Sexual Reproduction in the Human

Structure of the male reproductive system

Bladder Seminal Vesicle Prostate gland Cowper’s Gland Penis Sperm duct Urethra Epididymis Testis Scrotum

Testes A gonad is an organ that produces sex cell in animals. Male gonads are called testes Testes develop inside the body at first, but a few weeks before birth descend into the scrotum. This means they are kept at slightly lower than body temperature (35°) which is the ideal temperature for sperm production. Seminiferous tubules inside the testes are lined with sperm producing cells. Cells between the tubules produce the hormone testosterone.

Internal structure of testes Seminiferous tubule Sperm producing cells (2n) Sperm Sertoli cell – nourishes sperm Blood capillary Interstitial cells – produce testosterone

Epididymis All the seminiferous tubules join to form the epididymis. Sperm mature and are stored here.

Sperm duct and urethra The epididymis leads to the sperm duct (vas deferens) The sperm duct brings sperm to the urethra. The urethra is responsible for carrying sperm and urine out of the body

Glands in the male reproductive system Seminal Vesicle Prostate Gland Cowper’s Gland These glands produce seminal fluid which nourishes the sperm and provides a medium in which to swim. Seminal fluid + Sperm = Semen

Seminal vesicles, Cowper’s gland and Prostate gland Summary of functions of main parts of male reproductive system Part Function Testis Produces sperm and testosterone Epididymis Matures and stores sperm Sperm duct Carries sperm from the epididymis to the urethra Seminal vesicles, Cowper’s gland and Prostate gland Produces seminal fluid which feeds the sperm and allows them to swim. Sperm and seminal fluid are collectively called semen. Urethra Allows the passage of either urine or sperm. Penis Places sperm inside the body of a female Scrotum Keeps testes at a lower temperature (35°). This is the optimum temperature for Meiosis to occur.

Sperm Structure Acrosome (contains digestive enzymes) Head Nucleus (contains 23 chromosomes) Collar (contains mitochondria) Middle Flagellum (allows sperm to swim) Tail

Role of meiosis in sperm and egg production Sperm and egg producing cells are diploid i.e. they contain 46 chromosomes. They divide by meiosis to form sperm and egg cells. Each sperm and egg cell, therefore, has a haploid number of chromosomes i.e. they have 23 each

Role of meiosis in sperm and egg production As both a sperm nucleus and an egg nucleus are haploid they combine in fertilisation to form a diploid zygote i.e. the new zygote has 46 chromosomes. 23 chromosomes + 23 chromosomes = 46 chromosomes The zygote now grows by mitosis division ensuring that each new cell has a diploid number of chromosomes.

Role of meiosis in sperm and egg production

Male Hormone - Testosterone Testosterone is the male hormone responsible for the development of the primary and secondary male sexual characteristics The primary sexual characteristics are the presence of the male and female reproductive parts Secondary sexual characteristics refer to features that distinguish males from females, apart from the sex organs themselves

Male Secondary Sexual characteristics The growth of pubic, facial and body hair The enlargement of larynx and ‘breaking’ of the voice Increased muscular development and bone development A growth spurt at puberty An increased secretion of sebum in the skin

Structure of the female reproductive system

The Ovary These produce the eggs and female hormones. All the eggs in an ovary are present at birth. After puberty 20 eggs mature each month. Only one will be released from the ovary – the rest will die.

The Fallopian tube (oviduct) The Fallopian tubes are muscular and approx 12cm long. Funnels at the tip of each tube catch the egg after it is released from the ovary. The egg is moved along the tube by cilia and muscular peristalsis. The egg is either fertilised or dies in the fallopian tube.

The Uterus (womb) Muscular structure approximately the size of your fist. Outer wall made of involuntary muscle. Inner lining is called the endometrium This lining thickens each month with cells and blood vessels to nourish the embryo. The cervix separates the uterus from the vagina.

The Vagina Elastic muscular tube 10cm long. Allows entry of sperm. Is the birth canal for the exit of a baby. Lined with cells that produce mucous. This serves to protect against the entry of pathogens.

Fallopian tube (Oviduct) Structure of the Female reproductive system Fallopian tube (Oviduct) Funnel Ovary Ovarian Ligament Uterus Lining of uterus (endometrium) Cervix Wall of uterus Vagina Vulva

Summary of functions of main parts of female reproductive system Ovary To produce the egg (ova). To produce the hormones oestrogen and progesterone Fallopian tube (oviduct) Catches the egg from the ovary and transports it to uterus. Site of fertilisation. Uterus Site of implantation. Holds the developing embryo. Has a lining (endometrium) enriched with blood vessels to nourish the embryo. Forms the placenta. Vagina Allows entry of sperm and exit of baby at birth.

Female Hormones Oestrogen and progesterone are the female hormones A combination of oestrogen and progesterone at puberty causes the development of the secondary female characteristics: The maturing and enlargement of the breasts. The widening of the pelvis to allow for birth. The growth of pubic and underarm hair. A growth spurt.

The Ovary The ovaries produce the eggs and the female hormones oestrogen and progesterone. The ovaries of a female foetus contains all the potential eggs at birth. These eggs have not yet divided by meiosis and as a result are diploid After puberty a number of eggs are produced by meiosis each month. Usually only one egg continues to grow … the rest die

The Ovary Once meiosis is complete the egg is surrounded within a structure called the Graafian follicle. This structure produces the female hormone oestrogen When mature the follicle forms a swelling on the outside of the ovary. It bursts at ovulation to release the egg After ovulation the follicle fills with yellow cells and becomes the Corpus luteum (yellow body). This secretes the hormone progesterone

Summary of events in the menstrual cycle The menstrual cycle is a 28 day sequence of events that produces an egg and prepares the body for pregnancy. This cycle begins at puberty and continues until the menopause (the end of the woman’s reproductive life). Summary of events in the menstrual cycle Days 1 – 5 The old lining of the uterus (endometrium) breaks down and is shed from the body. The loss of this blood and tissue is called menstruation (period). A new egg is produced in the ovary by meiosis. This new egg is surrounded by the Graafian follicle.

- Oestrogen also prevents the development of any more eggs. The Menstrual Cycle Days 6 - 14 The hormone oestrogen is produced by the developing Graafian follicle. This has two functions: - It causes the lining of the uterus (endometrium) to build up again in preparation for implantation. - Oestrogen also prevents the development of any more eggs. Day 14 Ovulation. This occurs when the Graafian follicle bursts to release the egg into the fallopian tube.

- It causes the endometrium to thicken even further. Days 14 - 28 The Graafian follicle now develops into the Corpus Luteum (yellow body). This has two functions: - It causes the endometrium to thicken even further. - It also prevents new eggs from forming. The egg that was released will die by day 16 if it is not fertilised. Thus days 12 – 16 of the menstrual cycle are referred to as the Fertile Period. (Even though the egg is not released until day 14, sperm, which can survive for a period of time in the female body, may already be present. Thus the fertile period begins on day 12).

If fertilisation does not take place the Corpus Luteum starts to degenerate around day 22. This results in a reduction in progesterone levels. As a result the lining of the uterus breaks down again on day 28. The menstrual cycle begins again with day 1.

If fertilisation has not occurred the cycle begins again with the breakdown of the endometrium. The Menstrual Cycle 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 DAY 1-5 : Blood from the womb lining is shed from the body After day 5 the lining of the uterus repairs and builds up again Fertile period Ovulation occurs on Day 14 Implantation may happen

Inside the Ovary Developing Graafian follicle – secretes oestrogen Potential egg Egg is released from ovary (ovulation) Graafian follicle now changes to the Corpus luteum which secretes progesterone

Graafian follicle Corpus Luteum Hormones Endometrium 5 14 Days 28 Oestrogen________ Progesterone _____ Endometrium 5 14 Days 28