Sexual reproduction in humans

What does a male reproductive system consist of

a pair of testes in the scrotum, the p3nis, ducts connecting the testes to the p3nis and accessory glands that secrete fluids that mix with sperm to make semen

What does the mucus secreted by the seminal vesicles contain and why

fructose which is respired by the sperm for energy

Pathway of sperm

Testis → vasa efferentia —> vas deferens —> ejaculatory duct and prostate gland —> urethra —> p3nis

What is the purpose of the alkaline secretions from the accessory glands

maintain sperm mobility, provide nutrients for the sperm, neutralise acidity of urine in the urethra, neutralise acidity in the vaginal tract

Vas deferens function

carries sperm from the epididymis towards the penis

Seminiferous tubules function

spermatogenesis and testosterone production

Where are Leydig cells found and what is their function

in between the seminiferous tubules, produce testosterone

Where are Sertoli cells found and what is their function

in the seminiferous tubules, nourish and protect developing sperm from being attacked by the immune system

What does the female reproductive system consist of

2 ovaries, 2 oviducts with oviducal funnels, a uterus, cervix, vagina and vulva (+bladder and urethra)

What do oocytes develop from and where in the ovary are the cells they develop from found

oocytes mature in follicles which develop from cells in the germinal epithelium, around the periphery of the ovary

How do secondary oocytes enter the oviduct and then the uterus

Cilia at the entrance of the oviducal funnel sweep the secondary oocyte into the oviduct, the ciliated epithelial cells lining the oviduct sweep the secondary oocyte to the uterus

The 3 layers of the uterus wall

perimetrium (thin), myometrium (muscle), endometrium (mucous)

Endometrium purpose

a mucous membrane which is well supplied with blood, the layer that builds and sheds in a monthly cycle unless an oocyte is fertilised in which case the embryo implants in the endometrium and a pregnancy is established

Gametogenesis

the production of gametes in the sex organs

Spermatogenesis

the formation of sperm in the testis

Oogenesis

the formation of secondary oocytes in the ovary

Why should the gametes be haploid

so that at fertilisation, the diploid number is restored and the chromosome number doesn’t double every generation

Spermatogenesis process

• diploid cells of the germinal epithelium divide by mitosis to make diploid spermatogonia + more germinal epithelium cells

• spermatogonia divide lots by mitosis and enlarge, making diploid primary spermatocytes + more spermatogonia

• primary spermatocytes undergo meiosis I, making secondary haploid spermatocytes

• secondary spermatocytes undergo meiosis II, making haploid spermatids

• spermatids mature into spermatozoa/ sperm

Oogenesis before puberty

• diploid cells of the germinal epithelium divide by mitosis to make diploid oogonia + more epithelium cells

• oogonia divide lots by mitosis and enlarge, making diploid primary oocytes + more oogonia

• primary oocytes begin meiosis I but stop

• germinal epithelium cells divide to form diploid follicle cells which surround the primary oocytes, makes primary follicles

Oogenesis from puberty onwards

• hormones stimulate primary follicles to develop further. Just before ovulation, a primary oocyte completes meiosis I, making a secondary oocyte which contains most of the cytoplasm. First polar body also produced. Both are haploid

• primary follicle develops into secondary follicle (Graafian follicle when mature). Migrates to the ovary surface, bursts + releases secondary oocyte in ovulation

• secondary oocyte begins meiosis II but stops at metaphase II unless fertilisation takes place

Oogenesis after fertilisation

• after fertilisation, meiosis II is complete, making an ovum containing most of the cytoplasm + second polar body is produced

• after ovulation the Graafian follicle becomes the corpus luteum. If fertilisation occurs it produces hormones but otherwise regresses

Crossing over

The paternal and maternal chromosomes in a homologous pair exchange equivalent sections of DNA where they cross over, during prophase I

Independent/random assortment

The random alignment of homologous pairs during metaphase in meiosis is independent of the alignment of other homologous pairs.

What is the average length of 1 menstrual cycle

28 days

What hormones are involved in the menstrual cycle

follicle stimulating hormone (FSH), luteinising hormone (LH), oestrogen and progesterone

What hormone is secreted on day 0 of the menstrual cycle, from where and why

gonadotrophic releasing hormone, from the hypothalamus but secreted by the anterior pituitary gland, secretes FSH and LH

What hormone does FSH stimulate the production of

oestrogen

What are the roles of the hormone oestrogen

• trigger the rebuilding of the endometrium

• inhibit FSH secretion by negative feedback

• stimulate LH production by positive feedback

What hormones does the corpus luteum secrete

oestrogen and progesterone

What is the role of progesterone

maintains the newly rebuilt endometrium so that if a secondary oocyte is fertilised, there will be suitable tissue for an embryo to implant

What causes the endometrium to shed

if no embryo is implanted, FSH and LH concentrations decline, the corpus luteum degenerates and progesterone production declines. The endometrium is no longer being rebuilt by oestrogen or maintained by progesterone so it sheds

How does the menstrual cycle restart

oestrogen is low, FSH isn’t inhibited so primary follicles in the ovary are stimulated to develop

Stages of fertilisation

sperm reach secondary oocyte, capacitation, acrosome reaction, sperm head entry, cortical reaction, meiosis II, mitosis, first mitotic division

Capacitation

the removal of cholesterol and glycoproteins from the cell membrane over the acrosome in the sperm head. The membrane becomes more fluid and permeable to calcium ions

Acrosome reaction

acrosome releases proteases that digest the cells of the corona radiata. On contact with the zona pellucida the acrosome membrane ruptures and acrosin hydrolyses the zona pellucida around the secondary oocyte

Cortical reaction

reaction of the oocyte that produces the fertilisation membrane to prevent polyspermy. Sperm attaches to secondary oocyte, oocyte’s SER releases calcium ions into the cytoplasm. Cortical granules fuse with the cell membrane and release enzymes by exocytosis. Zona pellucida chemically modified, expands + hardens to create the fertilisation membrane

Cleavage process

embryo → morula → blastocyst

Implantation window

when the endometrium is receptive between 6-10 days after ovulation

What adaptation does the trophoblastic cells of the blastocyst have and why

trophoblastic villi that penetrate the endometrium, they increase the SA for the absorption of nutrients from the endometrium

How is the placenta grown

the trophoblast develops into the chorion, an outer membrane surrounding the embryo. Cells of the chorion move into the trophoblastic villi and form much larger chorionic villi. They acquire blood capillaries which are connected to the umbilical artery and vein that connect the embryo to the uterus wall through the umbilical cord

What are the major roles of the placenta

As an endocrine gland, to exchange products between the mother’s and foetus’s blood, as a physical barrier between the foetal and maternal circulation, providing passive immunity to the foetus and protection from the mother’s immune system

By what processes, through the wall of the chronic villi, does exchange between the mother’s and foetus’s blood occur

diffusion, facilitated diffusion, active transport, pinocytosis and osmosis

In what instances can the placenta not provide complete immunological protection

• some spontaneous abortions are equivalent to the rejection of a transplanted organ

• Rhesus disease in a foetus is the destruction of its blood cells by antibodies made by a Rhesus - mother against the blood cells of a Rhesus + foetus

• in the 2nd trimester, some women develop pre-eclampsia, when they have very high blood pressure. One cause is an abnormal immune response towards the placenta.

Which drugs can cross the placenta from the mother’s blood into the foetus’s blood

heroin and nicotine

Functions of the amniotic fluid

• maintains the foetus’s temperature

• provides lubrication to reduce risk of webbed hands/feet

• contributes to lung development

• allows movement so muscles and bones function before birth

• act as a shock absorber, protecting the foetus from injury from outside the uterus

what does hCG stand for, where is it produced and what does it do (pregnancy)

human chorionic gonadotrophin, produced by the blastocyst and chorion, maintains the corpus luteum’s secretion of progesterone which maintains the endometrium which is essential for embryonic development as it contributes to the structure of the placenta

Where is oestrogen produced from and what does it do (pregnancy)

secreted from corpus luteum + placenta

• inhibits secretion of FSH so no more follicles mature

• inhibits LH so ovulation is not possible

• inhibits prolactin so no milk is made

• stimulates the growth of the uterus to accommodate the foetus

• stimulates the growth of the mammary glands and increase their blood supply

Where is progesterone produced from and what does it do (pregnancy)

secreted from corpus luteum + placenta

• inhibits secretion of FSH so no more follicles mature

• inhibits LH so ovulation is not possible

• inhibits prolactin so no milk is made

• inhibits oxytocin, so the myometrium and muscles in the milk ducts don’t contract

Where is oxytocin secreted from and what does it do (birth)

secreted by the posterior pituitary gland, causes contractions of the myometrium in a positive feedback mechanism. The myometrium contacts from the top down so the foetus can be pushed out through the cervix

Where is prolactin secreted from and what does it do (birth)

secreted by the anterior pituitary gland, stimulates the glandular tissue in the mammary gland to synthesise milk