Lecture 23: Vertebrate Genitourinary System Pt. II Flashcards
1
Q
Explain the bipotentiality of vertebrate reproductive development
A
- Sexual reproduction is universal among vertebrates.
- There are functionally separate sexes in almost all vertebrate species.
- The reproductive system anatomically consists of gonads and their associated mesenteries and ducts that convey gametes to the exterior.
- Even though the reproductive system is either male or female in a given individual, it begins in the embryo as a bipotential system that has the ability to develop either into a male system or a female system.
- The fate of this bipotential system in a given individual is primarily under the control of the genetic sex of the individual, but other factors such as hormones and environmental factors may also play a role
- See Slide 5
2
Q
Bipotential System
A
- Like the excretory system, the reproductive system is entirely of mesodermal origin.
- The gonads arise by proliferation of epithelium along the mesonephric ridge (= gonadial or genetic ridge).
- The embryonic gonad is a “neutral” gonad which has the potential of forming either a testis or ovary.
- The mesonephric duct will also contribute to the reproductive system and is referred to in this capacity as the Wolffian duct.
- In addition, a second pair of ducts, the paramesonephric (Mullerian) ducts, also form in the mesonephric ridge parallel to the Wolffian ducts
- The gonads are the first part of the reproductive system to develop. They develop later after the embryonic kidneys have been established.
- The gonads are indifferent. They retain for some time the potential to form either an ovary or a testis. The “default” condition is female.
- The switch that determines whether the indifferent gonad will develop by default into an ovary or switch to a testis is carried on the Y chromosome on the SRY (sex-determining region) gene.
3
Q
Bipotential Gonads
A
- The indifferent gonad consists of an outer cortex and an inner medulla.
- Within the medulla, solid cords of cells develop. These are called the sex cords. The sex cords have the potential to give rise to the seminiferous tubules containing Sertoli cells and interstitial cells in the event of a genetic male.
- Anastomosing with the sex cords in the genetic male are the rete cords. In the genetic female the medulla regresses and the cortex forms the ovary.
- Separating the cortex from the medulla is a connective tissue layer called the tunica albuginea. In the adult gonad, the tuica albuginea will lie on the external surface of the testis but will be internalized in the ovary.
4
Q
Gonads and gametes
A
- Gonads are the anatomical structures that house the developing gametes.
- Gametes develop from reproductive cells that have the unique ability to undergo meiosis (in addition to mitosis) in order to form the haploid, genetically unique gametes.
- Gametes are of two types:
- Relatively large, often yolk-filled eggs
- Smaller, usually motile sperm
- The development and maturation of gametes occurs in the gonads.
- Fertilization, involving the fusion of a sperm and an egg, occurs either in the associated ducts or external environment.
5
Q
Testes
A
- In the male the gonads are the testes.
- Testes are typically paired structures:
- In cyclostomes they are fused.
- In sharks they may be fused posteriorly.
- Testes are made up of microscopic tubules called seminiferous tubules within which spermatogenesis takes place.
- The tubules are connected via a system of ducts to the actual ducts that convey sperm to the exterior.
- In most vertebrates:
- Testes occupy a coelomic position
- Testes are supported by a mesentery-like structure, the mesorchium.
- See slides 12-15
6
Q
Ovaries
A
- Ovaries are usually paired and located toward the cranial end of the trunk.
- In cyclostomes and some teleosts the ovaries are fused.
- Ovaries house developing eggs.
- Eggs will be released from the ovaries in a process called ovulation.
- They will then travel down the duct system (if present) and may become fertilized, either within the ducts or in the external environment.
- Ovaries tend to be oval-shaped at rest but can become very irregular and lobular in appearance when distended with eggs.
- Ovaries are suspended into the coelom by a mesentery-like membrane called the mesovarium.
- See Slide 17-18
7
Q
Bipotential Ducts
A
- While the gonads are developing, two pairs of ducts form in the lateral regions of the mesonephric ridges. One pair, the mesonephric or Wolffian ducts, form as described above. The second pair (paramesonephric or Mullerian ducts) form lateral to the mesonephric ducts as longitudinal invaginations along the mesonephric ridge. Both pairs of ducts empty into the cloaca.
- The mesonephric tubules in the vicinity of the developing gonads will anastomose with the rete cords.
- In females, the rete cords will later degenerate along with the rest of the medulla, but some vestigial mesonephric tubules may be retained.
- In males, the mesonephric tubules will be retained as part of the closed duct system that conveys sperm to the exterior. In the male, then, these retained mesonephric tubules are called efferent ductules.
8
Q
Reproductive Ducts Part I:
A
- The reproductive system in vertebrates also develops as a bipotential system that has the ability to become either a male or a female system.
- All vertebrate individuals, regardless of genetic sex, have two pairs of ducts:
- The archinephric or mesonephric ducts which may be used for sperm transport in the male and which degenerate in the female (or else or retained in both sexes for urine transport)
- The paramesonephric or Mullerian ducts are used in the female for egg transport. In the male the Mullerian ducts degenerate.
- In cyclostomes sperm and eggs are discharged directly into the coelomic cavity and from there move to the external environment via a urogenital pore.
- In all other vertebrates, however, sperm, and for the most part eggs, are conducted to the exterior via a closed duct system.
- See Slide 22-23
9
Q
Reproductive Ducts Part II
A
- Generally, the male transport system is a modified part of the urinary system.
- As the testis develops in a vertebrate it lies close beside and medial to the embryonic kidney.
- A short distance from the seminiferous tubules within the testis are the kidney tubules (nephrons) which empty into the archinephric (mesonephric) duct leading to the cloaca.
- If the short gap between the testis and the kidney is bridged by additional ductwork there becomes available a completely closed system by which the minute sperm could safely travel to the cloaca.
- The closing of this gap occurred in early gnathostomes and, in most vertebrates, the urinary system in the male has been more-or-less associated with the reproductive system.
10
Q
Reproductive Ducts Part III
A
In the most primitive condition, exemplified by frogs and sturgeons, the mesonephric duct transports both sperm and urine.
- Most vertebrates, however, have developed new ducts so that sperm and urine are kept separate, at least most of the way.
- In most bony fishes the kidney retrieves its original duct (the mesonephric duct), and a new duct develops on each side for sperm transport.
- This new duct probably develops from the central canal.
- In many vertebrates the connection between the seminiferous tubules and the nephrons occurs via the development of two longitudinal and parallel ducts (per kidney/gonad).
- One of these ducts (the central canal) connects the distal ends of the seminiferous tubules; the other (the lateral canal), the kidney tubules.
- Connecting these two parallel canals are numerous short tubules called the efferent ductules.
11
Q
Reproductive Ducts Part IV
A
- In most other vertebrates, including cartilaginous fishes and most tetrapods, the mesonephric duct is used mostly or exclusively for sperm transport.
- Urine conduction in elasmobranchs is found only in the caudal end of the mesonephric duct.
- When the mesonephric duct is used mostly or totally for sperm transport it is usually called a ductus deferens or a vas deferens.
12
Q
Reproductive Ducts Part V
A
- The Mullerian ducts degenerate in the male and form the oviducts in the female.
- Unlike the mesonephric ducts which link up with other ducts and ductules to form a closed transport system, the oviducts are open to the coelomic cavity at their cranial end.
- Typically the cranial end of the oviduct is expanded into a funnel-like structure called the infundibulum.
- This funneled opening receives the eggs when they are released (ovulated) from the ovary.
13
Q
Reproductive Ducts Part VI
A
- Modifications may develop along the walls of the ducts in the form of glands that secrete yolk and/or various types of membranes around the eggs as they are traveling down the duct.
- In some instances eggs are fertilized internally within the oviduct.
- Many fishes, however, transport the eggs to the exterior for external fertilization.
- In some fishes, like sharks, the fertilized eggs may be retained within the oviduct where they develop to maturity.
- The areas of the oviducts where fertilized eggs develop are usually referred to as uteri.
- See Slides 30-31
14
Q
Female System
A
- The cortex of the indifferent gonad will form the ovary.
- The rete cords and sex cords will degenerate along with the mesonephric tubules and ducts.
- The Mullerian ducts will give rise to the oviducts, uterus, and a portion of the vagina.
- In order to form the uterus, the Mullerian ducts fuse distally. This may be an almost nonexistent fusion as in the marsupials. In such a case the uteri remain unfused, and the condition is referred to as a bipartite (duplex) uterus.
- In most mammals there is some fusion that occurs, resulting in a uterus with two prominent expansions, called horns. This type of uterus is called a bicornuate uteus.
- Maximum fusion occurs in primates such as humans. This single uterus is called a simplex uterus.
- The proximal ends of the Mullerian ducts do not anastomose with any structures within the developing ovaries. Instead, the proximal ends of the Mullerian ducts become expanded as the ostia of the oviducts.
- When eggs are ovulated they are released from the Graafian follicles and almost immediately enter the ostia and begin their journey down to the uterus.
15
Q
Comparative Reproductive Anatomy in Anurans
A
- In male frogs, the archinephric ducts serve impartially for both urinary and genital functions.
- In many amphibians urine conduction is found, at most, only at the back end of the duct.
- Distally the archinephric duct may form sperm storage ampullae.
- Gonads are internal in both sexes.
- Both sexes have rudimentary structures of the opposite sex.
- Male toads have a Bidder’s organ which is a rudimentary ovary capable under certain conditions of becoming a functional ovary.
- In Ranayoung males show female tendencies and old females may shift to a male pattern.
