Embryogenesis

Question 1

Embryonic age

Answer 1

Time from fertilization

1. Hard to know clinically
2. Gestational -2 weeks

Question 2

Gestational age

Answer 2

Time from 1st day of last menstrual period

1. About 2 weeks before ovulation
2. Embryonic + 2 weeks

Question 3

Carnegie stages

Answer 3

First 8 weeks = 23 stages based on embryo appearance on structures
1. Can correlate w/ embryonic age

Question 4

Week 1: implantation

Answer 4

1. Fertilization
2. Zygote formation
3. Morula formation
4. Blastocyst formation
5. Implantation

Question 5

Fertilization

Answer 5

Day 1: Secondary oocyte + spermatozoan -> zygote
1. Secondary oocyte
A. Meiosis I completed
B. Zona pellucida - thick gel, prevents polyspermia
C. Corona radiata - signals to attract sperm
2. Capacitated spermatozoa: chem rxn to enable fertilization
3. Phase 3: after membrane fusion
A. Spermatozoan w/ acrosomal enzymes -> thru corona radiata
B. Sperm bind and penetrate zona pellucida
C. Membrane fusion -> sperm to cytoplasm
D. Oocyte changes to prevent polyspermy

Question 6

Zygote formation

Answer 6

Day 1:

1. Oocyte advances to meiosis II
2. DNA replicated -> pronuclei fuse
3. Mitosis

Question 7

Morula formation

Answer 7

Day 2-5:

1. Cells divide w/o growing
2. Morula at 16 cells (Day 3)
3. Outer mass
4. Inner mass
5. At uterine cavity

Question 8

Blastocyst formation

Answer 8

Days 5-6:

1. Fluid in -> swelling
2. Outer mass -> trophoblast
3. Inner mass -> embryoblast
4. Blastocoele - fluid surrounded by trophoblasts
5. Zona pullucida degenerates

Question 9

Implantation

Answer 9

Days 6-7:

1. Trophoblasts near pole adhere uterine epithelium -> invade endometrium
2. Blastocyst burrows into endometrium

Question 10

Embryonic dev. Days 8

Answer 10

1. Trophoblasts invade and differentiate -> layers
   A. Cytotrophoblast - inner layer, mononucleated cells
   B. Syncytotrophoblast- outer layer, multinucleate mass, no cell boundaries
2. Embryoblasts differentiate -> 2 layer embryonic disc
   A. Epiblast- columnar cells forming floor of amniotic cavity
   B. Hypoblast - cuboidal cells on ventral side
   
   • induction of ventral surface of embryo
3. Amnioblast - squamous cells, form roof of amniotic cavity
   A. NOT part of disc

Question 11

Embryonic dev. Day 9: lacunar stage

Answer 11

1. Trophoblast derivatives:
   A. Cytotrophoblasts and syncytiotrophoblasts proliferate
   B. Lacunae formation - spaces filled w/ maternal blood and debris
   *nutrients and O2 diffuse thru conceptus
2. Embryoblast derivatives:
   A. Amnioblasts proliferate - allows inc. amniotic cavity
   B. Epiblasts = unchanged
   C. Hypoblasts migrate -> inner surface of cytotrophoblast => exocoelomic cavity (primary yolk sac)
   *replaces blastocoele
3. Uterus
   A. Conceptus embedded in endometrium
   B. Fibrin coagulum plug forms

Question 12

Embryonic Dev. Days 10-12

Answer 12

1. Trophoblast derivatives:
   A. Proliferation and invasion cont.
   B. Trophoblastic lacunae + uterine sinusoids -> primitive uteroplacental circulation
2. Embryoblast derivatives:
   A. Amnioblasts and epiblasts unchanged
   B. Exocoelomic cells proliferate and migrate -> surround amniotic and exocoelomic cavities => extraembryonic mesoderm
   *multiple cavities form and fuse -> chorionic cavity
   1. Extraembryonic somatic mesoderm - lines inner cytotrophoblast
   2. Extraembryonic splanchnic mesoderm- surrounds amniotic and exocoelomic cavities
   3. Connecting stalk- connects 2 extraembryonic mesoderm layers
   C. Hypobalasts opposite stalk differentiate => anterior visceral endoderm (AVE) - induces cranial dev.
3. Uterus:
   A. Decidua rxn -> provide nutrients for conceptus

Question 13

Embryonic Dev. Day 13

Answer 13

1. Trophoblast derivatives:
   A. Primary villi form as columns of proliferating cytotrophoblasts that penetrate syncytoiotrophoblasts
   B. Inc. uteroplacental circulation
2. Embryoblast derivatives:
   A. Amnioblast + extraembryonic splanchnic mesoderm -> amnion
   B. Epiblasts = little change
   C. Hypoblasts - secondary proliferation and migration -> secondary yolk sac
   D. Chorion cavity contains:
   
   1. Embryo
   2. Secondary yolk sac (umbilical vesicle)
   3. Amniotic sac (secured by connection stalk)
3. Uterus:
   A. Decidua rxn continues throughout endometrium

Question 14

Placenta previa

Answer 14

Dev. Placenta bridges internal OS opening of cervix

1. Risk of life-threatening bleeding

Question 15

Ectopic pregnancy

Answer 15

Implantation outside uterine body

Question 16

Placental dev. Conceptus portion

Answer 16

1. Chorionic plate - thickening of extraembryonic somatic mesoderm
2. Villi - extension of cytotrophoblasts and extraembryonic somatic mesoderm form between lacunae of syncytiotrophoblasts
   A. Stem villi: chorionic plate to decidual plate
   B. Free villi: branches into intervillous space
3. Outer cytotrophoblast shell adheres decidua
4. Extraembryonic vessels form and connect w/ dev. Intraembryonic circulatory system

Question 17

Placental dev. Maternal portion

Answer 17

1. Decidua surrounds conceptus
2. Decidual plate forms
3. Uterine vessels into cytotrophoblast and intervillous space
   A. Spiral arteries deliver O2 and nutrient blood -> intervillous spaces
   B. Uterine veins remove waste and CO2

Question 18

Placental Dev from chorion and decidua near pole

Answer 18

1. Chorion frondrosum- growth and expansion of villi

2. Decidual basalis w/ decidual plate and septa

Question 19

Dizygotic twins

Answer 19

2 secondary oocyte release and fertilized -> 2 zygotes

1. Conceptus Ed genetically distinct and sexes mixed or matched
2. Embryos dev. In own amnion and have own umbilical vesicles
3. Separate placentas and chorion (may end up fusing)

Question 20

Monozygotic twins

Answer 20

One secondary oocyte + one sperm -> one zygote
1. Two blastocysts formed during cleavage
A. Own amniotic cavity and umbilical vesicles
B. Separate placenta and chorion (can fuse)
C. Similar to dizygotic but identical DNA

2. One blastocyst -> 2 embryoblasts
   A. Own amniotic cavity and umbilical vesicles
   B. Shared placenta and chorionic membrane
3. One blastocyst -> 2 partially separate embryo
   A. Shared amniotic and umbilical vesicle
   B. Shared placenta and chorion membrane
   C. May be conjoined
   *conceptuses identical and same sex

Question 21

Gastrulation

Answer 21

1. Primitive node and streak form (caudal, dorsal) epiblast
2. Anterior visceral endoderm (AVE) stops node and streak from dev. On hypoblast
3. Epiblasts migrate thru streak and node -> mesenchymal -> trilaminar disk
   A. Endoderm - replaces hypoblast, form ventral layer
   B. Mesoderm- notochord, prechordal mesoderm (xs cells from endoderm)
   C. Ectoderm- epiblast cells that don’t migrate thru primitive streak
4. Early embryonic structures
   A. Oropharyngeal and cloacal membrane at craniotomy/caudal regions -> openings to gut tube
   B. Allantios- invagination of endoderm lining of secondary yolk sac near connecting stalk
   C. Primordial germ cells- from epiblast -> endoderm -> embryo -> progenator gametes
5. Laterality established by gradient of signaling molecules (variation due to disruption)
   A. Situs solitus: normal left/right symmetry
   B. Situs inversus: mirror image of normal
   C. Situs ambiguous (heterotaxy): some organ reversed or asymmetric

Question 22

Neurulation

Answer 22

Signals fro notochord and prechordal mesoderm stimulation -> nervous system 1st to dev.
1. Neural plate along midline folds in on itself - 2 regions
A. Neural groove in midline
B. Neural folds (sides)

2. Neural tube forms from fusion of lateral edges of neural plate
   A. Cranially caudally
   B. CNS cells derived
   
   1. All somatic efferent neurons
   2. All presynaptic visceral efferent neurons
3. Neural crest cells from crests of neural folds
   A. Detach and migrate throughout mesoderm
   B. Gives rise to:
   
   1. Sensory neurons of PNS
   2. Postsynaptic neurons of ANS
   3. Neuroendocrine cells in adrenal gland medulla
   4. Non-neural tissues: melanocytes, facial bones, dermis of face, pharyngeal arch cartilage
4. Spina bifida: incomplete neural tube in trunk region

Question 23

Surface ectoderm layer

Answer 23

1. Epidermis of skin and appendages
2. Epithelial lining
   A. Oral and nasal cavities
   B. Distal anus
   C. Distal vaginal canal

Question 24

Mesoderm differentiation

Answer 24

Signaling in gastrulation stimulates:
1. Paraxial mesoderm - lines notochord bilateral -> more differentiation
A. Cranial
1. Mastication skeletal muscles
2. Facial expression and pharyngeal muscles
3. Bones of cranial vault
B. Somatic
1. Somites (dermatome, myotome, scleratome)

2. Intermediate mesoderm- cords lateral to paraxial in trunk region
   A. Urinary system- kidneys and ureters
   B. Reproductive system - supporting stromatolites of gonads and epithelial lining of genital tracts
3. Lateral plate mesoderm - extends lateral edges of flat embryo, continuous w/ extraembryonic mesoderm
   A. Splanchnic layer + endoderm = splanchnopleure
   1. Smooth muscle and supporting cells of viscera
   2. Visceral serous epithelium
   3. Primordial heart
   4. Blood and lymph vessels
   5. Spleen
      B. Somatic (parietal) + ectoderm = somatopleure
   6. Lining inner body walls
   7. Fibrous pericardium
      C. Coelomic (intraembryonic) cavity - continuous w/ chorionic cavity

Question 25

Endoderm differentiation

Answer 25

1. Epithelial lining A. GI tract, gallbladder, glandular ducts, parenchyma of liver and pancreas B. Tracheobronchial tree and parenchyma of lungs C. Bladder and urethra (from allantois)

Question 26

Embryonic folding (Midweek 3)

Answer 26

1. Lateral folding - lateral edges of somatopleure and splanchnopleure fuse at midline -> “tube w/in tube” A. Primitive gut tube = fusion of left and right splanchopleure B. Body wall encloses coelomic cavity by fusion of left and right somatopleure C. Vitelli new duct = connection between primordial mid-gut and secondary yolk sac 2. Cephalocaudal folding- cranial and caudal end folding -> fetal position A. Primordial heart moves to cervical region B. Septum transversum partially divides coelomic cavity superioinferiorly C. Allantois - partially incorporated into embryo - connected to primordial hind-gut 3. Amniotic sac- amniotic membrane pulled by somatopleure -> surrounds embryo and forms umbilical cord 4. Coelomic cavity division A. After cephalocaudal folding 1. Primordial heart and septum transversum repositioned 2. Septum tranversum demarcates thoracic and abdominal cavities 3. Left and right pericardioperitoneal canals still connected to coelomic cavities 4. Tracheobronchial diverticulum dev. Off foregut B. Pleural and pericardial cavity separation 1. Pleuropericardial folds dev and carry phrenic n. 2. Lungs grow to fill in cavity 3. Pleuropericardial folds grow between lungs and heart *fuse at midline and to roots of lungs *phrenic nerve between pleural and pericardial sac *ventral lung expansion between heart and ventral body wall C. Separation of pleural and peritoneal cavities 1. Septum transversum forms ventral partition 2. Esophagus and mesentary -> median partition 3. Pleuroperitoneal folds from posterolateral wall and span canals 4. Myoblasts from cranial somites 3-5 invade periphery of membrane