Early Embyrology

Question 1

fertilization - overview

Answer 1

*typically occurs in the ampulla of the fallopian tube
*haploid gametes meet in the fallopian tubes, creating a single-cell, diploid zygote

Question 2

transition from zygote → morula → blastocysts (cleavage)

Answer 2

*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

Question 3

what do the layers of the blastocyst become

Answer 3

*trophoblast → placenta
*inner cell mass → embryo/fetus

note - the blastocyst is surrounded by a shell called the zona pellucida

Question 4

implantation - overview

Answer 4

*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

Question 5

gastrulation - defined

Answer 5

*the process by which the bilaminar disc develops into a trilaminar disc

Question 6

bilaminar disc formation

Answer 6

*occurs during week 2
*inner cell mass differentiates into:
1. epiblast → further differentiates into fetal cells
2. hypoblast → forms the yolk sac

Question 7

anatomical planes of the bilaminar disc

Answer 7

1. 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]
2. 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)

Question 8

gastrulation - overview

Answer 8

*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

Question 9

gastrulation & cell migration - overview

Answer 9

*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

Question 10

gastrulation process - detailed

Answer 10

*involves a combination of cell migration and apoptosis to provide a space in which cells can migrate

1. 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
2. cells on the ventral edge replace hypoblast cells and become the endoderm
3. the dorsal-most cells (epiblast) will eventually become the ectoderm
4. the cells in the middle will become the mesoderm

Question 11

germ layers and their derivatives

Answer 11

*3 germ layers:
1. ectoderm → divided into surface ectoderm and neural ectoderm → CNS and PNS
2. mesoderm → gives rise to muscles, bones, connective tissue, blood/blood vessels, gonads, kidneys, etc
3. endoderm

Question 12

neurulation - overview

Answer 12

*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

1. the neural tube becomes the CNS (brain and spinal cord)
2. the neuroectoderm adjacent to the neural tube gives rise to the neural crest cells (which give rise to the peripheral nervous system)

Question 13

neural tube defects

Answer 13

*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)

Question 14

genes affecting the ectoderm

Answer 14

1. 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
2. Wnt-7 - organizes the limbs in a dorso-ventral plane
3. homeobox (Hox) - organizes the limbs in a cranio-caudal plane

Question 15

derivatives of the mesoderm germ layer

Answer 15

*in general, the mesoderm fives 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

Question 16

VACTERL syndrome/association: a disorder of the mesoderm

Answer 16

V - vertebral defects
A - anal atresia (imperforate anus)
C - cardiac defects
TE - tracheoesophageal fistula
R - renal defects
L - limb anomalies

Question 17

derivatives of the endoderm germ layer

Answer 17

1. 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
2. midgut & hindgut: small and large intestines (except the last portion of the rectum)

Question 18

timeline of embryogenesis & fetal development

Answer 18

*week 1: fertilization, cleavage, blastulation, and implantation
*week 2: formation of the bilaminar disc
*week 3: gastrulation (formation of trilaminar disc); organogenesis begins
*week 4: heart starts to beat, limbs start to form
*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

Question 19

embryonic period of fetal development

Answer 19

*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

Question 20

common teratogen & associated anomaly: lithium

Answer 20

*lithium → Ebstein anomaly

Question 21

common teratogen & associated anomaly: maternal diabetes

Answer 21

*maternal diabetes → caudal regression syndrome, macrosomia, increased risk for congenital heart defects

Question 22

common teratogen & associated anomaly: aminoglycoside antibiotics

Answer 22

*aminoglycoside antibiotics → hearing loss

Question 23

common teratogen & associated anomaly: tetracycline antibiotics

Answer 23

*tetracycline antibiotics → discolored teeth

Question 24

common teratogen & associated anomaly: phenytoin

Answer 24

*phenytoin → cleft lip & palate

Question 25

common teratogen & associated anomaly: thalidomide

Answer 25

*thalidomide → limb defects

Question 26

common teratogen & associated anomaly: androgens

Answer 26

*androgens → virilization of female external genitalia

Question 27

common teratogen & associated anomaly: folate antagonists

Answer 27

*folate antagonists (AEDs, chemotherapeutic agents, TMP) → neural tube defects

Question 28

common teratogen & associated anomaly: ACE inhibitors

Answer 28

*ACE inhibitors → renal dysplasia

Question 29

common teratogen & associated anomy: alcohol

Answer 29

*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

Question 30

development of the placenta

Answer 30

*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

Question 31

cytotrophoblast

Answer 31

*outermost cells of the blastocyst
*cells can either divide to form more cytotrophoblastic cells or fuse to add to the syncitiotrophoblast

Question 32

syncitiotrophoblast - functions

Answer 32

*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

Question 33

establishing the fetal blood supply

Answer 33

1. day 9 - the syncitiotrophoblast releases enzymes that break down maternal uterine vessels; areas between the pools of syncitiotrophoblast (lacunae) become filled with maternal blood
2. 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

Question 34

functions of the placenta

Answer 34

1. gas exchange
2. metabolism
3. endocrine functions (estrogen, progesterone, human placental lactogen, relaxin)

Question 35

placental gas exchance

Answer 35

*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

Question 36

twinning

Answer 36

*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