This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

Reproduction and development of sexual characteristics in vertebrates is controlled directly by the hormones produced in the gonads, estrogens, progesterone and testosterone.
Initially of course it is the genetic makeup of the individual that determines sex. Most vertebrates have a pair of chromosomes which are not identical (sex chromosomes).
In humans the chromosomes are called X and Y chromosomes.
Males have one X and one Y chromosome (heteromorphic).
Females have two X chromosomes (homomorphic).
In most vertebrate animals the male is heteromorphic but in some reptiles and birds the female is heteromorphic (ZW) and the males homomorphic (ZZ).

An interesting variation from the norm are a few species of fish that change sexes during their lifetime. This is generally in response to some environmental cue posibly temperature and /or the presence or lack thereof of the opposite sex.
The processes involved in phenotypic sex change is not well understood.

In humans there is a gene on the Y chromosome (SRY gene) that leads to the production of a protein TDF (testis determining factor).
Embryos are initially sexually indifferent and gonads can be testes or ovaries.
TDF appears to change the developmental pattern of the gonads to become testes.
In the absence of the Y chromosome (and TDF) the gonads develop as ovaries.

Developing testes produce androgens (mainly testosterone) which further promotes male development patterns and secondary male characteristics.
Developing ovaries produce estrogens (estradiol) and progesterone which promote female development and secondary characteristics.
Sex hormones stimulate development of sexual behavior patterns after physical determination.

Generally speaking males produce sperm after they mature.
Female patterns are somewhat different. In fish, amphibians and reptiles oocytes are produce throughout the reproductive life. In mammals and birds however all oocytes are produced by the time of birth (hatching) and remain dormant in the ovaries.
We will examine the mammal system first and then look at some variations in other classes.

Spermatogenesis
Sperm cells are formed from spermatogonia in seminiferous tubules in the testes.
Many mammals have testes located in a scrotum outside the body cavity. This is to reduce the temperature. Normal sperm development happens at about 2°C below normal body temperature.
Spermatogonia (2n) undergo continual mitosis to produce spermatocytes.
Spermatocytes undergo meiosis to produce spermatids that mature to become sperm cells.
Luteinizing hormone (LH) secreted by the anterior pituitary stimulate Leydig cells to secrete testosterone.
Leydig cells are located between seminiferous tubules.
Testosterone supports meiosis in the spermatocytes and the maturation of spermatids to spermatozoa (mature sperm cells).
Sertoli cells surround the developing sperm in the seminiferous tubules. Under the influence of follicle stimulating hormone (FSH) sertoli cells produce androgen binding protein (ABP) that concentrates the testosterone around the developing sperm.

Spermatogenesis takes about 8-10 weeks in humans and is continuous after puberty.
In many other animals spermatogenesis is seasonal.

Control of the whole process is based in the hypothalmus that releases gonadotropin releasing hormone (GnRH) that causes the release of LH and FSH from the anterior pituitary.
There are feedback mechanisms that maintain optimal levels of stimulation.
Testosterone has an inhibitory effect on the secretion of LH.
Sertoli cells secrete a peptide hormone called inhibin that supresses the release of FSH.

Ejaculation is the expulsion of sperm usually caused by the stimulation of the glans penis.
Peristalsis moves the sperm from the epididymus along the vas deferens. It is joined by fluid from the seminal vesicle and the prostate to form semen.
Seminal vesicles secretion makes up about 60% of semen volume. It is alkaline and contains fructose, ascorbic acid and prostaglandins.
Fructose provides fuel for sperm activity.
Ducts of the seminal vesicles connect to the vas deferens forming the Seminal vesicles secretion makes up about 60% of semen volume. It is alkaline and contains fructose, ascorbic acid and prostaglandins.
Fructose provides fuel for sperm activity.
Ducts of the seminal vesicles connect to the vas deferens forming the ejaculatory duct.
Prostaglandins decrease the viscosity of mucus at the cervix.
The prostate gland surrounds the beginning of the urethra as it leaves the bladder. It also surrounds the ejactulatory ducts.
The prostate secretes an alkaline fluid through a number of ducts which contains enzymes that activat sperm.
Prostate secretions make up about 30-35% of semen.

Alkalinity of semen counteracts acid environment of the vagina. Sperm are much more active in an alkaline environment.
Semen has antibiotic characteristics.
Clotting factors (fibrinogen) cause semen to coagulate soon after ejaculation and then fibrynolysin soon breaks down the clotting.
Ejaculate is about 2-6 ml containing about 50 - 100 million sperm per ml.

The female system
The ovaries of females has a dual role
1. To produce ova
2. To produce estrogens and progesterone
Estrogens is a collective term for a number of hormones the most active of which is estradiol.
Hormones are generally released cyclically and are under the control of the hypothalmus.

Regular cycles are often overcome in some animals from environmntal factors such as day length.
Other external cues such as pheromones, sexual intercourse and diet can also effect the cycle.

Oogenesis
During embryonic development oogonia divide by mitosis and form primary oocytes.
By birth, oogonia are no longer present and the mammalian (and bird) ovary contains its full complement of primary oocytes (about 2 million in humans of which only about 400-500 will be released).
Primary oocytes are still 2n and surrounded by follicular cells, together making up the primary follicle.

Primary oocytes are in an arrested stage of prophase I of meiosis.
Beginning at puberty a small number of primary oocytes begin development each month (ovarian cycle).
In humans normally only one oocyte from one of two ovaries will continue meiosis.
Meiosis II is only completed after sperm penetration!
Mechanism for choosing one to development is not understood.

Ovarian cycle
Three phases
1. Follicular phase
2. Ovulation
3. Luteal phase

Follicular phase
Follicular cells in primary follicle increase in number, differentiate and become granulosa cells. Granulosa cells secrete estrogen.
Secondary follicles are formed after proliferation of granulosa cells and production os fluid filled vesicles in the follicle. Primary oocyte still in prophase I.
Graafian follicle then develops after further growth, as the vesicles combine to form a single antrum. Primary oocyte completes meiosis I forming secondary oocyte.

Ovulation
Growing pressure from the expanding antrum rupture the wall of the ovary.
Secondary oocyte surrounded by some granulosa cells called the corona radiata are expelled into the peritoneal cavity.

Luteal phase
After ovulation the granulosa cells expand and change functionally to form an endocrine gland, a corpus luteum (yellow body). It secretes estrogen and progesterone.
If no fertilization occurs the corpus luteum degenerates in 10 days to become a corpus albicans (white body).

Hormonal Control of Ovulation
Start
Rising levels of GnRH (gonadotropin releasing hormone) from the hypothalamus result in increased secretion of FSH (follicle stimulating hormone) and LH (Luetinising hormone) from the anterior pituitary.
LH and FSH stimulate follicular development.
LH causes thecal cells (surround follicle) to secrete androgens which diffuse into the FSH activated granulosa cells where they are converted into estrogens. Granulosa cells secrete estrogens.
After initially inhibiting LH and FSH, a critical level of estrogen causes a surge of LH and a smaller surge of FSH to be releasesd from the anterior pituitary.
LH stimulates meiosis in the primary oocyte and rupture of the graafian follicle (ovulation).
LH surge also stimulates the change in the granulosa cells to form the corpus luteum.
Estrogen and progesterone levels from corpus luteum inhibit LH and FSH secretion.
If fertilization does not occur, corpus luteum degenerates due to falling LH, estrogen and progesterone fall, GnRH increases and cycle starts again.
Start again.

Menstrual cycle (only primates)
Menstruation or the shedding of the endometrium (uterine lining) following the degeneration of the corpus luteum and the drop in progesterone level.
Estrogen from growing follicle stimulates regrowth of the endometrium.
Progesterone from corpus luteum increases blood supply of endometrium and causes glycogen secretion in the uterus.

Estrous cycle in other mammals
Luteal stage is much shorter than in menstruating females.
The endometrium does not thicken greatly and it is not shed when progesterone levels drop.

Fertilization?
If fertilization occurs (normally takes place in the fallopian tubes):
The zygote (fertilized ovum) moves toward the uterus, where the endometrium has been developing to accept it since the previous menstrual phase and the influence of progesterone.
The blastocyst attaches to and buries into the endometrium (implantation).
Implantation stimulates the blastocyst to release human chorionic gonadotropin (HCG) which supports the continuing function of the corpus luteum.
Excess HCG is excreted in urine and is the indicator of many pregnancy tests.

The placenta develops and allows diffusion of nutrients and gases between the blood of the fetus and the blood of the mother (blood does not mix).
The placenta also takes on the endocrine function of producing estrogen and progesterone later in pregnancy.

Estrogen and to lesser extent progesterone inhibit the release of FSH and LH during pregnancy (no new follicular development) and hence their use as a contraceptive.

Oviparous animals have slightly different requirements during reproductive cycles.
In egg laying amniotes the oviduct secretes albumen (egg white) and the uterus secretes the egg shell.
Amphibian females often release 100,000's ripe eggs in a breeding cycle.
Cod fish can lay 4,000,000 ripe eggs in a season.
An ovisac is often present in the oviduct to store eggs after ovulation or developing young in ovoviviparous species.
Vitellogenesis is the process of accumulation of nutrients in the oocyte cytoplasm.
These nutrients (yolk) support embryonic development.
Example: African clawed frog, Xenopus laevis
1. GnRH stimulates release of FSH.
2. Growing follicles secrete estrogens.
3. Liver cells, primed by thyroxine, produce and release vitellogenin (lipoproteins) in response to estrogen.
4. Oocytes stimulated by FSH and LH take up vitellogenin.
5. LH induces ovulation.

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This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/reproduction.html
Last edit January 10, 2011.
Copyright Peter King