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Embryology (FETAL PERIOD (monthly changes (3rd month (limbs are relatively…

- - - - head is half of CRL
      - face becomes more human
      - eyes travel from lateral side to ventral side
      - ear come to lie close to their definite location
      - limbs are relatively short and underdeveloped
        
        lower limbs develop after upper limbs
      - Primary ossification centers
        
        long bones and skull
      - external genitalia seen in ultrasound in week 12
      - intestinal loops herniate in the umbilical cord
        
        end of 3rd month
        
        loops drawn into abdominal cavity
      - reflex movement detected
    - - head is quarter of CRL
      - lanugo hair
      - eyebrows and head hair
      - movement
    - - skin is reddish
        
        lack of underlying connective tissue (fat)
      - respiratory system and central nervous system underdeveloped
      - at the end of 6th month wieghs 1100 g
        
        90% chance of surviving
    - - subcutaneous fat
      - skin is covered with vernix caseosa
        
        sebaceous glands
    - - skull has largest d
      - CRL 36cm
      - weight 3000-3600
      - decreased testes
  - - - CRL measurement
      - biparietal diameter
      - abdominal circumfere
      - femur length
  - - - characteristics of trophoblast
        
        radial appearance
        
        secondary and tertiary villi
        
        extending from mesoderm of the chorionic plate to cytotrophoblast shell
        
        surface of the villi
        
        formed by the syncytium
        
        resting layer of the cytotrophoblastcells
        
        1 more item...
        
        villi cover the entire surface of the chorion
        
        as pregnancy advances
        
        villi on the embryonic pole continue to grow and expand
        
        chorion frondrosum
        
        villi on the abembryonic side
        
        degenerate
        
        by 3rd month this side is smooth
        
        1 more item...
        
        capillary system comes in contact with capillaries from chorionic plate and connecting stalk
        
        gives rise to extra embryonic vascular system
    - - stem villi give rise to free villi
        
        into surrounding lacunar or intervillous space
        
        first primitive
        
        developing to be more mature
        
        CT disappears
        
        maternal blood
        1.endothelium
        2.syncytium
        fetal blood
        
        syncytium becomes very thin, may break, blood lakes
        
        syncytial knots
        
        cytotrophoblast cells
        
        disappearing happens from small to larger
        
        maternal blood
        1.endothelium
        2.CT
        3.cytotrophoblast
        4.syncytium
        fetal blood
    - - beginning of 4th month
        
        two components
        
        fetal portion
        
        bordered by chorionic plate
        
        maternal portion
        
        bordered by decidual plate of desidua basalis
        
        formation of desidua septa
        
        cotyledons
      - functions
        
        exchange of nutrients and electrolytes
        
        transmission of maternal antibodies
        
        exchange of gases
        
        hormone production
  - - - decidua basalis
        
        decidual plate
        
        tightly connected to chorion frondrosum
        
        GIVES RISE TO PLACENTA #
      - decidua capsularis
      - decidua parietalis
        
        comes contact with chorion laeve
  - - - connective stalk
      - yolk sac
        
        in chorionic cavity
      - canal connecting the intraembryonic and extra embryonic cavities
- - - - steps
        
        (1) primitive streak formation
        
        narrow groove with slightly bulging regions on either side
        
        cells from the epiblast migrate here
        
        when arrive
        
        detach from the epiblast
        
        undergo shape change to flask-like form
        
        slip beneath the epiblast
        
        1 more item...
        
        migration and specification controlled by FGF8
        
        controls cell movement by down regulating E-cadherin
        
        1 more item...
        
        controls cell specification by regulating BRACHYURY (T) expression
        
        1 more item...
        
        streak cells themselves secrete FGF8 #
        
        (2) primitive node and primitive pit form
  - - - move forward cranially
        
        until they reach prechordal plate #
        
        cells become intercalated in the hypoblast
        
        hypoblast develops into endoderm
        
        cells from the notochord plate proliferate and detach and form a notochord
        
        signalling center for the axial skeleton
        
        elongation
        
        2 more items...
  - - - spreading laterally and cranially
        
        reach margin of the disc
        
        get contact with extra embryonic mesoderm
        
        cephalic region
        
        pass each side of the prechordal plates
        
        forms between
        
        2 more items...
        
        gives rise to forebrain #
    - - at the cranial end
      - of the endoderm layer
      - of the bilaminar disc
      - AVE
        
        expresses
        
        inhibitors for nodal
        
        TGF-ß family secretory factors
        
        lefty1
        
        cerberus
        
        absence of Cerberus and lefty 1 at the caudal region allows nodal expression to continue
        
        sirenomelia
        
        head formation genes
        
        OTX2
        
        LIM1
        
        HESX1
    - - FGF induces
        
        Nodal
        
        expression on the left side because of 5-HT
        
        activates MAD
      - midline FORMATION
        
        SHH
        
        LEFTY1
        
        ZIC3
        
        prevent
        
        NODAL to cross over right side
        
        in left lateral plate mesoderm
        
        initiates LEFTY2 to upregulate PITX2
        
        responsible of left-sidedness
      - right side
        
        SNAIL
  - - - BMP4
        
        together with FGF
        
        mesoderm ventralization
        
        antagonists
        
        CHORDIN
        
        activated by GOOSECOID
        
        over/under exp results in head region malformations
        
        noggin
        
        follistatin
        
        resulting in cranial mesoderm dorsalizing
        
        notochord, somites, somitomeres
    - - elongates
      - expansion of the cephalic region
        
        because of vigorous cell migration
      - invagination continues until the end of fourth week
        
        disappears after
      - caudal region develops a bit later than cephalic area
        
        cephalocaudal development
  - - - grow
        
        secondary villi
        
        capillaries and venules
        
        tertiary villi
        
        end of week 3
    - - types of villi
        
        villi extending from the chorionic plate to desidua basalis
        
        stem villi
        
        villi branching from the sides of the stem villi
        
        free villi
  - - - tightly adherent ectoderm and endoderm
        
        posterior wall of yolk sac forms a diverticulum
        
        extending into the connecting stalk
        
        allantois
        
        day 16
- - - - cytotrophoblast cells migrate to syn- #
        
        inner layer of mononucleated cells
        
        cells divide and migrate to syncytiotrophoblast
      - syncytiotrophoblast
        
        outer multinucleated cell mass without distinct boundaries
        
        produces hCG
    - - two layers
        
        hypoblast
        
        small cuboidal cells facing blastocyst cavity
        
        epiblast
        
        large, high columnar cells facing the amniotic cavity
        
        cavity within epiblast
        
        forming amniotic cavity
        
        epiblasts near cytotrophoblast #
        
        line the amniotic cavity
      - disc-like structure
  - - - vacuoles appear in the syncytium
    - - gives rise to yolk sac
  - - - eroding the maternal capillaries
        
        syncytial lacunae integrate with sinusoids
        
        establishes the uteroplacental circulation
    - - cells derived from yolk sac cells form
        
        extraembryonic mesoderm
        
        formation of extraembyonic cavity
  - - - as a result of increased blood flow into the lacunar spaces
      - menstrual cycle day 28
        
        confusion
    - - villi structures start to form
    - - migrate to other side
        
        forms secondary yolk sac
    - - connecting stalk
        
        umbilical cord
- - - - notochord and prechordal mesoderm
        
        induces thickening and formation of the neural plate
        
        neural plate cells=neuroectoderm
        
        neural crest cells on top of the groove
        
        undergo EMT #
        
        leaves ectoderm
        
        1 more item...
        
        molecular regulation of neural crest cells
        
        induction requires
        
        1 more item...
        
        initiation of neurulation
        
        neural plate forms neural tube
        
        lateral parts rise=neural fold
        
        approach each other and fuse from 5th somite
        
        open from head and tail
        
        2 more items...
        
        mid region descents =neural groove
        
        cephalic region
        
        otic placodes
        
        1 more item...
        
        lens placodes
        
        1 more item...
        
        regulative factors induce mesoderm to become notochord and paraxial mesoderm
        
        dorsalizes mesoderm
        
        only forebrain and midbrain development
        
        caudal region nerve tissue development
        
        WNT3a and FGF
      - regulation of neural plate induction
        
        upregulation of FGF :fire:
        
        inhibition of BMP :lock:
        
        up regulation of NOGGIN and CHORDIN
      - RA(=retinoic acid) organizes craniocaudal axis
      - gives rise to organs and structures maintaining contact with outside world
        
        epidermis
        
        hair
        
        nails
        
        subcutanous glands
        
        sensory epithelium
        
        ear
        
        nose
        
        eye
        
        mammary glands
        
        peripheral nervous system
        
        pituitary glands
        
        central nervous system
        
        enamel of the teeth
    - - Development
        
        mesodermal germ layer
        
        thickens and differentiates into
        
        paraxial mesoderm
        
        WEEK 3: begins to organize in segments
        
        somitomers
        (head region)
        
        2 more items...
        
        from the occipital region
        
        1 more item...
        
        intermediate mesoderm # #
        
        connects lateral and paraxial
        
        gives rise to urogenital structures
        
        lateral plate
        
        parietal mesoderm
        
        lines the intraembryonic cavity
        
        2 more items...
        
        sclerotome cells migrate #
        
        1 more item...
        
        visceral mesoderm
        
        surrounds the organs
        
        1 more item...
        
        together with embryonic endoterm
        
        1 more item...
        
        layers line a newly formed cavity
        
        intraembryonic cavity
        
        each side of the embryo
        
        blood and blood vessels
        
        vasculogenesis
        
        formation of the vessels from blood islands
        
        mesoderm cells induced to form hemangioblasts
        
        FGF2 :fire:
        
        angiogenesis
        
        sprouting from existing vessels
        
        maturation and modeling of the vasculature
        
        2 more items...
        
        patterning appears to involve guidance factors
        
        hematopoietic stem cells are derived from mesoderm surrounding aorta near mesonephric kidney
        
        Aorta-Gonad-Mesonephric region
        
        cells colonize the liver
        
        1 more item...
        
        VEGF :fire:
        
        neural crest
        
        differentiate into
        
        mesenchyme
        
        formation of
        
        facial bones
        
        1 more item...
        
        skull
        
        1 more item...
    - - germ layer covers ventral surface
        
        ventral body wall closes
        
        except in the umbilical region
        
        endoderm mainly for the formation of the gut tube
        
        formation divided in sections
        
        foregut
        
        foregut bounded by oropharyngeal membrane
        
        1 more item...
        
        midgut
        
        communicates with yolk sac
        
        1 more item...
        
        hindgut
        
        cloacal membrane
        
        1 more item...
      - role of yolk sac
        
        nutritive organ during the early stage
        
        contributes to the early blood cell formation
        
        houses germ cells that reside in its posterior wall
        
        ultimately will migrate to gonads
      - gives rise to
        
        reticular stroma of tonsils and thymus
        
        epithelial lining of urinary bladder+urethra
        
        parenchyma
        
        epitelial lining of tympanic cavity and auditory tube
        
        epithelial lining of respiratory tract
  - - - domain called ''homeodomain''
      - clustered in specific gene groups
        
        craniocaudal axis formation
        
        genes for cranial region in the 3' end and caudal region 5'
- - - - limbs
      - face
      - ears
      - nose
      - eyes
    - - forelimbs
        
        dorsal pericardial swelling
      - hindlimbs
        
        below umbilical cord attachment site
      - grooves
        
        formation of digits
  - - - grow ventrally
        
        closing the ventral body wall
        
        amnion is pulled ventrally
        
        embryo lies within the embryonic cavity
  - - - via vitelline duct
    - - via umbilical cord
- - - - visceral
        
        adjacent to ectoderm
        
        forming the gut tube
        
        continuous with the visceral layer of extra embryonic mesoderm covering the YOLK SAC
        
        together called SPLANCHOPLEURE
      - parietal
        
        adjacent to the surface ectoderm
        
        together called SOMATOPLEURE
        
        continuous with the extra embryonic parietal mesoderm layer over amnion
    - - 4th week
        
        sides of the embryo begin to grow ventrally
        
        forming two lateral body wall folds
        
        parietal layer of lateral plate mesoderm
        
        overlying ectoderm
        
        cells from paraxial mesoderm (somatic cells) migrating over somitic frontier
      - endoderm starts to fold also
      - FUSION
        
        lateral body wall folds meet in the midline and fuse to close the ventral body wall
        
        aided by the head and tail regions growth and embryo curves into fetal position
        
        closes completely except from the midgut region
        
        vitelline duct
        
        incorporated by umbilical cord
        
        degenerates 2-3 months of gestation
  - - - form parietal layer of serous membranes
        
        lining peritoneal, pleural and pericardial cavities
    - - form the visceral layer of serous membranes
        
        covering the abdominal organs, lungs and heart
    - - DOUBLE LAYER
        
        .
    - - results from thinning of mesoderm of the septum transversum
        
        block of mesoderm that forms connective tissue in the liver, central tendon of diaphragm.
        
        position
        
        between thoracic and abdominal cavities
        
        large opening (PERICARDIOPERITONEAL CANAL) each sides of foregut
        
        gives rise to lung buds
        
        rapid growth of lungs
        
        1 more item...
        
        PERICARDIOPERITONEAL CANALS
        
        caudal end projections
        
        1 more item...
      - DOUBLE LAYER
        
        provides a pathway for blood vessels, nerves and lymphatics to organs
- - - - ampullary region in fallopian tube
      - ovulation-induced chemoattractants
        
        produced by the cumulus cells surrounding the egg
    - - capacitation
        
        epithelial interactions between the sperm and the mucosal layer of epithelia in uterine tube
        
        7 hrs
      - acrosome reaction
        
        acrosome of the sperm induces a zona protein reaction
        
        results in enzyme release from cortical granules
        
        penetration of the zone pellucida with lytic enzymes
        
        XP3
    - - egg response to prevent polyspermy
        
        cortical and zonal reactions
        
        types
        
        zona pellucida alters its structure and composition
        
        preventing sperm binding
        
        release of cortical oocyte granules (contains lysosomal enzymes) #
        
        oocyte membrane becomes impermeable
        
        resumption of the second meiotic division
        
        22+X arrange as female pronucleus
        
        metabolic activation of the egg
        
        activating factor comes from the spermatozoon?
        
        activation regulates the initial cellular and molecular events
      - sperm moves near the female pronucleus
        
        tail detaches and degenerates
      - fusion
        
        pronuclei of both cells come into close contact and lose their nuclear envelope
        
        DNA replication
        
        chromosomes organize in mitotic spindle for mitotic division
        
        mitosis
        
        cytokinesis
        
        1 more item...
- - - - blastocele forms
        
        fluid cavity within the cell mass
    - - inner cells are now called embryoblast
      - outer cells are now called trophoblast
        
        form the epithelial wall of the blastocyst
    - - allows implantation, same as leukocyte capture in blood vessels
        
        selectin in trophoblast cells
        
        carb receptors in uterine wall
        
        further
        
        laminins
        
        promote attachment
        
        fibronectins
        
        promote migration