The Urogenital system

It is easiest to discuss the excretory and reproductive systems at the same time because they originate developmentally from adjacent tissues and utilize common ducts to allow passage of products outside of the body

Overview and function of the excretory system
Kidneys are the primary adult excretory organs and work in association with auxiliary structures such as the gills, lungs, skin, parts of the digestive system and salt glands

Together they perform two primary functions for the body:

  • remove nitrogenous waste products of protein metabolism and other harmful substances and eliminate them in the form of ammonia, uric acid or urea
  • eliminate controlled amounts of water and salts to help maintain the internal environment of the body
  • The kidneys originate from lateral plate mesoderm that differentiates into the nephric ridges which buds into blocks of tissue called nephrotomes, which differentiate into nephrons or renal tubules, the structural and functional units of the kidney

    Basic structure

  • the end of the renal tubule forms a cup-like structure called the Bowmanís capsule that surrounds a knot of capillaries called the glomerulus
  • blood flow to the glomerulus comes from the afferent arteriole and leaves through the efferent arteriole
  • if the capillary bed is surrounded by the renal capsule it is called an internal glomerulus; if the capillary bed discharges into the coelom it is called an external glomerulus - internal glomeruli are the derived trait among vertebrates
  • The primitive vertebrate kidney (holonephros) develops from all of the nephrotome and is currently found only in the larvae of hagfishes, where it develops from the entire nephric ridge

    The remaining kidney types differ in their placement in the body and the extent to which they run through the body

    The pronephros is the first formed kidney of a vertebrate embryo, which lies dorsal to the pericardial cavity and forms the archinephric duct - found in primitive fishes and in the larval and embryonic stages of some vertebrates; also called the head kidney

    The opisthonephros is the adult kidney of most anamniotes, in which most of the kidney tubules are concentrated caudally - all of the mesomere posterior to the pronephros forms one kidney, which taps into the pronephric duct

    The metanephros is the adult kidney of amniotes, which develops from the caudal part of the mesomere, is the most posterior of the kidneys and is the last to develop

    Early in development the ureteric bud grows out from the mesonephric duct, which later becomes the ureter, which drains urine produced by the kidneys, where it is collected in the renal pelvis and enters either the cloaca or the urinary bladder - within the kidney the ureteric bud forms collecting tubules that are continuous with the nephric tubules

    Osmoregulation
    The glomerulus is a group of capillary loops and anastomoses that hangs into the Bowmanís capsule - the capsule is cup-shaped, with an outer parietal wall and an inner visceral wall

    In the renal corpuscle, blood is driven through the capillaries at high pressure - capillaries are composed of a thin endothelium with pores, which creates a filter through which blood components can drain and be caught by the capsule

    Renal capsules are usually separated from nephric tubules by a narrow neck segment - nephric tubules are divided into a proximal tubule and a distal tubule, separated by an intermediate tubule:

  • the proximal tubule returns sugars, amino acids, vitamins and salts to the bloodstream
  • the distal tubule acidifies the filtrate and removes sodium and chloride ions; water is returned to the body by both tubules
  • Details of tubule function are varied and complicated, and related to environmental factors influencing the organism:
  • Freshwater fishes and amphibians - water constantly enters bodies from the environment, and salt is lost through urine; short proximal and distal tubules to return solutes; nitrogen released as ammonia or urea
  • Marine bony fishes - body fluids constantly leak away to the sea; pass very little urine, and drink freely to introduce more water (but also more salt); special cells in gills excrete salt; little or no distal tubules
  • Terrestrial species require active retention of water by the kidneys Urinary bladder and cloaca
    The urinary bladder provides temporary storage of urine:
  • in fishes, the urinary bladders are of small or moderate size
  • in amphibians, the bladder is a large ventral outpocketing of the cloacal
  • in reptiles and birds, bladders are absent, as the uric acid excreted is semisolid - the urethra empties directly into the cloaca
  • in mammals, the bladder is usually large and lined with epithelium and smooth muscle
  • Overview and function of the reproductive system
    The overall purpose of the reproductive system is to produce sex cells, bring egg and sperm cells together, provide for nourishment of the embryo or fetus until hatching or birth, and to release young from the maternal body

    Genital organs
    The early development of the genital organs is the same in both sexes, and is called the sexually indifferent stage; later, male structures atrophy and female structures develop if the embryo is female, and female structures atrophy and male structures develop is the embryo is male

    The gonads begin as a thickening of coelomic epithelium, which forms a genital ridge - the central part of the genital ridge forms the functional gonad

    The primordial germ cells migrate from their original location at the base of the yolk sac and into the germinal epithelium

    The gonad has a cortex of epithelium and a medulla formed from mesenchyme

    The primary sex cords then develop from cords of germinal epithelium that grow into the medullary tissue

    The reproductive ducts develop from a network of tubules in the medullary region of the gonad and interconnect the primary sex cords and the cranial mesonephric tubules - contribute to the sperm passages in males and regress in females

    Both sexes also begin to develop an oviduct during embryonic development, the Mullerian duct, which arises from the archinephric duct

    Testes and male genital ducts
    The testes develop earlier in embryonic life than the ovaries, and are mostly derived from the medulla of the indifferent gonad - the germinal epithelium becomes the covering of the testes

    The primary sex cords separate from the epithelium and become hollow to produce the coiled seminiferous tubules, or the tubules that produce sperm and are surrounded by interstitial cells of Leydig, which secrete testosterone. The wall of the tubule consists of sperm-forming cells and Sertoli cells, epithelial cells of the seminiferous tubules that play a role in the maturation of sperm

    Spermatogenesis proceeds through mitotic division of sperm-forming cells or spermatogonia, to form primary spermatocytes - these cells then undergo one meiotic division to form secondary spermatocytes, and then another division to form spermatids, that later mature and transform to become motile spermatozoa

    In most vertebrates, the testes lie in the body cavity, but in mammals the testes descend into the scrotum, guided by the gubernaculum, or cord of tissue that extends between the embryonic testes and the scrotum - descent is accompanied by an evagination of muscular and connective tissue of the body wall (cremasteric pouch) which are layers of the body wall that suspend the testes

    Rete cords become the sperm passage in males - in amniotes, sperm leaves the epididymis, where sperm matures and is stores; accessory glands associated with the male reproductive tract include the prostate gland (which surrounds the urethra), the vesicular gland, and the bulbourethral gland - these glands secrete seminal fluid that increases motility of sperm and neutralizes acids in the female reproductive tract

    Sperm and seminal fluid are released into the urethra during sperm transfer to the female - in male amniotes, the intromittent organ is the penis, which develops from the wall of the cloaca; in other species, sperm transfer occurs through cloacal apposition

    Ovaries and female genital ducts
    The germinal epithelium of the indifferent gonad becomes the thin covering of the ovary

    The ovary contains thousands of oogonia that become mature ova during ovogenesis

    The primary sex cords degenerate in females and are replaced by secondary sex cords, which form follicle cells that surround, nourish and support the developing ova - ova differ based on the amount of yolk present and whether the animal is oviparous, ovoviviparous or viviparous

    Mammalian ovaries are generally smaller than those of other species, and are connected by the gubernaculum to the body wall

    Eggs are found in primordial follicles (approximately 2 million in human females at birth) but only approximately 400 are ever actually ovulated - each follicle ruptures and releases ova at ovulation, and is converted to the corpus luteum, or hard yellowish body that develops from an ovulated follicle and acts as an endocrine gland

    The female reproductive ducts are more independent of the excretory system than are those of males - the oviduct (or Mullerian duct) in females continues to develop and grow posteriorly to the cloacal region to become the passage for removal of eggs

    The eggs of gnathostomes rupture into the coelom before entering the duct system - anterior ends of passages are the ovarian funnels or ostia, and eggs move into the duct system through peristaltic contraction of smooth muscles in the oviduct

    In cleidoic species, oviducts lead into shell glands that envelop eggs in albumen and a calcified shell

    In mammals, the genital tract is divided into three regions:

  • the oviduct conveys eggs from the ovary to the rest of the genital tract
  • the uterus houses the fetus during pregnancy and provides the maternal contribution to the placenta; composed of an endometrium (lining) and myometrium (smooth muscle layer), and is bounded caudally by the cervix
  • the duplex uterus is doubled, and is found in monotremes, marsupials, elephants and rodents
  • the bipartite uterus is y-shaped externally and divided internally (ungulates, carnivores)
  • the bicornate uterus is y-shaped but has no internal partition (some ungulates, whales)
  • the simplex uterus has only one uterine chamber (primates)
  • the vagina acts as a birth canal during parturition
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    Definitions Bowmanís capsule - dilated end of a kidney tubule that surrounds a knot of capillaries
    Corpus luteum - hard yellowish body that develops from an ovulated follicle and acts as an endocrine gland
    Cremasteric pouch - layers of the body wall that suspend the testes, or the scrotal wall apart from the skin
    Glomerulus - ball-like network of capillaries that is surrounded by Bowmanís capsule at the proximal end of a renal tubule
    Gubernaculum - cord of tissue that extends between the embryonic testis of mammals and the developing scrotum and guides the descent of the testes
    Holonephros - the hypothetical ancestral kidney that develops from all of the nephrotome
    Loop of Henle - that portion of the renal tubule of mammals and some birds that loops into the medulla of the kidney, and is essential for establishing the interstitial salt gradient needed for the production of concentrated urine
    Metanephros - the adult kidney of amniotes, which develops from the caudal part of the mesomere
    Opisthonephros - the adult kidney of most anamniotes; kidney tubules are concentrated caudally
    Pronephros - the first formed kidney of a vertebrate embryo, which lies dorsal to the pericardial cavity and forms the archinephric duct before it atrophies
    Rete cords - minute cords in the embryo that reconnect the primary sex cords and the cranial mesonephric tubules; they contribute to the sperm passages in males and regress in females
    Retroperitoneal - pertaining to structures that lie dorsal to the peritoneal cavity
    Seminiferous tubules - tubules within the testes that produce sperm
    Sertoli cells - epithelial cells of the seminiferous tubules that play a role in the maturation of sperm