Embryogenesis and the first-stage larva of Thelazia lacrymalis.

The research paper provides insights into the reproductive system and embryonic development of a nematode known as Thelazia lacrymalis. The study was conducted using light and scanning electron microscopy techniques.

Reproductive System of Thelazia lacrymalis

• The research reveals that the female T. lacrymalis has a didelphic and opisthodelphic type of reproductive system. This particular reproductive system includes paired ovaries, oviducts, and uteri, along with a single vagina and vulva.
• Throughout the stages of development, round and spindle-shaped primary oocytes are noted within the ovaries and oviducts, respectively.

Uteri and Fertilization Process

• The distal part of each uterus encompasses a fertilization chamber filled with spermatozoa, followed by a sphincter-like part.
• Anterior to this, the uteri broadens, housing dividing zygotes, small and large morulae, tadpole-stage embryos, and horseshoe-shaped embryos.

Development of Embryos and Larvae

• The horseshoe-shaped embryos grow in length and gradually become slim, forming into pretzel-shaped embryos and later, rolled-up larvae.
• These larvae continue to stretch until they finally lie straight, still encapsulated within their egg membrane in the vagina.
• The egg membrane, which fully encloses the larva, extends at the larva’s pointed tail, forming a bulb.

Detailed Observations at SEM Level

• Scanning Electron Microscopy (SEM) provided a detailed view of the first-stage larva, revealing a terminal mouth, three hooks facing the posterior, and a striated cuticle.
• Observations also revealed that morphologically identical larvae were found in lavages of the conjunctival sac of horses infected with T. lacrymalis.
• Based on these observations, the researchers concluded that the nematode species T. lacrymalis is ovoviviparous, effectively giving live birth to developed larvae.