MEDICAL EMBRYOLOGY 3

Neural induction

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.

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 givesrise to the brain, the more caudal portion gives rise

to the spinal cord.

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.

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.

• 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.

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.

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.

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.

• 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……..

•

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.

• 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.

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

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.

• 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.

• 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 .

• 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.

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 it’s 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

• 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)

• 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.

Amniotic fluid

• The amniotic cavity derives it’s 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 it’s amount.

• 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.

Amniotic fluid

• It contains:1-albuminUreaUric acid CreatinineLecithin and sphingomyelinProtiens, fats, fructose and bilirubinEpithelial cells ,enzymesLanugo and hair

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.

• Two arteries carry blood to chorionic villi from the embryo..

• One vein returns blood to the embryo.• Connective tissue called Whartson’s jelly

prevents compression of blood vessels and ensures continued nourishment of the embryo.

• umbilical cord is usually located centrally.

• (Maternity nursing. Lowdermilk berry, 2006)

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:

• 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.

• 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.

• 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.

• 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.

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

• 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.

• 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

• 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.