Reproductive System of Ascaris lumbricoides
Ascaris lumbricoides is a parasitic roundworm that infects the human intestine. It belongs to the phylum Nematoda, which is characterized by having a pseudocoelom, a complete digestive tract, and a cuticle covering the body. Ascaris lumbricoides is one of the most common helminthic infections in humans, affecting about 1.5 billion people worldwide.
One of the remarkable features of Ascaris lumbricoides is its reproductive system. Ascaris lumbricoides has separate sexes, meaning that there are male and female worms. The sexes are also dimorphic, meaning that they have different shapes and sizes. The males are smaller and have a curved tail with copulatory structures, while the females are larger and have a straight tail with an opening for egg laying.
The reproductive system of Ascaris lumbricoides consists of gonads, gonoducts, and accessory glands. The gonads are the organs that produce gametes, or sex cells. The gonoducts are the tubes that transport the gametes to the outside or to the site of fertilization. The accessory glands are the structures that secrete substances that aid in reproduction, such as lubricants, hormones, or nutrients.
The reproductive system of Ascaris lumbricoides is adapted to its parasitic lifestyle. The worms can produce a large number of eggs that can survive harsh environmental conditions and infect new hosts. The worms can also mate inside the host`s intestine or outside in the soil. The worms have a complex process of gamete formation that involves different zones of development and differentiation.
In this article, we will describe the male and female reproductive organs of Ascaris lumbricoides in detail. We will also explain how gamete formation occurs in this roundworm and what are the roles of the germinal, growth, and maturation zones in this process.
The male reproductive organs of Ascaris lumbricoides are located in the posterior half of the body and consist of the following parts:
- Testis: It is a single, long, thread-like, coiled tube that produces sperms. It has a solid core of cytoplasm called rachis, around which clusters of developing sperms are arranged. It is also called the growth zone, as the sperms grow and differentiate here.
- Vas deferens: It is a short and thick coiled tube that connects the testis to the seminal vesicle. It has a central lumen instead of rachis, which allows the passage of sperms.
- Seminal vesicle: It is a long, straight, and thick tube that stores the mature sperms. It lies below the intestine in the posterior third of the pseudocoel.
- Ejaculatory duct: It is a short, narrow, muscular and glandular tube that carries the sperms from the seminal vesicle to the cloaca. The cloaca is a common opening for both feces and sperms. The ejaculatory duct has prostatic glands that secrete a fluid to help in copulation.
- Penial setae: They are two club-shaped structures that are secreted and housed in two spicular pouches on the dorsal side of the cloaca. They can be protruded and retracted by special muscles. They help in opening the female gonopore during copulation.
The sperms of Ascaris lumbricoides are tailless, asymmetrical and amoeboid. They move by pseudopodia and fertilize the eggs inside the female uterus.
The male reproductive organs of Ascaris lumbricoides are shown in the following diagram:
The female reproductive organs of Ascaris lumbricoides are located in the posterior two-thirds of the body cavity and consist of two parallel tracts, each containing an ovary, an oviduct, and a uterus. This arrangement is called didelphic and is common in nematodes. The female reproductive organs have the following features:
Ovaries: These are long, thin, and coiled tubes that end blindly in the body cavity. They have no lumen and their wall is composed of a single layer of cuboidal cells that surround a cytoplasmic rachis. The rachis contains clusters of developing ova in various stages of growth. The ovaries are the site of oogenesis or egg formation.
Oviducts: These are short, thick, and twisted tubes that arise from the distal end of each ovary. They have a central lumen and their wall is similar to that of the ovaries. The oviducts transport the mature ova from the ovaries to the uteri.
Uteri: These are wide, straight, and thick tubes that receive the ova from the oviducts. They have a muscular wall with inner circular and outer oblique fibers and a layer of secretory cells that produce a protective shell around the fertilized eggs. The uteri store the eggs until they are ready to be expelled through the vagina.
Vagina: This is a short, muscular, and contractile tube that is formed by the union of the two uteri at the anterior one-third of the body. It has an inner lining of cuticle and opens to the outside by a slit-like opening called the vulva or female pore. The vagina serves as the copulatory organ and the passage for egg laying.
The female reproductive organs of Ascaris lumbricoides are adapted for producing large numbers of eggs that can survive harsh environmental conditions. The eggs are fertilized internally by the spermatozoa that enter through the vulva during copulation. The fertilized eggs undergo a second meiotic division and develop into embryos within the uteri. The embryos are then released into the soil through the vagina along with fecal matter. The eggs can remain dormant for several months or years until they find a suitable host to infect.
Gamete formation in Ascaris lumbricoides is a process of producing haploid sex cells (sperms and eggs) from diploid germ cells (gametogonia) in the gonads. Gamete formation involves two main steps: gametogenesis and maturation.
Gametogenesis is the formation and differentiation of gametogonia into gametocytes. Gametogonia are stem cells that are budded off from the terminal cell at the proximal end of the gonad. As they move towards the distal end of the gonad, they grow and differentiate into either spermatogonia or oogonia, depending on the sex of the worm. Spermatogonia and oogonia are the male and female gametocytes, respectively. They are attached to a cytoplasmic rachis, which is a central axis of the gonad.
Maturation is the transformation of gametocytes into gametes. Gametocytes detach from the cytoplasmic rachis and undergo meiosis, which is a type of cell division that reduces the chromosome number by half. Meiosis results in four haploid cells from each diploid cell. In males, each spermatogonium produces four sperms, which are amoeboid and motile. In females, each oogonium produces one egg and three polar bodies, which are small and non-functional cells. The egg is large and elliptical, and contains yolk granules for nourishment. The egg is actually a secondary oocyte, which has completed the first meiotic division but not the second. The second meiotic division occurs after fertilization by a sperm.
Gamete formation in Ascaris lumbricoides occurs in three distinct zones in the gonad: germinal zone, growth zone, and maturation zone. The germinal zone is where gametogonia are produced from the terminal cell. The growth zone is where gametogonia grow and differentiate into gametocytes. The maturation zone is where gametocytes mature into gametes. The maturation zone is followed by a gonoduct, which transports the gametes to the external environment for fertilization.
Gamete formation in Ascaris lumbricoides is an example of telogonic development, which is common in most nematodes. Telogonic development means that the germ cells are produced at the posterior end of the gonad and move towards the anterior end as they develop. This contrasts with progonic development, which occurs in some other animals, where the germ cells are produced at the anterior end of the gonad and move towards the posterior end as they develop.
Gamete formation in Ascaris lumbricoides involves the differentiation of germ cells into haploid gametes through meiosis. The germ cells are derived from a single terminal cell at the proximal end of the gonad, which buds off gametogonia. These gametogonia move towards the distal end of the gonad and undergo gametogenesis. This process can be divided into three zones: germinal, growth, and maturation.
- The germinal zone is also known as the proliferation zone. It lies at the proximal blind end of the gonad. Here, the gametogonia are attached to a cytoplasmic rachis, which is a central axis of cytoplasm surrounded by a single layer of epithelial cells. The gametogonia divide by mitosis and produce more gametogonia. This zone acts as a source of germ cells for gametogenesis .
- The growth zone follows the germinal zone. In this zone, the gametogonia grow in size and differentiate into gametocytes. The gametocytes are still attached to the cytoplasmic rachis, but they become more distinct from each other. The gametocytes are diploid cells that have undergone DNA replication and are ready to enter meiosis .
- The maturation zone is the distal-most zone, where the gametocytes separate from the cytoplasmic rachis and undergo meiosis. Meiosis consists of two consecutive divisions: meiosis I and meiosis II. In meiosis I, the gametocytes reduce their chromosome number from diploid to haploid by separating the homologous chromosomes. In meiosis II, the gametocytes divide again and separate the sister chromatids. The result is four haploid cells that develop into gametes .
The gametes of Ascaris lumbricoides are different in males and females. The male gametes are called sperms or spermatozoa. They are small, asymmetrical, and amoeboid. They have no flagella or cilia for locomotion, but they can crawl by pseudopodia . The female gametes are called ova or eggs. They are large, elliptical, and non-motile. They have a thick protective layer called vitelline membrane and a thin layer called zona pellucida around them . The ova are actually secondary oocytes that have completed meiosis I but not meiosis II. The second meiotic division occurs after fertilization by a sperm .
The gametes of Ascaris lumbricoides are produced in large numbers and released into the pseudocoelom (body cavity) of the worm. The male gametes are transferred to the female worm through copulation using penial setae (spicules). The female worm stores the sperms in a seminal receptacle until fertilization occurs . Fertilization leads to the formation of a diploid zygote, which develops into an embryo inside an egg shell. The eggs are passed out with the feces of the host and can infect new hosts by ingestion .
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