Reproductive System
Asexual reproduction creates genetically identical cells to the parent cell through mitosis. Parthenogenesis is when females produce an offspring from an unfertilized egg. Hermaphroditism is when an individual has both testes and ovaries, they might also change sex. Protogyny is female to male change and protandry is male to female change. In humans testosterone promotes male development. Vertebrate sexual production evolved in the ocean before vertebrates colonized on land. Marine bony fish use external fertilization, eggs and sperm are released into the water where union of free gametes occurs. Terrestrial vertebrates use internal fertilization, where the sperm are introduced into the female reproductive tract. Oviparity, fertilized eggs are deposited outside the mothers body to complete their development, Ovoviviparity, fertilized eggs are kept within mother to complete development, and young obtain food from egg yolk, viviparity, young develop within mother and obtain nourishment from her blood. Fish fertilize externally, thousands of eggs are fertilized but only a few individuals grow to maturity. In amphibians fertilization is also external, eggs of the species develop in water, there are however exceptions. Such as males carrying tadpoles on his back, froglets develop brood pouches, female carries developing larvae on her back or male holding the developing froglets in his vocal pouch. In reptiles and birds, fertilization occurs with the amniotic egg, which is an adaptation for life on land. Female reproductive cycles involve periodic release of mature ovum. Females sexually receptive to male only around the time of ovulation. Primates have menstrual cycles where females bleed when shedding inner lining of the uterus (menstruation). In mammals there are three types of reproduction: monotremes (lay eggs), marsupials ( give birth to incompletely developed fetuses which then mature to the mothers pouch), placentals ( retain young in uterus for long period of developments and they are nourished by the mothers blood.In males the seminiferous tubules are the site of sperm production. Sperm can be divided into three parts. The head (containing the nucleus), the body (provide energy and consist of many mitochondria), and the tail (which helps the sperm move). Sperm are delivered into the epididymis for storage and maturation, from there sperm enter the vas deferens and then the urethra. Semen is a mixture of fluids and sperm. Seminal vesicles produce a fructose rich fluid, which make up 60% of the semen volume. The prostate gland contributes 30% of the bulk of semen. The bulbourethral glands add secretions to make up the last 10% of semen and also lubricate the tip of the penis. The penis consists of erectile tissue. Ejaculation is the ejection from the penis of 2-5mL of semen containing an average of 300 million sperm. FSH stimulates the sertoli cells to facilitate sperm development; LH stimulates the leydig cells to secrete testosterone. Hormonal control is control by negative feedback. Ovaries develop more slowly than testes. Ovaries contain microscopic structures contain ovarian follicles, each follicle contain a potential egg cell called a primary oocyte and smaller granulosa cells. At puberty, granulosa cells secrete estrogen. At birth, ovaries contain about 1 million follicles. The human menstrual cycle last one month divided into follicular phase, ovulation and luteal phase. Methods of contraception include abstinence, sperm blockage and sperm destruction, prevention of ovulation. Infertility is the inability to conceive after one year of contraception free sexual intercourse.