Early Embryology I&II

Question 1

fertilization - what, where, to make what (super basic, not phases)

Answer 1

• development begins with this - the union of the sperm (male gamete) and the oocyte (female gamete) to forma zygote
• occurs in the ampulla of the uterine tube
• gametes unite to form a diploid number of 46 chromosomes

Question 2

each gamete contains….

Answer 2

• a haploid number of chromosomes (22 autosomes and one sex chromosome)

Question 3

First week of human development - high lights

Answer 3

1) fertilization in ampulla of uterine tube
2) sperm can only fertilize if undergoes capacitation and acrosome reaction
3) fertilization in 6 phases
4) cleavage
5) blastocyst formation

Question 4

capacitation

Answer 4

sperm, in order to reach and fertilize the egg, must undergo this period of conditioning that takes place in the female reproductive tract and lasts about 7 hours.

It involves an interaction between the sperm and the female reproductive tract. Without it, sperm cannot undergo the acrosome reaction

Question 5

acrosome reaction

Answer 5

• induced by zona proteins after the sperm binds to the zona pellucida covering of the ovum
• process releases enzymes that allow the sperm to penetrate the zona pellucida

Question 6

fertilization in 6 phases

Answer 6

1) passage of a sperm through the corona radiata of the oocyte
2) penetration of the zona pellucida
3) fusion of the plasma cell membranes of the sperm and the oocyte
4) completion of the second meiotic division of the oocyte. the nucleus of which becomes the female pronucleus and a second polar body
5) formation of the male pronucleus. within the cytoplasm of the oocyte the nucleus enlarges to form the male pronucleus, tail disappears. During growth the male and female pronuclei replicate their DNA
6) Breakdown of the pronuclear membranes and restoration of diploid number of chromosomes

Question 7

cleavage

Answer 7

• division of zygote approx. 30 hours after fertilization - zygote cells divide repeatedly, they are called blastomeres, becoming smaller with each division
• morula is the 16-32 cell stage

Question 8

blastocyst formation

Answer 8

• day 4 morula enters uterus and uterine fluid penetrates through the zona pellucida into the intercellular spaces of the inner cell mass
• fluid filled spaces coalesce to form the blastocystic cavity
• blastomeres are separated into 2 parts
• inner cell mass/ embryoblast gives rise to tissues, which forms the embryo proper
• outer cell mass/trophoblast contributes to formation of the placenta
• implantation occurs approx. 5 days after fertilization
• zona pellucida degenerates and disappears, allowing the blastocyst to attach to the endometrial epithelium

Question 9

Day 8 of embryological development

Answer 9

• trophoblast proliferates rapidly and differentiates into two layers
  
  • cytotrophoblast - inner layer of cells
  • syncytiotrophoblast - outer layer of cells (multinucleated)
• syncytiotrophoblastic cells over the embryonic pole of the blastocyst begin to penetrate endometrial epithelium
• embryoblast differentiates to form a bilaminar embryonic disc composed of two layers
  
  • epiblast - thicker layer
    
    • small cavity appears within epiblast, enlarges to form amniotic cavity
    • epiblast differentiate to form the amnion (thin membrane) which encloses the amniotic cavity (epiblast forms the floor)
  • hypoblast - thinner layer

Question 10

Day 9 and 10 of embryonic development

Answer 10

• blastocyst becomes fully embedded in endometrium at day 10
• includes the lacunar stage and primitve yolk sac formation

Question 11

Days 11-12 of embryologic development

Answer 11

• uteroplacental circulation is established
• lacunae appear in syncytiotrophoblast, which erode endothelial lining of maternal capillaries
  
  • capillaries become congested and dilated to form sinusoids
• maternal blood enters the lacunar system of spaces
  
  • represents begining of uteroplacental circulation
• extraembryonic mesoderm and its caitations are formed

Question 12

Day 13-14 of embryologic development

Answer 12

• Formation of primary villi
  
  • cytotrophoblast proliferate and penetrate syncytiotrophoblast, forming cellular columns that are surrounded by syncytia
  • together these form chorionic villi through which maternal and fetal exchange of materials (oxygen and nutrients) occurs
• syncytiotrophoblast produce hCG which by the end of the second week can be detected (pregnancy test)
  
  • hCG provides positive feedback to corpus luteum for progesterone secretion
    *

Question 13

gastrulation

Answer 13

• occurs in third week
• establishes 3 germ layers: ectoderm, mesoderm, endoderm
• begins with formation of primitive streak
  
  • narrow groove with elevations on either side
  • on caudal part of the epiblast
• cells of the epiblast migrate toward the primitive streak, detach, and slip underneath the epiblast in a process called invagination
• some invaginated cells displace the hypoblast and form the embryonic endoderm
• other cells migrate to a position between the now-formed endoderm and the epiblast to form the mesoderm
• remaining cells of teh epiblast become the ectoderm

Question 14

Formation of the notochord

Answer 14

• in third week of development
• begins at cranial end of the embryo
• notochord cells proliferate near the prechordial plate and the notochordal plate in-folds to form the notochord
• the prechordal plate will be important in the development of the forebrain while the notochord will be the basis for the formation of parts of the axial skeleton and spinal cord

Question 15

Third week of embryologic development in highlights

Answer 15

• gastrulation
• formation of the notochord
• highly sensitive period for tertogenesis and malformations

Question 16

3-8 weeks of embryologic developments - high lights

Answer 16

• period of organogenesis
• most gross structural birth defects occur

Question 17

derivatives of the ectoderm layer

Answer 17

• 3-8 weeks
• contacts the outside world
  
  • CNS, PNS, sensory epithelium of the eye, ear, nose, epidermis and epidermal derivatives
• cells of the neural plate make up the neural ectoderm
• neurulation (further development) - by the end of week 3, the neural plate forms lateral elevations called neural folds with a depression between called neural groove.
• neural folds fuse to form the neural tube
  
  • neuroectodermal cells at its lateral border separate to form the neural crest cells
• in trunk area, neural crest cells migrate along a dorsal pathway to form melanocytes in the skin and a ventral pathway to form sensory, sympathetic and enteric ganglia and Schwann cells
• in the cranial area, neural crest cells migrate to form parts of teh craniofacial skeleton, cranial nerve ganglia, and parasympathetic ganglia

Question 18

derivatives of the mesoderm layer

Answer 18

• median notochord with paraxial mesoderm lateral to it
• lateral to that is intermediate mesoderm, then forming the lateral margin of teh embryo is the lateral plate mesoderm
  
  • by day 21 the lateral plate mesoderm has split into a more medial layer
    
    • splanchnic mesoderm
  • and a lateral layer
    
    • parietal mesoderm
• much of mesoderm will form mesenchyme (embryonic connective tissue)
  
  • lots of ground substance
  • capable of ameboid locomotion
  • makes segmental blocks called somites
  • less distinct blocks in the head region, termed somitomeres
  • both become mesenchyme
  • somite number can be used as a proxy for developmental stages
• spleen is derived from mesoderm

Question 19

derivatives of the endoderm layer

Answer 19

major contribution of the endodermal germ layer is to the formation of the GI tract and its derivatives

Question 20

third month to birth (9 W to birth)

Answer 20

• characterized by maturation of organs and rapid growth of body length measured as CRL - crown rump length or CHL - crown heel length
• greatest length increase in months 3,4,5
• greatest weight increase in months 8 and 9
• length of pregnancy = 38 weeks (266 days) after fertilization
  
  • approx. 40 weeks (280 days) after onset of last menstrual period