Separation of Cloaca Into Urogenital Sinus and Rectum https://app.lecturio.com/#/article/3809
The primitive urogenital sinus arises in the fourth week of gestation from the cloaca, an epithelial chamber common to both the gastrointestinal and urogenital systems located in the pelvic region (Kromer, 1996).
During the fifth to sixth weeks of gestation, the urorectal septum partitions the cloaca into the primitive urogenital sinus ventrally and the anorectal canal dorsally (Fig. 2).
Differentiation 103 (2018) 66–73
The bladder forms from the primitive urogenital sinus above the confluence with the Wolffian (mesonephric) ducts, which serves as the demarcation between the cranial vesicourethral canal and the caudal “urogenital sinus”.
Unfortunately, the terminology is a bit confusing. When the cloaca is partitioned into the “primitive” urogenital sinus and the anorectal canal, the primitive urogenital sinus is in continuity with the allantois that extends cranially up the anterior body wall to end blindly in the umbilical cord.
The cranial portion of the primitive urogenital sinus above the junction with the Wolffian (mesonephric) ducts forms the bladder. The caudal or pelvic portion of the primitive urogenital sinus in males forms the prostate, prostatic urethra, bulbourethral glands and the membranous urethra. This caudal portion of the primitive urogenital sinus is simply referred to as the “urogenital sinus”.
Thus, one must be cognizant of this unfortunately confusing terminology.
In females the urogenital sinus forms the urethra below the bladder neck and in the fetus also forms the sinovaginal bulbs that are confluent with the Mullerian-derived uterovaginal canal (Cunha et al., 2018; Robboy et al., 2017).
The junction between the Mullerianderived uterovaginal canal and the urethra occurs near the future introitus (Robboy et al., 2017).
Thus, in humans, urogenital sinus epithelium appears to form the hymen and contributes to the vaginal vestibule (Robboy et al., 2017; Cunha et al., 2018).
The bladder and ureterovesical junction form primarily during the fourth to eighth weeks of gestation, and arise from the primitive urogenital sinus following subdivision of the cloaca.
The human bladder is formed from the cranial portion of the primitive urogenital sinus, and collects and stores urine from the kidneys and until it can be expelled at a socially appropriate moment.
In males the prostate develops from epithelial buds arising from the urogenital sinus (urethra) immediately below the bladder (Cunha et al., 2018).
混同しないこと：urogenital sinusは子宮にはならない
In females, the urogenital sinus develops into the urethra and vaginal vestibule, not a part of the uterus. The uterus develops from the paramesonephric (Müllerian) ducts, which fuse and connect with the urogenital sinus at the sinus tubercle. The urogenital sinus is a structure that is distinct from the uterus, and its development is separate from that of the uterus.
Here’s a more detailed explanation:
Urogenital Sinus:
During development, the urogenital sinus is a structure formed from the cloaca, which also contributes to the rectum. In females, the pelvic portion of the urogenital sinus forms the urethra and the vaginal vestibule (the area surrounding the vaginal opening).
Paramesonephric Ducts (Müllerian Ducts):
These ducts are a separate structure that develops alongside the urogenital sinus. In females, these ducts fuse to form the uterus and fallopian tubes.
Connection:
The fused paramesonephric ducts (the uterovaginal primordium) connect to the dorsal aspect of the urogenital sinus at a structure called the sinus tubercle. This connection is crucial for the development of the lower vagina.
Therefore, while the urogenital sinus and the paramesonephric ducts are closely related during development, they are distinct structures that contribute to different parts of the female reproductive and urinary systems.
（Google検索結果 AI Overview)
尿道 Urethra の発生
AI Overview
The male and female urethra develop from different regions of the urogenital sinus during embryological development, leading to distinct structures and functions. In males, the urethra is longer and more complex, while in females, it is shorter and simpler.
Here’s a more detailed breakdown:
Early Development (Indifferent Stage):
Urogenital Sinus:
Both male and female embryos initially have a single structure called the cloaca 排泄腔, which later divides into the urogenital sinus 尿生殖洞 (anterior) and the anal canal 肛門直腸管 (posterior).
Bladder 膀胱 Formation:
The upper part of the urogenital sinus develops into the bladder.
Pelvic/Middle Part:
The pelvic (middle) part of the urogenital sinus contributes to the urethra in females and the prostatic and membranous urethra in males.
Phallic/Caudal Part:
The phallic (caudal) part of the urogenital sinus forms part of the female reproductive tract and the spongy urethra in males.
Male Urethra Development:
Prostatic Urethra:
The prostatic urethra develops from the pelvic part of the urogenital sinus and is lined with transitional epithelium.
Membranous Urethra:
The membranous urethra, which passes through the pelvic floor, also develops from the pelvic part.
Spongy Urethra:
The spongy urethra, located within the penis, develops from the phallic part.
Epithelial Changes:
The epithelium lining the male urethra undergoes changes, with the prostatic urethra developing prostate buds that form the prostate gland.
Female Urethra Development:
Entirely from Urogenital Sinus:The entire female urethra develops from the pelvic (middle) part of the urogenital sinus.
Shorter and Simpler:Compared to the male urethra, the female urethra is shorter and simpler.
Paraurethral Glands:The upper part of the female urethra also contributes to the formation of paraurethral glands, which are analogous to the prostate in males.
External Opening:The external urethral orifice opens dorsal to the clitoris.
Key Differences:
Length and Complexity:
The male urethra is significantly longer and more complex than the female urethra.
Epithelium:
The male urethra has different types of epithelium lining different sections (transitional, pseudostratified columnar, and stratified squamous), while the female urethra is primarily lined with transitional epithelium.
Associated Structures:
The male urethra is intimately associated with the prostate gland and bulbourethral glands, while the female urethra is associated with paraurethral and Bartholin’s glands.
Developmental Origin:
While both arise from the urogenital sinus, the specific regions and subsequent development differ, leading to the distinct structures.
図引用元：Singh, Vishram. Textbook of Clinical Embryology, 3rd Edition
The nephrogenic cord and the urogenital ridge are closely related embryological structures, but they are not exactly the same. Here’s a clear distinction:
🔹 Urogenital Ridge
A longitudinal elevation of intermediate mesoderm on each side of the dorsal aorta.
Forms during early embryonic development.
It gives rise to two major components:
Nephrogenic cord (or nephrogenic ridge) – related to the urinary system.
Gonadal ridge – related to the genital (reproductive) system.
🔹 Nephrogenic Cord (Nephrogenic Ridge)
A part of the urogenital ridge, specifically the lateral portion.
Gives rise to the three successive kidney structures:
Pronephros
Mesonephros
Metanephros (becomes the permanent kidney)
✅ In summary:
Nephrogenic cord = urinary portion of the urogenital ridge
Yes, the urogenital ridge runs from head (cranial) to tail (caudal) along the posterior wall of the embryonic abdominal cavity.
To elaborate:
The urogenital ridge is a longitudinal elevation of intermediate mesoderm.
It forms on either side of the dorsal aorta.
It gives rise to two key structures:
The nephrogenic cord (lateral part), which develops into the urinary system (kidneys, ureters).
The gonadal ridge (medial part), which develops into the reproductive organs (testes or ovaries).
So yes — it is an elongated structure that extends craniocaudally, reflecting the segmental development of the embryo.
(ChatGPT 4o)
AI による概要
Yes, the urogenital ridge, a structure in embryonic development, does run from the head to the tail (cranially to caudally) of the developing embryo.This ridge gives rise to both the urinary and genital systems.Specifically, the intermediate mesoderm, which forms the urogenital ridge, develops into three sets of nephric structures (from head to tail): the pronephros, mesonephros, and metanephros.
Here’s a more detailed breakdown:
Urogenital Ridge Formation:
The urogenital ridge arises from the intermediate mesoderm, located on either side of the aorta.
Cranio-caudal Development:
The three sets of nephric structures (pronephros, mesonephros, and metanephros) develop sequentially from the urogenital ridge, starting from the head (cranial) and progressing towards the tail (caudal) of the embryo.
Pronephros:
The first to appear, located in the neck region.
Mesonephros:
Located in the mid-section of the embryo, developing into mesonephric tubules and ducts.
Metanephros:
The final kidney structure, developing from the caudal end of the mesonephric duct and a condensation of nearby mesoderm.
泌尿器系の発生の講義動画 Development of the Urogenital System – Urinary System and Kidney Development by John McLachlan, PhD Urogenital ridgeについての解説 1:39~
https://app.lecturio.com/#/article/3809
自分：Does the intermediate mesoderm change its name to the urogenital ridge, and further change the name when it turns to three sets of nephric structures (from head to tail): the pronephros, mesonephros, and metanephros?
ChatGPT 4o：
Great question — you’re touching on the dynamic naming and regional specialization of the intermediate mesoderm during development. Here’s a precise explanation:
🌱 1. Intermediate Mesoderm:
Initially, a longitudinal column of mesoderm located between the paraxial and lateral plate mesoderm.
It appears bilaterally along the dorsal body wall.
This structure is simply called intermediate mesoderm in early stages (around week 3–4 of human development).
🪴 2. Urogenital Ridge:
As development progresses, the intermediate mesoderm bulges into the coelomic cavity, forming the urogenital ridge (starting ~week 4).
The urogenital ridge is a broader term that includes:
Nephrogenic cord (lateral part) → forms the urinary system.
Gonadal ridge (medial part) → forms the genital system.
Yes, you can think of the urogenital ridge as a regionally specified derivative of the intermediate mesoderm.
🌊 3. Pronephros → Mesonephros → Metanephros:
These are successive nephric structures derived from the nephrogenic cord (i.e., the urinary part of the urogenital ridge), and they appear in a cranial-to-caudal sequence:
Region
Structure
Fate
Cranial
Pronephros
Rudimentary; regresses quickly
Intermediate
Mesonephros
Temporary kidney; forms mesonephric duct (Wolffian duct)
Caudal
Metanephros
Becomes the definitive kidney
So, in essence:
✅ Intermediate mesoderm →
➡️ Urogenital ridge (as it bulges into body cavity) →
➡️ Nephrogenic cord (for urinary structures) →
➡️ Pronephros → Mesonephros → Metanephros (in sequence, cranial to caudal)
Summary Answer:
Yes, the intermediate mesoderm functionally and regionally transforms — first becoming the urogenital ridge, then differentiating further into specific nephric structures (pronephros, mesonephros, metanephros) along the cranio-caudal axis, as well as the gonadal ridge for the reproductive system.
Let me know if you’d like a diagram to go with this!
Embryological Differences Between the Upper and Lower Urinary Tracts
The upper and lower urinary tracts have different embryological origins. The upper urinary tract (kidneys and ureters) is primarily derived from the mesoderm, while the lower urinary tract (bladder and urethra) is primarily derived from the endoderm.
The kidneys develop from a part of the mesoderm called the intermediate mesoderm. Specifically, they form through three successive stages: the pronephros, mesonephros, and finally the metanephros, which becomes the permanent kidney.
The ureters develop from the ureteric bud, which branches off from the mesonephric duct.
The majority of the bladder and the upper part of the urethra develop from the urogenital sinus, which is formed when the cloaca (the terminal end of the primitive gut, derived from the endoderm) is divided.
However, a part of the bladder, known as the bladder trigone, is formed by the incorporation of the mesoderm-derived ureters into its wall. Therefore, it is not composed entirely of endoderm.
Thus, the upper and lower urinary tracts originate from completely different germ layers, which contributes to differences in their structure, function, and the types of diseases that affect them.
