Embryogenesis and Sexual Differentiation Flashcards
differentiation
Process by which a primitive group of unspecialized cells
develop a functional and specialized group of cells that provide a common function
how many germ layers does differentiation involve
3
Ectoderm, mesoderm, endoderm
embryonic tissue
forms all adult tissues and organs
Ectoderm
forms exterior tissues
- skin, hair, sweat glands
- mammary glands
- hypothalamus. anterior/posterior pituitary
- part of the reproductive tract (male and female)
- vestible/outer vagina
mesoderm
forms structural tissue
- Muscle, skeletal system, blood vessels
- Reproductive system
a. gonads, uterus, cervix, part of vagina, accessory sex glands - Renal system (urinary system)
- Skeletal system
endoderm (nothing to do w/ reproductive tract)
form internal organs
- digestive system, liver and lungs
- majority of glands
genetic differentiation
- An individual’s sex is genetically determined by the presence of a Y chromosome
- Genetic differentiation takes place at fertilization when a sperm delivers either an X
(female) or Y (male) chromosome to the oocyte - SRY gene: Sex determination gene located on the Y chromosome
i. Causes the undifferentiated gonad to develop into the testis - SRY gene controls the expression of Testis Determining Factor (TDF), which is
secreted by the sex cords. - TDF controls the pathway towards either male or female development.
SRY gene
- sex determination gene located on the Y chromosome
- controls the expression of Testis Determining Factor (TDR), which is secreted by the sex cords
gonadal differentiation
- Development of primordial germ cells in the yolk sac (first 15% of gestation)
- Migration of primordial germ cells from the yolk sac into the genital ridge
- Genital ridge gives rise to undifferentiated/bipotential gonad
- Genital ridge with stimulation of sex cords give rise to renal system
- Development of the urinary system
- Development reproductive tract
urinary system
pronephros
mesonephros
metanephros
pronephros
primitive kidney
mesonephros
closely associated with the undifferentiated gonad
metanephros
becomes the functional kidney
reproductive tract
mesonepheric ducts
paramesonepheric ducts
REMEMBER! BOTH the mesonephric and paramesonephric ducts are present at the
SAME TIME, called the SEXUALLY INDIFFERENT STAGE. The undifferentiated gonads
thus need a signal to differentiate into either female or male gonads.
mesonepheric ducts
Wolffian Ducts; MALE reproductive tract
efferent ducts, epididymis, vas deferens
paramesonepheric ducts
Müllerian Ducts; FEMALE reproductive tract
oviducts, uterus, cervix, vagina
Male Gonadal Differentiation
I. Presence of Y chromosome/ SRY gene
II. Presence of Testis Determining Factor (TDF)
III. Development of undifferentiated gonad to testes and Sertoli cells
IV. Secretion of Anti-Müllerian Hormone (AMH) by sertoli cells
V. Degeneration of Müllerian ducts (paramesonephric) and development of Wolffian ducts
(mesonephric)
VI. Differentiation of interstitial Leydig cells
VII. Secretion of testosterone from Leydig cells and development of male reproductive duct
system
Female Gonadal Differentiation
I. No Y chromosome/ Absence of SRY gene II. No Testis Determining Factor (TDF) III. Development of ovaries IV. No Sertoli cells V. Absence of Anti-Müllerian Hormone (AMH) VI. VII. Regression of Wolffian ducts and differentiation of Müllerian ducts VIII. IX. Development of female reproductive duct system
Hypothalamic/Brain differentiation
Pre-knowledge: The hypothalamic GnRH surge center is necessary for initiation of the estrous cycle
and follicular ovulation in the female
male hypothalamic differentiation (defeminization of the brain)
Pre-knowledge: Hypothalamic GnRH surge center is necessary for initiation of the estrous cycle and
follicular ovulation in the female
I. Testosterone from the fetal testis crosses the blood-brain barrier and reaches the brain
II. Testosterone is converted to estradiol by aromatase enzyme in the hypothalamus
III. Regression of the hypothalamic GnRH surge center by estradiol
IV. Defeminization of the hypothalamus (no surge center)
female hypothalmic differentiation (feminization of the brain)
I. Estradiol from fetal ovaries binds with a protein called alpha-fetoprotein (produced by the liver)
II. Alpha-fetoprotein prevents estradiol from crossing blood brain barrier
III. Estradiol cannot affect the hypothalamic surge center
IV. Surge center retained in females
three phases of testicular descent
Transabdominal: 1. Growth and elongation of the body away from the stationary testes.
Inguinal-Scrotal: 2. Rapid growth of the distal gubernaculum
a. Rapid growth results in the testes being pulled from the region of the tenth
thoracic vertebra to the inguinal ring
Inguinal-Scrotal: 3. Shrinkage of the gubernaculum within the scrotum pulls the testes through the
inguinal ring.
a. Regression continues and gubernaculum situates the testes within the
scrotum.
growth and regression of the gubernaculum (descent of testis)
- Testosterone (from Leydig cells) and insulin like-3 (Insl-3) (also called descendin)
synthesized by the fetal pancreas needed for gubernacular growth - Intra-abdominal pressure may play a larger role in the movement of the testes out of
the abdomen and into the scrotum
what happens when the testis fail to descend?
- Cryptorchidism
- Hormone production is NOT decreased in cryptorchid males
a. Will exhibit secondary sex characteristics and normal reproductive behavior - Cryptorchidism is heritable-therefore cull
a. It is possible to surgically or with pharmaceuticals lower the retained testis;
however, it is most likely that fertility will be compromised - Descent of testes from the body cavity into the scrotum occurs by:
a. mid-gestation (bull and ram)
b. last quarter of gestation (boar)
c. just before birth/ at birth (stallion)
Cryptorchidism
Undescended testes or testis (“Crypt” = hidden “Orchid” = testis)
bilateral cryptorchidism
both testis retained in the body cavity: male is sterile
-increases temperature, decreases sperm (TEMP REGULATION)
unilateral cryptorchidism
one testis retained: male is fertile-sub-fertile
why are bilateral cryptorchids sterile?
Temperature regulation: The temperature requirements for normal spermatogenesis is specific. If you increase the temperature of the scrotum and the testes cannot lower to allow for cooling (thermoregulation) spermatogenesis is impaired (increased # of abnormal sperm=decreased fertility)
freemartinism: (“free” = sterile “martin” = bovine)
Common blood supply between twins during gestation
Testosterone and AMH from male fetus influences female fetus
Paramesonephric ducts in female do not develop correctly
Results in canalization and a “blind” reproductive tract in the female
Ovaries do not develop properly and do not secrete estradiol secrete testosterone instead
Often see male-like behavior in freemartin heifers
why keep freemartin heifers in herd?
Normally we should not, but it may useful for detecting estrus in other females due to malelike sexual behaviors!
3 parts of sexual differentiation
genetic
gonadal
brain
oocyte
always have x chromosome (haploid- n)
sperm
x or y (haploid- n)
oocyte+sperm
becomes diploid (2n)
xx
female
xy
male
absence of SRY and TDN
female
TDF
stimulates male development
GnRH
Gonadotropin Releasing Hormone
no surge center
has to do with blood-brain barrier
estradiol is responsible for
regression of GnRH surge center
recombination of sex chromosomes at fertilization
1) chromosomal sex (genotype)
2) Hermaphrodite
in females one of the
x chromosomes is deactivated
appears as the barr body in somatic cells
abnormal
aneuploidy
XXY
Klinefelter’s syndrome
sterile: testicular hypoplasia
XO
Turners syndrome
sterile: inactive ovaries
true hermaphrodite
contain combination of ovaries and testis
male pseudohermaphrodite
has testis but has female external genetalia
lack androgen receptor for sexual development
female pseudohermaphrodite
- has incomplete ovaries, masculinized external genetalia
- male hormone pass into female in twin pregnancy
- – exchange of blood cells from male to female, stimulates male duct development
