Lecture_15_Comparative Anatomy_The Urogenital System

Lecture_15_Comparative Anatomy_The Urogenital System - •...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: • • Comparative Anatomy Lecture 15 – The Urogenital System General Introduction o Although urinary and reproductive functions are quite different we will discuss them together as the urogenital system because both share many of the same ducts. o Anatomically, the urinary system includes the kidneys and ducts that carry away their product, urine. o The genital system includes the gonads and their ducts that carry away the products they form, sperm or eggs. o Embryologically, urinary and reproductive organs arise from the same or adjacent tissues and maintain close anatomical association throughout the organism’s life. Urinary System o Structure of the Mammalian Kidney  The vertebrate kidneys are a pair of compact masses of tubules situated dorsal to the abdominal cavity.  Urine produced by the tubules is ultimately released into the cloaca or its derivative, the urogenital sinus.  A slice down the long axis of the kidney reveals its two regions: an outer cortex surrounding a deeper medulla. • Urine produced by the kidney enters the minor and then the major calyx, which joins the renal pelvis, a common chamber leading to the urinary bladder via the ureter. Elimination of urine from the body occurs through the urethra.  The uriniferous tubule is the functional unit that forms urine within the kidney. • The uriniferous tubule consists of two parts: the nephron (nephric tubule) and the collecting tubule into which the nephron empties. o The number of nephrons, which forms urine, varies from only a few hundred in the kidneys of cyclostomes to over a million per kidney in mammals. o The collecting tubule affects the concentration of urine and conveys it to the minor calyx, the beginning of the excretory duct.  The renal artery, a major branch from the dorsal aorta, delivers blood to the kidneys. This artery branches numerous times (the number is species dependent) before eventually forming tiny capillary beds known as glomeruli. Each glomerulus is associated with a renal capsule (Bowman’s capsule)  constituting the first portion of the nephron. Collectively, the glomerulus and renal capsule form the renal corpuscle. In short, an ultrafiltrate without blood cells and proteins is forced through the capillary walls and collects in the renal capsule before it passes through the proximal convoluted, intermediate, and distal convoluted tubules of the nephron before entering the collecting tubule. Different substances (e.g. sodium, glucose, etc.) are reabsorbed (into specialized arteries) at different points along the filtrates path through the nephron (see General Physiology). o Embryonic Development  Nephrotome to Nephric Tubules • The kidneys form within the intermediate mesoderm located in the dorsal and posterior body wall of the embryo. o At the onset of its differentiation, the afore mentioned posterior region of the intermediate mesoderm expands, forming a nephric ridge that protrudes slightly from the dorsal wall of the body cavity. o The next structure to appear is the paired nephrotome, the embryonic forerunner of the uriniferous tubule.  The nephrotome is often segmental and contains the nephrocoel, a coelomic chamber that may open via a ciliated peritoneal funnel to the coelom. • The medial wall of the nephrotome widens into a thin ­walled renal capsule into which grows the glomerulus. • The lateral end of the nephrotome grows outward and fuses with similar outgrowths to form successive nephrotomes to form the common nephric duct. At this point, the modified nephrotome can be described as a uriniferous tubule even though it may remain connected with the coelom via a peritoneal funnel. • In summary, the fundamental plan underlying the excretory system is envisioned to consist of paired and segmented uriniferous tubules that open on one end to the coelom and on the other end to the nephric duct, with a glomerulus in between. The ciliated peritoneal funnel seems to drive fluid from the coelom into the tubule, the associated glomerulus adds fluids from the blood, and the tubule itself modifies this collected fluid before it flows into the nephric duct. It should be noted here that although this structure represents the primitive or fundamental plan of excretory tubule organization, tubules opening to the coelom are rarely found in the kidneys of adult vertebrates because they fuse early in development with the nephric ridge.  Tripartite Concept of Kidney Organization • Theory o The tripartite concept envisions formation of the nephric tubules in one of three locations (anterior or pronephros, middle or mesonephros, posterior or metnephros) within the nephrogenic nephric ridge with the subsequent loss, merger, or replacement of these tubules constituting the developmental basis for the definitive adult kidneys. o In addition to positional differences, the three regions vary with respect to their connections to the coelom.  Tubules retain their connections to the coelom through the peritoneal funnel in the pronephros.  Tubules arising within the middle or posterior regions are not connected to the coelom. • Pronephros o Usually only a transient embryonic developmental stage in all vertebrates. o Pronephric tubules – tubules that appear within the anterior portion of the nephric ridge which join to form a common pronephric duct. o The pronephric duct grows posteriorly in the nephric ridge, eventually reaching and opening into the cloaca. o Fluid filters from glomeruli that may protrude into the coelom or make direct contact with the pronephric tubules. Pronephric tubules then take up this coelomic fluid through the ciliated peritoneal funnels, act on it, and eventually excrete the fluid as urine. o Pronephric tubules are functional in the embryos of most vertebrates, but as they regress during development that are replaced by the mesonephros. • Mesonephros o Tubules of the mesonephric kidney (mesonephric tubules) arise in the middle of the nephric ridge and tap into the preexisting pronephric duct. To be consistent, the pronephric duct is then renamed to be the mesonephric duct. o The mesonephros often becomes functional in the embryo, but if it persists into the adult, it is modified by the incorporation of additional tubules arising within the posterior nephric ridge. It is then called the opisthonephros. o The opisthonephros is found in most adult fishes and amphibians, but gets replaced in amniotes by the metanephros. • Metanephros o The earliest hint of metanephros formation is the appearance of the metanephric duct, which appears as a ureteric diverticulum arising at the base o the preexisting mesonehpric duct. o The ureteric diverticulum grows dorsally into the posterior region of the nephric ridge where it enlarges and stimulates the growth of metanephric tubules that come to make up the metanephric kidney. o The metaneprhos becomes the adult kidney in amniotes and the metanephric duct is called the ureter. o Kidney Function and Structure  Tetrapod Kidney Structure • Most amphibians, sharks, and teleosts have opisthonephric kidneys whose anterior kidney segments transport sperm (see notes on reproduction below). • • In amniotes, the anterior end of the nephric ridge rarely produces pronephric tubules. The predominate embryonic kidney is a mesonephros, but in all amniotes, it is supplemented in later development and completely replaced by a metanephros drained by a ureter. Metanephric ducts tend to be long with well ­ differentiated proximal, intermediate, and distal regions. o In mammals, the intermediate sections are elongated to form the loop of Henle.  Positionally, the loop includes the part of the nephron that departs from the cortex and dips into the medulla (a descending limb), makes a sharp turn, and returns to the cortex (the ascending limb).  Structurally, three regions contribute: • the straight portion of the proximal tubule • the thin ­walled intermediate region • the straight portion of the distal tubule The terms thick and thin refer to the height of the epithelial cells forming the loop. • Cuboidal cells are thick and squamous cells are thin.  The length of the loops determine how capable the organism is of concentrating urine (long in desert species, short in species where water is in abundance). In general, vertebrate kidneys contribute to the maintenance of a constant, or near constant, internal environment (termed homeostasis) such that active cells are not stressed by radical departure from optimum operating conditions. This is accomplished by two fundamental physiological functions: excretion and osmoregulation.  •  Excretion: Removing the Products of Nitrogen Metabolism • Most excreted components in urine are metabolic byproducts that collect within the organism and must be voided so that they will not interfere with the organism’s physiological balance. • •  Metabolism of proteins and nucleic acids produces nitrogen, usually in the reduced form of ammonia (NH3). Ammonia is highly toxic and therefore must be removed from the body quickly, sequestered, or converted into a nontoxic form to prevent accumulation in tissues. Three routes for eliminating ammonia: o ammonotelism – direct excretion of ammonia; most common in animals living in water; NH3 is very water soluble o uricotelism – excretion of nitrogen in the form of uric acid; common in birds and lizards; uric acid is not very water soluble; uric acid is transported via the ureters to the cloaca where it precipitates out to form a salt (K, NA, or ammonium) o ureotelism – excretion of nitrogen in the form of urea; common in mammals; urea is much less toxic than NH3 and more water soluble; urea is concentraited in urine and excreted. o Note: in some species, the route of ammonia elimination can change depending on the amount of water available. For example, turtles excrete primarily ammonia in aquatic habitats, but eliminate urea or uric acid when on land. Osmoregulation: Regulating Water and Salt Balance • General Points o Osmoregulation involves the maintenance of water and salt levels. The kidneys regulate the constant volume and composition of blood and lymph in terrestrial vertebrates. In aquatic vertebrates, the gill epithelium and digestive tract are as important as the kidneys in addressing the problem of salt balance. • Water Balance o General Points  On land, the kidneys, cloaca, and possibly the urinary bladder are water conservers, meaning that they recover water before nitrogen is eliminated from the body.  Organisms in water must deal with water fluxes, movement of water into or out of the body. In freshwater fishes, the osmotic problem results from a net tendency for an inward shift of water. Relative to fresh water, the body of the fish is hyperosmotic. In this case, fish kidneys are designed to excrete large quantities of dilute urine (~10x the amount of their marine counterparts!). • In saltwater fishes, there is a tendency for a net outward flux of water from the body tissues, dehydrating them. Relative to the salt water, the bodies of most marine fishes are hyposmotic. To aid in water conservation, the kidneys are designed to excrete very little water, thus reducing water loss. To address the problem of excess salt, the gills and sometimes special glands (e.g. salt glands) will excrete salt. The body of some animals is isosmotic and are therefore called osmoconfomers (e.g. hagfishes, chondrichtyans). •  • Reproductive System o Structure of the Mammalian Reproductive System  Ovary Overview • In mammals, each ovary consists of an outer connective tissue capsule (tunica albuginea) encloses a thick cortex and deeper medulla. • The ova (eggs) occupy the cortex and are wrapped in layers of follicle cells derived from connective tissue. An ovum plus its associated follicle cells is termed a follicle. • While some follicles remain in this rudimentary state, others pass through a series of maturation stages at the end of which the ovum and some of its clinging follicle cells are cast out of the ovary in the process of ovulation. At this point they are ready for fertilization. o If fertilization occurs, the ovum continues down the oviduct and becomes implanted in the wall of  the prepared uterus, where subsequent growth of the embryo occurs. o If fertilization does not occur, the undeveloped ovum continues down the oviduct and is flushed out of the uterus during the next menstruation. Testis Overview • Each mammalian testis also consists of an outer tunica albuginea, which encloses the seminiferous tubules that produce sperm. • Within the walls of the seminiferous tubules, stem cells multiply and grow into sperm that eventually are released into the lumen. • The coiled seminiferous tubules straighten, forming tubuli recti just before they join the rete testis. The rete testis joins the epididymis, where sperm are temporarily stored, via the efferent ductules. • Upon ejaculation, sperm travel along the vas deferens (ductus deferens) into the urethra. Along the way, three accessory sex glands (the seminal vesicle, prostate, and bulbourethral (Cowper’s) gland) add their secretions as sperm move from the testes to the urethra. The fluid and sperm it contains constitute seminal fluid, or semen. o Embryonic Development  Gonads and Gametes • The paired gonads arise from the genital ridge, initially thickening in the splanchnic mesoderm to which adjacent mesenchyme cells contribute. At this point, this germ cell containing gonad shows neither male nor female characteristics and is therefore called an indifferent gonad. • Interestingly, the germ cells do not arise in the genital ridge or adjacent mesoderm; in fact, they do not arise from the embryo at all. Germ cells arise from the extraembryonic endoderm! o In females, germ cells establish residence in the cortex. o In males, arriving germ cells establish residence in the medulla, which develops into the seminiferous tubule.  Reproductive Tracts • Parts of the embryonic urinary system are salvaged by or shared with the genital system. o In female mammals, the mesonephric duct (wolffian duct) drains the embryonic mesonephros, but it regresses later in development when the metanephros and its ureter become the kidney of the adult. However, a parallel Mullerian duct arises next to the embryonic mesonephric duct before it regresses. The Mullerian duct becomes the oviduct, uterus, and vagina. o In male mammals, the mesonephric tubules and some associated ducts contribute to the epididymis. Although a rudimentary Mullerian duct arises in males, it never assumes a significant role in the adult. o Female Reproductive System  Ovary • The ovary produces both hormones and mature ova • Oogenesis is the process of egg maturation, which occurs from the time of its appearance in theory until it completes meiosis. • The ovary is suspended from the dorsal wall of the coelom by a mesentery, the mesovarium. • Except for cyclostomes, in which eggs escape through secondary pores in the body wall, vertebrate eggs travel through genital ducts after they are released from the ovaries. • In most vertebrates, the ovaries are paired; however, in cyclostomes, most birds, the platypus, and some bats, only a single ovary is functional.    Genital Ducts (see lecture presentation) Oviduct (see lecture presentation) Uterus (see lecture presentation) o Male Reproductive System  Testis • Except in cyclostomes and some teleosts, testes are paired, and each is suspended from the dorsal wall of the coelom by a mesentery, the mesorchium. • The testes have two functions – sperm production and hormone secretion. • The hormones of the testes are steroids collectively called androgens. The principle androgen is •   testosterone, secreted primarily by the interstitial cells (Leydig cells) of the testes. Sertoli cells embrace and nutritionally support spermatids, perhaps promoting further maturation. Genital Ducts (see lecture presentation) Copulatory Organs (see lecture presentation) o Colaca (see lecture presentation) o Urinary Bladder References: Kardong, K (2011). Vertebrates: Comparative Anatomy, Function, Evolution. McGraw ­ Hill: New York, USA. Liem, K, Bemis, W, Walker, WF. (2010). Functional Anatomy of the Vertebrates: An Evolutionary Perspective. Brooks Cole: New York, USA. ...
View Full Document

This note was uploaded on 01/16/2012 for the course BI 200 taught by Professor Potter during the Fall '11 term at Montgomery College.

Ask a homework question - tutors are online