embryology Flashcards
journey of an egg
Sperm can survive for 5 days (travels cervic, uterine cnal, fallopian tube)
The oocyte can survive for 24 hours
step 1 - egg leaves ovary and enters fallopian tube
step 2 - sperm enters egg and united with nucleus
step 3 - fertilised egg divides
step 4 - cells attach to uterus
fertilisation
1 - sperm makes contact with egg
2 - acromosome reacts with zona pellucida
3 - acrosome reacts with periviterlline space
4- plasma membrane of sperm and egg fuse
5 - sperm nucleus enters egg
early embryo development
Day 1 – Formation of pronuclei (two nuclei in fertilised egg one to egg and one to sperm)
Day 2 – First cell division
Day 3 – Cell division continues (rapid increase number –> ball of cells (morula))
Day 4 –Formation of Morula
Day5 - Blastocyst forms
implantation process
Early embryo enters the uterine cavity in morula phase which converts to blastocyst on day 5.
Hatching- trophoectoderm cells produce protease to dissolve the zona in preparation for implantation
Apposition- first connection between blastocyst and endometrium, apposes to microvilli like structure called pinopodes expressed on receptive endometrium
Adhesion- trophoblast of the blastocyst adheres to the epithelial layer of maternal endometrium, embryonic tissue starts to actively secrete hCG
Invasion- trphoblast proliferation, differentiation, crossing of the epithelial basement membrane and invasion of endometrial stroma to form the placenta. Uterine spiral arteries remodelled by the invasive trophoblast.
hatching of the blastocyst
hatching of the embryo from sona pellucida 5th day after fertilisation
inability to hatch results in infertlity and premature hatching can result in abnormal implantation in the uterine tube
decidulisation change prior to invasion
Pre-requisite for trophoblast inasion and placentation -involves transformation of stromal cells of maternal endometrium every month under the effect of hormone progesterone.
Changes-
Stromal cell differentiation (elongated fibroblast like cells) converted into decidual( rounded epithelial cells).
Angiogenesis - new vessel formation, increase vascular permeability
Increased macrophages, lymphocytes and decidual leukocytes(uterine natural killer cells) for maternal immune tolerance
These changes transform the endometrium into a vascular receptive tissue for blastocyst invasion.
invasions of thropoblast
placenta isnt the mothers property mainly composed of the fetus
placentation
Invading trophoblast form primary chorionic villi
With infiltration of extra-embryonic mesoderm these become secondary villi
When capillaries form they become tertiary villi
Invasion of cytotrophoblast remodels spiral arteries to increase blood flow
mature placenta components
Chorionic villi
The intervillous space
Placental Septae
Cotyledons (subunits of the
placenta)
functions of the placenta
Acts as an immunological barrier
Gas exchange
Nutrient exchange Waste excretion
Endocrine functions
embryogenesis
embryogenesis is the first eight weeks of embryo development post fertilisation.
Fertilised egg forms blastocyst on day 5 with two organised cell groups- inner cell mass(embryoblast) and trophoblast which develop to form the baby and placenta respectively
embryoblast development
Week 2-3- formation of bilaminar disc and development of amniotic sac, yolk sac, extraembryonic mesoderm and chorionic cavity
Week 3-4- differentiation of bilaminar disc into trilaminar structure- three germ layers ( endoderm, medoderm and ectoderm) through process of gastrulation and neural tube formation
Week 5-8 - organogenesis.
blastocyst cell group
Pluripotent cells
bilaminar disc consisting of Epiblast and hypoblast.
Epiblast undergoes gastrulation to form the three germ layers - ectoderm, mesoderm and endoderm
Hypoblast forms extraembryonic mesoderm
Blastocoele
Inner layer cytotrophoblast forms chorionic villi
inner cell mass differentiation
The inner cell mass differentiates into bilaminar epiblast (columnar epithelial cells) and the hypoblast (cuboidal epithelial cells). Hypoblast is the layer facing the blastocoel, while the epiblast is on the other side
Two cavities form, yolk sac on the side of the hypoblast and amniotic cavity on the side of epiblast
epiblast differentiation / gastrulation
Epiblast differentiates into the three germ layers- ectoderm , mesoderm and endoderm in a process called gastrulation (D13 to 16 of embryo development)
Gastrulation starts with a groove appearing in the caudal end of epiblast called primitive streak
Epiblast cells migrate into the hypoblast layer displacing them- forming the endoderm
Further epiblast cells migrate through primitive streak between the epiblast and the hypoblast/endoderm to form the mesoderm
Remaining epiblast becomes the ectoderm
The newly formed trilaminar disc with three layers is called gastrula and forms the different organ systems
ectoderm purpose
Epidermis, central nervous system, peripheral nervous system, hair, nails, neuroendocrine organs (adrenal medulla, pituitary gland), enamel of teeth
mesoderm purpose
Dermis, musculoskeletal structures, cardiovascular system, kidneys, ureters, trigone of the bladder, gonads (not germ cells), adrenal cortex, visceral and parietal linings (pleura, pericardium, peritoneum),
endoderm purpose
Lining of the gastrointestinal tract, the parenchyma of the liver, pancreas, thyroid, parathyroid, tonsils and thymus, the bladder (excluding the trigone) and urethra
mesoderm subdivisions
Paraxial Mesoderm
Intermediate Mesoderm
Lateral Plate Mesoderm
whaT does intermediate mesoderm allow to develop
kidneys ureters and gonads
multiple pregnancy
Dizygotic- Two or more eggs fertilising and implanting, non-identical
Monozygotic- One embryo Splitting
Timing of embryo splitting determines the nature of pregnancy
identicle twins
non-identical is more common
Risk of multiple pregnancy in natural conception 1-2%
Fertility treatments have increased the rates of multiple pregnancy, target to keep <10%. Strategies to reduce- One at a time.
types of twins
Dichorionic diamniotic (separate placenta and amniotic sac)
monochorionic diamniotic (share placenta, separate amniotic sac)
monochorionic monoamniotic (share placenta and amniotic sac)
conjoined twins (as above, joined up organs)
What three major units is mesoderm subdivided into
paraxial Mesoderm
Intermediate Mesoderm
Lateral Plate Mesoderm
link between urinary system and gonads
Part of urinary system (Kidney and ureter) and reproductive system develop from the urogenital ridge in the intermediate mesoderm, therefore same origin
Urogenital ridge differentiates into gonadal ridge medially which gives rise to gonad and nephrogenic cord laterally which forms the kidney and ureter
Urinary system develops ahead of the reproductive system from 4th week of embryo development
what is the urinary system comprised of
Kidney, ureter, bladder and urethra
when do different parts of the urinary system develop
Kidney and ureter develop from three overlapping systems in nephrogenic cord portion of the urogenital ridge - pronephros, mesonephros and metanephros.
Kidney develop in the pelvis and then ascends into the abdomen with the final position taken by 12th week.
Bladder and urethra are formed from urogenital sinus (part of cloaca) which also give rise to parts of female and male reproductive tract.
what are the three stages of development for the nephorgenic chord
The pronephros (non-functional)
The mesonephros (functional but transient)
The metanephros (final kidney)
Metanephric mesenchyme forms kidney and ureteric duct forms the ureter
The nephric duct drains into the cloaca which is a common temporary outlet for digestive, urinary and genital tract in embryonic life.
whaT are common renal anomolies
renal agenesis
horseshoe kidney
pelvic kidney
duplex kindeys
duplex ureter and cloacal system
describe development of the cloaca
The caudal end of the enfolded yolk sac is known as the cloaca. Its covered by the cloacal membrane which is formed by fusion of ectoderm and endoderm
The cloaca is divided by the urorectal septum to form
- The urogenital sinus
- The anal canal
The urogenital sinus will form the bladder and urethra
The Mesonephric duct below the ureteric bud is incorporated into the bladder as the trigone
development of the gonadal ridge
The indifferent gonad develops in the gonadal ridge
The gonadal ridge is closely related to the mesonephros
Primordial germ cells do not originate in the gonadal ridge but migrate to the gonads from endoderm lining of yolk sac via hindgut around 6 weeks of gestation
Epithelium of gonadal ridge forms primitive sex cords
Combination of germ cells and primitive sex cord forms the indifferent gonad which is capable of developing into a testis or ovary
gonad differentiation
The Y chromosome contains the SRY gene (sex-determining region Y)
SRY produces testis determining factor / SRY protein
This protein acts on the indifferent gonad to promote the formation of the testis
In rare cases, following a translocation error, a male can be born with a 46 XX Karyotype (1:20,000)
development of ovary
The migrating germ cells enter the ovary
The primitive sex cords extend into the medulla but degenerate
Secondary sex cords (cortical cords) develop and surround the germ cells to form the ovarian primordial follicles
Ovarian follicle pool (determining female ovarian reserve established at 20 weeks of gestation in fetal life
developent of testes
The migrating germ cells enter the testis
The primitive sex cords extend into medulla and form testis cords (medullary cords) which transform into future seminiferous tubules and rete testis
Sertoli cell are derived from surface epithelium under effect of AMH
Leydig cells are produced from intermediate mesoderm and produce testosterone from 8 week onwards
tesTicular descent
The gonads originate in the posterior abdominal wall
The testes reach the deep inguinal ring by 7 months of pregnancy
By term 97% of male infants will have fully descended testes
Undescended testicle, also known as cryptorchidism requires early surgery for correction
development of reproductive tract
There are two sets of genital ducts
Mesonephric duct next to the gonad
Paramesonephric duct laterally
The presence or absence of AMH determines which develops and which regresses
In the male the presence of AMH causes the paramesonephric ducts to regress and these do not form any part of the adult
male reproductive tract
Mesonephric ducts , also called Wolffian ducts drain the mesonephros into the cloaca
In both males and females it forms the trigone part of the bladder
Rest of the duct only persists in the males under the effect of testosterone and forms the epididymis, vas deferens and seminal vesicles
In females, the duct regresses and remnants might remain as epoophoron, paraophoron (small cystic structures lateral to ovary) and gartners duct cyst (benign lesions on the lateral vaginal wall)
female reproductive tract
In the absence of AMH, the paramesonephric ducts continue to develop and the mesonephric ducts regress
Paramesonephric ducts grow medially and fuse. The fused portions of the paramesonephric ducts canalises to form the uterus and upper 2/3 rd of vagina. The unfused portions give rise to the fallopian tubes
Urogenital sinus forms the lower 1/3rd of vagina
Paramesonephric ducts fuse with the urogenital sinus at the sinus tubercle and point of fusion forms the hymen
DEVELOPMENT OF EXTERNAL GENITALIA
On either side of the cloacal membrane folds develop – The urogenital folds
Anteriorly these fuse to form the genital tubercle
Lateral to this are the labioscrotal swellings
As the cloaca is split by the urorectal septum the anus is separated off
MALE EXTERNAL GENITALIA
In the male the genital tubercle elongates to form the penis
The labioscrotal swelling folds fuse posteriorly
The urogenital folds fuse posterior to anteriorly to form the penile urethra (spongy urethra)
The glans penis will canulate at the tip to form the final part of the urethra
FEMALE EXTERNAL GENITALIA
In the female the urogenital folds form the labia minora
The labioscrotal folds form the labia majora
The genital tubercle forms the clitoris
MALE REPRODUCTIVE TRACT ANOMALIES
Penile- micropenis (genital tubercle does not elongate fully), hypospadias (urethral development problem causing opening to be abnormally located)
Testicular- Absence, undescended (cryptochordism)
Absence of vas deferens (cystic fibrosis)
Absence of seminal vesicles
FFEMALE REPRODUCTIVE TRACT ANOMALIES
Anomalies of the female reproductive tract are common
4-7% of women
No one agreed classification system:
ASRM (AFS)
ESHRE/ESGE
There is an association with renal tract anomalies
OTHER CONGENITAL ANOMALIES IN FEMALE
Vaginal anomaly
Vaginal Septa Can be longitudinal or transverse (1:80,000) Imperforate Hymen (1:2000) Vaginal Agenesis Absent or underdeveloped uterus and vagina
MKRH Syndrome (Mayer–Rokitansky–Küster–Hauser) or Mullerian agenesis syndrome (1:4500)
Failure of the paramesonephrict ducts to develop normally
