Sexual vs. Asexual Reproduction
Asexual Reproduction.
There are many forms of asexual reproduction exhibited throughout the animal kingdom.
Sexual Reproduction
Reproductive Patterns
Most animals have their reproductive patterns linked to the changing seasons. This allows for the best possible environment to ensure that their offspring will survive. Since some animals do not exhibit separate sexes in their species, some lower forms of invertebrates contain both sexes in the same body. This is called hermaphroditism. This allows the organism who has difficulty finding a suitable mate the chance to reproduce. Many hermaphrodites exchange sperm with a mate increasing the potential of producing offspring. Another form of sexual reproduction is parthogenesis. Parthogenesis is the development of an egg without fertilization. Adults developed by parthogenesis are haploid.Some social insects develop in this way. The worker bees are diploid while the drones (males) are haploid. Sequential hermaphroditism occurs in some species. Here an animal can change it s sex. This reversal is connected with the age and size of the animal. Clams and some fish are able to carry out this process.
Major Mechanisms of Fertilization.
Embryo Protection.
Externally produced embryos are protected in several ways: they may be covered with a jelly-like coat which allows for free gas exchange with the environment. Large numbers of zygotes are produces to insure that some reach maturity. The water they are released in is suitable for optomim development.
Internally produced embryos may be protected by a hard shell, as in reptiles and birds. Placental mammals retain their offspring in special structures called the uterus. Internally protected zygotes have a higher rate of survival with a minimum of eggs being produced.
Human Reproduction
Structure and Function
Located in the scrotum out side the body. composed of tightly coiled tubules that produce sperm. The surrounding tissue produces the male hormones
Newly produced sperm is stored in this tube just outside and to the rear of each testicle.Here the sperm will develop.
Vas deferens
A muscular duct that connects the epididymis and the ejaculatory duct. It carries sperm out toward the urethra during ejaculation.
Ejaculatory duct
the area formed by the union of the two vas deferens ducts (one from each testis) and the duct from the seminal vesicles.
The tube that runs through the penis and drains the excretory and reproductive systems.
Seminal Vesicles
Gland located behind the bladder and empty into the ejaculatory duct. Mucus, amino acids, fructose and prostoglandins are found making up the fluid. Makes up 60% of seman volume.
Prostate Gland
Large gland that surrounds the upper portion of the urethra. Gives off a thin milky secretion containing several enzymes. It nutralized the acidic urine residue in the urethra and activates sperm.
Bulbourethral Glands
Glands below the prostate that empty into the urethra at the base of the penis. Unclear function
Composed of 3 cylinders of spongy erectile tissue.
Structure and Function
Each ovary contain many follicles (one egg surrounded by follicle cells). Formed at birth. Produces estrogen. Starting at puberty one follicle matures and releases an egg during each menstrual cycle.
Fallopian Tubes
Do not touch the ovaries. Funnel like opening surrounded with cilia. Houses the mature egg and is the site of fertilization. Distil end connects to the uterus.
Thick muscular organ shaped like an inverted pear. 7 cm long and 4-5 cm wide. The lining is the endometrium, it is richly supplied with blood vessels.
Neck of the uterus which opens to the vagina.
Thin walled chamber serves as a repository for the seman and forms the birth canal.
Chamberlike area formed by 2 pairs of skin folds covering the vaginal and uretheral openings.
Bulb of erictile tissue at the front edge of the vestibule.
Bartholin's Gland
Small gland located near the opening of the vagina. Secretes a mucus used in the lubrication of the vagina during sexual arousal.
Female Menstrual Cycle
Menstrual Flow Phase
During this time most of the endometrium is being lost from the uterus. It lasts for only a few days. The first day of this phase is designated as Day 1 of the cycle.
Proliferative Phase
Lasts for 2 or 3 weeks. It involves the regenerastion and thickening of the endometrium.
Secretory Phase
Lasts about 2 weeks. The endometrium continues to thicken and become more vascularized, and produces glands that produce glycogen rich fluid. If the embryo is not implanted the flow phase begins.
Ovarian Cycle
An ovarian cycle parallels the menstrual cycle. This cycle consists of three steps.
Hormones Involved
Follicular phase
Several follicles within the ovary begin to grow. The egg cells within the follicles enlarge and only one continues to grow while the others degenerate. A fluid filled cavity develops in the maturing follicle causing a bulge on one side of it.
GnRH is secreted by the hypothalamus which stimulates the anterior pitutiary to secrete small amounts of FSH and LH. FSH stimulates the follicles to produce estrogen. As estrogen increases it causes GnRH secretion to increase as well as FSH and LH.
Ovulatory phase
This is the time ovulation occurs. The follicle and the adjacent wall of the ovary rupture and release the egg.
High concentrations of LH stimulates final maturation and ovulation.
Luteal phase
The follicular tissue remaining in the ovary forms a corpus luteum. This is an endocrine gland that secretes female hormones.
High concentrations of LH cauase the follicle to form the corpus luteum. The corpus luteum secretes estrogen and increasing amounts of progestrone. This inhibits the production of GnRH which also decrease the production of LH and FSH. Low LH causes the corpus luteum to degenerate resulting in lower estrogen and progesterone levels.