Urogenital System

Updated 4 November 2004





A.   Urogenital (Excretory + Reproductive) develops embryonically from same or adjacent tissues; maintain close morphological association throughout ontogeny; slide 1 (urogenital ridge)

B.    Kidney develops first (functional; reproductive later)

C.   Reproductive system takes over parts of excretory system


Excretory System

A.   Functions: excrete nitrogenous metabolites, osmoregulation (water balance)

B.    Functional unit - uriniferous tubule; slide 2

1.     Structure

a.      glomerulus (arteriole, capillary bed); high pressure filter

b.     nephron (renal capsule, proximal, intermediate & distal convoluted tublules); selective reabsorption

c.     collecting tubule; common collection and transport

2.     Development - gross morphology of kidney conservative (reflects phylogeny)

a.      nephrotome (=intermediate mesoderm - segmental); slide 3

b.     anterior®posterior development; complexity (# tubules/segment) increases


A.   Kidney types (tripartite model); slide 4

1.     Pronephros (functional in larval fishes & amphibians, adult hagfish, and a few adult teleosts)

a.      few tubules/segment

b.     short-lived (head kidney)

c.     connected to coelom (nephrostome =peritoneal funnel)

d.     initiates duct formation (archinephric =pronephric duct); slide 5

e.      tubules degenerate (duct remains)


2.     Mesonephros (embryonic only)

a.      elongate (functional in most)

b.     not connected to coelom

c.     modified in most adult nonamniotes (opistonephros; “extended” mesonephros)

d.     trends: concentrate function in more posterior portions (anterior portions incorporated into reproductive system), increased # tubules/segment

e.      degenerates in amniotes

f.       tubules tap on to existing archinephric duct (=mesonephric duct); slide 5

g.     mesonephric duct transports sperm in males (=vas deferens), degenerates in females


3.     Metanephros (amniotes)

a.      concentrated at extreme posterior (bird - last segment); at level of pelvis - move anterior later (cf position in cat)

b.     complex (thousands of tubules/segment)

c.     not connected to coelom

d.     ureter (metanephric duct); slide 5


D.   Structure and function (microstructure of uriniferous tubules reflects physiological function; readily adaptive – cf gross morphology)

1.     Freshwater fish/amphibians (physiological problem: hyposmotic - too much water)

a.      little drink, water and salt in (gills, mucous membranes)

b.     excrete continuously

2.     Marine teleosts (problem: hyperosmotic - too little water)

a.      drink, salt intake

b.     excrete occasionally; also urea excreted by gills

c.     salt glands

3.     Marine hagfish, elasmobranchs, coelacanths (isosmotic - osmocomformer)

a.      little drink

b.     gills impervious to urea (water in)

c.     salt glands


4.     Terrestrial vertebrates (problem - limited water)

a.      loose water through lungs, skin, excretory wastes; gain water primarily through drinking

b.     conservation of water primarily through kidneys

c.     excretory products (breakdown of proteins and nucleic acids)

1)     ammonia (highly toxic, highly soluble); excreted by kidneys, gills, and most mucous membranes in many aquatic forms where water is not limiting

a)     e.g., fishes, amphibians, turtles, crocodilians

2)     urea (relatively non-toxic, highly soluble; converted from ammonia)

a)     e.g., amphibians, some turtles, mammals

b)    some birds and all mammals able to concentrate urine (cf blood; long nephrons with specialized intermediate portion (looped) – man reclaims 169 of 170 l/d (~45 gal)

c)     birds w/o loops? (kidneys are like those of reptiles)

d)    concentration varies (e.g., vampire bat vs. K-rat [25X cf blood])

3)     uric acid (non-toxic, insoluble [does not exert osmotic pressure – sequester]; costly to convert)

a)     e.g., many reptiles, birds; marine species (limited water)

b)    often with associated salt glands (nasal, crocodile tears)


5.     Summary (phylogenetic, adaptive, acclimation effects) – slide 6 


Reproductive system

A.   Life cycles (vertebrates - diplontic)

B.    Sexual reproduction (recombination alternating with meiosis)

1.     Parthenogenetic (some fish, salamanders, lizards)

2.     Gonochoristic (separate sexes)

3.     Hermaphroditic (hagfish, some teleosts)

a.      simultaneous: common in deep-sea fishes

b.     sequential:  protogyny (F®M; 14 families)

c.     protandry (M®F; 8 families)

d.     some normally M or F vertebrates have potential for developing opposite sex

4.     Embryology: undifferentiated®differentiated (all vertebrates have potential for developing into either sex [early in mammals])

a.      examples: toads (Bidder’s organ [undifferentiated gonad®ovary])

b.     chicken (1 ovary + 1 undifferentiated gonad)


C.   Structures

1.     Gonads (produce gametes – eggs/sperm)

2.     Ducts (transport gametes within body)

3.     Copulatory organs (transfer sperm to female)

a.      claspers, gonopodium (fin ray)

b.     hemipenes, penis (erectile tissue, bone)

4.     Secondary sexual characters (vertebrates often sexually dimorphic; often important in sexual behavior, e.g., courtship)


D.   Gonads

1.     Female ovary

a.      discrete or diffuse, subject to seasonal change in size

b.     cyclic oogenesis slide 7

c.     modifications for elongate bodies (fusion, deletion, ant-post positioning)

2.     Male testis

a.      more discrete, less subject to seasonal change in size

b.     spermatogenesis slide 8

c.     modifications for endothermy (spermatogenesis ceases >36.5C)

d.     birds (cool with air sacs)

e.      some mammals (extend outside of general body cavity - scrotum)

3.     Ducts

a.      agnathans - no ducts (abdominal pores); also some teleosts

b.     most other males - closed system derived from embryonic excretory system; testis (seminiferous tubules)®efferent ducts®epididymis®vas deferens

c.     most other females - open system derived from embryonic excretory system and mullerian duct; ovary®coelom®oviduct


Representative urogenital systems

A.   Females slide 9, slide 10

B.    Males slide 11, slide 12

C.   Specializations of distal oviduct

1.     Ovisac (dilation, egg storage) – fishes

2.     Jelly gland (albumin) - amphibians

3.     Shell gland (albumin, shell) – amniotes

4.     Uterus - muscular, glandular (cyclic growth) slide 13

5.     Vagina - reception of penis (mammals)

D.   Termination of ducts (both sexes); cloaca

1.     Embryonic cloaca (proctodeum, urodeum [=U-G sinus], coprodeum, urorectal septum); DRAW

2.     Adult structure – highly variable

3.     Cloacal differentiation in embryonic mammals slide 14


Reproductive strategies

A.   Fertilization (transfer of sperm)

1.     External (behavioral rituals; amplexus [frogs], cloacal apposition [birds])

2.     Internal (behaviorial [salamanders - spermatophore], or structural [copulatory organ])

3.     Amniote [shelled egg] - must be fertilized early

B.    Parental care

1.     Classical categories

a.      oviparous (no maternal nourishment, develop outside of maternal body)

b.     ovoviviparous (no maternal nourishment, develop inside maternal body)

c.     viviparous (maternal nourishment, develop inside maternal body)

2.     Problems - intermediate conditions (examples)

3.     Current: formally recognize endpoints of continuum

a.      oviparous (incubation period; oviposition; may brood eggs)

b.     viviparous (includes all forms of live-bearing; gestation period; birth; parental care); found in most vertebrate groups (major exceptions: birds, turtles, crocodilians)

4.     Nourishment: oviducal secretions®placenta (continuum)

5.     No. independent origins of viviparity (Shine 1994)















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