REPRODUCTIONTopics 6.6, 11.4

ASSESSMENT STATEMENTS 6.6.1 Draw and label diagrams of the adult male and

female reproductive systems. 6.6.2 Outline the role of hormones in the menstrual

cycle, including FSH (follicle stimulating hormone), LH (luteinizing hormone), estrogen and progesterone.

6.6.3 Annotate a graph showing hormone levels in the menstrual cycle, illustrating the relationship between changes in hormone levels and ovulation, menstruation and thickening of the endometrium.

6.6.4 List three roles of testosterone in males. 6.6.5 Outline the process of in vitro fertilization (IVF). 6.6.6 Discuss the ethical issues associated with IVF.

HUMAN REPRODUCTION

Process includes a male gamete (sperm) fertilizing a female gamete (egg or ovum)

Union ensures that half of the genetic makeup of the resulting zygote is derived from each parent

Ensures genetic variation in the species

MALE REPRODUCTIVE SYSTEM

Roles of testosterone in males

Determines the development of male genitalia during embryonic development

Ensures development of secondary sex characteristics during puberty

Maintains the sex drive of males throughout their lifetime

FEMALE REPRODUCTIVE SYSTEM

MENSTRUAL CYCLE Begins at puberty in females Each complete cycle lasts about 28 days Purpose is to time the release of an egg or

ovum (ovulation) for possible fertilization and later implantation into the inner lining of the uterus

Implantation must occur when the uterine lining (endometrium) is rich with blood vessels

Vascularization is not maintained if there is no implantation

Breakdown of the blood vessels leads to the menstrual bleeding (menstruation) of a typical cycle

Sign that no pregnancy occurred

HORMONES FROM THE BRAIN

Hypothalamus produces gonadotrophin releasing hormone (GnRH)

Target tissue is the pituitary gland and it results in the pituitary gland producing and secreting two other hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH)

Target tissue for these hormones are the ovaries

HORMONAL SUMMARY OF THE MENSTRUAL CYCLE

EFFECTS OF FSH AND LH ON THE OVARIES

Increases the production and secretion of oestrogen Oestrogen enters the bloodstream and targets

the endometrium of the uterus Result is an increase in the blood vessels in the

endometrium of the uterus Production of structures within the ovaries

known as Graafian follicles

OVARY EVENTS DURING A SINGLE MENSTRUAL CYCLE

Spike in the level of FSH and LH leads to ovulation (release of the oocyte from the Graafian follicle)

Oocyte is accompanied by the inner ring of follicle cells and a glycoprotein membrane coat known as the zona pellucida

Entire structure is known as a follicle and typically enters the Fallopian tube soon after ovulation

Outer ring of follicle cells remains within the ovary and begin to produce/secrete progesterone

Cells of outer ring begin to divide and fill in the area left by ovulation (now known as corpus luteum)

Corpus luteum continues to produce progesterone for 10-12 days after ovulation

Progesterone maintains the thickened endometrium As long as progesterone is produced, endometrium

will not break down and an embryo will still be able to implant

High levels of oestrogen and progesterone are a negative feedback signal to the hypothalamus

Hypothalamus does not produce GnRH, so FSH and LH remain at levels not conducive to production of another Graafian follice during this time

Assuming there is no pregnancy, the corpus luteum breaks down and levels of progesterone and oestrogen drop

Capillaries and small blood vessels begin to rupture and menstruation begins

Hypothalamus begins producing GnRH and thus another menstrual cycle begins

FERTILIZATION

Natural fertilization typically occurs in one of a female’s Fallopian tubes 24-48 hours after ovulation

The resulting zygote begins to divide by mitosis and takes several more days to travel down the Fallopian tube to the endometrium of the uterus

The embryo will then implant in the highly vascular tissue of the endometrium

INFERTILITY

Some couples are unable to bear children for a wide variety of possible reasons, including: Males with low sperm counts Males with impotence (failure to achieve or

maintain an erection) Females who cannot ovulation normally Females with blocked Fallopian tubes

Reproductive technologies have been developed to help overcome these situations.

Most common of these new technologies is in-vitro fertilization

IN-VITRO FERTILIZATION (IVF) Woman is injected with FSH for about 10 days This will ensure the development of many Graafian

follicles within her ovaries Several eggs (oocytes) are then harvested surgically Man ejaculates into a container to obtain the sperm cells

that are needed for fertilization Harvested eggs are mixed with sperm cells in separate

culture dishes Microscopic observation reveals which ova are fertilized

and if the early development appears normal and healthy Usually two or three healthy embryos are introduced into

the woman’s uterus for implantation Very expensive Introducing more than one embryo increases the

likelihood that at least one will implant successfully

ETHICAL ISSUES CONCERNING IVF Arguments for IVF:

Enables couples to have a family Embryos that are visibly not healthy in early stages of

development can be eliminated from consideration Genetic screening is possible before implantation to eliminate

the chance of passing on genetic diseases Arguments against IVF:

Embryos produced, but not implanted are either frozen or destroyed

Legal issues concerning use of frozen embryos when couples split up

Genetic screening could lead to society choosing desirable characteristics

Bypasses natures way of decreasing genetic frequency of that reproductive problem

Multiple births and the problems associated with a multiple births are more likely with IVF than with natural conception

ASSESSMENT STATEMENTS 11.4.1 Annotate a light micrograph of testis tissue to show the location and function of interstitial

cells (Leydig cells), germinal epithelium cells, developing spermatozoa and Sertoli cells. 11.4.2 Outline the processes involved in spermatogenesis within the testis, including mitosis, cell

growth, the two divisions of meiosis and cell differentiation. 11.4.3 State the role of LH, testosterone and FSH in spermatogenesis. 11.4.4 Annotate a diagram of the ovary to show the location and function of germinal epithelium,

primary follicles, mature follicle and secondary oocyte. 11.4.5 Outline the processes involved in oogenesis within the ovary, including mitosis, cell growth,

the two divisions of meiosis, the unequal division of cytoplasm and the degeneration of polar body. 11.4.6 Draw and label a diagram of a mature sperm and egg. 11.4.7 Outline the role of the epididymis, seminal vesicle and prostate gland in the production of

semen. 11.4.8 Compare the processes of spermatogenesis and oogenesis, including the number of

gametes and the timing of the formation and release of gametes. 11.4.9 Describe the process of fertilization, including the acrosome reaction, penetration of the egg

membrane by a sperm and the cortical reaction. 11.4.10 Outline the role of HCG in early pregnancy. 11.4.11 Outline early embryo development up to the implantation of the blastocyst. 11.4.12 Explain how the structure and functions of the placenta, including its hormonal role in

secretion of estrogen and progesterone, maintain pregnancy. 11.4.13 State that the fetus is supported and protected by the amniotic sac and amniotic fluid. 11.4.14 State that materials are exchanged between the maternal and fetal blood in the placenta. 11.4.15 Outline the process of birth and its hormonal control, including the changes in

progesterone and oxytocin levels and positive feedback.

LIGHT MICROGRAPH OF TESTIS

(Leydig cells)

(germinal epithelium cells)

SPERMATOGENESIS: PRODUCTION OF MALE GAMETES BY MEIOSIS

Occurs within testes which are located outside of the human body to provide the cooler temp. necessary for production of sperm cells or spermatozoa

Occurs within seminiferous tubules of each testis

Near the outer wall of the seminiferous tubules lie germinal cells called spermatogonia

Each spermatogonium may be undergoing either mitosis or meiosis at any given time

MITOSIS

Spermatogonia undergo mitosis in order to replenish their numbers

Mitosis replaces the cells that have become spermatozoa

Sperm cells production begins at puberty and continues throughout life

Millions of sperm cells can be produced in a single day

1st replicate DNA within diploid cell Mitosis will result in two half-size cells,

capable of growing again for another cell division

MEIOSIS

Spermatogonia undergo meiosis to produce spermatozoa

Reduces the diploid number of chromosomes in spermatogonia to the haploid number in spermatozoa

In humans, 23 homologous pairs of chromosomes becomes 23 individual chromosomes

Meiosis will result in four spermatozoa, each containing a haploid number of chromosomes

SPERMATOGENESIS

Spermatagonia (2n) contain 23 homologous pairs of chromosomes (total of 46)

DNA replication occurs and each of the 46 chromosomes now exists as a pair of chromatids

Meiosis I occurs and two half-sized cells result, each with the haploid number of chromosomes (23) because homologous pairs have been separated

Each chromosome exists as a pair of chromatids, so then meiosis II must occur

During meiosis II, the chromatids separate 4 haploid cells, each containing 23

chromosomes are created

DIFFERENTIATION INTO SPERMATOZOON

After meiosis, cells stay within the interior of the seminiferous tubule while they form a flagellum for motility and an acrosome (enzyme needed for fertilization)

Developing cells stay attached to Sertoli cells which nourish them

Once spermatozoa have completed formation of flagella, they detach from Sertoli cells and are carried through the lumen with the movement of fluid

Each sperm cell is swept to the epididymis of the testis where it is stored

HORMONAL CONTROL OF SPERM PRODUCTION

Leutinizing hormone (LH) stimulates Lyedig cells to produce testosterone

Follicle stimulating hormone (FSH) and testosterone stimulate the meiotic divisions of spermatogonia into spermatozoa

ROLE OF THE EPIDIDYMIS, SEMINAL VESICLES, AND PROSTATE

Epididymis Sperm cells are stored and gain motility On sexual arousal, sperm move from the

epididymis into the vas deferens Seminal vesicles

Add large volume of fluid into vas deferens with sperm

Fluid contains fructose (provides energy for sperm cells)

Prostate gland Adds more fluid that is alkaline and helps the

spermatozoa survive the environment within the female’s vagina

OVARY

OOGENESIS

Produces female gametes by meiosis Only one of the four end products (cells) are

used as gametes Gamete is known as ovum; others are known

as polar bodies Within the ovaries of a female fetus, cells

called oogonia undergo mitosis repeatedly in order to build up the numbers of oogonia within the ovaries

Oogonia grow into larger cells called primary oocytes (diploid)

Primary oocytes begin meiosis, but stops during prophase I

Other cells called follicles repeatedly undergo mitosis

A single layer of these follicle cells surrounds each primary oocyte and the entire structure is then called a primary follicle

Females’ ovaries contain 400,000 primary follicles

Primary follicles remain unchanged until puberty and menstrual cycles begin

MENSTRUAL CYCLE During each menstrual cycle, a few primary follicles

finish meiosis I One cell is very large and is a secondary oocyte One cell is very small and is called 1st polar body (may

divide again; both degenerate) The single ring of follicle cells begin dividing and

forming a fluid Two rings of follicle cells are formed with a fluid-filled

cavity separating them Secondary oocyte begins meiosis II, but stops during

prophase II Entire structure is now called a Graafian follicle Increased fluid between layers creates a bulge on the

surface of the ovary and eventually leads to ovulation

Secondary oocyte with the inner ring of follicle cells is released from the ovary at ovulation

Meiosis II is not completed until fertilization Another polar body is created (degenerates) Fertilized secondary oocyte is now a zygote

MATURE SPERM AND OVA

Both gametes are haploid Spermatozoon is a very small cells with a

flagellum for motility and mitochondria to provide ATP for swimming

At its anterior end, each sperm cell contains an organelle called an acrosome which contains hydrolytic enzymes which help with the fertilization process

Egg is the largest cell in the body by volume Unequal division of cytoplasm ensures that

one cell would receive all of the cytoplasm, nutrients, and organelles

Nutrients are known as yolk Cytoplasm contains small vesicles called

cortical granules which release calcium and prevents other sperm from entering the egg

Just outside the plasma membrane is a layer of glycoproteins called the zona pellucida which disappears to permit implantation in the uterus

Spermatogenesis Oogenesis

Millions of sperm cells are produced every day

Typically, on secondary oocyte is ovulated per menstrual cycel

Four gametes are produced for each germinal cell which begins meiosis

One gamete is produced for each germinal cell which begins meiosis (plus polar bodies)

The resulting gametes are very small

The resulting gametes are very large

Occurs within testis (gonad tissue)

Occurs within ovaries (gonad tissue)

Spermatozoa are released during ejaculation

Secondary oocyte is released during ovulation

Haploid nucleus results from meiosis

Haploid nucleus results from meiosis

Spermatogenesis continues all through life (starting a puberty)

Ovulation starts at puberty, occurs with each menstrual cycle, then stops during menopause

FERTILIZATION

As a result of sexual intercourse, millions of sperm are ejaculated into a female’s vagina

Spermatozoa absorb fructose for their long journey

Some spermatozoa die immediately upon entering the female due to high acidity

Others find their way to the cervical opening and enter the uterus

Some of these enter the openings of the Fallopian tubes

If the female is near the middle of her menstrual cycle, there may be a secondary oocyte within one of the tubes

Many sperm cells are needed to push their way through the follicle cells surrounding the oocyte

Sperm cells gain access to the zona pellucida (glycoprotein gel layer) and release the hydrolytic enzymes contained in the acrosomes

One sperm cell reaches the plasma membrane of the secondary oocyte first

The plasma membranes of the two gametes fuse together initiating a series of events called the cortical reaction

Cortical granules within the secondary oocyte fuse with the oocyte’s membrane and release enzymes to the outside

A chemical change occurs in the zona pellucida making it impermeable to any more sperm cells

This reaction takes place within a few seconds and ensures that only one sperm cells actually fertilizes the oocyte

The secondary oocyte now completes meiosis II and produces another polar body

Resulting fertilized ovum is now referred to as a zygote

PREGNANCY

Fertilization triggers the zygote to begin a mitotic division which occurs 24 hours after fertilization

The early embryo continues to divide for about 5 days and moves within the Fallopian tube towards the uterus

By the time it reaches the uterus it is approximately 100 cells and is ready to implant itself into the endometrial wall

Ball of cells known as a blastocyst: Surrounding layer of cells called a trophoblast (helps

form placenta) Inner cell mass (becomes body) Fluid-filled cavity

ROLES OF HORMONES

Ovulation of the secondary oocyte left an outer ring of follicle cells within the ovary

This remainining layer (corpus luteum) divides mitotically and begins secreting estrogen and progesterone for about 14 days

These hormones maintain the thickened endometrium in case there has been fertilization

If no fertilization occurs, the corpus luteum ceases hormone production and the endometrium begins to break down

HUMAN CHORIONIC GONADOTROPHIN (HCG)

If fertilization occurs, the embryo enters the uterus and begins implantation about a week after fertilization

Corpus luteum is still active and therefore endometrium is still thick

Soon after implantation, the embryo begins to secrete HCG

HCG enters the mother’s bloodstream targeting the corpus luteum and maintaining its secretory function

ROLE OF THE PLACENTA

During the 1st two weeks, the cell divisions that occur create an embryo of 100 or more cells but the overall size is no larger than the original cell

The nutrients stored within the egg have been used for metabolism, not growth

Upon implantation, it is running out of yolk The embryo and maternal endometrium soon

begin to create a structure known as the placenta

Forms from the trophoblast layer of the blastocyst When fully formed, two fetal blood vessels within

the umbilical cord carry fetal blood to the placenta

Materials passed from fetus to mother within the placenta

Materials passed from mother to fetus within the placenta

Carbon dioxide Oxygen

Urea Nutrients (glucose, amino acids, etc.)

Water Water

Hormones (e.g. HCG) Hormones

Vitamins, minerals

Alcohol, many drugs, nicotine

Some viruses, such as German measles, HIV

At no time does the blood of the fetus and the blood of the mother actually mix

When the corpus luteum stops production of estrogen and progesterone, the placenta has already begun producing and secreting these hormones

ROLE OF AMNIOTIC FLUID Some of the tissue of a developing embryo is

used to create extraembryonic membranes, such as the amniotic sac

Amniotic sac extends all of the way around the fetus and is filled with amniotic fluid in which the fetus floats as it continues to grow and develop

Functions: Cushions fetus should a blunt force be applied to the

mother’s abdomen Provides an environment in which the fetus has free

movement and therefore well balanced exercise for all developing muscles and skeleton

Provides thermal stability Sampled during amniocentesis which can be

cultured for chromosome abnormalities

HORMANAL EVENTS ASSOCIATED WITH BIRTH

Physiological events associated with a woman’s body preparing for a birth are collectively called parturition

1. Major hormone changes Drop in progesterone Oxytocin released stimulating contractions

within uterus Contractions signal posterior lobe of pituitary to

produce more oxytocin which intensifies and increases frequency of contractions (positive feedback)

2. Opening of cervix to 10 cm3. Baby head-first, face down

4. Shoulders of baby (widest part) pass through the birth canal

5. Afterbirth (placenta) is expelled after birth6. Lactation (birth milk production) begins

after birth