Pathways to men and women Flashcards
Which two entirely different components can the urogenital system be divided into?
Functionally, the urogenital system can be divided into two entirely different components: the urinary system and the genital system.
What is the urinary system and the genital system developed from?
Embryologically and anatomically the urinary system and the genital system are developed from the intermediate mesoderm
Name the kidney systems
Pronephos, Mesonephros and Metanephros
Which kidney system is the genital system derived from?
Mesonephros
What happens during the fourth to seventh weeks of development?
The cloaca divides into the urogenital sinus anteriorly and the anal canal posteriorly.
What does the tip of the urorectal septum form?
The perineal body
Which parts can the urogenital sinus be distinguished in?
The upper and largest part is the urinary bladder
- Initially, the bladder is continuous with the allantois, but when the lumen of the allantois is obliterated, a thick fibrous cord, the urachus, remains and connects the apex of the bladder with the umbilicus. In the adult, it forms the median umbilical ligament
The narrow canal, the pelvic part of the urogenital sinus
- In the male it gives rise to the prostatic and membranous parts of the urethra
The phallic part of the urogenital sinus
- Development of the phallic part of the urogenital sinus differs greatly between the two seces
How are the ejaculatory ducts formed?
During differentiation of the cloaca, the caudal portions of the mesonephric ducts are absorbed into the wall of the urinary bladder. Consequently, the ureters, initially outgrowths from the mesonephric ducts, enter the bladder separately. As a result of ascent of the kidneys, the orifices of the ureters move farther cranially; thos of the mesonephric ducts move close together to enter the prostatic urethra and in the male become the ejaculatory ducts.
Which germ layer does the ureters originate from?
Mesoderm
Which germ layer does the ejaculatory ducts originate from?
Mesoderm
Which germ layer does the ejaculatory ducts originate from?
Mesoderm
Which germ layer does the ejaculatory ducts originate from?
Mesoderm
Which germ layer does the ejaculatory ducts originate from?
Mesoderm
What is the inside of the bladder lined with?
Endodermal epithelium
What does the urethra in both sexes originate from?
The epithelium of the urethra in both sexes originates in the endoderm
What is the surrounding connective and smooth muscle tissue of the urethra derived from?
Visceral mesoderm
What is the key to sexual dimorphism?
The key to sexual dimorphism is the Y chromosome, which contains the testis-determining gene called the SRY (sex-determining region on Y) gene on its short arm.
What protein does the SRY gene transcript that determines the fate of rudimentary sexual organs?
The SRY protein is the testis determining factor (TDF); under its influence, male delopment occurs; in its absense, female development is established.
When is the sex of the embryo detemined genetically?
The sex of the embryo is determined genetically at the time of fertilization
When is the sex of the embryo detemined genetically?
The sex of the embryo is determined genetically at the time of fertilization
When do gonads acquire male or female morphological characteristics?
The gonads acquire male or female morphological characteristics in the seventh week of development.
Gonads appear initially as a pair of longitudinal ridges, the genital or gonadal ridges.
They are formed by proliferation of the epithelium and a condensation of underlying mesenchyme.
Which cells have an inductive influence on development of the gonad into ovary or testis?
Primordial germ cells
What happens to the gonads if the embryo is genetically male?
If the embryo is genetically male, the primordial germ cells carry an XY sex chromosome complex. Under influence of the SRY gene on the Y chromosome, which encodes the testis determining factor, the primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis (medullary cords).
Toward the hilum of the gland, the cords break up into a network of tiny cell strands that later give rise to tubulus of the rete testis.
During further development, a dense layer of fibrous connective tissue, the tunica albuginea, seperates the testis cords from the surface epithelium.
In the fourth month, the testis cords become horseshoe-shaped, and their extremities are continuous with those of the rete testis. Testis cords are now composed of primitive germ cells and sustentacular cells of Sertoli derived from the surface epithelium of the gland.
Interstitial cells of Leydig, derived from the original mesenchyme of the gonadal ridge, lie between the testis cords. They begin delopment shortly after onset of differentiation of these cords.
- By the eight week of gestation, Leydig cells begin production of testosterone and the testis is able to influence sexual differentiation of the genital ducts and external genitalia.
Testis cords remain solid until puberty, when they acquire a lumen, thus formning the seminiferous tubules. Once theseminiferous tubules are canalized, they join the rete testis tubulus, which in turn enter the ductuli efferentes. These efferent ductules are the remaining part of the excretory tubules of the mesonephric system. They link the rete testis and the mesonephric or wolffian duct, which becomes the ductus deferens.
What happens to the gonads if the embryo is genetically female?
If the embryo is genetically female, the primordial germ cells carry an XX sex chromosome complex. Primitive sex cords dissociate into irregular cell clusters and these clusters, containing groups of primitive germ cells, occupy the medullary part of the ovary. Later, they disappear and are replaced by a vascular stroma that forms the ovarian medulla.
The surface epithelium of the female gonad, unlike that of the male, continues to proliferate. In the seventh week, it gives rise to a second generation of cords, cortical cords, which penetrate the underlying mesenchyme but remain close to the surface.
- In the third month, these cords split into isolated cell clusters. Cells in these clusters continue to proliferate and begin to surround each oogonium with a layer of epithelial cells called follicular cells. Together, the oogonia and follicular cells constitute a primordial follicle.
Draw the influence of primordial germ cells on indifferent gonad
Which genital ducts does both male and female embryos contain?
Initially, both male and female embryos have two pairs of genital ducts: mesonephric (wolffian) ducts and paramesonephric (müllerian) ducts.
How does the mesonephric (wolffian) ducts and paramesonephric (müllerian) ducts evovle during the indifferent stage of the genital ducts?
The paramesonephric duct fuse to form the uterine canal. The caudal tip of the combined ducts projects into the posterior wall of the urogenital sinus, where it causes a small swelling, the paramesonephric or müllerian tubercle.
The mesonephric ducts open into the urogenital sinus on either side of the müllerian tubercle.
How does autosomal gene SOX9 induce testes differentiation?
SOX9 is known to bind to the promotor region of the gene for antimüllerian hormone (AMH; also called müllerian inhibiting factor (MIF)) and probaby regulates this gene’s expression.
Describe the development of genital ducts in the male
Initially, SRY and/or SOX9 induce the testes to secrete FGF9 that acts as a chemotactic factor that causes tubulus from the mesonephric duct to penetrate the gonadal ridge. Without penetration by these tubules, differentiation of the testes does not continue.
Next, SRY either directly or indirectly (through SOX9) upregulates production of steroidogenesis factor 1 (SF1) that stimulates differentiation of Sertoli and Leydig cells.
- SF1 working with SOX9 elevates the concentration of MIF leading to regression of the paramesonephric (müllerian) ducts.
In Leydig cells, SF1 upregulates the genes for enzymes that synthesize testosterone. Testosterone enters cells of target tissues where it ma yremain intact or be converted to dihydrotestosterone by a 5-alpha reductase enzyme.
Testosterone and dihydrotestosterone bind to specific high-affinity intracellular receptor, and this hormone receptor complex is transported to the nucleus where it binds to DNA to regulate transcription of tissue-specific genes and their protein products.
- Testosterone receptor complexes mediate virilization of the mesonepthric ducts to form the vas deferens, seminal vesicles, efferent ductules and epididymis.
- Dihydrotestosterone receptor complexes modulate differentiation of the male external genetalia.
Describe the development of genital ducts in the female
Estrogens are involved in sexual differentiation and under their infulence, the paramesonephric (müllerian) ducts are stimulated to form the uterine tubes, uterus, cervix amd upper vagina. In addition, estrogens act on the external genitalia at the indifferent stage to form the labia majora, labia minora, clitoris, and lower vagina.
What makes the paramesonephric (müllerian) ducts degenerate?
Under the influence of müllerian inhibiting factor produced by sertoli cells, paramesonephric (müllerian) fucts in the male degenerate except for a small portian at their cranial ends, the appendix testis.
What happens after the paramesonephric ducts reaches the urogenital sinus?
Shortly after two solid evaginations grow out from the pelvic part of the sinus. These evaginations, the sinovaginal bulbs, proliferate and form a solid vaginal plate. Proliferation continues at the cranial end of the plate, increasing the distance between the uterus and the urogenital sinus. By the fifth month, the vaginal outgrowth is entirely canalized. The wing-like expansions of the vagina sround the end of the uterus, the vaginal fornices, are of paramesonephric origin.
Where is the vagina originated from?
The vagina has a dual origin, with the upper portian derived from the uterine canal and the lower portion derived from the urogenital sinus.
Which parts of the mesonephric (wolffian) ducts remains in the developed female genitalia?
Epoophoron, paroophoron and Garner’s cyst
What will lack of fusion of the paramesonephric ducts result in?
In its extreme form, the uterus is entirely double (uterus didelphys); in the least severe form, it is only slightly indented in the middle (uterus arcuatus). One of the relatively commeon abnormalities is the uterus bicornis, in which the uterus has two horns entering a common vagina.
What will complete or partial atresia of the paramesonephric ducts result in?
In patients with complete or partial atresia of one of the paramesonephric ducts, the rudimentary part lies as an appendage to the well-developed side. Since its lumen usually does not communicate with the vagina, complications are common (uterus bicornis unicollis with one rudimentary horn).
If the atresia involves both sides, an atresia of the cervix may result. If the sinovaginal bulbs fail to fuse or do not develop at all, a double vagina or atresia of vagina, rescpectively, results. In the latter case, a small vaginal pouch originating from the paramesonephric ducts ususally surrounds the opening of the cervix.
Describe the external genitalia during the indifferent stage
In the third week of development, mesenchyme cells originating in the region of the primitive streak migrate around the cloacal membrane to form a pair of slightly elevated cloacal folds.
- Cranial to the cloacal membrane, the folds unite to form the genital tubercle.
- Caudally, the folds are subdivided into urethral folds anteriorly and anal folds posteriorly.
In the meantime, another pair of elevation, the genital swellings, becoms visible on each side of the urethral folds.
What does the genital swellings later form in the male?
The genital swellings later form the scrotal swllings in the male
What does the genital swellings later form in the female?
The genital swellings later form the labia majora in the female
What is the development of the male external genitalia characterized by?
Development of the male external genitalia is characterized by rapid elongation of the genital tubercle, which is now called the phallus. During this elongation, the phallus pulls the urethral folds forward so that they form the lateral walls of the urethral groove. This groove extends along the caudal aspect of the elongated phallus but does not reach the most distal part, the glans. The epithelial lining of the groove, which originates in the endoderm, forms the urethral plate.
How and when is the penile urethra formed?
At the end of the third month, the two urethral folds close over the urethral plate, forming the penile urethra. This canal does not extend to the tip of the phallus. This canal does not extend to the tip of the phallus. This most distal portion of the urethra is formed during the fourth month, when ectodermal cells from the tip of the glans penetrate inward and form a short epithelial cord. This cord later obtains a lumen, this forming the external urethral meatus.
What is hypospadias?
Hypospadias is incomplete fusion of the urethral folds, which results in abnormal openings of the urethra along the inferior aspect of the penis, usually near the glans, along the shaft, or near the base of the penis.
What is epispadias?
Epispadias is a rare abnormality in which the urethral meatus is found on the dorsum of the penis.
What is the development of the female external genitalia characterized by?
The genital tubercle elongates only slightly and forms the clitoris; urethral folds do not fuse, as in the male, but develop into the labia minora. Genital swellings enlarge and form the labia majora. The urogenital groove is open and forms the vestibule.
What is ambiguous genitalia?
Ambiguous genitalia may appear as a large clitoris or a small penis. Thus, a child may be born with a typically female appearance, but with a large clitoris (clitoral hypertrophy) or typically male with a small penis that is open on its ventral surface (hypospadius).
In some cases, these abnormalities result in individuals with characteristics of both sexes and they may be called hermaphrodites. However true hermaphrodites have both male and female gonadal tissues and such individuals have not been described in humans.
