Embryonic period

Embryonic period

1. inner cell mass

   • 2 layer embryonic disk called:
     • epiblast
       • becomes embryo
       • embryonic epiblast (tissue)
         • gastrulation
           • - two layers to three layers - pluripotency to multipotency - 
           • primitive streak
             • - nodes form that emit growth factors and signals that direct cells to differentiate and proliferate cells migrate toward and through primitive streak 
             • embryonic endoderm
               • displaced hypoblast - this layer formed first 
               • digestive system, liver, pancreas, lungs
             • embryonic mesoderm
               • second layer migrates and fills in middle layer (makes mesoderm)  - formed second 
               • circulatory system, lungs, skeletal, muscular
                 • formation of heart
               • somitogenesis
                 • segment of mesoderm forms trunk - axial skeleton and muscles  - somites = for the vertebrae and axial skeleton/ skeletal muscles 
                 • somites
                   • - mesoderm splits into temperary structures called somites  -  give rise to  1. vertebral column 2. muscles of ribs 3. limbs  4. dermis of skin
                   • vertebral column
                   • muscles of ribs
                   • limbs
                   • dermis of skin
               • notochord
           • embryonic ectoderm
             • cells that did not migrate through the primitive streak - formed last 
             • epidermis, neuron or brain, pigment cell
               • neurulation
                 • formation of neural tube from top layer or neuroectodermal cells 
                 • notochord
                   • - signals overlying ectroderm in dorsal midline to thicken and form the neural plate 
                   • neural plate
                     • - edges of neural plate (neural folds) bend towards each other  - due to sytoskeletal changes in cells 
                     • - folds meet and the neural tube disconnects 
                     • neural crest
                       • - disconection of neural tube frees special cells (neural crest) - 4th germ layer 
                       • - migrate all over to form important structures (population of migratory cells left over from neuralation
                       • - contribute to  1. peripheral nervous system (spinal ganglia)  2. septum partitions outflow tracts of heart into two major vessles - vessles separate systemic and pulmonary circulation
                       • - heart defects (most common birth defects) has to do with migration problems 
                     • neural tube
                       • - brain and spinal cord from from the neural tube  - just involves ectoderm 
                       • neural tube defects
                         • - deficiency in maternal folate in 1st few weeks 
                         • spina bifida
                           •  - compatible with life, protrusion in spin  - can be detected by ultrasound  - spinal tissue protruding  - disruption of blood supply to limbs resulting in paralysis  - fluids can build up in brain 
                         • anencephaly
                           • - failure of posterior neuropore closure  - not compatible with life (death in utero)  - no functional brain
                 • neuronal folding
       • amnionic ectoderm
     • hypoblast
       • becomes yolk sac
       • blastocyst cavity (splits off)
         • extraembryonic ectoderm (tissue)
           • yolk sac
             • - supplied nutrients absorbed by trophoblasts - primitive circulation 2 and 3 week - after week 4, reduces in size (placenta takes over)  - source of blood cells/primordial germ cells 
             • allantois
               • - primitive excretory duct (will become part of bladder)
             • extraembryonic mesoderm (tissue)
       • embryonic folding
         • - from flat disc to 3D cylinder by end (tube in a tube)  - expansion, folding and fusion of edges of embryo - inner tube is future gastrointestinal tract  - lateral to sagital folding
         • - amnion encases whole process - yolk sac is cut off and forms the primitive gut 
         • - involves all primary germ layer  - helps the heart fold as well 
         • formation of heart
           • - cranial mesodermal cells condense into paired tubes that fuse to form single heart tube (due to embryonic folding  - rudimentary tube loops upon itself
           • - first fully functional system to develop (starts beating at week 4) - begins to beat and pump blood - RBC first formed in liver and then comes from bone marrow 
           • - partioning form 4 rudimentary chambers (right and left atria, right and left ventricles)  - ventricle is main pump (right to lungs, left to body) - septa separates left and right side (think of it right now as two separate pumps)
         • at 7 weeks
           • - GI systems develop too rapidly fro abdomen, intestines temporarily hook into umbilical cord
           • - webbing removed from digits on paddles shaped hand and feet 
           • - facial structures includes nostrils, outer ears and lenses of eyes
           • at 8 weeks
             • - major organs and external body structures have begun to form  - babies heart beats with a regular rhythm 
             • - ambitious external genetalia (can't determine if boy or girl) - umbilical cord visible
             • - eyes moved forward on face and eyelids have formed  - looks more human  - 3cm long and 8.5 g
   • amnionic cavity
     • - amniotic fluid (filtrate of maternal plasma)  - fetus can swallow to practice digestion  - cushion, shock absorber  - temp regulator 
     • fluid filled

2. trophoblast

   • cytotrophoblast
     • - adheres to endometrium, burrows in to uterine wall - attracts blood vessels (secretions) - remodels blood vessels (enzymes degrade walls of blood vessels, bleeding where nutritian exchange happens)
   • syncytiotrophoblast
     • multinucleate body, digests uterine tissue
   • chorion
     • - embryonic contribution to placenta - membrane that surrounds everything 
     • placenta
       • - two arteries that carry deoxygenated blood and waste from fetus  - single vein that carries nutrients and oxygen from mom and fetus
       • - simple diffusion: O2, CO2 and lipid soluble (rapid diffusion)  - facilitated diffusion of carbs and vitamins  - active transport amino acids, iron
       • made from tissues from chorion + mesoderm + mom
         • - chorionic membrane with finger like villi- burrow into endometrium and remodel maternal blood vessels (mediates a lot of gas exchange - decidual basalis portion of endometrium
         • - mesenchymal cells derived from mesoderm fill villa and differentiate into blood vessels
         • surrounds entire embryo
           • umbilical vein
             • brings in O2 (red not blue)
           • umbilical artery
             • - brings waste back to mom
       • functions
         • nutrition and digestion
           • - mediates diffusion of maternal glucose, vitamins, minerals  - stores nutrients during early stages for later stages in pregnancy (when baby needs more nutrients)
           • - excretes and filters fetal waste (nitrogenous waste) into maternal blood
         • respiration
           • - mediates maternal to fetal O2 transport - mediates fetal to maternal CO2 transport (waste)
         • endocrine function
           • - secretes hCG, estrogens, progestrone to maintain pregnancy  - mediates maternal hormones to fetal blood (vice versa)
       • health issues with placenta
         • placenta privia
           • - an embryo that implants too close to cervix opening  - placenta partially or completely covered cervix  - 0.5% of time 
           • - no prenatal care = haemorraging and death  - can detect through ultra sound  - a little bleeding before birth  - C section or bed rest
     • chorionic epithelial cells
       • - no direct intermingling of maternal and fetal blood - maternal blood encased by chorionic epithelial cells - fetal blood encased by fetal connective tissue and endothelial cells  - keep out maternal hormones too 

3. twins

   • - some twins can take more nutrients  - one might be larger  - often born prematurely 
   • monozygoic (identical)
     • - egg fertilized by sperm  - splitting of blastocyte  - genetically identical 
     • - 33% identical, separate chorions and amnions (back to back configuration) - 66% identical, common chorion and separate amnions  - 1% identical conjoined, common chorion and amnion (ICM never splits completely)  
   • dizygotic (fraternal)
     • non genetically identical  - two eggs fertilized by two sperm   - separate placenta  - separate chorions

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