MEDICAL EMBRYOLOGY 3

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MEDICAL EMBRYOLOGY 3

• Neural induction

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Neural induction

• The ectoderm germ layer gives rise to organs and
  structures that maintain contact with outside
  world.
• 1- CNS
• 2- peripheral NS
• 3- epidermis including hair and nails
• 4-subcutaneous glands, mammary glands, pituitary
  gland, and teeth enamel.

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Derivatives of the ectoderm germ layer.

• At the beginning of third week, ectoderm layer
  has the shape of disc.
• Appearance of notochord and prechordl mesoderm
  induces the overlying ectoderm to thicken and
  forms new palate.
• Cells of plate make up neuroectoderm and their
  induction represents the initial process of
  neurulation.
• The anterior portion of the neural tube gives
• rise to the brain, the more caudal portion gives
  rise to the spinal cord.

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Neural induction

• By the end of the embryonic period, the main
  organ systems have been established making the
  major features of external body by the end of the
  second month.
• At the beginning of the third week of development
  the ectoderm germ layer has the shape of disc
  that thickens and form the neural plate.
• Cells of the plate make up the neuroectoderm, and
  their induction represents the initial process of
  neurulation.

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neurulation

• Neurulation the process by which neural plate
  develops into a neural tube.
• Once induction has occurred, the elongated
  slipper shaped neural plate gradually expands
  toward the primitive streak.
• By the end of the third week, lateral edges of
  the neural plate become more elevated to form
  neural folds, and the depressed mid region forms
  the neural groove.

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• Gradually the neural folds approach each other in
  the midline where they fuse.
• Fusion begins in the cervical region and proceeds
  cranially and caudally.
• As a result, the neural tube is formed.
• At approximately, 25 day, neurulation is complete
  and the CNS is represented by a closed tubular
  structure with a narrow caudal portion (spinal
  cord) and a cephalic portion characterized by a
  number of dilations (brain vesicles).

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• Crest cells from the trunk region leave the
  neural folds after closure of the neural tube and
  migrating along the along one of two pathways.
• 1) dorsal path way through the dermis, where they
  will enter ectoderm to form melanocytes in skin
  and hair follicles.
• 2) ventral path way through the anterior half to
  become sensory ganglia, sympathetic neurons and
  cells of adrenal medulla.

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Origins of the Neural Crest

• The neural crest originates from cells located
  along the lateral margins of the neural plate.
• Neural crest cells are specified as the result of
  an inductive action by the nonneural ectoderm
  (possibly mediated by bone morphogenetic protein
  BMP and BMP) on the lateral cells of the neural
  plate.

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Neural crest cells derivatives

• Neural crest cells also form and migrate from
  cranial neural folds, leaving the neural tube
  before closure in this region.
• These cells contribute to the craniofacial
  skeleton, as well as for cranial ganglia. glial
  cells, meninges, cells of thyroid, melanocytes
  and other cell.
• Induction of neural crest cells needs interaction
  between adjacent neural and overlying ectoderm.

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Regulation of neural tube induction

• A gradient of bone morphogenitic proteins ( BMPs)
  secreted by non neural ectoderm, with the help of
  (FGFs) fibroplast growth factor work together
  initiate the induction process with unknown
  mechanism.
• So, the fate of entire ectoderm is dependent up
  on BMPs concentration.
• High levels result in epidermis formation.

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• Lower levels at the border of neural plate and
  nonneural ectoderm induce neural crest.
• Crest cells give rise to hetrogenous array of
  tissue like connective tissue of and bones of
  skull and face, cranial nerve ganglia, ,septum in
  heart, dermis in face and neck, melanocytes,
  etc..

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Ectoderm becomes the primary Neural Tissue

• Neural tube closure starts in neck region
  proceeds anteriorly and posteriorly
• Anterior and posterior neuropores temporary
  connections with amniotic cavity.
• When the neural tube is closed, two bilateral
  endoderml thickenings occur, the otic placodes,
  and the lens placodes become visible in the
  cephalic region of the embryo.

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• Otic placodes develop to form otic vesicles which
  well develop in to structures needed for hearing
  and maintenace of equilibrium.
• Lens placodes also develop and form eyes lenses
  in the fifth week.

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• Establishment of the basic embryonic body
  plan-------early vessels, placenta and
  extraembryonic membranes

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Changes in trophoplast

• By the beginning of the second month, trophoplast
  develops greater number of secondary and tertiary
  villi.
• Stem villi extend from mesoderm.
• the surface of viili is formed by syncytium,
  resting on a layer of cytotrophoplast cells that
  in turn covers a core of vascular mesoderm.

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• The capillary system developing in the core of
  the villous stems soon comes in contact with
  capillaries of chorionic plate and connecting
  stalk, thus gives rise to extraembryonic vascular
  system.
• Maternal blood is delivered to the placenta by
  spiral arteries in the uterus.

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• Erosion of these maternal vessels to release
  blood into intervillous space is accomplished by
  endovascular invasion by cytotrophoplastic
  tissue.
• These cells are released from ends of villi
  invade the terminal ends of spiral arteries.
• This creates hybrid vessels containing both fetal
  and maternal blood cells .

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• During the following months numerous small
  extensions grow out from existing stem villi and
  extends as free villi into the surrounding
  lanucar or intervillous space.
• The cynsytium and endothelial wall of the blood
  vessels are then the only layer that separate
  maternal and fetal circulations.

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membranes

• At the time of implantation, two fetal membranes
  membranes begin to form
• 1- the chorion develop from the trophoplast and
  contains the chorionic villi on its surface.
• The villi burrow into the decidua basalis and
  increase in size and complexity, and develop into
  placenta.
• Villi degenerate a by the third month, chorion
  becomes smooth

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• The chorion becomes the covering of the fetal
  side of the placenta.
• It contains the major umbilical blood vessels
  that branch over the placenta.
• ( lowder milk Berry, 2006)

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• 2- amnion develops from the interior cells of
  the blastocyst.
• The cavity that develops between this inner cell
  mass and outer layer of cells (trophoplast) is
  the amniotic cavity.
• As it grows, the amnion forms .
• The developing embryo draws the amnion around it
  self to form a fluid filled sac.

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• The amnion becomes the covering of the umbilical
  cord and covers the chorion of the fetal surface
  of the fetal surface of the placenta.
• As the embryo grows larger, the amnion grows to
  accommodate and surrounding amniotic fluid.
• The amnion comes in contact with the chorion
  surrounding the fetus.

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Amniotic fluid

• The amniotic cavity derives its fluid by
  diffusion from surrounding maternal vessels.
• It increases weekly.
• 800-1200 ml is present at term.
• T6he fetus swallows fluid, and fluid flows into
  and out of fetal lungs.
• Fetal urine increases its amount.

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• It keeps embryo from tangling with the membranes,
  facilitating symmetric growth of the fetus.
• The volume of amniotic fluid is important in
  assessing fetal well being.
• Less than 300ml is associated with fetal renal
  anomalies.
• More than 2l is associated with gastrointestinal
  and other malformations.

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Amniotic fluid

• It contains
• 1-albumin
• Urea
• Uric acid
• Creatinine
• Lecithin and sphingomyelin
• Protiens, fats, fructose and bilirubin
• Epithelial cells ,enzymes
• Lanugo and hair

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Umbilical cord

• During the third week, blood vessels develop to
  supply the embryo with maternal nutrients and o2.
• During the fifth week, embryo has curved inward
  on it self from both ends.
• The connecting stalk becomes compressed from both
  sides by the amnion and forms the narrow
  umbilical cord.

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• Two arteries carry blood to chorionic villi from
  the embryo..
• One vein returns blood to the embryo.
• Connective tissue called Whartsons jelly
  prevents compression of blood vessels and
  ensures continued nourishment of the embryo.
• umbilical cord is usually located centrally.
• (Maternity nursing. Lowdermilk berry, 2006)

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placenta

• Structure
• Begins to form at the time of implantation during
  the third week after conception.
• The trophoplast cells of the chorionic villi
  continue to invade decidua basalis.
• When uterine capillaries are tapped, endometrial
  spiral arteries fill with maternal blood.
• The chorionic villi grow into spaces with two
  layers of cells

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• 1- the outer syncytium layer,
• 2- inner cytotrophoplast.
• A third layer develops dividing the projecting
  decidua in to separate areas called cotyledons.
• Each cotyledon is a functional unit, the whole
  structure is the placenta.

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• The maternal placental embryonic circulation is
  in place by day 17, when embryonic heart start
  beating.
• By the end of the third week, embryonic blood is
  circulating between the embryo and the chorionic
  villi.

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• It functions as means of metabolic exchange.
• Permeability increase as the cytotrophoplast
  thins and disappears.
• Only single layer of syncitium is left between
  fetal and maternal capillaries.

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• The syncytium is the functional layer of the
  placenta.
• the structure of the placenta is complete by the
  twelfth week.
• It continues to grow wider until 20 weeks when it
  covers half of the uterus.
• Then it continues thicker.

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Functions of placenta

• 1- endocrine function syncytium produces four
  important hormones important to maintain the
  pregnancy and support the fetus.
• HCG-preserves the function of corpous luteum
• HCS- ( human chorionicsomatomammotropin)similar
  to growth hormone.
• Progesterone-maintains endometrium, decreases
  contractility of the uterus, stimulate
  development of breast alveoli, and maternal
  metabolism

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• By 7th week, steroid h.( estrogens) are produced.
• Estriol is the major estrogen produced by the
  placenta.
• Ovaries produce mainly estradiol h.
• Estrogen stimulate uterine growth and
  uteroplacental blood flow.
• Stimulate myometrial contractility.

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• 2- metabolic functions are respiration
• ( lungs for the fetus)
• -nutrition ( cho, protiens, ca, and ironare
  stored in pla centa for ready access to meet
  fetal needs.,
• -excretion ( metabolic waste products of fetus
  cross placenta and is excreted by mat. Kidneys
• and storage

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• If there is interference with placentl
  circulation, it cannot supply embryo with
  nutrients.
• Decreased uterine circulation, may lead to
  intrauterine growth restriction. And infants whop
  are born small for gestational age.