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Chapter 21 General Embryology


---a science which study the processes and regulations of development of human fetus.(from 1 cell(zygote) to (5-7)X1012 cells )---total 38 weeks: preembryonic period: before 2 weeks. embryonic period: 3-8 weeks fetal period: after 9 weeks *perinatal stage: 26 weeks to birth*Teratology: abnormal development of fetus .

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Chapter 21 General Embryology

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Chapter 21 general embryology

Chapter 21 General Embryology


---a science which study the processes and regulations of development of human fetus.(from 1 cell(zygote) to (5-7)X1012 cells )

---total 38 weeks:

  • preembryonic period: before 2 weeks


  • embryonic period: 3-8 weeks

  • fetal period: after 9 weeks

    *perinatal stage: 26 weeks to birth

    *Teratology: abnormal development of fetus


1 fertilization

1.Fertilization

---definition: the process by which the male and female gametes(sperm and ovum) unite to give rise to zygote

---place: in the ampullary region of the fallopian tube


---conditions:

1) maturation of oocyte

2) maturation of spermatozoon:

/ductus epididymus: 2-3 weeks, forward motility protein


3)capacitation of spermatozoa:

  • decapacitation factor: exist in epididymus, to inhibit the release of acrosome enzymes

  • capacitation factor: secreted by epi. cells of oviduct, capacity of releasing acrosome enzymes


4)quality and quantity of spermatozoa:

  • 2-6 ml, 100,000,000/ml,

  • <1.5 ml; or <10,000,000; abnormal sperm >30%; or capacity for mobile< 70%

    5) meeting of sperm and ovum

  • sperm: 20-24h

  • ovum: 15-18h(1-3ds)


---processes of fertilization:

1)acrosome reaction:

/hyaluronic acidase: dissolve corona radiate

* interact with ZP3(receptor glycoprotein, exist in zona pellucide) molecules, release

/acrosin: dissolve zona pellucide

/enter perivitelline space


2) spermatozoa fuse with the membrane of ovum: nucleus enter cytoplasm of ovum

3) monospermy:

  • cortical reaction: cortical granules(located in cytoplasm of ovum) are released into perivitelline space, and cause

  • zona reaction: change of ZP3 molecules in zona pellucide

    4) formation of zygote(fertilized ovum)

  • secondary meiosis complete: form second polar body

  • male pronucleus fuse with female pronucleus, to restore the 2n chromasome


---significance of fertilization:

1) a new life begin(initiation of cleavage)

2)restoration of the diploid number of chromosomes: cross-over

3) determination of the sex of the new individual:

  • ova(22+X) + X-bearing spermatozoon = girl

  • ova(22+X) + Y-bearing spermatozoon = boy

    *klinefelter’s syndrome: 47XXY, 48XXXY


2 blastocyst formation and implantation

2. Blastocyst formation and implantation


1) cleavage and blastocyst formation

①cleavage

---blastomere: cells formed by cleavage

/30h: 2

/40h: 4

/72h: 12-16- morula


②formation of blastocyst: by 5th day, blastomere reach to 107, small space appear between them

---blastocoele: filled with liquid

---trophoblast

---inner cell mass

---polar trophoblast


2) Implantation

---definition: the process by which the blastocyst is embedded in endometrium

---time: 5th–6th day begin; 11th–12th day complete


---processes: zona pellucide disappear → polar trophoblast touch the endometrium→secrete proteolytase →dissolve the endometrium → embedded into endometrium→coagulation plug seal the space


* trophoblast become into two layers when blastocyst is embedding into endometrium

  • syncytiotrophoblast

  • cytotrophoblast


---place:

/posterior wall of fundus and body of uterus

  • placenta praevia

  • ectopic pregnancy

    ---decidual response of endometrium

    /stroma cell→predecidual cell→decidual cell(cell become larger and rich in glycogen and lipid droplet)


/endometrium →decidua:

-decidua basalis

-decidua capsularis

-decidua parietalis

---conditions:

/endometrium is in secretory phase

/morula reach the cavity of uterus on time

/zona pellucide disappears in time


3 formation and differentiation of trilaminar germ disc

3. Formation and differentiation of trilaminar germ disc

1) formation of endoderm and ectoderm: early of 2 weeks, inner cell mass differentiate into two layers of cells

---bilaminar germ disc: epiblast(columnar) +hypoblast(cuboidal)


---epiblast: primary ectoderm

/amniotic membrane: amnioblast

/amniotic cavity

/amniotic fluid

/amnion


---hypoblast: primary endoderm

/extraembryonic endoderm→exocoelomic membrane→primary yolk sac → exocoelomic vesicle


/extraembryonic mesoderm: → extraembryonic cavity: chorionic cavity

-visceral layer

-parietal layer

/secondary yolk sac: yolk sac

---body stalk: formed by extraembryonic mesoderm


2) formation of mesoderm: early of 3 weeks

---primitive streak: cells of epiblast proliferate to form a longitudinal arranged cell cord

---primitive groove

---primitive node

---primitive pit


---mesoderm: intraembryonic mesoderm

---endoderm: hypoblast cells are replaced by epiblast cells

---ectoderm: epiblast changed the name into ectoderm

* trilaminar germ disc: endoderm + mesoderm + ectoderm

* determination of head and tail of germ disc


---head process→notochordal tube → notochord

---buccopharyngeal membrane

---cloacal membrane


3)differentiation of trilaminar germ disc: 4th–8th weeks

---differentiation: same cells which are primordial and inmuture differentiate into different cells which have specific structure and function

---induction: some tissues effect the differentiation, and determine the differentiating orientation of another tissue


①differentiation of ectoderm: from 18th–19th days

---neural plate: neuro-epithelium(neural ectoderm): pseudostratified columnar epi.

---neural fold

---neural groove

---neural tube: →CNS

/anterior neuropore: closed by 25th days

/posterior neuropore: closed by 27th days

---neural crest(mesoectoderm): two lines of cell cords→ganglion


②differentiation of mesoderm: 17th days

---paraxial mesoderm

  • somite: 20th days, 3 pairs/per day, 42-44 pairs by the end of 5th weeks

    -sclerotome: →bone, cartilage

    -myotome: →skeletal muscle

    -dermatome: dermis and hypodermis


---intermediate mesoderm: →kidney and reproductive gland

  • nephrotome: segmentation

  • nephrogenic cord:


---lateral mesoderm:

  • intraembryonic coelom: →body cavity

  • somatic or parietal mesoderm: →muscle, CT, parietal layer of pleura, peritoneum and pericardium

  • splanchnic or visceral mesoderm: →muscle, CT of digestive tract, visceral layer of pleura, peritoneum and pericardium

    ---mesenchyme: →cardiovascular and lymph system


③differentiation of endoderm:

---primitive gut: →digestive, respiratory and urinary system


4 the development of fetal membrane and placenta

4. The development of fetal membrane and placenta


1) chorion:

---formed by trophoblast and extraembryonic mesoderm


---primary stem villus: projections of cytotrophoblast and syncytiotrophoblast

---secondary stem villus: extraembryonic mesoderm enter the primary stem villus

* chorionic plate: trophoblast + extraembryonic mesoderm

* chorion: secondary stem villus + chorionic plate


---tertiary stem villus: extraembryonic mesoderm differentiate into CT and BV

  • free villus: branches

  • anchoring villus

    ---cytotrophoblastic cell column: →cytotrophoblastic shell

    ---chorion leave: 6 weeks later

    ---chorion frondosum

    ---hydatidiform mole

    ---chorion carcinoma


2) yolk sac:

---blood island: primitive blood cell- derived from extraembryonic mesoderm on the wall of yolk sac

---primordial germ cell: derived from endoderm of yolk sac


3) amnion:

---amniotic membrane: amniotic epi. + extraembryonic mesoderm

---amniotic fluid:

/secrete by amniotic epi.

/slight basic fluid: 500-1000ml

-polyhydramnios: >2000 ml, abnormal digestive system or CNS

-oligohydramnios: <500 ml, abnormal urinary system


/function:

-intraenvironment

-protecting

-preventing from adherence

-wash germ tract


4) allantois

---allantoic A: paired, →umbilical A

---allantoic V: paired

  • right: degenerate

  • left: umbilical V


5) umbilical cord

---cylindrical structure

---surface: amniotic membrane

---cord: mucous CT, umbilical A,V, yolk sac and allantois

---40-60 cm long, 1.5-2.0 cm in D

---> 80 cm, or < 35 cm


6) placenta:

---the structure by which exchange of material between fetus and mother takes place

---size: round, disc-shaped, 15-20 cm in D, 2.5 cm thickness, 500g in weight


  • fetal surface: smooth, covered by amniotic membrane

  • mother surface: rough, 15-30 cotyledons


---structure:

/chorionic plate

/chorion and chorion space

-chorion: 60 chorion stalks→branches

-chorion space: space between chorion, filled with mother blood


/basal plate: cytotrophoblastic shell + deciduas

-placental septa: separate the chorion into cotyledon


---blood circulation of placenta

  • fetus: umbilical A →cap. of chorion →umbilical V

  • mother: spiral A → chorion space → uterus V


* placental barrier:

/the structure between fetal and maternal blood

/components:

-endothelium of chorion capillary and its basal lamina

-CT in the core of the villus

-trophoblast epithelium and its basal lamina


---function:

a. the exchange of material between the maternal and fetal blood streams

b. defense barrier

c. the production of hormones:

/human chorionic gonadotropin, HCG:

-begin: end of 2nd week

-highest lever: 9th–11th week

-lowest lever: 20th week, until birth


/human placental lactogen, HPL

-similar to HGH

-highest lever: 36th–37th week

/human placental progesterone, HPP and human placental estrogen, HPE

-begin: 4th month

/other hormones: human chorionic thyrotropin, HCT; human chorionic adrenocorticotrophic hormne, HCATH; prostaglandin