OBL HOME | OBL REFERENCES | |||||||||||||
THE REPRODUCTIVE SYSTEM
|
||||||||||||||
Reproduction existed long before sex; asexual
reproduction may have predated sexual reproduction by 1.5 billion years
or more. Although sexual reproduction
does have the advantage of creating diversity in a species, it is certainly
not a requirement for life in general.
Asexual reproduction is the predominant form of reproduction in
bacteria and in many protists and fungi. It can occur in plants and animals as well,
including vertebrates. Although
sexual reproduction is the primary form of reproduction in animals, the
process of meiosis which is the basis for sex was essentially complete
before the first animals evolved. Although animals would make sexual reproduction
more complex (by enclosing meiotic cells together to form gonads, developing
tubes through which gametes could travel, adapting the process to occur
inside the female body in many groups, and recruiting the nervous system
to involve sexual drives and courtship behaviors) the cellular mechanisms
of sexual reproduction have changed little since the evolution of eukaryotic
protists. In sponges, sexual reproduction occurs without
gonads—sex cells may originate from anywhere any part of the animal (Beklemishev, vol. 2). In
sponges, ova move like amoeba and may perform phagocytosis
(Barrington, p. 386). Spermiogenesis occurs to modify the shape of the male gamete,
although no acrosome is formed on the sperm
(Harrison, Vol. 2). Oocytes
are large cells, about ten times the size of the oogonia
from which they developed (Harrison, Vol. 2).
|
||||||||||||||
![]() |
![]() |
|||||||||||||
In cnidarians, reproductive cells can divide
to produce a small groups of cells, forming the
most primitive ovaries and testes (Beklemishev,
vol. 2). These “gonads” cannot be
considered as organs, they are simply small aggregations of reproductive
cells (Hyman). It is interesting that in both these cnidarian “gonads” and the first true gonads found in higher
flatworms, the sex cells must migrate to the gonads rather than originate
there, since this is also a characteristic of higher animals as well (Hyman,
p. 431; Hickman). In craniates from
hagfish to humans, spermatogonia and oogonia are produced in yolk sac and migrate to the gonads
(Romer, p. 421).
|
||||||||||||||
HYDRA |
![]() |
|||||||||||||
![]() |
The most primitive flatworms are equivalent
to coelenterates with regards to their reproduction. Acoela possess no gonads since spermatogonia
and oogonia are dispersed throughout the body. Eggs lack yolk (Hickman, Beklemishev,
vol. 2). Higher flatworms do possess
gonads and in nemertine worms, the gonads are
lined by epithelium (Hickman). In
lancelets, the gonads are segmented unlike the situation in higher chordates.
Lancets have 26 pairs of gonads, starting in
the middle of the pharynx. In jawless and cartilaginous fish, the secondary
oocytes are released into the body cavity.
Even though females in higher vertebrate groups possess an oviduct
to transport the secondary oocytes, they are still released into the body cavity first
(causing a risk of ectopic pregnancy in humans)
(Weichert, p. 280) Many invertebrates are hermaphrodites, possessing
both male and female gonads. This
condition even exists in primitive chordates: almost all ascidians and
some lancelets are hermaphrodites. In
lampreys, 2 larval gonads fuse to form 1 which is attached to a mesovarium. In hagfish,
the anterior portion of the gonad becomes ovarian tissue and the posterior
portion becomes testicular, although these do not function simultaneously.
In most fish, the two ovaries fuse. In cartilaginous fish, the left ovary degenerates
later in life (Weichert). |
|||||||||||||
SHARK![]() |
![]() |
|||||||||||||
BOWFIN |
FROG |
|||||||||||||
FOLLICLES The
simplest animals, sponges and cnidarians, lack follicular cells which
surround the ova. In some flatworms the ovary is compacted and divided
into layers so that oocytes may be surrounded
by follicular cells. In hemichordates
and urochordates, follicle cells surround oocytes
unlike the condition in males which possess no such cells. In both
hemichordates and urochordates, oocytes are released from the ovary before meiosis is complete.
Just as in vertebrates such as humans, an ova
is technically not produced until fertilization (and thus a female which
has not conceived has technically never produced a female gamete). (Benito,
form Harrison 1997, p. 93; Burighel, from Harrison,
1997, p. 282). In the most primitive flatworms, eggs lack yolk
(Hickman, Beklemishev, vol. 2) while in others,
entolecithal egg production occurs (Dougherty). In tunicates, some of the follicular cells
remain as a “corpus luteum” or follicular residuum
after ovulation. (Burighel, from Harrison, 1997,
p. 288) In a number of teleost
fish, a corpus luteum forms after ovulation
(Weichert, p. 281). In
amphibians, the ova are released from the external portion of the ovary
and the center portion functions in the lymphatic system. Corpus lutea exist
in some amphibians, all reptiles, and all mammals (Weichert,
p. 283; Romer). In the Indian elephant, blue antelope, and in
horses, accessory corpora lutea form during
pregnancy (Weichert, p. 286). |
||||||||||||||
AMPHIOXUS |
||||||||||||||
OOCYTES |
||||||||||||||
SALAMANDER |
||||||||||||||
CHICKEN |
![]() |
|||||||||||||
![]() |
||||||||||||||
OPOSSUM |
||||||||||||||
CAT |
PIG |
|||||||||||||
MONKEY |
![]() |
|||||||||||||
HUMAN MODEL |
||||||||||||||
Marsupials are similar to placental mammals
in their estrus cycle which includes the cyclical growth and loss of the
endometrium (Stonehouse, 1977).
In a number of mammals, ovulation only occurs
after copulation. (Weichert, p. 285) SPERM |
||||||||||||||
HYDRA |
||||||||||||||
![]() |
||||||||||||||
In hemichordates
and cephalochordates, the sperm consist of a head, a midpiece
containing few mitochondria, and a flagellar
tail just as in chordates (Benito, form Harrison 1997, p. 95
). In urochordates and
lancelets, intercellular bridges exist between spermatocytes
during spermatogenesis, just as in vertebrates (Burighel,
from Harrison, 1997, p. 295)
In lancelets, spermatocytes progress
towards lumen as meiosis occurs and Sertoli
cells exist (Ruppert, from Harrison, 1997, p.
491). In both lancelets and jawless fish, sperm are
released into the coelom (as are ova). Amphioxus |
||||||||||||||
![]() |
||||||||||||||
SHARK |
||||||||||||||
LUNGFISH |
![]() |
|||||||||||||
FROG |
||||||||||||||
TURTLE |
ALLIGATOR |
|||||||||||||
CAT |
||||||||||||||
MONKEY |
||||||||||||||
![]() |
||||||||||||||
HUMAN MODEL |
||||||||||||||