Chapter 46: Animal Reporduction Flashcards
Parthenogenesis
The development of a new individual from an unfertilized egg
Mainly observed in invertebrates such as wasps and ants
Hermaphroditism
Each individual in a species has male and female reproductive systems
Two hermaphrodites can mate and some hermaphrodites can self-fertilize
Individuals of some species undergo sex reversals
Male reproductive anatomy
Internal organs:
- Gonads which produce sperm and hormones
- Accessory glands that secrete products needed for sperm movement
- Ducts that carry sperm and glandular secretions
Testes
The testes consist of highly coiled tubes surrounded by connective tissue
Sperm form in these seminiferous tubules
Leydig cells produce hormones and are scattered between the tubules
Male reproductive ducts
Sperm pass from the seminiferous tubules of a testis into the coiled duct of the epididymis
During ejaculation sperm are propelled through a muscular duct called the vas deferens which extend up and around the bladder
The vas deferens joins a duct from the seminal vesicle behind the bladder forming a short ejaculatory duct before exiting the penis through the urethra
Male reproductive accessory glands
Semen composition
Semen is composed of sperm plus secretions from three sets of accessory glands
The two seminal vesicles contribute about 60% of the total volume of semen
- Fluid is thick, yellowish, and alkaline
- Contains mucus, fructose, coagulating enzymes, ascorbic acid, and prostaglandins
The prostate gland secretes its products directly into the urethra through several small ducts
- Fluid is thin and milky
- Contains anticoagulant enzymes and citrate
The bulbourethral glands secrete a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra
Penis anatomy
Composed of three cylinders of spongy erectile tissue
The head of the penis is called the glans and has a thinner skin covering than the shaft and is more sensitive to stimulation
The glans is surrounded by a fold of skin called the prepuce or foreskin
Female reproductive anatomy
External reproductive structures include the clitoris and two sets of labia which surround the clitoris and vaginal opening
The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and fetus
Ovaries
The outter layer of the ovary contains many follicles which consist of a partially developed egg, called an oocyte, surrounded by support cells
Oviducts and uterus
The egg cell travels from the ovary to the uterus via an oviduct, or fallopian tube
- Cilia in the oviduct conveys the egg to the uterus by drawing fluid from the body cavity into the oviduct
The endometrium is the uterine lining and is richly supplied with blood vessels
The uterus is a thick muscular organ that narrows the base and forms the cervix which opens into the vagina
Vagina and vulva
The vagina opens to the outside at the vulva, the collective term for the external female genitalia
The vulva consists of the:
- Labia majora- a pair of thick fatty ridges that enclose and protect the rest of the vulva
- Labia minora- Internal pair of slender skin folds
- Hymen- a thin piece of tissue that partly covers the vaginal opening at birth
- Clitoris- has a head called a glans and is covered by the prepuce, a small hood of skin
Spermatogenesis
Oogenesis
Hormonal control of the male reproductive system
FSH stimulates the activity of Sertoli cells in the seminiferous tubules to nourish developing sperm
LH regulates Leydig cells scattered in the connective tissue between the tubules to produce testosterone and other androgens which in turn promote spermatogenesis
Two negative feedback mechanisms control sex hormone production in males
- Testosterone regulates the production of GnRH, FSH, and LH through negative feedback mechanisms
- Sertoli cells secrete the hormone inhibin which reduces FSH secretion from the anterior pituitary
The ovarian cycle
Follicular phase
Follicular phase (day 0−14)
- The hypothalmus secretes GnRH which stimulates the anterior pituitary to secrete a small amount of FSH and LH
- FSH stimulates follicle growth aided by LH and the cells of the growing follicle start to make estridiol
- As estridiol levels continue to rise they exert positive feedback on the hypothalmus and stimulate increased GnRH secretion and sensitivity
- The maturing follicle forms a bulge on the surface of the ovary and ruptures in response to a a peak in LH levels
- Ovulation marks the end of the follicular phase
The ovarian cycle
Luteal phase
Luteal phase (days 15−28)
- LH stimulates the remaining follicular tissue to form the corpus luteum, a glandular structure
- LH stimulates the corpus luteum to secrete progesterone and estradiol which exert negative feedback on the hypothalamus and pituitary
- Negative feedback greatly reduces LH and FSH secretion which prevents maturation of another egg
- If no pregnancy, low GnRH causes the corpus luteum to disintegrate which triggers a sharp decline in estradiol and progesterone, removing negative feedback on hypothalmus and pituitary
- The pituitary can then secrete enough FSH to stimulate the growth of new follicles
The uterine cycle
Also known as the menstrual cycle
Proliferative phase (days 6−14)
- Estradiol secreted by the growing follicle signals the endometrium to thicken
- Uterine thickening occurs in coordination with the follicular phase
Secretory phase (days 15−28)
- Estradiol and progesterone secreted by the corpus luteum after ovulation stimulate maintenance and further development of the uterine lining
- Stimulates the enlargement of arteries and growth of endometrial glands that can sustain an early embryo
- Occurs in coordination with the luteal phase
Menstrual flow phase (days 1−5)
- Once the corpus luteum has disintegrated a rapid drop in ovarian hormone concentration causes arteries in the endometrium to constrict
- Deprived of circulation the uterine lining largely disintegrates, shedding endometrial tissue and fluid
- A new set of ovarian follicles begins to grow during this phase
By convention the first day of flow is designated as day one of the new uterine and ovarian cycle
Sexual response cycle
The sexual response cycle has four phases
- Excitement
- Plateau
- Orgasm
- Resolution
Direct stimulation of genitalia maintains the plateau phase and prepares the vagina for receipt of sperm
Conception and implantation
- Conception (fertilization) occurs in the oviduct
- The resulting zygote begins to divide by mitosis in a process called cleavage about 24 hours after conception
- After day 4 gives rise to a sphere of cells surrounding a central cavity called a blastocyst
- A few days later the embryo implants into the endometrium of the uterus about seven days after conception
The first trimester
The implanted embryo secrets human chorionic gonadotropin (hCG) that acts like pituitary LH
- Maintains secretion of progesterone and estrogens by the corpus luteum through the first few months of pregnancy
The first trimester is the main period of organogenesis
After eight weeks all the major structures of the adult are present an embryo is called a fetus
Placental development
During the first 2−4 weeks the embryo obtains nutrients directly from the endometrium
Meanwhile, the outer layer of the blastocyst called the trophoblast grows outward
Mingles with the endometrium eventually helping form the placenta
Umbilical arteries- 2 smaller tubes that take blood from the fetus to the placenta
Umbilical vein- single larger tube that takes blood from the placenta to the fetus
The second trimester
The fetus grows and is very active
The mother may feel fetal movements
The uterus grows enough for the pregnancy to become obvious
The placenta produces progesterone
Hormonal regulation of labor
Once labor begins local regulators (prostaglandins) and hormones (chiefly estradiol and oxytocin) induce and stimulate further contractions
Uterine contractions create a positive feedback loop that stimulates secretion of oxytocin which in turn stimulates further contractions
Stages of labor
- Dilation
- Expulsion
- Delivery of the placenta
