Lecture Twelve - Reproduction Flashcards
Describe the similarities and differences between sexual and asexual reporoduction.
Animals may repoduce sexually and/or asexually.
In asecual reporoduction, all the genes come from teh same parent (cloning).
In sexual reproduction, the creation of offspring involves the fusion of male and female gametes (i.e. sperm and egg).
What are the mechanisms of asexual reporoduction?
Asexual reproduction enables anmals to reproduce rapidly without the need to find a mate. It also allows perpetuation of successful genotypes.
A wide variety of mechanisms of asexual reproduction exist amoung animals.
Fission - Involves the sepatation of a parent into two or more indiciduals of approximately the same size.
Budding - Involves the production of new individuals from outgroths of existing ones.
Fragmentation - Involves the formation of new individuals from bits broken of from the main body. The lost body part also regenerates.
Parthenogenisis - Is the process by which an egg develops without being fertilized.
Some animals alternate between parthenogenisis and sexual reproduction depending on environmental conditions.
In some taxa, one sex is produced by parthenogenisis while the other develops from a fertillized egg. (E.g. male bees are produced parthenogenically).
Hermaphrodistism - In some species, each individual has both male and female parts. Hermaphroditism involves sexual reproduction. In some species, individuals fertilise themselves but most mate with another individual.
Two types:
1) Sequential hermaphroditism is where an individual changes sex during its life time.
2) Simultaneous hermaphroditism is where an individual has male and female parts at the same time.
What are the mechanisms of sexual reproduction?
Sexual reproduction involves the union of gametes.
In some species, eggs are fertilized externalls.
Other species have internal fertilization.
To reproduce sexually, animals must have systems that produce gametes and make the avaliable to the gametes of the opposite sex.
Amoung animals, reproductive systems are highly varied in terms of their complexity.
What is the female reproductive anatomy?
External structures comprise the clitoris and two sets of labia.
The internal structures consist of a pair of gonads (ovaries) and a system of ducts and chambers.
Ovaries:
The ovaries are the female gonads and lie in the abdominal cavity.
Each ovary contains many follicles.
A follicle consists of one egg cell surrounded by one or more layers of follicle cells.
Follicles produce the primary female sex hormones - estrogen.
Follicle cells nourish and protect the developing egg cell.
A woman is borne with about 400,000 follicles but only several hundered will release eggs during a females reproducive life time.
Usually one follicle matures and releases its egg during each menstral cycle.
After ovulation, the remaining follicular tissue develops into a solid mass called the corpus lutenum.
The corpus lutenum secretes estrogens and progesterone and maintains the unterine lining during pregnancy.
If pregnancy does not occur, the corpus lutenum disintergrates.
What is oogenesis?
Oogenesis begins in female embryo with differentiation of primordial germ cells into oogonia (i.e. ovary-specific stem cells).
Oogonium multiplies by mitosis and begins meiosis.
But stops at prophase of meiosis I as a ‘primary oocyte’ where it remains dormant until puberty.
Beginning at puberty, FSH stimulates a follicle to grow and induces its primary oocyte to complete meiosis I and start meiosis II.
But stops at metaphase of meiosis II as a
‘secondary oocyte.’
Secondary oocyte is released when follicle breaks open at ovulation.
Meiosis is completed when sperm penetrates the oocyte (i.e. oogenesis is complete, producing an ovum).
Ruptured follicle develops into the corpus luteum (if released oocyte is not fertilized, the CL degenerates).
At birth an ovary may contain all of the primary oocytes it will every have.
Unequal cytokenesis results in a single large secondary oocyte and smaller polar bodies.
Oogenesis has long resting periods.
Describe the oviducts and uterus
and the vagina and vulva.
The egg is released into the abdominal cavity near the opening of the oviduct or fallopian tube.
Cillia in the tube convey the egg to the uterus.
The vagina is a thin-walled chamber that serves as the repository for sperm during copulation.
It also serves as the birth canal though which a baby is born.
The vagina opens to the outside at the vulva which includes the hymen, vestibule, labias and clitoris.
Describe male reproductive anatomy.
External organs comprise of the penis and scrotum.
The internal organs consists of the gonads, sex glands and ducts.
Penis :
Composed of three cylinders of spongy erectile tissues which fills with blood during arousal.
Pressure seals off the veins that otherwise drain the penis.
Erection is essential to insertion of the penis into the vagina.
In some mammals, the penis may also contain a bone called a baculum to help stifin the penis.
Some animals pensis are equipped with spikes.
Testes:
The testes are the male gonads and consist of highly coiled seminferous tubules surrounded by connective tissues.
The seminiferous tubules produce testosterone and other androgens.
Scrotum:
Testes are held in the scrotum, where the temperaure is lower than the rest of the body. This allows normal sperm to develop.
Ducts:
From the seminderous tubules of the testes the sperm pass into the coiled tubules of the epididymis.
During ejeculation, sperm are propelled throug the muscular vas edferens, the ejaculation duct, and exit the penis though the urethra.
Glands:
Three sets of accessory glands add secretions to the semen, the fluid that is ejeculated.
Seminal vesicles contribute 60% of the volume of semen and contains mucous, fructose, coagulating enzymes and prostaglandins.
Prostate gland is the largest of the semen secreting glands. Prostatic fluid contains anticoagulant enzymes and citrate.
Bulbourethral glands setrete a clear mucus that neutralises any acidic urine.
Explain the process of spermatogenisis.
Mature sperm is produced in a process known as spermatogenesis.
Begins in male embryo with differentiation of primordial germ cells into spermatogonia (i.e. testes-specific stem cells).
As spermatogonia differentiate into spermatocytes and then into spermatids, meiosis reduces chromosome number from diploid to haploid.
As spermatogenesis progresses, the developing sperm cells move from the wall to the limen of the seminiferous tubules, and then to the epididymis, where they become motile.
Sperm is produced throughout a mans life time.
There are no ‘rest periods’ (spermatogenesis occurs in uninterrupted sequence).
Equal cytokenesis means all found products of meiosis become mature sperm.
What are androgen secretion and sperm production controlled by?
Both androgen secretion and sperm production are controlled by hormones from the hypothalamus and pituitary glands.
What does semen do in the female reproductive tract?
A male usually ejeulates about 2-5ml of semen.
Each mL contains about 50-130 million sperm.
In the female reproductive tract, prostaglandis in the semen thin the mucus at the opening of the uterus and stimulates uterine contractions.
Alkalinity of semen helps nutralize the acidic environment of the vagina.
When first ejaculated, semen coagulates, making it easier for uterine contractions to move it along.
Anticoagulants then liquify the semen, and the sperm begin swimming through the female tract.
What is the female reproductive cycle (menstrual and estrous cycles)?
In females, the sectretion of hormones and the reproducive events they regulate are cyclic.
Two different types of cycles occur in female mammals.
1) Menstrual cycle.
Found in humans and certain other primates. Other mammals hae estrous cycles.
In both cases, ovulation occurs at the same time in the ccle after the endmetrium (lining of the uterous) has started to thicken and develop a rich blood supply, preparing the uterus for the possible implantation of an embryo.
2) Estrous cycle.
Menstrual Vs Estrous cycles:
In menstrual cycles, if pregnancy does not occur, the endometrium is shed through the cervix and vagina.
In estrous cycles, if prgnancy does not occur, the endometrium is reabsorbed by the uterus.
Animals that undergo estrous cycle also tend to exhibit more pronounced behavioural changes (e.g. coming into heat).
How is the female reproductive cycle intergrated using the uterus and the ovaries?
The female reproductive cycle involves the uterus and the ovary.
It is orchestrated by cyclic secretions of GnRH from the hypothalamus and by FSH and LH from the anterior pituitary.
GnRH, FSH and LH regulated by negitive feedback by androgens.
1) Cycle beins with the release of GnRH.
2) This stimulates pituitary to secrete small amounts of FSH and LH.
3) FSH stimulates follicle growth, aided by LH.
4) Cells of growing follicle starts to make estrogen. During the follicular phase (i.e. phase when follicles are growing and oocytes are maturing), several follicles begin to grow but only one matures. Low estrogen levels at this stage inhibit secretion of FSH and LH.
5) When the secretion of estrogen begins to rise steeply, FSH and LH levels suddenly shoot up (low estrogen inhibits pituitary gonadotrophins; high estrogen stimulates gonadotrophin secretion by acting on hypothalamus to increase output of GnRH).
6) Hence the peak in FSH and LH corresponding with high estrogen. LH induces final maturation of the follicle.
7) Maturing follicle develops a fluid filled cavity, grows large and formes a bulge near the surface of the ovary.
Follicular phase ends about 1 day after LH surge with ovulation: follicle and adjacent wall of ovary ruptures and releases the secondary oocyte.
8) Next, during the luteal phase, LH stimulates the remaining follidular tissue to form the corpus luteum).
CL produces progesterone and estrogen, the combination of which, inhibits secretion of LH and FSH (negitive feedback).
Near the end of the luteal phase, CL disintergrates causing progesterone and estrogen to decline which allows secretion of FSH and start the growth of new follicles initiating new ovarial cycle.
9) Estrogen and progesterone stimulates continued development and maintenance of endometrium.
10) Rapid drop of ovarian hormones when CL disintergrates leads to disintergration of enometrium, resulting in menstruation.
