Biology, SS 2 Week: 10

Topic: Reproductive System

Introduction

The reproductive system or genital system is a system of sex organs within an organism which work together for the purpose of reproduction. Reproduction is a process involving the coming to life of new organisms either from one parent or a pair of parent organisms. Reproduction allows continuity of life. It may be sexual or asexual.

  •  Sexual reproduction involves the fusion of two different sex cells which usually come together from two different parents to produce an offspring. Gametes are formed by a kind of cell division called meiosis.
  • Asexual reproduction involves only one parent. An individual produces an offspring by itself. There is no fusion of nuclei, and the cells which give rise to the offspring usually divide by means of mitosis.

Reproductive Systems in Fish, Reptiles, Birds and Mammals

The Reproductive System of Fish

All fish species reproduce sexually. This means that a new individual is created by the combination of an egg and a sperm. The unifying of egg and sperm is known as fertilization, and the juvenile fish develop within the egg. However, there are some variations among fish in the methods of reproduction.

Sexual Organs: The two organs responsible for producing eggs and sperm in fish are ovaries and testes respectively. Usually, the female has ovaries and the male has testes, however a few species of fish are hermaphrodite, containing both sets of sexual organs in one individual.

Oviparous Fish: Over 97 percent of fish species are classified as oviparous reproducers. This means that the development of the embryos inside the eggs occurs outside of the parent’s body. This is the same reproductive method used by insects, amphibians, birds, reptiles and arachnids

Oviparous Reproductive Method

Female oviparous fish release unfertilized eggs (either laying them on a rock or plant surface) which are then fertilized by the male either by rubbing his sexual organs on the eggs, releasing his sperm, or emitting his sperm into the water so that fertilization takes place in the zooplankton layer.

Ovoviviparous Fish: Some species of fish notably rays and chimaeras are classed as ovoviviparous reproducer. This means that fertilization and development of the embryonic fish take place inside the female. The fish hatch inside the female’s body, being born as live young. Unlike in mammals, the eggs do not receive extra nutrition from the mother’s body, developing with energy solely from the egg’s yolk.

Viviparous Fish: Like ovoviviparous reproducers, viviparous fish develop their young within the female’s body. However, the eggs hatch inside the mother and remain in the placenta where they receive oxygen and nutrients from the mother’s biological system via an umbilical cord. When mature they are also born as live young.

Reproductive System in Reptiles

Most reptiles lay eggs. The act of laying eggs is called oviposition. Reptiles that lay eggs are called oviparous. Some reptiles bear live young and the term for this is viviparous. Technically, a female that lays eggs is said to be gravid when she is holding eggs inside of her.

The male Agama lizard has two testes which are located in the abdomen. The right testis is slightly higher in position than the left testis. From the testes, the sperm are conducted to tiny tubes known as vasa deferentia which lead them to the coiled Epididymis found along the outer edge of the testis. The sperm cells are finally passed on to the two protrusible penises. Each penis releases sperms into the female for internal fertilization.

In the female reproductive system, the two ovaries are located in the abdomen. The eggs are released into the abdominal space and by the action of some cilia the eggs are moved into the oviduct. Albumin and shell are deposited on the eggs as they move down the oviduct. The oviduct opens at the posterior end into the cloaca near the openings of the ureter. As mating between the male and female lizard takes place, fertilization takes place internally.

Reproductive System in Birds

The male reproductive system of domestic fowl consists of two large and round ovoid testes which are attached to the outer edges of the kidneys. From each testis, a vas deferens leads towards the hind end, alongside the ureter. Birds generally do not have penis. In many birds, the sperm ducts expand at their posterior ends to form seminal vesicles. Sperms accumulate here during copulation and are transferred from the male to the female when their cloaca comes in contact.

In the female reproductive system of domestic fowl, only the left ovary is functioning. The single ovary produces ova (eggs) in capsules attached to the ovary by short stalks. The ovary also produces the yolk. The egg is finally formed in the uterus after which the egg is laid through the cloaca. Fertilization of the egg can take place as soon as the egg enters the oviduct when spermatozoa are present. Fertilization occurs before the formation of the albumen.

Female reproductive organs of birds

Reproductive system in Mammals

Mammals generally are unisexual and reproduce sexually. Reproduction in mammals e.g. man is viviparous, that is, they give birth to living young ones that develops from fertilized egg.

Structure and Functions of Female Reproductive System

The female reproductive system is designed to carry out several functions. It produces the female egg cells called the ova for reproduction. The system is designed to transport the ova to the site of fertilization, the fertilization of an egg by a sperm, normally occurs in the fallopian tubes. The fertilized egg is implant into the walls of the uterus (womb), beginning the initial stages of pregnancy. If fertilization and/or implantation do not take place, the system is designed to menstruate. In addition, the female reproductive system produces female sex hormones that maintain the reproductive cycle. The human female reproductive system contains three main parts: the vagina, which leads from the vulva, the vaginal opening, to the uterus; the uterus, which holds the developing fetus; and the ovaries, which produce the female’s ova.

  1. Vagina: The vagina is a canal that joins the cervix (the lower part of uterus) to the outside of the body. It also is known as the birth canal.
  2. Uterus (womb): The uterus is a hollow, pear-shaped organ that is the home to a developing fetus. The uterus is divided into two parts: the cervix, which is the lower part that opens into the vagina, and the main body of the uterus, called the corpus. The corpus can easily expand to hold a developing baby. A channel through the cervix allows sperm to enter and menstrual blood to exit.
  3. Ovaries: The ovaries are small, oval-shaped glands that are located on either side of the uterus. The ovaries produce eggs and hormones.
  4.  Fallopian tubes or oviduct: These are narrow tubes that are attached to the upper part of the uterus and serve as tunnels for the ova (egg cells) to travel from the ovaries to the uterus. Conception, the fertilization of an egg by a sperm, normally occurs in the fallopian tubes. The fertilized egg then moves to the uterus, where it implants into the lining of the uterine wall.

Structure and Functions of Male Reproductive System

The male reproductive system includes the scrotum, testes, spermatic ducts, sex glands, and penis. These organs work together to produce sperm, the male gamete, and the other components of semen. These organs also work together to deliver semen out of the body and into the vagina where it can fertilize egg cells to produce offspring.

  1. Testes: are two oval-shaped organs present in the lower part of the abdomen. The 2 testes, also known as testicles, are responsible for the production of sperm and the male sex hormone, testosterone.
  2.  Scrotum: This is a sac-like organ made of skin and muscles that houses the testes. It protects the testes and epididymis.
  3.  Epididymis: It is a long (6m) coiled tube on the outside of testes. It is for the temporary storage of sperm after production until when matured.
  4.  Sperm duct or vas deferens: is a narrow tube which leads from the epididymis to the seminal vesicle. They join with the tube from the bladder to form the urethra. It carries urine or sperms though never both together.
  5.   Prostate gland, Seminal vesicle and Cowper’s gland: are along the sperm tubes and urethra. They secrete fluid (seminal fluid) containing food and enzymes to activate the sperms as well as make them mobile (fluid + sperm = semen).
  6.   Penis: is an prominent organ for introducing sperms into the female reproductive organ (vagina). The penis contains spongy tissues which can be filled with blood and become erected. It introduces the sperm into the vagina of the female and for urination.
  7.  Urethra: is a narrow tube which prolonged into the penis. The urethra is urinogenital in function, that is, it serves as a means of reproduction as well as excretion. It aids the passage of sperm into vagina and the passage of urine out of the body

Sex cells

The reproductive sex cells are also known as gametes. The formation of gametes called gametogenesis takes place in the gonads (Testes and Ovaries)

Male gametes or Sex cells called sperms are produced in the testes by a process called spermatogenesis. The gamete is unicellular in nature. The sperm consists of a head which contains the nucleus, a m