Avian Male Reproductive Tract - Anatomy & Physiology

Introduction

Paired reproductive tracts lie along the dorsal body wall. Each tract consists of a testis, a rudimentary epididymis and a highly convoluted deferent duct running alongside the ureter. The testes are connected to the body wall by a mesochorium. This peritoneal fold serves as an attachment for the testes and also a conduit for nerves and blood vessels.

Testes

Dissection of the Male Avian Reproductive Tract- Courtesy of Andrew Crook, Copyright RVC 2008

• Bean-shaped, paired
• Lie near the cranial pole of the kidney
• Medially, they lie close to the aorta and caudal vena cava.
• Each testical suspended by a short mesochorium and surrounded medially by the abdominal air sac.

• Left tends to be larger than right in immature birds.
• Dimensions increase rapidly with sexual activity.
• In the non-breeding season, testes shrink to almost nothing and become hard to visualize.
• Dormant testes light brown/yellow in colour, turn white when sexually active.
  • In some psittacine species, immature or dormant testes may appear black due to melanocytes located in the interstitium.
• Tunical Albiguinea thinner than in mammals.
• No Pampiniform plexus.

• Convoluted semniferous tubules comprised of spermatogonia (germ cells) and Sertoli cells make up the bulk of the testes. These are responsible for spermatogenesis.
• Interstitial Leydig cells produce male androgens.
• Mature spermatozoa exit into the rete testes, which connects to the cranial epididymis.
  • Rete testes not present in all birds.
  • Epididymis considered vestigial in birds.
    • Sperm maturation occurs in the ductus deferens.
• Epididymis lies along the dorsomedial aspect of the testes.
• Spermatazoa exit the epididymis and enter the ductus deferens.
  • Ductus deferens is closely associated with the Ureter in the dorsomedial midline coelom, distinguished by its zig-zag appearance.
  • Under hormonal control, more convoluted in the breeding season.
• Ductus deferens enters the dorsal wall of the cloacal urodeum.
  • Straightens and abruptly widens at ats junction with the cloaca.
    • Structure known as the receptacle in Passerines and Budgerigars.
    • Receptacle appears bean-shaped when engorged with semen.
    • Birds that do not have the receptacle structure have little sperm storage capacity.

Phallus

• Most birds lack a true phallus.
• Analogue of the mammalian penis.
• Consists of a small median tubercle flanked by a pair of large, lateral phallic bodies.
• When present, the avian phallus is soley reproductive and becomes engorged by lymph fluid instead of blood during erection.
• Owing to the lack of accessory sex glands, avian semen has low volume.
  • Some lymph may contribute to the seminal fluid.
• Sperm remains viable in the female tract for much longer than in mammals.
  • May survive for 5-6 days.

Absence of Phallus

• Psittacines, Passerines, Pidgeons and birds of prey all have no phallus.
• Copulate by transferring semen from the everted Cloaca directly into the female oviduct.

Non-Protrusible Phallus

• Rudimentary non-protrusible phallus is seen in male Turkeys and Chickens.
• Lies on the ventral lip of the vent.
• Consists of a small medial tubercle intimately associated on each side with lymphatic folds and vessels.
• When erect with lymph, the phallus develops a median groove.
  • Median groove permits passage of ejaculate down into the everted female oviduct.

Protrusible Phallus

• Ratites and Anseriformes
• Elongated, capable of true intromission into the female cloaca.
• Distal end lies enclosed in a cavity on the floor of the cloaca and becomes engorged with lymphatic fluid.
  • Anseriformes have a curved, fibrous phallus that conveys semen via a spiral groove.

Accessory Sex Organs

The male avian has no accessory sex glands that are seen in mammals. Instead, they have accessory reproductive organs:

• Paracloacal Vascular Bodies
• Dorsal Procotodeal Gland
• Lymphatic Folds

These are either in proximity to, or are an integral part of the cloaca.

Paracloacal Vascular Bodies

• Found alongside the receptacle of the vas deferens.
• Contribute to the lymphatic erection of the either cloacal or phallic tissue.
• Release a lymph-like transparent transudate when engorged.

Dorsal Procotodeal Gland

• Found on the dorsal proctodeum.
• Develop to varying degrees.
• Undergo hypertrophy in response to steroid sex hormones.

Lymphatic Folds

• Found within the wall of the procotodeum.
• Contribute to the lymphatic erection of the either cloacal or phallic tissue.
• Release a lymph-like transparent transudate when engorged.