Embryology Flashcards
Three phases of human prenatal development
- Pre-embryonic period –> gametogenesis and fertilization
- Embryonic period –> blastogenesis (1-2) and embryogenesis (3 to 8)
- Fetal period –> fetogenesis (9th week to birth)
Sheaths of oocyte
Plasma membrane with microvilli
Zona Pellucida
Corona Radiata
Zona peullucida
- secreted by follicular cells and oocyte
- composed of glycoprotein, ZP3 is sperm receptor
- for fertilization and implantation
Corona radiata
- follicular cells
- protection, Nutrition and growth Regulation
Sperm parts
Head: haploid nucleus, covered by acrosome
neck: contains pair of centrioles (9x3 centrioles)
tail:
- middle piece: axoneme in Center (central pair and 9 doublets of microtubules), 9 outer dense fibers, sheath of mitochondria
- principal piece: axoneme and 9 outer fibers, fibrous sheath
- end piece: axoneme, no outer fibers
Fertilization phases
Phase I:
Penetration of CR, binding to ZP3
Acrosome reaction:
Plasma membrane of sperm and acrosome fuse
Phase II.
Penetration of ZP
Phase III:
Fusion of oocyte and sperm Plasma membranes
Cortical reaction
Release of cortical granules with Enzymes of oocyte
Zona reaction
Alteration of ZP3, prevents ither sperm binding
Male and female pronuclei form, pronuclei fuse and form zygote
Results of Fertilization
Finishing the secnid meioitc division of oocyte Diploid number of chromosomes Determination of sex Variation of human Initiation of cleavage
Cleavage or segmentation
- part of blastogenesis
- series of mitotic divisions
- cells are blastomeres
Purpose: to generate a mutlicellular embryo
Migration inside uterine tube
- emrbyo is carried passivly by cilia and contraction if tube
- Nutrition: yolk and secretions of tube
- on 3rd day: 16 cell Morula
- two types of blastomeres: inner and outer cell mass
Entering the uterine cavity
- blastocyst Formation: on 4th day
Parts of Blastocyst
1- Trophoblast: flattened outer cell, develops into Chorion and placenta
2- Embryoblast: inner cell mass at one pole, develops into Embryo, Amnion and yolk sac
3- Blastocyst cavity
Blastocyst inside uterus
Migration until 6th day
covered in Zona pellucida
Implantation stages
site: Fundus or corpus of Uterus
- Hatching from blastocyst from the zona pellucida
- Attachment of blastocyst to uterine Endometrium
- Penetration of blastocyst into Endometrium
–> trophoblast differentiates into:
>cytotrophoblasts (inner)
>syncytiotrophoblast (outer) - Embedding of the blastocyst in endometirum and Proliferation of trophoblast
>lacunae appear in STB and maternal blood enters
Penetration is stopped in compact layers
Functions of the STB
- produces Enzymes that erode Endometrium
- resolves the destroyed products
- produes Hormones hCG
- will later form placental barrier
Decidual reaction of endometrium
- Response to Implantation
- Endometrium becomes more succulant and edematous
Decidual cells
- large with Lipids and glycogens
- Nutrition of Embryo, Regulation of development of Embryo, protection of agressive penetration
During Implantation, embryoblast differentiates into two layers
- Epiblast layer
- Hypoblast layer
–> Bilaminar germ Disc
two extraembryonic cavities: Amnion, Yolk sac
Amnion formation
- small Cavity within epiblast -> primitive Amnion
- 11-12th days: primitive Amnion covered by extraembryonic Mesoderm –> definite mesoderm
yolk sac formation
- flat cells of Hypoblast cover Surface of CTB –> primitive yolk sac
- covered by extraembryonic Mesoderm –> definitive yolk sac
Extraembryonic mesoderm
- 12th day: cells derive from primitive yolk sac
- extraembryonic cavity: cavity within the extraembryonic Mesoderm
- lines the trophoblast: froms Chorion
- Embryo remains connected to trophoblast by connecting stalk (forms umbilical cord)
Gastrulation
- start of Embryogensis in 3rd week
- process during which the bilaminar germ Disc develops into a 3 laminar germ Disc
- Ectoderm
- Mesoderm
- Endoderm
beginning of 3rd week
Epiblast is source of all germ layers
Primitive streak: at caudal midline of epiblast, contains primitive groove
Cranial end of primitive streak: primitive node with primitive pit
Invagination/ ingression
- Migration of epiblast cells towards the primitive streak
- enterinf through primitive streak
- detaching from epiblast
- migrating beneath it
Formation of Endoderm
- some invaginated cells invade and displace the hypoblast
Formation of Mesoderm
- invaginated cells between epibalst and newly created endoderm
- cells invaginated in primitive node: definitive notochord
- cells invaginated in primitive streak: paraxial Mesoderm, intermediate Mesoderm, lateral Mesoderm, cardiogenic mesiderm
Trilaminar germ Disc contains of three layers, except:
- Oropharyngeal membrane
2. Cloacal membrane
Derivatives of ectoderm
Neurulation (3-4 weeks) induced by notochord
- Surface ectoderm
- Neuroectoderm (neural tube, neural crest)
Neurulation
neural plate –> neural groove –> neural fold –> neural tube and neural crest
- Fusion Begins in cervical regionand proceeds in borth directions
- cranial and caudal Ends are open until Fusion is complete (anterior and posterior neuropores)
Derivatives of neural tube
CNS
Retina
Neurohpophysis
Epiphysis
Dervatives of neural crest
PNS
Melanocytes
Adrenal medula
Mesenchyme of head
Derivatives of Surface ectoderm
- dermal Epithelium
- hair, nail, Skin Glands
- ectodermal thickenings or placodes in head –> sensory epithelium
Derivatives of mesoderm
- axial Mesoderm (notochord)
- Paraxial Mesoderm (somites)
- Intermediate Mesoderm (nephrotomes)
- Lateral plate Mesoderm (2 layers - parietal and visceral)
Notochord
Axis of germ Disc
Inductor of neurulation adn somitogenesis
Contributes to nucleus pulposus and vertebral disc
Paraxial mesoderm
- middle of 3rd week: divides into Segments –> SOMITOMERES
- Formation Begins in cephalic Region
- 7 pairs of SMM in the head –> Mesenchyme of head
- from occipital Region caudally: somites (3 pairs a day)
Derivatives of the somite
Sclerotome: mesenchymal cells surround notochrod and neural tube –> vertebral column
Myotome: mesenchymal cells form skeletal muscles
Dermatome: mesenchymal cells form the dermis of back
Intermediate Mesoderm and its drivatives
- connects paraxial and lateral plate Mesoderm
- consists of segmented nephrotomes (form urinary System and Glands)
Lateral plate Mesoderm and its derivatives
- Parietal layer: forms dermis of bidy, Skeleton of limbs
2. Visceral layer: forms CT and muscles of internal organs
Primordial heart
- cardiogenic Region is anterior to buccopharyngeal membrane
- 21 to 22 day: heart tube Begins to beat
Coleom
- between parietal and visceral layers of lateral plate Mesoderm
- transforms into serous cavities
Derivatives of endoderm
- gut tube –> forms during Embryo folding
Embryo folding
- 4th week
- Head and tail fold –> cephalocaudal folding
- lateral Body wall folds –> lateral folding
Embryo separates from yolk sac
Gut tube forms
Gut tube
Ends are boundedn by buccopharygeal and cloacal membranes
foregut
midgut
hindgut
Derivatives form gut tube
Epithelium of digestive System
Epithelium of respiratory sytstem
Epithelium of liver, pancreas
Epithelium of urinary bladder
Features of the external Body by the end of 8th week
human appearance shape of face and extremities tail has disappeared bended Body Begins to traighten head takes half of Body length 33 mm
Fetal membranes and adnexa
- Chorion
- Amnion
- Allantois
- umbilical cord
- decidua
- placenta
Chorion
extraembryonic Mesoderm lines CTB of trophoblast –> Chorion forms
Wall: trophoblast and extraembryonic mesoderm
Chorionic villi
emerge from Chorion and invade Endometrium
Primary (11 to 13th day): CTB core, covereby by STB layer
Secondary (13 to 15th): extraembryonic Mesoderm penetrates core of Primary villi
Tertiary (18th to 22nd): blood vessel in extraembryonic mesodermal core
in embryonic period:
- villi cover entire Surface of Chorion
in fetal period:
- only on embryonic pole –> Chorion frondosum
Amnion
- sac that contains Embryo and amniotic fluid
- inner lining of amniotic cells
- outer layer of extraembryonic Mesoderm
Amniotic fluid
fluid is derived from:
- maternal blood
- fetal blood
- produced by amniotic cells and fetal respiratory epithelium
Amniotic fluid recirculation
4th to 5th month: Fetus swallows it
- -> blood circulation
- -> placenta or urine
Vernic caseosa
- fatty substance that covers the fetal Skin
- from 5th to 6th month
- protects Skin from maceration
Lanugo hair
- anchor to hold vernix caseosa to the Skin
- before birth: replaced by vellus hair
- fetur consumes Lanugo hair together with amniotic fluid
Meconium
- earliest stool of an Infant
- consists of: intestinal epithelial cells, Lanugo hair, water, bile
Function of amnion
- absorbs jolts
- allows fetal movement
- insures bod temperature, symmetrical growth and normal development of the fetal lungs
Yolk sac
- inner layer: extraembryonic endoderm
- outer layer: extraembryonic Mesoderm
end of 3rd month: yolk sac shrinks but stays connected by vitalline duct
Functions of the yolk sac
- hematopoietic stem cells areise in vitelline vessels (3rd to 4th week)
- Primoirdial germ cells in 3rd to 4th week
Vitelline duct
- Yolk stalk
- later becomes vitelline ligament between Ileum and umbilicus of fetus
Allantois
- diverticulum of yolk sac
- 3rd week
- rudimentary
- becomes fibrous cord, (urachus) during mebryo folding
- becomes median umbilical ligament after birth
Formation of the umbilical cord
Amnion envelops connecting stalk and yolk stalk –> primitive umbilical cord
Physiological umbilical hernia
6th to 7th week: abdominal cavity is too small for intesines –> some of them are in umbilical cord
11th week: intestines returs into body
Decidua
Functional layer of the Endometrium
- decidua basalis: at site of Implantation, over Chorion frondosum
- decidua capsularis: decidual layer of implanted Embryo
- decidua parietalis: at opposite site of Uterus
3rd month: decidua capuslaris fuses with decidua parietalis
Placenta
- maternal Portion
- decidua basalis
- decidual Septa - fetal Portion
- chorionic plate
- chorionic villi
Lacunae
- filled with maternal blood
- separated by decidual septa
Maternal blood circulation
- spiral arteries open into Lacuna
- oxygenated blood bathes the villi
- blood flows back by endometrial veins
Chorionic plate
- covered by Amnion
Cotyledon
- structural and functional unit of placenta
- composed of stem villi in Lacuna
- 15 to 30 in one placenta
Placental membrane until 4th month
STB
CTB on b.m.
Connective tissue in villus core
Wall of fetal vessel
Placental membrane after 4th month
Fetal vessel come in contact with STB
All CTB was used to make STB
Functions of the placenta
- Exchange of metabolic products
- Exchange of gaseous products
- Protection from bateria, maternal Hormones..
- Hormone production
- Depot of the Vitamins, glycogen, Fe
- Immunological barrier
5 types of birth defects
structural behavioral functional metabolic disorfer present at birth
Teratology
science that studies the causes, mechanisms and Patterns of abnormal development
Types of abnormalities during different prenatal periods
Gametopathies
Blastopathies
Embryopathies
Fetopathies
Types of abnormalities according to different mechanisms
- malformations (abnormal organogenesis)
- disruption (morphological alterations of already formed structures)
- Deformation (mechanical Forces press in Fetus)
- Chromosomic disorders
Causes of birth defects
- unknown (40-45%)
- genetic factors (28%)
- Multifactorial (20-25%)
- Environmental (3-4%)
- Twinning (1%)
Genetic factors to abnormalities
numerical or structural abnormalities of chromosomes
Non-Disjunction
- failure of homologous chromosomes to separate in meiosis I
or - failrue of sister Chromatids to separate during meiosis II or mitosis
- -> death of gametes
- –> death of Embryo ir Fetus
- -> Trisomies or monosomies
Environmental factors /teratogens
Teratogens are able of Crossing placental membrane and causing a brith defect
infectious agents Pharmaceutical drugs and chemical agents Maternal nutritional deficiencies heavy metals radiation
Principle of teratology
1- Susceptibility depends on genotype of Embryo
2- Susceptibility depends on stage
3- Dose and Duration of exposure
3- Mechanism of exposure
5. manifestations are detah, malformation, growth retardaton anf functional disorders
Dizygotic twinning
- 90%
- two oocytes are fertilized by different spermatozoa
- two blastocysts implant individually
Monozygotic twinning
a single fertilized Ovum and result of Splitting of Zygote and Embryo at various stages
early spitting: individual Implantation –> dizygotic Twins
Twins with the same Chorion and Amnion (monochorionic and monoamniotic) –> Separation in bilaminar germ Disc stage –> MoMo twins
