Intermediate Mesoderm — gives origin to kidneys, ureters, gonads and genital ducts
• intermediate mesoderm is positioned between paraxial mesoderm (which gives rise to somites) and
lateral mesoderm (which becomes splanchnic and somatic mesoderm); all mesoderm is
derived from primary mesenchyme that penetrates the primitive streak
• intermediate mesoderm & adjacent coelomic mesothelial cells form a urogenital ridge, consisting of:

- a lateral nephrogenic cord which gives rise to kidney and ureter
- a medial gonadal ridge which gives rise to an ovary or testis
• in addition to kidney & ureter and gonad & genital tracts, intermediate mesoderm gives rise
to adrenal cortex

Kidneys — excrete urine (waste molecules and water extracted from blood filtrate)
• three kidneys develop chronologically from the nephrogenic cord, in cranial-caudal order:
- pronephros is the earliest, most cranial, and least functional of the three kidneys

- mesonephros is the second of the three kidneys; it develops caudal to the pronephros
- metanephros is the last-formed and most caudal of the three kidneys; it becomes the adult kidney
• urine production essentially requires:
- an increased capillary surface area, e.g., a spherical mass of capillaries (glomerulus);
- epithelial tubules to collect blood filtrate & retrieve desirable constituents from the filtrate; and
- a duct system to convey the ultimater product (urine) away from the kidney and body
Note: Although kidneys may become functional in-utero, they are not essential because the placenta is
capable of removing waste molecules from fetal blood.

Pronephros — consists of 7-8 primitive tubules
• begins with segmentation of the cranial end of the nephrogenic cord; segments become tubules
• tubules fuse to create a pronephric duct that grows caudally and empties into the cloaca
• tubules soon degenerate; the pronephros is non-functional (except temporarily in sheep)
Note: The term "nephro" comes from the Greek word for kidney; "ren" = Latin for kidney

Mesonephros — consists of 70-80 tubules
• begins with segmentation along the nephrogenic cord; segments transform into tubules
• tubules are induced to form by the pronephric duct which is renamed: mesonephric duct
• tubules surround glomeruli on arteral branches from the aorta and drain into the mesonephric duct
• the mesonephros is temporarily a functional kidney in mammals
• eventually, mesonephric tubules degenerate (some tubule cells become incorporated into the gonad)
• the mesonephric duct becomes ductus deferens and epididymis in male embryos
Note: The mesonephros remains the functional kidney of fish and amphibians.

Metanephros — becomes the adult kidney and ureter of mammals, birds, and reptiles
• the metanephric kidney originates caudally in the pelvic region; by differential growth, it
becomes re-positioned cranially into the abdomen
• the metanephros (metanephric kidney) is initially lobulated; subsequently, it becomes externally
smooth and apparently unilobar most species (except, e.g., cattle where it remains lobulated)
• the metanephros originates from two sources:
- a ureteric bud grows out of the mesonephric duct near the cloaca (induced to grow by neural tube)
- a metanephrogenic mass, another name for the caudal region of the nephrogenic cord mesoderm
• the ureteric bud grows into the metanephrogenic mass and develops into the ureter, renal pelvis,
and numerous collecting ducts (the latter are induced to form by the metanephrogenic mass)
• metanephrogenic mass gives rise to nephrons in the following manner (induced by collecting ducts):
- adjacent to collecting ducts, mesodermal cells proliferate and form solid cell-cords
- the cell-cords canalize, elongate, become S-shaped metanephric tubules, and eventually nephrons
- one end of each metanephric tubule expands and surrounds a capillary glomerulus; the other end
must connect to a collecting tubule & duct (failure to connect results in polycystic kidneys)
- between the two ends, each metanephric tubule differentiates into regions characteristic of a
nephron (proximal segment, thin loop, and distal segment)
Note: Nephrons develop from deep to superficial in the kidney. Many early nephrons degenerate as a
normal occurrence. Nephrons continue to form and mature postnatally (except in the bovine);
thereafter, nephrons cannot be replaced if they are damaged.


Cloaca — a common chamber for digestive, urinary, and genital tracts
• the hindgut terminates in a temporary cloaca (the cloaca is permanent in birds, reptiles & amphibians)
• a urorectal septum divides the cloaca into a dorsal digestive chamber and a ventral urogenital sinus
• the urachus (intraembryonic allantois) connects to the cranial end of the urogenital sinus
• the urogenital sinus and the urachus (allantois) give rise to the urinary bladder and urethra

Urinary Bladder — stores urine received from ureters; expels urine through the urethra
• develops from expansion of the urachus (allantois) and the cranial end of the urogenital sinus
• the expansion incorporates the mesonephric duct (future ductus deferens) and ureter such that each
opens separately into the dorsal wall of the urogenital sinus (bilaterally)
• differential growth of the dorsal wall results (bilaterally) in the mesonephric duct and ureter becoming
repositioned, so that the ureter opens cranial to the bladder neck and the future ductus deferens
empties caudally into the urethra
• this differential growth and repositioning creates a new region in the dorsal wall of the bladder neck:
bladder trigone (which plays a role in anchoing ureters to the bladder neck & urethral crest)
• failure to reposition properly can result in an ectopic (misplaced) ureter, a source of postnatal
incontinence when the ureter opens in the caudal urethra or vagina, distal to urinary sphincters
• failure of the urachus to close postnatally (patent urachus) can lead to urine leakage at the umbilicus;
also, urachal persistence as a blind pouch (vesicourachal diverticulum) can predispose to cystitis
Note: The urinary bladder & urethra originate from endoderm, except for the trigone region which is
derived from mesonephric duct (mesoderm). Smooth muscle of the urinary tract
originates from splanchnic mesoderm.

Urethra — urethral development is gender specific
• the female urethra and male pelvic urethra develop from the mid region of the urogenital sinus
(in females, the caudal region of the urogenital sinus becomes adult vestibule & eventually vagina)
(in males, the caudal region of the urogenital sinus elongates and becomes penile urethra)

Note: In-utero, fetal urine is discharged into the allantoic cavity through the urachus and into the amniotic cavity through the urethra and urogenital orifice (future vestibule or penile urethra).


Note: Named for its position beside the kidney, the adrenal gland consists of an adrenal medulla
surrounded by an adrenal cortex. Although they are combined grossly, the cortex & medulla
are embryologically, histologically, and functionally different glands.

Adrenal Cortex — secretes glucocorticoid and mineralcorticoid hormones to regulate metabolism
• arises from mesoderm cells of mesonephric tubules; during mesonephros degeneration, future
adrenal cortex cells dissociate from tubules and migrate to the location of the adrenal gland
• the cortex is stimulated to secrete by Adrenocorticotropic Hormone (ACTH) from the pituitary gland

Adrenal Medulla — secretes epinephrine (adrenalin) especially during "fight or flight" situations
• derived from ectoderm and neural crest (like autonomic ganglia neurons)
• future adrenal medulla cells migrate to the location of the adrenal gland and become incorporated
within the developing adrenal cortex
• adrenal medulla cells are modified postganglionic neurons and they are innervated by sympathetic
preganglionic neurons