Embryology Flashcards
What is the embryonic period?
Weeks 1-8
Development of structues
What is the fetal period?
Week 9-38 (birth)
Growth
What are PGCs?
Primordial germ cells develop at wk 4
Develop into gonads and gametes
Remain dormant in males until puberty
What are the stages of spermatogenesis?
PGCs arrested at wk 4
Begin to differentiate into type A spermatogonia in puberty
A few type A spermatogonia differentiate into type B which form primary spermatocytes. They divide into secondary spermatocytes. They enter meiosis II to form 2 spermatids.
Each B spermatogonia results in 4 spermatids
What are the steps of spermiogenesis?
Starts with spermatids
Adds acrosome gap, forms flagella, loose organelles and a majority of cytosol
Final product of spermatocyte
What are the steps in oogenesis?
By 5th month maximum number of primary oocytes formed and arrested in prophase of meiosis I until puberty
At puberty one primary oocyte completes meiosis I a month to form a secondary oocyte and polar body
Secondary oocyte enters meiosis II but is arrested at metaphase until fertilization
What is the ovarian cycle?
Menstrual phase
Follicular/proliferative phase
–> LH surge
Progestational/secretory phase
Menstrual phase if not fertilized or gravid phase
What is capacitation?
Conditioning of sperm in female reproductive tract for 7 hours
Interaction between mucosal surface of fallopian tube and sperm
What are the steps of fertilization?
Sperm penetrates the corona radiata, then the zona pellucida
Acrosome reaction occurs
Merges with oocyte cell membrane
What is the acrosome reaction?
After sperm binds to zona, zona proteins are released to prevent dispermy (acrosin and trypsin like)
What is oocyte response to fertilization?
Completes meiosis II and forms female pronucleus which fuses with male pronucleus to form zygote - diploid again
Where does fertilization take place?
Ampulla of fallopian tube
What occurs in the first 72 hours after fertilization?
Cleavage begins 30 hours post fertilization into two cells
Once into 8 cells forms blastomere
16 cell morula forms 72 hours after fertilization and enters uterus
There is no cell growth
What is the blastocyst and when does it form
Outer cell mass of trophoblasts that will form placenta
Inner cell mass of embryoblasts
From morula on day 5
What does trophoblast differentiate into and when?
Cytotrophoblasts
Syncytiotrophoblasts
End of wk 1
When does implantation begin?
End of week 1
What are the beginning steps of implantation?
During progestational phase of menstrual cycle blastocyst implants into functional layer of endometrium
Into the posterior or anterior uterine wall
Zona pellucida must degenerate first
Embryonic pole of blastocyst implants first
What is an ectopic pregnancy and where does it occur typically?
Abnormal site of implantation
Ampulla of fallopian tube
Rectouterine pouch
What are clinical signs of ectopic pregnancy?
Abnormal uterine bleeding
Sudden onset abdominal pain with missed period
What is placental previa and its effect?
Implantation at internal os of cervix, bridging the opening
Causes severe bleeding in second part of pregnancy and with delivery
What is hCG and when is it produced?
Glycoprotein produced by syncytiotrophoblast starting on day 6
What is the function of hCG?
Stimulates progesterone production by corpus luteum
Causes uterine endometrium to grow and proliferate
Prevents menstrual cycle
What can abnormal hCG levels cause or indicate?
Low may cause spontaneous abortion or indicate ectopic pregnancy
High may indicate multiples, hydatidiform mole, or gestational trophoblastic disease
What is are the two types molar pregnancy (hydatidiform mole)?
Partial mole
Complete mole
What is a partial mole and its effects?
One oocyte fertilized by 2 sperm (Triploidy)
Trophoblastic elements and some fetal parts
Abnormally small placenta
Vaginal bleeding in first trimester
What is a complete mole and its effects?
Fertilization of enucleate ovum (no female pronucleus) by normal sperm
No embryo
Sperm will duplicate its chromosome
Only trophoblastic elements develop
Tissue appear as a cluster of grapes
What are the germ layers and when do they form?
Endoderm - hypoblast
Ectoderm - epiblast
Day 8
How does the amniotic cavity form?
Between epiblast and trophoblast
Derived from epiblast cells
How does the primary yolk sac form and when?
Flattened cells arise from hypoblast line the blastocyst cavity
Day 9
What is the extraembryonic mesoderm?
2 layers between embryo and the cytotrophoblast
What are the layers of the extraembryonic mesoderm and where are they located?
Inner splanchnic lines yolk sac
Outer somatic lines the cytotrophoblast
What will the outer somatic mesoderm become?
Connecting stalk and chorionic plate
What is the function of the primitive streak and when does it appear?
It appears in the week 3 and defines body axis
Where is the primitive node?
Cephalic end of primitive streak
What is gastrulation?
From epiblast of bilaminar disc
Forms 3 germ layers - endoderm, mesoderm, and ectoderm
What occurs at the cranial end of embryonic disc during week 3?
Prechordal plate induces forebrain development
Oropharyngeal membrane develops
Mesoderm spreads except to prechordal plate and cloacal membrane
What is the cloacal membrane?
Caudal end of embryonic disc
What is the notochord and when does it develop?
Grows forward from primitive node between ectoderm and endoderm until it reaches the prechordal plate
Future site of vertebral column
Days 16-18
How does the notochord form?
Prenotochordal cells migrate through primitive streak. they become intercalated in endoderm to form notochordal plate and then detach to form definitive notochord. Cranial to caudal.
What is the role of the notochord?
Defines primordial axis or embryo
Acts as primary inductor
Induces formation of neural plate
What happens to the notochord?
Will degenerate and mostly disappears as vertebrae form
Remnants as nucleus pulposus in adults
What is neurulation and when does it occur?
Notochord signals ectoderm to begin neurulation
Neural plate to neural groove to neural tube
Will develop into spinal cord and brain
Closure of neural tube begins in the middle
End of wk 3 and week 4
What arises from neural crest cells?
Sensory and autonomic ganglia
What is sirenomelia/caudal dysgenesis?
Insufficient mesoderm formed in caudal most region during gastrulation
Legs fused together
What is a sacrococcygeal teratoma?
Tumor associated with gastrulation at base of coccyx derived from remnants of primitive streak
Contains hair, skin, bone, and liver
What is a chordoma?
Tumor associated with notochord - slow-growing
What is holoprosencephaly?
Caused by high doses of alcohol during third week
Kills anterior midline of germ disc - deficiency in craniofacial structures
Small forebrain - no hemispheres
eyes are close together
What are neural tube defects?
Failure of closure of neural tube
What are the two types of NTDs?
Anencephaly - at head region
Spina bifida - at lumbosacral region
What is the intra-embryonic mesoderm?
Differentiation of mesoderm by week 3 into three parts
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
What arises from the paraxial mesoderm?
Organized into somites by end of week 4
42-44 somites by end of 5th week
What are the differentiations of somites and what do they become?
Sclerotome - cartilage and bone
Myotome - segmental muscle component
Dermatome - skin
What does the intermediate mesoderm become?
Urogenital system
What are the two layers of lateral plate mesoderm?
Somatic/parietal layer - covering related to amnion
Splanchnic/visceral layer - related to yolk sac
What are the major derivatives of the endoderm?
Epithelial lining and glands of GI and respiratory tracts
What are the major derivatives of the somatic mesoderm?
Parietal serosa
Dermis of ventral body
connective tissues of limbs
What are the major derivatives of the splanchnic mesoderm?
Wall of GI and respiratory tracts
Visceral serosa
Heart
Blood vessels
What are the major derivatives of the ectoderm?
Epidermis, hair, nails, glands of skin
Brain and spinal cord
Neural crest and its derivatives including adrenal medulla
What is the folding of the embryo and when does it occur?
Flat disc becomes cylindrical with head fold, tail fold, and lateral folds
Margins bound by primitive umbilical opening
End of week 3
What causes the folds?
Head and tail by neural tube growth
Lateral by somite growth
What are ventral body wall defects and some examples?
Failure of body wall to close involving one or both lateral body wall folds
Thorax - ectopia cordis
Abdomen - gastroschisis
Pelvis - bladder exstrophy or cloacal exstrophy
What is gastroschisis?
Defective development of anterior abdominal wall with herniated abdominal contents into amniotic fluid
What are the components of the placenta?
Maternal portion of endometrium - decidua basalis
Fetal portion of tertiary chorionic villi
What are the stages of placenta development in week 2?
Trophoblast differentiate into inner cytotrophoblast and outer syncytiotrophoblasts
Lacunae develop in syncytiotrophoblasts
Maternal sinusoids eroded by syncytiotrophoblasts
Maternal blood enters lacunar network
Establishes primitive utero-placental circulation
Formation of primary chorionic villus
What is a primary chorionic villus made of?
Cytotrophoblast core covered by a syncytial layer
What are the stages of placenta developments in week 3?
Mesodermal cells penetrate primary villi to make secondary chorionic villus
Mesodermal core capillaries arise forming tertiary chorionic villus/definitive placental villus
Villous capillaries make contact with capillaries in chorionic plate and connecting stalk
Intraembryonic circulatory system
What is the structure of a secondary chorionic villus?
mesoderm core, cytotrophoblasts, then syncytial covering
When does the heart begin to beat?
Week 4 - intraembryonic circulatory system already formed
What changes occur in the tertiary chorionic villi after the fourth month and why?
Cytotrophoblasts begin to degrade - easier diffusion
What are the functions of the placenta?
Gas exchange
Nutrient exchange
Transmission of maternal antibodies
Hormone production to maintain pregnancy
Detox some drugs
What is placenta accretia?
Implantation into myometrium instead of endometrium
Possible viable pregnancy, but requires hysterectomy to remove placenta after delivery
Vaginal bleeding at second trimester or later
Massive maternal hemorrhage during delivery
What is the amnion and when is it formed?
Large cavity filled with amniotic fluid
At week 2 from epiblast
What is the function of amniotic fluid?
Fetus suspended inside by umbilical cord
Absorbs jolts
Allows for fetal movement
Prevents adherence of embryo to surrounding tissues
What are normal levels of amniotic fluid throughout pregnancy?
30 mL at 10 weeks
450 mL at 20 weeks
800-1000 mL at 37 weeks
What is polyhydramnios/hydramnios and its cause?
Excess amniotic fluid - 1500-2000 mL
GI abnormalites
What is oligohydramnios and its effects?
Less than 400 mL
Renal abnormalities, renal agenesis
What is an amniocentesis?
Withdrawal of amniotic fluid to detect alpha fetoproteins
What is the length of a full-term umbilical cord and what does it contain?
50 cm
Right and left umbilical arteries - from fetus to placenta
One umbilical vein - placenta to fetus
Wharton’s jelly
How to dizygotic twins occur and how to relate?
Fertilization of two oocytes by two sperms
Diamniotic and dichorionic
How to monozygotic twins occur and how do they relate?
Fertilization of oocyte by one sperm that splits into two at blastocyst stage
Typically diamniotic and monochorionic, but may be dichorionic as well
