From StudyingMed

< BGDB‎ | Pracs
Jump to: navigation, search
  • http://php.med.unsw.edu.au/embryology/index.php?title=BGDB_Practical_-_Sexual_Differentiation
  • Story of three phases: 1) Gonad differentiation 2) Internal genital tract 3) External genitalia. 2) and 3) are completely dependent on 1) -- if that hasn't occurred you'll have problems with 1) and 2).
  • Hasn't completed its development until puberty
  • Females have puberty before males (by a couple of years)
  • Often described as the urogenital system. Today we'll talk a little bit about the urinary system as well.
  • The absolute days aren't needed, but the order is important
  • 24 days - intermediate mesoderm, pronephros primordium
    • Important for formation of the kidney
  • 28 days - mesonephros and mesonephric duct
  • 35 days - uteric bud, metanephros, urogenital ridge
  • 42 days - cloacal divison, gonadal primordium (indifferent)
  • 49 days - paramesonephric duct, gonadal differentiation
  • 56 days - paramesonephric duct fusion (female)
  • 100 days - primary follicles (ovary)

Sex determination

Sex determination (male/female) at the biological level is determined by the presence or absence of the Y chromosome. Initially, we did not know what this factor was and it was designated the "testis determining factor" (TDF). We now know (since 1990) that TDF is the protein product encoded by the SRY gene on the Y chromosome. Without this gene/protein the potential sex is female (see Male below).

  • If you don't get expression of TDF from SRY locus then you'll get expression as female (genotype differs from phenotype)
  • SRY results in the differentiation of male PENOtype

For some time, female was considered the "default" sex in the absence of SRY, we now know this is not the case, with several genes specifically required for ovary formation. In females, sex determination involves at least one X chromosome gene, DAX1 encoding a nuclear hormone receptor. Another critical genetic issue is related to the presence of two X chromosomes, "gene dosage", and in the case of mammals this is regulated by inactivating one of those X chromosomes in each and every cell (see Female below).

  • Primordial germ cells are not important in sexual differentiation -- that is the role of tissues that respond to SRY
  • The "default" is not a female - there are additional X-chromosome proteins that must be expressed for a female to be generated
  • There a whole lot of really important genes on the X-chromosome other than just sex-related (e.g. causing Turner's syndrome, Fragile X, colour blindness, Duchenne's dystrophy)
  • For females, one of the X chromosomes needs to be inactivated (to prevent excess genetic material)
    • We have maternal and paternal X chromosome and there is random activation of one or the other throughout the embryo
  • The inactive one is packed up by "XIST", and is packaged to the edge of the nucleus, and has no gene expression. Can identify this as a small projection at the periphery of the nucleus of a white blood cell in females (Barr Body).

Primordial germ cells

  • Are the very first cells that migrate through the primitive streak during gastrulation
  • Junctional region between the yolk sac and hindgut is where the primordial germ cells are
  • They sit around and wait for the genital ridge to form (much like stem cells wait for bone marrow)
  • When the intermediate mesoderm begins to differentiate in the 4th-5th week, they begin to migrate into the genital ridge
  • Mouse gonad at early stage of development - green cells = primordial germ cells that migrated in, and the red cells around it are the Sertoli cells, supporting it. These Sertoli cells express SRY, which result in the differentiation of the male gonad.
  • There will be a third population of cells outside of this are the Leydig (interstitial cells) produce testosterone, and are involved in the later differentiation of the internal and external genital tract

Early embryo

  • Begin to see gonadal differentiation
  • Second image - Stage 13 embryo (end of 4th week of development). By this stage, the intermediate mesoderm has undergone key stages of development (bilateral yellow structures, running down the entire length of the embryo). On the ventral surface of these is the mesonephric duct. This yellow structure is the mesonephros (a transient structure). This blue duct fuses to the cloaca. It will persist in males and mostly lost in females.
  • Later in development (not visible in 5th week), the paramesonephric ducts (Mullerian; medial at one point, lateral at another) will form parallel to the mesonephric duct (Wolffian)
  • We see swellings running down the side where the mesonephros is located
  • In the neck region an earlier kidney formed in the intermediate mesoderm; called the pronephros. Then a duct begins up there and grows down the IM. Early in the 4th week, the pronephros degenerates and is loss. Later, a further mesonephros forms, and extends down the length of the embryo, fusing with the cloaca at the time where the cloaca is being divided into two halves. It opens at the cloacal space.
  • The mesonephros will degenerate and be lost
  • Down the very bottom, the mesonephric duct gives off a little but at the level of the hindlimbs, called the uteric bud (lateral buds growing into the intermediate mesoderm). This is the beginning of the drainage system for the adult kidney, which is forming down at the bottom of the embryo (metanephros). There is an induction between how the buds come off and grow into the intermediate mesoderm. These uteric buds will become the ureters that run to the urinary bladder
  • Hindgut: at the beginning of week 4, we have a single cavity ending at the cloacal membrane. During 4th-5th week, a septum grows down through the cloacal cavity, dividing it into an anterior urogenital and dorsal rectal part. It comes down and also from the lateral walls, growing down and bifurcating the cloaca. It will fuse with the cloacal membrane before it grows down. Then we have a rectum, a bladder, and an allantois which extends out into the umbilical cord.

Late embryo

  • Begin to see internal genital tract differentiation
  • Start at section 29 and down (just cutting through the forelimb bud). As we go up in numbers we're going down the embryo
  • Note: Mesonephric duct, mesonephric tubules and genital ridge
    • Mesonephros (on either side), directly between them is the dorsal aorta. Each mesonephros has its own duct which runs down the length of the embryo bilaterally.
    • Mesonephros is retroperitoneal
  • Mesonephric duct = dark epithelial wall, and is round, has blood vessels associated
  • Mesonephros forms primitive glomerular structures (not a fully functioning kidney)
    • Neither the mesonephros and pronephros actually form anything in the adult
  • Hindgut, note portal vein, mesenteric artery
  • Note the mesentery of the midgut (dorsal mesogastrium)
  • Either side of the hindgut down the bottom is the thickened urogenital ridge
    • Germ cells migrate in along the mesentery to the hindgut and populate the urogenital ridge
    • Note the thickening of the epithelium, which proliferates to populate the urogenital ridge. Chemotactic factors attract germ cells to the genital ridge. Can lose their way in abnormalities -- source of tumours (germ cell tumours). Can differentiate into a whole lot of different structures (teratoma)
    • About 1000 primordial germ cells migrate into genital ridges. These populate the genital ridge
    • Genital ridge = thickened epithelial stuff posteromedially to the mesonephric tubules, between the mesonephric duct and the hindgut
  • Urogenital sinus (cloaca) is pinched into a posterior rectal compartment and anterior urogenital region. This is septation of the hindgut into these 2 bits. Can see the ureters opening into this in some of the sections
  • Next 2 animations show the genital ridge differentiating from week 4 onwards, in both male and female.
  • Male
    • Peritoneal cavity is to the right.
    • Fingers are the primary sex cords (extending from a proliferation of the mesothelium)
    • Primordial germ cells have been attracted, and come down and populate the cords
    • Purple duct is the mesonephric duct
    • From week 5 onwards, the duct beside it (in orange) is the paramesonephric duct
    • If SRY is expressed in the Sertoli cells, it will differentiate the cells beside it into testes. The Leydig cells secrete testosterone and result in differentiation of the mesonephric duct (rete testes and seminiferous tubule, ductus deferens). The Sertoli cells also secrete the Mullerian Inhibitory Factor (MIF) to make the Mullerian duct regress. The testosterone will maintain the mesonephric (Wolffian duct). The entire internal male genital tract comes from the Wolffian duct. We lose the Mullerian ducts in male
  • Females is the same, with germ cells in sex cords
    • But lack of SRY stops differentiation into Sertoli cells, so they make follicular cells
    • Because the Leydig cells are not secreting testosterone, the Wolffian duct regresses
    • Because the sustentacular (Sertoli/follicular) cells are not secreting MIF, the Mullerian duct regresses (forms the Fallopian Tubes and Uterus; vagina has a subtley different origin)
    • Medullary differentiation of seminiferous tubules (end up in centre of testes), and cortical differentiation of the follicles in the ovary (they go around the periphery)
  • Sertoli/follicular cells = support cells = sustentacular cells. These form from the epithelium/mesothelium lining the peritoneal cavity.
  • In the ovary, through foetal period/late embryo, the primordial germ cells proliferate to form a large number of edge (2.5 million primordial oocytes/follicles to populate the cortical region of the ovary in the newborn). All the primordial follicles will form prenatally, and after birth there will be a significant loss of these.
  • In the male, the primordial germ cells in the seminiferous tubules, until puberty. At puberty, the spermatogonia (diploid cells from the primordial germ cells) that populate the periphery of the seminiferous tubule begin to divide by mitosis. They undergo meiosis at puberty. The seminiferous tubule is not mature during childhood at all.
  • Image shows the embryo in week 8 male, showing the urogenital region, colour coded
    • Dark red = adrenal
    • Orange = metanephros (kidney)
    • Green = gonad (testes)
  • Note that both the adrenals and testes are larger than the kidney at this stage (the kidney will grow throughout the foetal period).
  • The gonads will descend and the kidneys will ascend (they separate)
  • Kidneys have ureters, which drain into the posterior wall of the urinary bladder (orange). Superior end of bladder goes to the allantois. The inferior urinary bladder goes down to the urinary sinus (cloaca) at the bottom of the embryo
  • Embryo cross sections, week 8
    • Level of liver -- occupies whole anterior part of the embryo's body
    • Large structure bilaterally is adrenal gland. Will be replaced by the adult adrenal gland in the foetal period
    • Stomach
    • At E9, we see still the adrenal gland on the left anteriorly, but sitting beneath it posteriorly on the left is the kidney (metanephros)
    • Sections beneath it cut through the kidney (retroperitoneal), and appearing beneath the kidney is the gonad
    • Developing gonad is attached to the kidney by the mesorchium, a little mesentery (early foetal period)
    • Down in the pelvis region (G1), we see limb buds, and allantois (or superior end of urinary bladder in the midline), with gonad sitting next to it.
    • G2: urinary bladder, mesonephric ducts and the rectum
    • High res: Note vertebral body, rectum, sex cords (in testes), mesorchium, mesonephric duct and paramesonephric duct (still here in week 8 in the male embryo)
  • In the ovary, through foetal period/late embryo, the primordial germ cells proliferate to form a large number of edge (2.5 million primordial oocytes/follicles to populate the cortical region of the ovary in the newborn). All the primordial follicles will form prenatally, and after birth there will be a significant loss of these.
  • In the male, the primordial germ cells in the seminiferous tubules, until puberty. At puberty, the spermatogonia (diploid cells from the primordial germ cells) that populate the periphery of the seminiferous tubule begin to divide by mitosis. They undergo meiosis at puberty. The seminiferous tubule is not mature during childhood at all.
  • Image shows the embryo in week 8 male, showing the urogenital region, colour coded
    • Dark red = adrenal
    • Orange = metanephros (kidney)
    • Green = gonad (testes)
  • Note that both the adrenals and testes are larger than the kidney at this stage (the kidney will grow throughout the foetal period).
  • The gonads will descend and the kidneys will ascend (they separate)
  • Kidneys have ureters, which drain into the posterior wall of the urinary bladder (orange). Superior end of bladder goes to the allantois. The inferior urinary bladder goes down to the urinary sinus (cloaca) at the bottom of the embryo
  • Embryo cross sections, week 8
    • Level of liver -- occupies whole anterior part of the embryo's body
    • Large structure bilaterally is adrenal gland. Will be replaced by the adult adrenal gland in the foetal period
    • Stomach
    • At E9, we see still the adrenal gland on the left anteriorly, but sitting beneath it posteriorly on the left is the kidney (metanephros)
    • Sections beneath it cut through the kidney (retroperitoneal), and appearing beneath the kidney is the gonad
    • Developing gonad is attached to the kidney by the mesorchium, a little mesentery (early foetal period)
    • Down in the pelvis region (G1), we see limb buds, and allantois (or superior end of urinary bladder in the midline), with gonad sitting next to it.
    • G2: urinary bladder, mesonephric ducts and the rectum
    • In the "early foetus" section, we'll see the development of the internal genital tract in the female
  • Mesonephric ducts are lateral to the paramesonephric ducts, which in the female fuse to form the uterus.
    • Both these duct systems lie behind the bladder
  • Need to change where the mesonephric duct is connected into the urinary system. The mesonephric duct fuses at the posterior wall of the urogenital sinus (does so when septation is occurring).

Dorsal view of the bladder: pale structures coming off the dark mesonephric ducts are the uteric bud, which will be inducted into the kidney laterally. The mesonephric duct with the uteric duct are the 3 elements that form the trigone of the bladder.

  • In the male, the mesonephric duct is carried down on the posterior wall of the bladder and opens into the inferior end where the urethra will form, and the prostate will differentiate here (where the ductus deferens enters before the it becomes the penile urethra later).
    • In the female, this structure is lost. The paramesonephric ducts fuse together and to the epithelium of the bladder (?)
  • Week 10 female (planes A-D)
    • 4 sections are highlighted regions of a week 10 female foetus
    • Note the kidney (retroperitoneal in the posterior body wall), renal pelvis, adrenal gland, degenerating mesonephros, ovary, mesonephric duct and paramesonephric duct. Inguinal cone = muscular structure on the anterior body wall
    • Ovary is attached to the posterior body wall
    • At the bottom of the inguinal cone is the round ligament (female only)
    • Note the rectum (hindgut), with the lumen of the urinary bladder and uterus between them
  • In the female foetus we can identify the genital tubercle, extending from the surface (also visible in the week 8 foetus of the male; in the male it forms the glans of the penis, in the female it forms the clitoris). The GT is the only part of the external genitalia at this stage.
  • Paramesonephric ducts are at first lateral, then cross over to become medial (midline). Middle forms uterine body, lateral forms uterine horns (Fallopian tubes)
    • Paired paramesonephric ducts fuse in the midline; don't fuse to the urogenital sinus (just to epithelium). There is an interaction between the origenital sinus and the paramesonephric ducts to form the vagina (has a complex embryonic origin)
    • Junctional region represents the level of the cervix
    • Abnormalities in female genital tract occur if this fusion isn't complete (e.g. bicornate uterus, unicornate uterus, septated uterus)
    • Antrum forms in the urogenital sinus which forms the vagina; and later will open up to the uterus. The external surface of the urogenital remains as the hymen
    • Epithelium of the uterus responds to hormonal changes in the menstrual cycle (proliferative and secretory phases), unlike the vagina
    • The uterus becomes 6cm in circumference with a 2cm lumen

Foetal

  • External genitalia differentiation
  • Male and female begin the same
  • Opening in midline = where the cloacal membrane was (will divide ventrally and posteriorly)
  • In the midline ventrally is the genital tubercle
  • Folds laterally (larger) form the 1) scrotum or 2) outer labia
  • Inner folds either sides of the cloacal membrane are the urogenital folds, which form the 1) shaft of the penis 2)labia minora (inner labia); do not fuse in the midline
    • Major difference in the males is that the urogenital folds fuse in the midline in the male (under the influence of testosterone that is converted into DHT by enzymes in the epithelium -- transformation of the external genitalia)
      • This is the origin of hypospadias (if the genital folds do not fuse correctly; the opening of the urethra can be anywhere along the shaft of the penis, right back to the scrotum)
  • Testes descent only occurs in the 3rd trimester (week 8-38, birth)
    • Descends through the abdominal wall
    • Gubernaculum either aids or guides the descent of the testes through the anterior body wall, descending into the scrotal sac
    • As it descends, it pulls the serosal layer down, and during development, there is the processus vaginalis: the source of abnormalities associated with males.
      • If it doesn't close, you get herniation, particularly in newborn boys (also because the pathway of the descent is weakened)
      • If it closes with fluid, you get hydrocoel
  • Cryptoorchidism = undescended testes

Postnatal

  • There will be other classes about this

Abnormalities

  • 18-19% of all abnormalities are associated with the urogenital system
  • Graph shows you that different parts of the system are affected by teratogens at different times
  • Cryptoorchidism can either be true (if on the pathway where it should descend) or ectopic (if the testes lose its way and descends down a different pathway e.g. femoral, scrotal or pre-penile region)
    • Occurs in up to 30% of preterm infants and 4% of term infants
  • Weakness in the anterior wall - can have part of the gut herniating through the pathway of descent. You get part of the intestinal tract in the scrotal sac
  • Hypospadias are classified based on where the urethra opens (A-F)
  • Uterine duplication, unicornate uterus, bicornate uterus, septate uterus, cervical, vaginal absence
  • Most common types of abnormalities associated with the genital system are chromosomal (e.g. Turner's syndrome and SRY-absence etc).

A note on the mesonephric duct

  • Male development

In a male, it develops into a system of connected organs between the efferent ducts of the testis and the prostate, namely the epididymis, the vas deferens, and the seminal vesicle. The prostate forms from the urogenital sinus and the efferent ducts form from the mesonephric tubules. For this it is critical that the ducts are exposed to testosterone during embryogenesis. Testosterone binds to and activates androgen receptor, affecting intracellular signals and modifying the expression of numerous genes. In the mature male, the function of this system is to store and mature sperm, and provide accessory semen fluid.

  • Female development

In the female, in the absence of testosterone support, the Wolffian duct regresses, and inclusions may persist. The epoophoron and Skene's glands may be present. Also, lateral to the wall of the vagina a Gartner's duct or cyst could develop as a remnant.

The derivatives of the Mesonephric duct can be remembered using the mnemonic, "Gardener's SEED" for Gartner's duct, Seminal vesicles, Epididymis, Ejaculatory duct and Ductus deferens.[

  • The gubernaculum forms the vas deferens in males and the round ligament of the uterus in females