of
chromosomal, gonadal, or anatomic sex is atypical
DSDs Classification: 46,XX DSD
virilized female
– Disorders of androgen
excess
– Disorders of gonadal
development
46,XY DSD
undervirilized male
– Disorders of androgen action
or synthesis
– Disorders of gonadal
development
Ovotesticular DSD
– Ovarian & testicular
tissue
– abnormal vagina
– hypoplastic uterus
– Usually 46,XX
Mixed gonadal dysgenesis
– 45,X/46,XY mosaicism
» 42% = phenotypic ♀(Turner syndrome)
» 42% = ambiguous external
genitalia, asymmetrical gonads
» 15% = phenotypic ♂(incomplete)
– Phenotype dependent on % of blood
& gonadal tissue genetic variation
Klinefelter syndrome
47, XXY
– ♂ phenotype, typically
Turner syndrome
(45,XO)
– ♀ phenotype, typically
Precursors of external genitalia =
urogenital tubercle, swelling, & folds
