Reproductive System of Ascaris lumbricoides- Wandofknowledge
Sexes in Ascaris are separate and sexual dimorphism is well defined. Males are smaller than females. They possess a recurved tail with pre- and post anal papillae, a cloaca, and a pair of spicules called penial setae. Gonads are typically long, coiled and tubular. They are attached at the genital pore in female and at cloaca in male.
[I] Male reproductive organs
These are confined to the posterior half of pseudocoel. It includes following parts:
There is a single testis, a condition termed monorchic in contrast to the diorchic or two testis condition found in some nematodes. It extends to the middle of body and has the form of a long, thread-like, highly twisted tube. Wall of testis is made up of a single layer of cuboidal cells covered by basement membrane. Central axis of testis is in the form of a solid cytoplasmic rachis, around which are clusters of amoeboid sperms in various stages of development. There is no lumen.
Distal part of testis continues into a short and thick twisted tube, the vas deferens. It is distinguished from testis in having a lumen in place of the central cytoplasmic rachis.
Vas deferens is followed by a much thicker, wider, somewhat muscular straight tube, the seminal vesicle. It lies in the posterior third of pseudocoel below intestine.
Terminal part of seminal vesicle narrows to form a highly muscular ejaculatory duct. It joints rectum to from the cloaca. Ejaculatory duct contains a number of prostatic glands whose secretions help in copulation.
It is situated on the dorsal side of the cloaca, and formed by evagination of the latter, and is a pair of small penial sacs or spicular pouches. Each spicular pouch secretes and houses a club-shaped spicule or penial seta, enclosed in a spicular sheath and consists of a cytoplasmic core surrounded by thick cuticle. The spicules, each are 2 to 3.5 mm long, can be protruded and retracted through the cloacal aperture by special
protractor and retractor muscles. Spicules help in opening the female gonopore for copulation.
[II] Female Reproductive Organs
These lie in the posterior two third of pseudocoel and comprise two parallel tracts, each consisting of an ovary; an oviduct and a uterus. The didelphic or two tract condition is most common in nematodes, though monodelphic (one tract) and polydelphic (many tracts, upto 10 or 11) conditions are also met with.
These are two in number. Each ovary of Ascaris is a long, thread-like, much twisted and blind tubule. Its wall consists of a single layer of cuboidal epithelial cells lined externally by a basement membrane. Its central axis is in the form of a cytoplasmic rachis, around which are groups of ova. There is no lumen.
From the distal end of each ovary arises a thicker, wider and twisted oviduct, with a similar wall as that of the former and a lumen instead of solid cytoplasmic rachis.
Each oviduct continues into a much wider, thicker and almost untwisted tube, the uterus. Their wall is composed of tufted cells, surrounded by a muscle layer with inner circular and outer oblique fibres. The uteri serve to store fertilized eggs enclosed in shells.
Near the anterior one third of body, the two uteri unite to form a median, short and highly muscular tube, the vagina, having an inner lining of cuticle. It opens to exterior through female gonopore or vulva, situated mid-ventrally at about one third distance from the anterior end.
[III] Gamete formation
In Ascaris, gametogonia are budded off from the blind proximal end of gonad and undergo gametogenesis as they move towards the distal end. This mode of germ cell formation, termed telogonic, occurs in most nematodes. The alternative mode termed hologonic, is confined to only two orders, Trichuroidea and Dioctophymoidea. In this latter mode, the germ cells proliferate along the entire length of gonad.
In a telogonic gonad, three zones can be distinguished:
- Germinal zone: Also known as proliferation zone, it lies at the proximal blind end of gonad. Usually a single large terminal cell constitutes this zone. A large apical cell which is in fact an epithelial cell of the gonad wall, lies in front of the terminal cell, from which gametogonia arise by budding.
- Growth zone: It follows the germinal zone. Here gametogonia, which lie attached to the cytoplasmic rachis, grow and differentiate into amoeboid gametocytes.
- Maturation zone: It is the distal most zone, where gametocytes separate from rachis and undergo maturation to form gametes. It is followed by the gonoduct. Spermatozoa (sperms) are amoeboid, while the ova are elliptical. Ova at this stage are actually secondary oocytes, having resulted from oogonia by a single maturation division. Second maturation division occurs after fertilization.
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