REPRODUCTIVE SYSTEM

I. PRIMARY REPRODUCTIVE ORGANS - GONADS

A. Develop from genital ridges (mesoderm); invaded by primordial germ cells which originate elsewhere in embryo.
B. Cortex and medulla develop differently depending on sex.
C. Sexual homology - organs of the 2 sexes are derived from identical primordia (indifferent gonad), yet become different in structure and function later in development. This equivalence is different from phylogenetic homology. Also referred to as sexual bipotentiality.
D. Ovaries:

1. Cortex of indifferent gonad is invaded by PGCs, forms germinal epithelium.
2. Produce ova (in follicles) and hormones.

E. Testes:

1. Develop from medulla of indifferent gonad, and tend to develop a little earlier than ovaries.
2. Sperm produced in seminiferous tubules, testosterone produced by Leydig cells.
3. Some mammals: testes descend into scrotal sacs, suspended outside body, but connected to coelom via inguinal canal.

II. ACCESSORY SEX ORGANS: ducts, glands, and copulatory organs which provide transport for gametes.

A. Urogenital ducts: (may be shared with or borrowed from excretory system)

1. Males - usually use archinephric duct, which may double as urinary tracts.

a. Cyclostomes - opisthonephric ducts carry only urine. Sperm released into coelom, leave via genital pores.
b. Primitive gnathostomes - uriniferous tubules in anterior opisthonephros connect with testes, so that opisthonephric duct carries urine and sperm.
c. Chondricthyes - archinephric ducts carry sperm only, accessory urinary ducts develop to carry urine.
d. Teleosts - new sperm duct develops so that opisthonephric duct carries only urine.
e. Amphibians - some have primitive arrangement in which opisthonephric duct carries both; some have new urinary ducts to drain kidney.
f. Amniotes - ureter drains metanephric duct, mesonephric duct forms ductus deferens.
g. Different solutions to problem of separating systems illustrates principle of 'unrepeatability of evolution'.

2. Females - variation less extreme than in males, since urinary and genital systems are more independent of each other. Why is this separation more important for females than for males?

a. Cyclostomes - no genital ducts, ova leave via genital pores.
b. Primitive gnathostomes - at first probably used urinary ducts to transport eggs, but separate ducts were acquired by splitting original ducts. Evidence: development of elasmobranchs - oviducts arise by splitting off from urinary ducts, enlarged nephrostome forms ostium.
c. All living gnathostomes have separate oviducts; in no case does urinary duct carry ova.
d. Oviducts generally do not connect directly with ovaries, instead enter ciliated ostium (probably a retention of a modified nephrostome). Exception: Teleosts secondarily acquire direct connections with ovaries, so that eggs won't clog body cavity.
e. Oviduct may be differentiated into regions for shell secretion, development of embryos, or reception of penis.
f. Trend toward fusion of posterior ends of paired oviducts; vagina remains paired in monotremes and marsupials, but is a single fused tube in eutherians.
g. Degree of fusion of uterus varies among mammals.

B. Copulatory organs:

1. Cyclostomes, bony fishes, most anurans - lay eggs in water, external fertilization.
2. Chondricthyes - internal fertilization by using claspers, which are modifications of pelvic fins, so that sperm is transported along grooves to female cloaca.
3. Some teleosts - modified anal fins or erectile genital papillae.
4. Urodeles - internal fertilization by cloacal apposition or transfer of spermatophores.
5. Squamata: hemipenes = evertible sacs near cloaca which bear grooves for sperm transport; usually only one inserted into female cloaca.
6. Unpaired penis - turtles, crocodilians, few birds, and mammals. Consists of erectile tissue with dorsal groove in first 3 groups; in mammals, distal part of urogenital sinus becomes incorporated into penis to form canal for sperm and urine conduction.
7. Penis may have bone to aid with erection (eg. os penis, bacculum).

III. TRENDS IN EVOLUTION OF REPRODUCTIVE SYSTEM

A. External fertilization to internal fertilization (necessary in terrestrial verts).
B. Transition from close structural relationship of excretory and reproductive systems to complete separation.