Chapter 25 Aspects of Human Reproduction. Human reproduction Female urogenital system.

Chapter 25

Aspects of Human Reproduction

Human reproduction• Female

urogenital system

Human reproduction- male urogential system

Gametogenesis

• Gametogenesis is the production of gametes– Men: spermatozoa are made in the testes– Women: eggs are made in the ovaries

Spermatogenesis

• Spermatogenesis: Production of sperm in the testis

• Begins ~11years of age

• Around 100-200 million sperm made each day

Spermatogenesis

• Testes made up of many seminiferous tubules; spermatogenesis takes place in these walls

• Spermatogenesis begins in the germinal epithelium of seminiferous tubules, and newly produced cells form towards the inner edge

Spermatogenesis

• Cells that begin spermatogenesis are called spermatagonia (singular: spermatagonium)– Diploid cells that divide by mitosis to make more

diploid cells• Some of these grow into spermatagonia ,

where others grow much larger and are called primary spermatagonia

Spermatogenesis

• Meiosis begins with primary spermatocytes which divide into haploid cells called secondary spermatocytes (one division)

• After a few days, secondary spermatocytes go through a second division to form two haploid spermatids– Spermatids do not undergo further divisions

Spermatogenesis

• Spermatids must undergo differentiation to form spermatozoa (singular: spermatozoan), which are referred to as sperm cells

• They line up with their heads attached to the wall of the tubule and their tails hanging free in the lumen

Spermatogenesis• Throughout spermatogenesis, the developing sperm

cells are protected by large cells known as Sertoli or nurse cells

• These cells nourish and protect the developing sperm cells, and interact with pituitary hormones to regulate sperm production

Oogenesis

Oogenesis• Oogenesis: production of eggs, or ova• Early stages of oogenesis take place in the

ovary, but an ovum is not fully formed until fertilization

Oogenesis

• While a girl is an embryo in her mother’s womb, germinal epithelial cells in her developing ovaries divide to from diploid oogonia (singular oogonium)

• After a few weeks, these oogonia begin to divide by meiosis, however, they do not complete meiosis I and only get to prophase I– These are called primary oocytes– At birth, females have ~400,000 primary oocytes

Oogenesis

• Each month, one of the girl’s secondary oocytes is released into the oviduct

• If fertilized, it continues division by meiosis– Technically, it is not really an ovum until this has

happened• The ‘egg’ that is released by an ovary during

ovulation is actually a secondary oocyte, not an ovum

Oogenesis

• Developing oocytes are inside follicles in the ovary– Follicles are also produced by the germinal

epithelium• The wall of a follicle contains several types of

cells, including granulosa cells which surround and protect the oocyte and secrete hormones

Oogenesis

The human menstrual cycle

• In women, there is an approximately 28 day cycle, the menstrual cycle, of activity in the ovaries and uterus, in contrast to the steady production of sperm that occurs in men

• The menstrual cycle usually begins between the ages of 10 and 14, and continues until the women reaches menopause approximately 40 years after her first menstrual cycle


• The hormones that control spermatogenesis in men also controls the production of eggs in women

• These cycle is considered to begin with the onset of menstruation– Usually lasts 4-8 days


• During menstruation, the anterior pituitary gland secretes luteinizing hormone (LH) and follicle-stimulating hormone (FSH)– The concentration of these hormones gradually

increase at the onset of menstruation

The human menstrual cycle• In the ovary, on follicle becomes the ‘dominant’ one• In the presence of LH and FSH, cells surrounding the

follicle secrete estrogen • The presence of LH and FSH in the blood induces a

negative feedback effect which decreases the concentration of these two hormones


• Estrogen produced by the ovaries stimulates the endometrium (lining of the uterus) to proliferate- meaning to thicken and develop numerous capillary beds


• When estrogen concentration in the blood has increased 2x-4x its concentration at the start of menstruation, it stimulates a surge in LH (and to a lesser extent surge in FSH)

• This LH surge causes the dominant follicle to burst and shed its secondary oocyte into the oviduct (usually around 14-36 hours are LH surge)


• The follicle then collapses to form the corpus luteum (‘yellow body’)

• The corpus luteum is formed from granulosa cells, which secrete progesterone

• Progesterone secreted from the corpus luteum maintains the lining of the uterus, making it ready to receive the embryo if fertilization occurs.

• If fertilization does not occur, the drop in progesterone levels causes the uterine lining to be shed, starting the cycle ALL OVER AGAIN

Birth Control

• Control over reproduction– Contraception: preventing fertilization when

sexual intercourse takes places– Also may prevent embryo from implanting into the

lining of the uterus• Termed anti-implantation methods. Include intra-

uterine devices (IUDs) and the ‘morning after’ pill

The birth control pill

• Developed in 1960’s• Currently available only for women, although

oral contraceptives for males currently being research

• Has contributed to the rise in incidence of sexually transmitted diseases (STIs) including HIV/AIDS because more people have unprotected sex with multiple partners


• Contains steroid hormones that suppress ovulation

• Usually synthetic hormones are used rather than natural ones because they are not broken down as rapidly in the body so they have a prolonged action


• Some pills contain progesterone only, but most contain progesterone and oestrogen and are known as ‘combined oral contraceptives’

• With most oral contraceptives, the woman takes one pill daily for 21 days and a placebo or sugar pill for 7 days, during which menstruation occurs


• Both estrogen and progesterone suppress the secretion of FSH and LH from the anterior pituitary gland

• Example of negative feedback• Taking the pill daily, starting at he end of

menstruation, keeps estrogen and progesterone levels high, thus suppressing the LH surge that stimulates ovulation


• After the 21st pill, estrogen and progesterone concentrations fall to the point where the uterine lining is no longer maintained – Menstruation occurs, reassuring the woman

taking the pill that she is not pregnant• Combined oral contraception over 97%

effective when pill is taken DAILY


• Combination of progesterone and estrogen can also be given through a skin patch, injection, or implant under the skin that is effective for several months– With these methods, no menstruation takes place


• Pills containing only progesterone may allow ovulation to occur

• They work by reducing the ability of sperm to fertilize the secondary oocyte and by making the mucus secreted by the cervix more viscous and so less easily penetrated by sperm

The morning after pill

• Intended to be taken after a women has unprotected sexual intercourse and suspects she may be pregnant

• It works for up to 72 hours after unprotected sex

The morning after pill

• Contains a synthetic like hormone that works by either reducing the chances of sperm reaching and fertilizing an egg or, in most cases, stopping a fertilized embryo from implanting in the uterus

Biological, social and ethical implications of birth control

• Taking the combined pill over a long period of time has various biological consequences. Benefits include:– Regular menstruation– Reduced risk of developing ovarian cysts or

ovarian/uterine cancers– Reduced risk of bacterial infections of the uterus

Biological, social and ethical implications of birth control

• Long-term consequences include:– Increase in blood pressure and risk of blood clots– Adverse reaction to synthetic hormones (nausea,

headaches)– Increased risk of developing breast cancer– Greater rate of infection of STIs (b/c increased rate

of unprotected sexual intercourse)

Arguments against widespread use of oral contraceptives

• Sexual freedom conferred by the pill may have contributed to the breakdown of a higher percentage of marriages than up to the 1960’s, causing more children to be brought up by one parent and sometimes causing difficulties for the parent and child

• Increased availability of effective contraception appears to have led to an increase in promiscuity among people of all ages

Arguments against widespread use of oral contraceptives

• Parents are concerned that their children may be sexually active without their knowledge

• Some people see use of anti-implantation pills as being equivalent to ‘unsupervised abortion on demand’

• Some religions maintain that sexual activity is for reproduction only and sexual activity for any other purpose is unacceptable

Arguments in favor of the widespread use of oral contraceptives

• Early access to contraceptives prevents unwanted pregnancies and many terminations that could cause distress to women

• Have given women increased control over their bodies

• There is an urgent need to try to reduce world population growth

Fertility

• A couple is described as infertile if they have unsuccessfully tried to get pregnant for 12 months

• While the incidence of infertility has remained fairly constant, the number of couples seeking medical help with infertility has tripled in the past 20 years– Due in part to many couples waiting until the age

of 25 to reproduce

Fertility

• In all cases of fertility:– 50% attributed to woman’s reproductive system– 35% attributed to man’s reproductive system– 15% unexplainable

• When root cause is accurately diagnosed, 50% will eventually achieve pregnancy following treatment.

• 60% with no obvious cause will achieve pregnancy even without treatment

In-vitro fertilization

• If a couple are infertile and investigation suggest that the yare producing some normal eggs and sperm, then in-vitro fertilization, or IVF, may be tried

• In IVF, a woman’s oocytes and her partner’s sperm are mixed in a dish, where fertilization takes place

In-vitro fertilization procedure

1. The activity of the woman’s anterior pituitary gland is suppressed

2. Ovulation is stimulated by injecting hormones that have a similar action as FSH and LH

3. This causes superovulation- multiple dominant follicles to be produced in each ovary


4. About 36 hours later, the oocytes are collected via ultrasound guided needles

5. The oocytes are microscopically examined to ensure they have research metaphase II

6. The same say, semen is collected from the man and the sperm is separated from the seminal solution

7. Each oocyte is placed in a separate dish with appx. 100,000 motile sperm– Alternately, a single sperm may be injected into the egg

to fertilize it


8. Between 12 and 18 hours after being mixed with sperm, each oocyte is examined to see if it has been fertilized– This is usually accomplished by looking for a second polar body,

indicating that meiosis II has been completed

9. The fertilized eggs are assessed to make sure they are dividing normally, during which time the woman is being given progesterone injections to prepare the endometrium for implantation

10. Finally, two embryos are chosen and inserted into the uterus using a tube passed through the vagina and cervix. Two embryos are usually selected to increase the likelihood that one will implant, while avoiding the risk of having triplets or even more babies.


• Multiple pregnancies tend to have more complications

• The babies are often born early, and such premature babies, with low birth weights, may need intensive care


• The fate of the ‘unused’ embryos raises ethical issues– Some see fertilized embryos as human beings,

who should be treated as such– It could also be argues that since many embryos

conceived naturally are lost, so the loss of embryos after IVF mirrors the natural process


• IVF may also be used when both parents are at risk of genetic disorders– In this instance, embryos are allowed to reach the

8 cell stage and are genetically screened using PGD

• IVF may also be used to implant a surrogate mother with the embryo of a couple when the genetic mother has uterine abnormalities, or no uterus

Ethical considerations of IVF• Consider whether or not it is acceptable to:

– Use unnatural methods of conception– Use procedures that are expensive and only available to a small number of people

worldwide– Freeze gametes or embryos, when the long-term effects of freezing are not known– Discard surplus IVF embryos– Use surplus IVF embryos for stem cell research– Donate surplus embryos to infertile couples– Perform PGD on embryos– Select embryos whose tissue type matches older siblings who need tissue donation– Allow post-menopausal women to become pregnant– Allow single-sex couples to have children– Involve the use of surrogate mothers– Use any procedure that increases the number of babies born in an over-populated

world

Frozen embryo replacement

• Using liquid nitrogen, frozen embryos can be stored for several years and can be thawed and transferred to a uterus when needed (70% embryos survive freezing)

• Usually happens when mother has to undergo therapy that affects her ovaries, like chemotherapy or radiotherapy as cancer treatment


• Ethical issues similar to those of IVF, but with the added issue of male consent– After several years, man may not want child

anymore or he could have died


• Another risk is that of embryo mix-up• Several women have been implanted with

embryos not belonging to them• Causes obvious distress to parents

Intracytoplasmic sperm injection

• Also known as ICSI• Modification of the normal IVF procedure in

which a single sperm is injected into an egg instead of allowing the sperm to fertilize the egg unaided

• Usually performed when the man’s sperm do not swim normally, are not fully active, or when the man has a low sperm count

Intracytoplasmic sperm injection

• With this procedure, there is an increased risk of fetal abnormalities

• Normally, only ‘fit’ sperm fertilize an egg• With ICSI, it is possible for a less than perfect

sperm to pass on congenital abnormalities to offspring

Gamete intrafallopian transfer

• Aka GIFT• Similar to IVF, (sperm and eggs are extracted

and examined) but fertilization occurs inside the oviduct instead of in a dish outside the body

• Raises fewer ethical concerns than IVF because fertilization is internal

Sperm bank$

• Frozen samples of sperm can survive freezing and thawing over several years

• Sperm banks contain samples of sperm that can be used in infertility treatments– Technique of storing frozen cells is called

cryopreservation

Sperm banks

• Usually used for IVF, but could also be used for intra-uterine insemination

• Removes sperm from semen, which would removed the chance of passing HIV from father to embryo

Sperm banks

• Most sperm used for fertilization comes from a woman’s partner, but when a male has infertile sperm, a donor may be used

• Donors’ background are throughoutly screen to create a donor profile- including health, family history, interests, and education

Sperm banks

• A woman who received donated sperm is given a several donor profiles, but not names, and she can choose which donor she prefers

Chapter 25 Aspects of Human Reproduction. Human reproduction Female urogenital system.

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Transcript of Chapter 25 Aspects of Human Reproduction. Human reproduction Female urogenital system.