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HD exam 2 summary
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Resume of lectures and lecture notes I took (it was primed and reviewed before the exam). I passed the course with a final mark of 8.
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Body cavitiesFolding embryo:
The folding from head to toe (cephalocaudal) will make sure the heart gets in place as well the
cavities surge. This folding, also lateral, gives rise to the amniotic sac that will enclose the embryo. At
this point, embryo turns from lamina to an entity. The yolk sack stops growing (not like in the chicken,
that continues forming🡪 cloaca) and then is invaginated and the intra embryonic body cavity is
formed.
● At week 2, the heart already started to develop and is located ventrally
● From week 3 the septum transversum (thick mass of cranial mesenchyme) starts to form.
This septum is initially ranial to the pericardial cavity, finally is wedged between the heart and
the neck of the yolk sac. It will give rise to the diaphragm cavity and the ventral mesentery
(abdominal cavity) from the 5th to about 6th/7th week. It grows dorsally from ventrolateral
wall. It divides then, them bottom and upper to-be cavities.
● Once the heart appears, the both pleuro-pericardial folds starts to join themselves together,
thus forming the cavity lining the heart (the pericardium cavity) and its membrane.
● When the lungs start to grow in the pericardioperitonial canal, they grow ventrally and
laterally. They grow at the same time that the peritoneal cavity is being formed.
● First the pericardial cavity forms and then the pleural (lungs) 🡪 5th to 6th week
● Heart is envolved at this time by pericardial sac
● Diaphragm is created
Sex determination
Gonads, at first instance, are bipotential. The embryo develops two ducts at the beginning of 7th
week - close to their urinary system - that are called Mullerian duct and Wolffian duct. One of this
duct is either degraded by itself without the presence of signaling or induced to be destroyed by a
signaling. The wolffian duct is directly attached to the primordial kidney. The external genitalia is
constituted by the tuberculum genitalis, cloacal fold and cloacal membrane, which will persist
undifferentiated until the 7th week. Bellow you see in each sexes these processes take place and how:
⮚ Male
● When the fertilized egg contains the Y chromosome, inside its short arm, next to its p
arm, there is a region called SRY (also TDF) which will stimulate a localized gonadal
(primitive testis) generation of Sertoli and Leydig cells, from the 8th week. The Leydig
cells will produce testosterone that guarantees the remaining of Wolffian ducts, also
mesonephric ducts (internal male structures: vas defends, epididymis, ejaculatory
ducts and seminal vesicles) and its conversion to DHT (external male characteristics:
penis, urethra, testis and prostate)
● Primordial germ cells migrate from the yolk sac to gonadal ridge. At week 7, male
characteristic start to gradually be visible in this region. This is stimulated altogether by
the combination of SRY and autosomal SOX9 which then (FGF9)
● Testosterone is also slightly converted into estradiol and brought to the brain to make
the process of masculinization of the same (ex.: some structures may be larger than
others, like the amygdala or hippocampus, or even have different /variate receptors)
● At week 10, scrotum an penis start to develop (external carachteristics)
,*cascades: SRY 🡪 SOX9 🡪 (promoter region of)MIH: Mullerian inhibiting hormone
SRY 🡪 SOX9 🡪 FGF9: testes formation
NOTE: the seminiferous cords have no light! They will only be called seminiferous tubules at puberty
when they are channeled! Gonocytes will also remain quiescent until puberty.
- seminiferous cords connect to the testicular network and this, in turn, connects to the efferent ducts
(derived from the mesonephric tubules)
- the gonad takes on an oval shape
- the white tunic appears, separating the seminiferous cords from the celomic coating epithelium
Differentiation of the attached glands
- the bulbourethral glands and the prostate are derived from the urethra: there are evaginations from
the urethra to the surrounding mesenchyma to form such glands. The endoderm of the urethra forms
the inner lining of the glands and the lateral splanchnic mesoderm forms its muscles and connective
tissues
- seminal vesicles originate from the mesonephric (Wolffian) duct
- the genital tubercle turns into a phallus
- the urethral plaque grows and stimulates the development of the phallus, which becomes the penis
- the urogenital folds merge in the midline, involving an orifice in the center (orifice of the penile
urethra)
- the lip-scrotum protrusions grow and merge to form the scrotum
Heads up!!! The urethra inside the penis initially opens in the ventral region. But the opening of the
urethral orifice must be at the end! As the distal end of the urethra has a different origin than the rest
(ectodermal origin), a plaque is formed that channels, forming an urethra at the ectodermal end,
which merges with the endodermal urethra. Thus, the opening of the urethra happens to be at the
extremity
Descent of the gonads
- gonads form in the thoracic region (gonadal ridge)
- go down to the scrotum
- its descending process takes place through a ligament called the gubernacle, which attaches the
lower pole of the gonad to the body wall
- the gubernacle shortens and pulls the gonad down
- in the 3rd month of pregnancy, the testicles reach the region of the inguinal canal, then the descent
is interrupted
- the final descent into the scrotum only happens at the end of pregnancy
- if the testicles do not descend into the scrotum (cryptorchidism) until the age of two, it is necessary
to lower them surgically, otherwise the death of germ cells and consequent impairment of the
individual's fertility will occur
- this descent of the testicles is partially controlled by hormones, among them testosterone
⮚ Female
● Without the presence of testosterone, the wolffian ducts will spontaneously
degenerate, and without MIH the Mullerian ducts will develop to internal female
structures (fallopian tubes, ovaries, uterus, upper vagina).
● External genitalia also develops.
● In females, the estradiol product are avoided to reach the brain, but in contrast, it
makes female characteristics when directed to other regions of the body.
Differentiation of the gonads
, - as the cells of the primary sex cords do not have a Y chromosome (karyotype XX), there is no
production of a determinant factor in the testis
- primary sex cords break (are destroyed) and germ cells accumulate on the periphery of the gonad
(germ cells do not die)
- a new series of cords appears (from the celomic epithelium) (secondary or cortical sexual cords)
- the secondary sex cords (which are much smaller than the primary ones) grow, surround the germ
cells and break, so that each germ cell is isolated in a follicle
- the supporting cells (follicular cells) will produce factors that stimulate the differentiation of
gonocytes (primordial germ cells).
- the gonocytes divide and form the egg cells, which initiate meiosis and interrupt it in prophase I,
forming the primary oocytes. These primary oocytes are surrounded by follicular cells, constituting
primordial follicles
- the gonad medullary region (very vascularized) is formed by the differentiation of a filling connective
tissue
- the interstice between the follicles is formed by the ovarian stroma, which derives from the genital
crest
Duct differentiation
- as there is no production of the anti-Müllerian hormone, the paramesonephric duct (or MULLERIAN
DUCT) remains and differs in uterine tubes and in the uterus-vaginal primordium (fused part), which
will give rise to the uterus and the upper part of the vagina
Formation of the vagina
- the vagina is an organ with double formation: the upper part comes from the uterus-vaginal
primordium and the lower part comes from the vaginal plaque, which originates in the urogenital
sinus (therefore, it derives from the endoderm)
NOTE: The entire lining of the vagina is derived from the endoderm
- the uterus-vaginal primary touches the urogenital sinus
- a thickening forms in the urogenital sinus (sinus tubercle)
- the sinus tubercle emits projections (sino-vaginal bulbs), which merge to form the vaginal plaque,
which is channeled
Heads up! The lower portion of the vagina would open up in the urethra and this cannot happen!
Therefore, the lower portion of the vagina is displaced inferiorly until its opening separates from the
urethral opening.
Differentiation of external genitalia
- having no hormones to act on the external genitalia, it differs very little
- urethral protrusions do not grow and do not merge, persist to form the small lips
- the lip-scrotal protrusions do not grow and do not merge; will form the big lips
- the genital tubercle involutes (decreases in size) and forms the glans of the clitoris
NOTE: A tumor in the adrenal gland causes growth of folds and the formation of an ambiguous
genitalia
Descent of the gonads (less pronounced in females)
- the ovaries descend to the pelvis
- its descent occurs through the gubernaculum ligament, which connects the ovary to the
paramesonephric duct (MULLERIAN DUCT)
- the gubernaculum is divided into two parts: ovarian ligament (connects ovary to uterus) and round
uterine ligament (connects uterus to large labia)
● - the gubernaculum decreases in size and pulls the ovaries to the pelvic region
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