Early Embyrology Flashcards
fertilization - overview
*typically occurs in the ampulla of the fallopian tube
*haploid gametes meet in the fallopian tubes, creating a single-cell, diploid zygote
transition from zygote → morula → blastocysts (cleavage)
*the zygote makes a 4-day journey from the fallopian tube to the uterine wall, dividing along the way (5 divisions, to form the morula [16 cells] at day 4)
*morula continues to divide until the blastocyst forms
*water enters the developing blastocyst and pushes an inner clump of cells to one pole; the fluid becomes the blastocoel
*compacted cells form the inner cell mass, which is surrounded by the trophoblast
note - the zona pellucida remains
what do the layers of the blastocyst become
*trophoblast → placenta
*inner cell mass → embryo/fetus
note - the blastocyst is surrounded by a shell called the zona pellucida
implantation - overview
*the blastocyst makes its way to the uterine cavity and must attach/implant itself into the endometrium to remain viable
*typically occurs at the start of week 2 of gestation (days 6-10)
*blastocyst must shed the zona pellucida
*the trophoblast invades the endometrium and begins to form fetal villa
gastrulation - defined
*the process by which the bilaminar disc (epiblast & hypoblast) develops into a trilaminar disc (3 germ layers formed from epiblast: ectoderm, mesoderm, endoderm)
bilaminar disc formation
*occurs during week 2
*inner cell mass differentiates into:
1. epiblast → further differentiates into fetal cells
2. hypoblast → forms the yolk sac
anatomical planes of the bilaminar disc
- dorso-ventral plane (back to belly):
-epiblast faces the dorsal (back) axis
-hypoblast faces the ventral (belly) axis [recall that the yolk sac contributes to developing the gut to remember this] - cranio-caudal plane (head to tail):
-formation of the primitive streak [a thickened cord of epiblast cells] begins at the caudal (tail) end and progresses cranially (towards the head)
gastrulation - overview
*the process by which the 2-layer bilaminar disc forms the 3-layer trilaminar disc
*the single-layered epiblast transforms into a multilayered and multidimensional structure (ectoderm, mesoderm, endoderm)
*this process begins at week 3 (week 3 = 3 layers)
gastrulation & cell migration - overview
*an indentation, called the primitive streak, starts to form at the caudal end of the embryo and progress in a cranial direction
*epiblast cells at the center of the primitive streak die off and leave a potential space (primitive groove) for other cells to move through
gastrulation process - detailed
*involves a combination of cell migration and apoptosis to provide a space in which cells can migrate
- cells on the edge of the primitive streak secrete fibroblast growth factor (FGF8) and allows epiblast cells to migrate to and through the primitive streak
- cells on the ventral edge replace hypoblast cells and become the endoderm
- the dorsal-most cells (epiblast) will eventually become the ectoderm
- the cells in the middle will become the mesoderm
germ layers and their derivatives
*3 germ layers:
1. ectoderm → divided into surface ectoderm (becomes epidermis of skin) and neural ectoderm → CNS and PNS
2. mesoderm → gives rise to muscles, bones, connective tissue, blood/blood vessels, gonads, kidneys, etc
3. endoderm → epithelial linings of GI & respiratory tracts
neurulation - overview
*definition: the process by which we form the central nervous system
*the notochord induces thickening and differentiation of the overlying ectoderm into neuroectoderm, which invaginates to create the neural tube during week 3
- the neural tube becomes the CNS (brain and spinal cord)
- the neuroectoderm adjacent to the neural tube gives rise to the neural crest cells (which give rise to the peripheral nervous system)
neural tube defects
*occur due to incomplete closure of the neural tube
1. anencephaly (due to incomplete CRANIAL closure of the neural tube)
2. spina bifida (due to incomplete CAUDAL closure of the neural tube)
genes affecting the ectoderm
- sonic hedgehog (Shh) - important for establishing the left and right sides of the brain
-separates the eye plate into 2 separate eyes
-failure can → holoprosencephaly - Wnt-7 - organizes the limbs in a dorso-ventral plane
- homeobox (Hox) - organizes the limbs in a cranio-caudal plane
derivatives of the mesoderm germ layer
*in general, the mesoderm gives rise to:
1. muscles, bones, and connective tissues
2. blood, blood vessels, and lymphatics
3. gonads & kidneys
4. dermis
5. adrenal cortex
6. intervertebral discs
VACTERL syndrome/association: a disorder of the mesoderm
V - vertebral defects
A - anal atresia (imperforate anus)
C - cardiac defects
TE - tracheoesophageal fistula
R - renal defects
L - limb anomalies
derivatives of the endoderm germ layer
- foregut:
a. GI tract - esophagus, stomach, liver, gallbladder, pancreas
b. respiratory tract - entire respiratory system (lungs, bronchi, bronchioles, and alveoli)
c. endocrine glands - thyroid glands, epithelial lining of the thymus
d. auditory system - epithelium of the eustachian tube & inner ear
e. urinary system - bladder and part of the urethra - midgut & hindgut: small and large intestines (except the last portion of the rectum)
timeline of embryogenesis & fetal development
*week 1: fertilization, cleavage, blastulation, and implantation
*week 2: formation of the bilaminar disc (2 layers in week 2)
*week 3: gastrulation (formation of trilaminar disc); organogenesis begins (3 layers in week 3)
*week 4: heart starts to beat, limbs start to form (4 limbs & 4 heart chambers in week 4)
*week 6: brain & lungs begin to form; limbs lengthen & digits start to form
*week 8: fetal movement begins
*week 10: genitalia starts to take on sex-specific characteristics
embryonic period of fetal development
*weeks 3-8 are known as the embryonic period
*when organogenesis occurs
*highly susceptible to errors from genetic defects & teratogens
*note - a woman may not even know she is pregnant during the embryonic period
common teratogen & associated anomaly: lithium
*lithium → Ebstein anomaly
common teratogen & associated anomaly: maternal diabetes
*maternal diabetes → caudal regression syndrome, macrosomia, increased risk for congenital heart defects
common teratogen & associated anomaly: aminoglycoside antibiotics
*aminoglycoside antibiotics → hearing loss
common teratogen & associated anomaly: tetracycline antibiotics
*tetracycline antibiotics → discolored teeth
common teratogen & associated anomaly: phenytoin
*phenytoin → cleft lip & palate
common teratogen & associated anomaly: thalidomide
*thalidomide → limb defects
common teratogen & associated anomaly: androgens
*androgens → virilization of female external genitalia
common teratogen & associated anomaly: folate antagonists
*folate antagonists (AEDs, chemotherapeutic agents, TMP) → neural tube defects
common teratogen & associated anomaly: ACE inhibitors
*ACE inhibitors → renal dysplasia
common teratogen & associated anomy: alcohol
*alcohol → fetal alcohol spectrum disorder (nervous system development & micrognathia)
*alcohol crosses to fetus through the placenta
*affects development of nervous system
*can also lead to poor growth
*no known safe amount during pregnancy
development of the placenta
*placenta arises from cells of the trophoblast
*the trophoblast invades the endometrium and begins to form fetal villi in order to use maternal circulation to obtain nutrients and eliminate waste
*the trophoblast gives rise to 2 cell populations:
1. syncitiotrophoblast
2. cytotrophoblast
cytotrophoblast
*outermost cells of the blastocyst
*cells can either divide to form more cytotrophoblastic cells or fuse to add to the syncitiotrophoblast
syncitiotrophoblast - functions
*releases enzymes that break down the surrounding endometrium, allowing for further invasion
*secretes hCG (human chorionic gonadotropin)
*hCG stimulates the corpus luteum to continue secreting progesterone
*progesterone maintains the uterine lining, preventing menstruation and loss of the embro
establishing the fetal blood supply
- day 9 - the syncitiotrophoblast releases enzymes that break down maternal uterine vessels; areas between the pools of syncitiotrophoblast (lacunae) become filled with maternal blood
- day 12 - extra-embryonic mesoderm cells proliferate between the hypoblast and the cytotrophoblast
*the layer surrounding the embyro = amnion
*the layer on the placenta = chorion
functions of the placenta
- gas exchange
- metabolism
- endocrine functions (estrogen, progesterone, human placental lactogen, relaxin)
placental gas exchange
*oxygen travels from the maternal circulation to the fetus through passive diffusion
*deoxyhemoglobin has an increased affinity for CO2
*CO2 transfers the maternal circulation, which decreases the pH and shifts the curve to the right
*fetal Hb has a higher affinity for O2 compared to adult Hb
twinning
*can occur if:
1. there are multiple ovulated oocytes
2. one oocyte separates
*the earlier the split, the more separate the twins
*split after hatching → monochorinoic
*split after implantation → monoamniotic
