Embryology

Embryology

pre-emryonic phase
1. week 1
  1. zygote forms + divides to form blastocyst
    1. morula forms
      1. solid ball of cells
    2. blastocystic cavity develops
    3. blastocyst
      1. cells accumulate at one end to form inner cell mass
      2. outer lining of cells = trophoblast
      3. nutrition provided by endometrium
        blood vessels lining uterus
    4. fertilisation in uterine tube
      1. ciliated epithelium
        cilia beat ova/zygote towards uterus
        secretes mucous which catches ovum
  2. zygote moves through unterine tube to uterine cavity
    1. blastula forms in uterine cavity
    2. each successive division is quicker
2. week 2
  1. implantation occurs in uterine endometrial layer
    1. blastocyst burrows into endometrium
      1. trophoblast divides to form two layers
        chorionic villi with enzymes
        chorion
        helps with burrowing
        takes part in formation of placenta
        secretes human chorionic gonadotrophine
        tells ovaries to keep secreting hormones so blastocyst survives
        used to detect pregnancy
    2. formation of embryo
      1. maternal blood vessels are not in contact with fetal blood vessels
        diffusion occurs
      2. inner mass of cells in blastocyst form two balimar discs
        epiblast on top
        amniotic cavity is above
        hypoblast on bottom
        yolk sac below
        provides nutrition
        waste products go into alliotic cavity
      3. development of placenta
        matures by 18-20 weeks
        functions
        foetal nutrition
        transport of waste + gases
        immunity
3. week 3
  1. gastrulation
    1. formation of germ layers
      1. epiblast proliferates
        cells displace hypoblast
        ectoderm
        mesoderm
        endoderm
  2. primitive streak between epiblast + hypoblast
    1. axis of embryo forms
      1. cells become specialised
  3. development of somites
    1. lateral plate mesoderm splits to form somatic + splanchnic mesoderm
      1. paraxial mesoderm
        segments form somites
      2. intermediate plate mesoderm
        kidneys + reproductive system
      3. lateral plate mesoderm
        body cavities + coverings
  4. early development of cardiovascular system
  5. notochord formation
    1. cells sink down to form solid tube
      1. notochord
        releases chemicals that act on mesoderm
        formation of somites
    2. neurulation
      1. neural plate sinks down
        induces ectodermal cells in midline to form neural tube
embryonic period
1. neural tube
  1. brain + spinal cord development
2. heart
  1. starts to beat on day 24
3. urogenital system formation from intermediate mesoderm
4. 43 pairs of somites
  1. each one divides into 3
    1. dermatome
      1. dermis of skin
    2. myotome
      1. muscles
    3. sclerotome
      1. bones inc vertebrae
    4. when migrate take never with them
5. limb buds form
6. neck development
  1. pharyngeal arches
7. teratology
  1. abnormal development
  2. teratogens
    1. environmental factors which cause abnormal development
      1. risk posed depends on exposure during critical periods of development
      2. genetic constitution of embryo
      3. dosage of drug / chemical / factor
8. lateral folding of embryo into tube
  1. endoderm
    1. forms gut, branches at head to form lungs + trachea
  2. ectoderm
    1. forms outer layer of embryo
development of respiratory system
1. week 4 of gestation
  1. respiratory diverticulum
    1. lung bud
    2. gives rise to epithelium + glands of bronchi
2. upper respiratory tract
  1. pharyngeal arches derived from neural crest cells
    1. 1st +2nd arches = facial structures
    2. 4 (+2 rudimentary)
    3. 3rd arch = hyoid bone
    4. 4th +6th arches
      1. muscle of pharynx, larynx, upper oesophagus + laryngeal cartilages
    5. pharyngeal pouches lie in between arches
      1. give rise to head + neck structures but not to respiratory structures
      2. 4th lies between 4th + 6th arches
      3. respiratory primordium develops in midline caudal to 4th pouch
      4. larynotracheal groove
        gives rise to laryngotracheal diverticulum
        becomes invested with mesoderm + enlarges to form lung bud
        ventral outgrowth from foregut
        at 28 days gestation
3. lung development
  1. at 4 weeks of gestation
  2. from ventral wall of foregut
4. development of trachea + bronchi
  1. lining of lung bud (endoderm)
    1. gives rise to glands + epithelium of respiratory tract
  2. cartilage + muscle of lower respiratory tract from mesoderm investing laryngotracheal tube
    1. bronchial buds arise from growing tips of lung bud
      1. grow into primordia of pleural cavities
        pericardioperitoneal on each side
        splanchnic mesoderm gives rise to visceral pleura
        somatic mesoderm gives rise to parietal pleura
      2. give rise to primary bronchi
        bud to give secondary bronchi
        week 7 - surrounding mesenchyme (mesoderm) forms partitions around buds of tertiary bronchi
        gives rise to bronchopulmonary segments
        10 in each lung
  3. 17 orders of branching by week 24
    1. form respiratory bronchioles
      1. additional 7 orders of branching develop after birth
  4. bronchial walls
    1. epithelium + glands form endoderm
    2. walls develop from investing mesoderm
      1. muscle, cartilage, capillaries, CT support
5. diaphram
  1. originates from
    1. septum transversum
      1. located opposite to C3-C5 somites
        brings with it C3-C5 spinal nerves
    2. muscular body wall
    3. mesentery of oesophagus
    4. leuraperitoneal folds
6. abnormal development
  1. failure of separation of trachea + oesophagus
    1. connected
    2. blind ended
  2. surfactant
    1. Respiratory Distress Syndrome
  3. ectopic lung lobes
  4. abnormal bronchial divisions
    1. no functional significance

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	Created by fiona.medic about 10 years ago