Embryology Flashcards
What occurs in gastrulation
The embryo forms 3 layers
Called the trilaminar disc
When does gastrulation occur
Week 3 of gestation
Where is the urogenital system derived from
The intermediate mesoderm
How are the endoderm and mesoderm formed
Invagination of epiblast cells through the primitive streak
How does cranial-caudal folding affect the embryo
It rolls up like a scroll from each end to give the foetal position
Describe the function of the pronephros, mesonephros and metanephros
The pronephric system will eventually disappear – doesn’t function in humans
Mesonephros is not the end kidney but does have some functional capability in the foetus – some urine production
Metanephric system is the end kidney
Where do the excretory ducts of the forming UG system enter
They join with the cloaca to eventually form an output
Which cells will go on to form gametes
Primordial germ cells
Describe the progression of the primordial germ cells
They form in the yok sac
They work their way into the embryo into the developing gonads (intermediate mesoderm)
When they get in, they stimulate epithelium to start proliferating – forms a gonadal/genital ridge
The proliferating epithelium will form somatic support cells which envelop to the germ cells
These support cells will eventually become the follicular cells or Sertoli cells
Which two ducts form the urogenital system
Mesonephric (Wollfian) duct
Paramesonephric (Mullerian) duct
Where is the mesonephric duct found
Connects to the posterior wall of the urogenital sinus and the cloaca
Joins the paramesonephric duct at the urogenital sinus
Where is the paramesonephric duct found
It is blindly open at the top and opens into the peritoneal cavity
Joins the mesonephric duct at the urogenital sinus at the caudal end of the foetus
When does sexual differentiation occur
From week 7 of gestation onwards
What triggers the differentiation into a male
The presence of the SRY protein transcription factor
This is the sex determining region of the Y chromosome
Describe the effect of SRY on the early gonads
Triggers the somatic support cells to become Sertoli cells and they form medullary cords
These cords engulf the PGCs
Rete testis connect the mesonephric tubules to the testis cords
What happens to the paramesonephric duct in males
It disappears
Sertoli cells secrete anti-Mullerian hormone to cause the degeneration of the paramesonephric duct
How is testosterone production started in the early male
The Sertoli cells stimulate the formation of Leydig cells which secrete testosterone
What is the role of testosterone in the developing male gonads
influences the mesonephric duct to form the structures of the male repro system – epididymis , vas deferens etc
Dihydrotestosterone (DHT) is more potent and stimulates formation of external genitalia
What is persistent Mullerian duct syndrome
When part of the female reproductive tract is present in a biological male
Due to a failure of the anti-Mullerian hormone or its receptors
The paramesonephric duct doesn’t regress
What is cryptorchidism
Failure of descent of one or both testes
Where do the testes originate
Level of T10 on the abdominal wall
How do the testes move down to the scrotum
Tissue called the gubernaculum shortens and pulls the testes down and through the body wall
Should be in scrotum by birth - sitting high up
What is the vaginal process in men and its significance
Vaginal process is a small pocket in front of the descending testes
It should close off and form a space that allows for testicular movement
If it stays open you are at increased risk of inguinal hernias
How do the seminal vesicles develop
They are outpouchings of the mesonephric ducts
How do the bulbourethral glands and prostate develop
Form from endodermal invaginations from the urethra
What triggers the embryo to develop into a female
The absence of SRY transcription factor = no Y chromosome
How are the ovaries formed
Germ cells differentiate into oogonia and then into primary oocytes.
Somatic support cells differentiate into granulosa cells and surround the primary oocytes.
This forms primordial follicles in the ovary.
What does the paramesonephric duct go on to form in females
Uterine tubes
Uterus
Superior vagina
What is the role of oestrogen in the early development of a female foetus
Stimulates the formation of the external female genitalia and the development of the paramesonephric ducts
How do the paramesonephric ducts form the uterine cavity
Ducts from each side fuse together
The bits that are touching (middle) will regress to form the bigger cavity
Still have an independent uterine tube coming from either side
Describe the hymen
Thin tissue that closes off the vaginal opening
At birth there are little holes in the hymen and it degrades with activity
How does the uterus connect to the vagina
Forms a tubercle when it meets the urogenital sinus
Starts to elongate and forms a plate with sinovaginal bulbs
What can happen if the paramesonephric ducts don’t join properly
Double uterus/vagina if they completely fail to meet
Double uterus – they have come together at the bottom (one vagina) but middle of cavity hasn’t regressed so get 2 uteruses
What is cervical atresia
Where the uterus and vagina are not connected
Cannot get fertilisation with cervical atresia as there is no way for sperm to meet eggs
What causes hypospadias
Malfunctioning in the zipping mechanism on the ventral surface (underneath) of the penis
The urethral groove doesn’t close up properly leading to the outlet being in an abnormal place
What is hypospadias
When the urethral opening lies in an abnormal position on the penis
Usually underneath (ventral surface)
Comes in various severity - on glans, shaft or even scrotum
Can be fixed surgically
How do the external genitalia develop - prior to sex differentiation
The cloaca forms a general outlet
Septum (from mesenchyme) splits the cloacal area into a urogenital sinus and anorectal sinus
Separates anus from the UG system
Mesenchymal cells at the top of the urogenital sinus proliferate and form the genital tubercle
What does the genital tubercle go on to form in males and females
Males - penis
Females - clitoris
How does the penis form
Growth of th genital tubercle
The urethral groove starts to ‘zip up’ towards the tip of the phallus - forms the spongy urethra
should have a outlet on the glans
A circular ingrowth of ectoderm around the glans forms the foreskin
What is meant by monozygotic twins
Develop from one egg and therefore will be identical
What is meant by dizygotic twins
Develop from two eggs and therefore will be non-identical
What is the most common subtype of monozygotic twins
Dichorionic
What are dichorionic monozygotic twins
Twins which develop from the same egg but cleavage occurs before implantation
Each twin has it’s own placenta
What are monochorionic diamniotic twins
Twins which develop from the same egg (monozygotic) but cleavage occurs between day 6-8
The twins share a placenta but have separate amniotic sacs
What are monoamniotic twins
Twins which develop from the same egg (monozygotic) but cleavage occurs after day 8
The twins share an amniotic sac
In the development of twins, what is the latest form of cleavage
Conjoined twins - separate very late
Rarest form
List risk factors for dizygotic twins
Aged 35-40, Family history Previous multiple birth High BMI Smoking Summer and autumn conceptions. induced ovulation and in-vitro fertilisation race e.g. Afro-Caribbean
Monoamniotic monozygotic twins are associated with which poor outcomes
ncreased spontaneous miscarriage, perinatal mortality rate
increased malformations, IUGR, prematurity
twin-to-twin transfusions: recipient is larger with polyhydramnios
