Determination of Sex Flashcards
What is a germ cell?
A biological cell that gives rise to the gametes of an organism to reproduce sexually
What is a gamete?
A mature haploid male or female germ cell, which is able to unite with another of the opposite sex in sexual reproduction to form a zygote
What does the female gamete always contain?
X chromosome
What does the male gamete contain?
Either X or Y chromosome
What is the result of the male gamete containing either an X or Y chromosome?
Sperm decides the sex of offspring
What is found on the Y chromosome?
A gene known as SRY gene
What does the SRY gene code for?
Transcription factors that are members of the SOX family
What do the SOX transcription factors do?
Induces expression of another family of transcription factors
What does the action of male derived transcription factors do?
Determines that the gonad that develops will be a testis
What are primordial germ cells?
A special population that are the ‘seed’ for the next generation
When are the primordial germ cells allocated?
Shortly after gastrulation
What is the significance of the primordial germ cells being allocated shortly after initiation of the current generation?
They are recruited and specialise very early
Where do primordial germ cells arise?
In the yolk sac, near a structure called the allantois
What happens to the primordial germ cells once they have arisen?
They migrate
What course to the primordial germ cells take during migration?
They migrate into the retroperitoneum, in through the umbilicus, move up through the dorsal mesentery, and populate the gonads
What is the problem with the migration of primordial germ cells?
They have to migrate widely along the posterior abdominal wall, which may cause them to get lost or die
What can sometimes occur due to errors in primordial germ cell migration?
Get germ cell tumours along pathway of migration
What is true of the gonad at the point of migration of primordial germ cells?
It is indistinguishable between male and female
Where do male germ cells (XY) colonise the gonad?
The medulla
Where do female germ cells (XX) colonise the gonad?
The cortex
What is the ‘default’ position of the gonad?
Female
What happens to germ cells after they have colonised the gonad?
- Proliferate by mitosis
- Reshuffle genetically and reduce to haploid
- Cytodifferentiate into mature gametes
How do germ cells reshuffle genetically and reduce to haploid once they have colonised the gonad?
By meiosis
What are the functions of meiosis in the gonad?
- Reduces the chromosome number in the gamete to 23, giving a haploid gamete
- Ensures every gamete is genetically unique
When is meiosis used?
Only in the production of sperm and eggs
What happens in meiosis?
There are two successive divisions, meiosis 1 and 2
What do the divisions in meiosis 1 and 2 produce?
4 daughter cells
What happens to the daughter cells from meiosis in the females?
Only one develops into mature oocyte, others from polar bodies
What does genetic variation arise from in meisois?
- Crossing over
- Random segregation
- Independant assortment
What happens in crossing over?
Exchange of regions of DNA between 2 homologous chromosomes
When does crossing over occur?
Prophase 1 of meiosis 1
When is crossing over visible?
Not until metaphase 1
What happens in random segregation?
Distribution of chromosomes among four gametes
What happens in independent assortment?
Two homologous chromosomes of a pair must go into seperate gametes
How does cytodifferentiation into mature gametes differ between the sexes?
Timing and scale varies
What produces the ovum?
Oogenesis
What produces the sperm?
Spermatogenesis
How do oogenesis and spermatogenesis differ?
- In oogenesis, there are very few gametes. In spermatogenesis, there are a huge number of gametes
- In oogenesis there is intermittent production. In spermatogenesis, production is constant
How many gametes are produced from oogenesis?
About 400 in a lifetime
What does each ovum have?
1/400 of the potential to pass on gene to next generation
What is the result of each ovum having 1/400 of the potential to pass on genes to next generation?
Only the best cells are selected to survive
How often are ovums produced?
˜1 per month
How many sperm are produced per day?
˜200million
Where does spermatogenesis occur?
In seminiferous tubules
What happens once spermatogenesis has occured in the seminiferous tubules?
They coalesece in the rete testis
What happens to sperm in the rete testis?
They are concentrated
When does the initial development of gonads occur?
In the indifferent stage
What causes development of a male or female gonad following the indifferent stage?
Y influence, or lack of it
What happens to the indifferent gonad in males?
- Medually cords develop
- No corticol cords
- Thick outer covering, called the tunica albuginea
What does the tunica albuginea do?
Encloses the gonad quite tightly
What happens to the indifferent gonad in females?
- Medually cords don’t develop
- Corticol cords develop
- No tunica albuginea
Why is there no tunica albuginea in females?
Facilitates ovulation
What is the urogenital ridge?
A region of intermediate mesoderm
What does the urogenital ridge give rise to?
Both embyonic kidney and gonad
What is the gonad derived from?
Both intermediate mesoderm and primordial germ cells (extragonadal)
What is meant by the primordial germ cells being extragonadal?
They have to move into supporting tissues
What does the gonad start off as?
An indifferent stage gonad
What is the indifferent stage gonad?
Just supporting tissue
What is the purpose of the indifferent stage gonad?
Provides environment for gametogenesis to occur
What embryonic tissues does the vagina have contributions from?
The endoderm and mesoderm
What does the internal genitalia of the male consist of?
- Testis
- Duct system
- Seminal vesicles
- Prostate gland
- Bulbo-urethral glands
What does the duct system consist of in the male?
- Epididymis
- Vas deferens
- Urethra
What is the purpose of the epididymis?
Main storage of sperm
What does the external genitalia of the male consist of?
- Penis
- Scrotum
What does the internal genitalia of the female consist of?
- Ovaries
- Duct system
What does the duct system of the female consist of?
- Fallopian tube
- Uterus
- Cervix
- Vagina
What does the external genitalia of the female consist of?
- Vagina
- Vestibule
- Labia minora
- Labia majora
- Clitoris
Why is development of the reproductive tracts complex?
Because at some point, you have to choose a path
What is the path chosen by development of the reproductive tract regulated by?
The karyotype of embyro (XX or XY)
What does the karyotype of the embryo (XX or XY) do?
Sets chain of events that leads to development of specific genitalia
What does the indifferent stage of the gonad encompass?
Development of the gonad and duct system
Where does structural development of the reproductive system occur?
In utero
What does functional develoment and maturation of the reproductive system occur?
After birth
Describe the reproductive system in babies and childhood
- Human babies are born physically immature
- Childhood is a period of complete immaturity, with no sexual reproduction possible
When does sexual maturation and puberty occur?
In adolesence
What develops in sexual maturation and puberty?
Systems are activated that cause secondary sexual characteristics to develop
What is the purpose of secondary sexual characteristics?
- Allows function of system
- Behavioural characteristics
What are the male sexual secondary characteristics?
- Larger body size
- Body composition and fat distribution
- Hair and skin
- Facial hair, male pattern baldness
- Central nervous effects
- Skin
What are the female sexual secondary characteristics?
- Smaller body size
- Subcutaneous fat distribution
- Hair and skin
- Breast development
- Central nervous effects
What does the secondary sexual characteristic of subcutaneous fat distribution relate to?
Ability to support pregnancy
What is the purpose of the secondary sexual characteristic of breast development?
For lactation
What must be true for a genetic male to be created?
The male gamete (from the father) must carry the Y chromosome, therefore an XY male conceptus, thus primordial germ cells carry Y chromosomes
What genes drive the development of a genetic male?
SRY genes
What does the gonad become in the male?
Testis
What does the internal genitalia develop into in the male?
The male duct system
What is the develoment of the external genitalia of the male under the influence of?
Testistoride hormones
What must happen for a genetic female to be produced?
The male gamete (father) must carry the X chromosome, therefore XX female conceptus, therefore primordial germ cells do not carry the Y chromosome
What leads to the development of a female?
Absence of SRY chromsomes (and other factors)
What is the gonad in the female?
The ovary
What does the internal genitalia develop into in the female?
The female duct system, including uterine tubes and uterus
In what gender do the mesonephric ducts and paramesonephric ducts develop in?
Both male and female
What are the mesonephric ducts?
The ducts of the embryonic kidney
Where do the mesonephric and paramesonephric ducts end?
At the cloaca
What is the cloaca?
A single structure that the GI, urinary, and reproductive tract end at early in development
How do paramesonephric ducts appear?
As invaginations of the epithelium of the urogenital ridge
What do the paramesonephric ducts do caudally?
Make contact with the cloaca
What do the paramesonephric ducts do cranially?
Opens into abdominal cavity
What do the paramesonephric ducts do in the abdominal cavity
Grow into the abdominal cavity, try to meet each other in the midline
What happens when the paramesonephric ducts try to meet in the midline of the abdominal cavity?
To start with, there is a septum, but this disappears and ducts fuse
What ultimately happens to the mesonephric and paramesonephric ducts?
What happens to them depends on the effect of the gondal hormones
What effect to the gondal hormones from the testis have on the mesonephric and paramesonephric ducts?
- Androgen secretion supports mesonephric duct
- This is a positive act to maintain the duct
- Testis secrete Mullerian Inhibiting Substance, which causes the degeneration of the paramesonephric duct
What can be said of androgen secretion maintaining the mesonephric duct?
It is a positive act to maintain the duct
What does the mesonephric duct develop into?
- Vas deferens
- Epididymis
- Seminal vesicles
Why does the testis need to secrete Mullerian Inhibiting Substance?
Because you need to actively suppress the paramesonephric ducts, as its natural program is to develop
What is Mullerian Inhibiting Substance secreted from?
Sertioli cells
What happens to the mesonephric and paramesonephric ducts in the female?
- No androgen means mesonephric duct degenerates
- No testis derived Mullerian Inhibitng Substance means paramesonephric duct develops
What does the paramesonephric duct develop into?
- Fallopian tubes
- Cervix
- Uterus
- Upper 1/3 of vagina
When are the basic components of the external genitalia derived?
In the indifferent stage
What are the basic components of the external genitalia?
- Genital tubercle
- Genital folds
- Genital swellings
What do the genital folds surround?
The urogenital sinus
What do the genital folds become in males?
The urogenital sinus
What do the genital folds become in females?
Labia minora
What do the genital swellings become in males?
Scrotum
What do the genital swellings become in females?
Labia majora
What happens to the genital tubercle in males?
The GT elongates and genital folds fuse to form spongy urethra
What is the elongation of the genital folds in the male under the influence of?
Testis-derived androgen hormones, including dihydrotestosterone
What must be true for the genital tubercles to respond to dihydrotestosterone?
They must have working receptors
What happens to the genital tubercle in the female?
No fusion occurs, and the urethra opens into the vestible
Where do the testis arise in males?
In the upper lumbar region
How are the testis secured in the upper lumbar regions?
They are tethered to the labioscrotal folds (future scrotum) by the gubernaculum
What happens to the testis as the body grows?
The relative position of the testis becomes more caudal
What happens to the scrotum as it develops?
A musculo-fasical layer evaginates the scrotum, together with the peritoneal membrane
What is formed by the invagination of the scrotum with a musculo-fascial layer and the peritoneal membrane?
Processus vaginalis
What happens to the testis between 25 and 28 weeks gestation?
The testis migrate over the pubic bone, behind the processus vaginalis
When do the testis reach the scrotum?
Between 34-40 weeks
What happens above the testis once they have reached the scrotum?
The fascia and peritoneum become closely apposed
What is the spermatic cord formed by?
- Fascial layers
- Obliterated stem of processus vaginalis
- Vas deferens
- Testicular vessels and nerves
Where does the spermatic cord occupy?
The inguinal canal
What is the scrotal ligament?
The vestigial remnant of gubernaculum in males
Where does the ovary originate?
On the posterior abdominal wall
What is the ovary attached to?
The labio-scrotal folds, inferiorly
What attaches the ovary to the labioscorotal folds?
The gubernaculum
What happens to the ovary after it has originated on the posterior abdominal wall?
It descends into the pelvis
What does the gubernaculum become in females?
- Ovarian ligament
- Round ligament of uterus
What does the ovarian ligament do?
Connects ovary to uterus
Where does the round ligament of the uterus run?
In the inguinal canal, the only structure occuping the inguinal canal in females
What does the round ligament of the uterus do?
Connects uterus to labia
What are the common abnormalities in genital development?
- Genotype-phenotype mismatches
- Structural defects
What can happen if external genitalia are ambiguous at birth?
Investigations can be undertaken
What investigations can be undertaken if external genitalia are ambigous at birth?
- Investigate internal genitalia
- Test for sex hormones
- Genetic testing using cells from cheek
- Karyotyping
- Biopsy of glands
How can internal genitalia be investigated?
- Endoscopy
- Abdominal X-ray
- Pelvic ultrasound
What sex hormones can be tested for?
- Androgens
- Oestrogens
Give 3 genital structural defects?
- Clocal partioning defects
- Hypospodias
- Uterine structural defects
What structures produce reproductive hormones?
- Hypothalamus
- Posterior pituitary
- Anterior pituitary
Gonads (ovaries and testes) - Placenta
What reproductive hormones does the hypothalamus produce?
- Peptides releasing factor
- GnRH (gonadotrophin releasing hormone)
- PRH (prolactin releasing hormone)
- PIH (prolactin inhibiting hormone)
What reproductive hormones does the posterior pituitary release?
Posterior pituitary hormone
What does posterior pituitary hormone do?
Produces oxytocin
How does posterior pituitary hormone produce oxytocin?
Through neural control from hypothalamus
What type of reproductive hormones does the anterior pituitary gland produce?
Gonadotrophins
What gonadotrophins does the anterior pituitary gland produce?
- FSH (follicle stimulating hormone)
- LH (lutenizing hormone)
What reproductive hormones to the gonads produce?
- Gondal steroids
- Inhibin
What are the gonadal steriods in the male?
Testosterone
What are the gondal steroids in the female?
- Oestrogens
- Progesterone
What are the different types of oestrogens?
- Principally oestradiol
- Also oestrone and oestriol
What reproductive hormones does the placenta produce?
- Human chorionic gonadotrophin (hCG)
- Human placental lactogen (hPL)
- Oestrogens
- Progesterone
