UNIT 4

Flowering plants

called angiosperms. Most flowering plants are diploid

Meiosis takes place within the reproductive tissues and produces haploid spores which contain the gametes

male spores are pollen grains produced in the Anther

female spores are the embryo sac produced in the Ovule, in the ovary

Hermaphrodic

flowers have 4 sets

Contains both male and female parts

flowers are four sets of modified leaves arising from the receptacle at the base of the flower

Flower diagram

calyx. corolla, stamen, carpel

• Calyx: comprises the sepals they are usually green and protect the flower in the bud

• Corolla: inside the sepals, “a ring of petals”. There may be a nectary at the base releasing nectar which is scented and attracts pollinators

• Stamen: each statemen consists of a filament supporting an anther which produces pollen grains. The filament contains vascular tissue which transports sucrose, mineral ions and water to the developing pollen grains. The anther usually contains four pollen sacs arranged in two pairs side-by-side. When mature the pollen sacks dehisce and they open to release its pollen

• Carpel: female parts, contains developing ovules, ovary and the stigma

Pollinators

Attracted to large-coloured petals by scent and nectar

they use their long tongues to reach the sugary nectar at the base of the petals. As the insect enters the flower the anthers brush against its thorax and legs leaving sticky pollen behind

when the insect enters another flower it brushes some of the pollen against the ripe stigma and cross-pollination takes place

Wind pollinated flowers

Do not need bright scented flowers that attract insects

the anthers hang outside the flower so that the wind can blow away the small, smooth and light pollen

The feathery stigmas hang outside the flower and provide a large surface area for catching pollen grains that are blown into their path

Dehiscence

The opening of the anther, releasing pollen grains

Development of the male gamete

in the pollen sac

meiosis

In the pollen sac of the anther, diploid pollen mother cells undergo meiosis. Each forms a tetrad containing four haploid cells which become four pollen grains.

The tapetum a layer of cells around the pollen sack provide nutrients and regulatory molecules to developing pollen grains. It has a significant role in the formation of a pollen cell wall which is tough and resistant to chemicals.

The cell wall resist desiccation so the pollen grains can be transferred from one flower to another without drying out. UV lights cannot penetrate the pollen cell wall so the DNA in pollen that is carried at high altitude is protected from mutation

Inside the pollen grain

The haploid nucleus undergoes mitosis to produce two nuclei: a generative nucleus and tube nucleus

The generative nucleus producers to male nuclei by mitosis

When the pollen is mature

outer layers

opening

The outer layers of the anthers dry out causing tension in lateral grooves

eventually occurs in which the tension pulls the walls of the anther apart and the edges of the pollen sacs curl away

The opening called stoneium exposes the pollen grains and they are carried away by insects or wind

Development of the female gamete

• the ovary contains one or more ovules

• in each ovule: a mega-spore mother cell undergoes meiosis making 4 haploid cells

• 3 disintegrate

• the remaining cell undergo 3x mitosis, producing 8 haploid nuclei (1 is female gamete)

• two of the haploid nuclei fuse to make a diploid nucleus, called polar nucleus

Purpose of the nuclei

surrounded by nucleus

The nuclei are in the embryo sac surrounded by the nuclleus, a layer of cells which provide nutrients

around the nucleus are two layer of cells integuments. a gap in the interguments is the micropyle

As with the formation of the male gamete that type of cell division that directly produces the female gametes mitosis

8 nuclei form

• One forms the haploid female gamete

• Two form the haploid synergids

• Two will fuse to form a polar nucleus that is diploid.

• Three form 3 antipodal nuclei.

ovule diagram

Pollination

of the same species

The transfer of pollen grains from the anther to the mature stigma of a plant of the same species

postandry

The stamens of the flower ripen before the stigmas

self and cross pollination

• self pollination: The pollen from the anther of the flower is transferred to the mature stigma of the same flower or another flower on the same plant

• cross pollination: pollen is transferred from the anther of one flower to the mature stigma of another flower on another plant of the same species

self pollination leads to

genomes of gametes

self fertilisation- resluts to inbreeding

• self fertilized species depend only on independent assortment and crossing over during meiosis and on mutation to bring about genetic variation in the genomes of the gametes. They display less genetic variation than cross fertilized species

• greater chance of two potentially harmful recessive alleles being brought together at fertilization

• the advantage of inbreeding is that it can preserve those successful genomes that are suited to a relatively stable environment

Cross pollination can lead to

evolutionary signficabce

Cross fertilization- results in outbreeding

• outbreeding combined gametes from two individuals, in addition to events in meiosis and mutation, and so it generates more genetic variation

• outbreeding reduces the chance of producing harmful allele combinations

• outbreeding is of great evolutionary significance, because in a particular environment some genomes are more successful than others. It may allow a species to survive in a changing environment as there are always likely to be some members of a population with suitable combination of alleles

Ensuring cross pollination

dichogamy, protandy, protogyny

anther is below

genetic incompatibity

sepreate

• Dichogamy: eg. Protandy. Or protogyny (stigma opens first)

• the anther is below the stigma so pollen cannot fall onto it eg. Pin eyed primrose

• genetic incompatibility eg. red clover. pollen cannot germinate on the stigma of the flower that produced it

• separate meal and female flowers on the same plant eg. Maizev

• separate male and female plants eg. Holly

Fertilisation

fusion

The Fusion of a female and male gamete

producing a zygote

After pollination

up a gradient of

• When a compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube

• the pollen tube nucleus is at the tip of the tube with two male nuclei behind

• the pollen tube grows out of the pollen grain through a gap in the cell wall (Pit) and down the style up a gradient of chemoattractants

Tube nucleus

codes for production

pollen tn disentegrates

• The pollen tube nucleus codes for the production of hydrolases including cellulases and proteases, and it digests its way through the tissues of the style. The products of digestion are used by the growing pollen tube

• the pollen tube grows through the gap in the integument, the micropyle and passes into the embryo sac

• the pollen tube nucleus disintegrates, presumably having completed its function of controlling the growth of the pollen tube

Two male gametes released

tip of the pollen tube opens

• The tip of the pollen tube opens, releasing the two male gametes into the embryo sac

• the male and female gametes are haploid one of the male gametes fuse with the female (the oosphere), to form the zygote, which is diploid

• the other male gamete fuses with the diploid polar nucleus to form a triploid nucleus. This triploid nucleus is the endosperm nucleus. When it divides repeatedly by mitosis, it generates the endosperm tissue which takes over from the nucleus in providing nutrition for the developing embryo

Double fertilisation

Starts when the pollen tube grows through the gap

Fusions have occurred to form the zygote and to form the endosperm

Fruit, structure d

ovary wall

A structure developing from the ovary wall, containing one or more seeds

Seed

fertilized ovule

Structure developed from a fertilized ovule, containing an embryo and food store enclosed within a testa

Development of the seed

zygote endosperm integuments

micropyle

• Diploid zygote divides by mitosis, becoming an embryo which consists of a plumule ( developing shoot) a radical ( developing root) and one or two cotyledons (seed leaves)

• Triploid endosperm nucleus develops into a food store, providing food for their developing embryo

• outer integuments to dry out, harden and become waterproof, with deposits of ligon they become the seed coat/testa

• the micropyle remains as a pore in the seed

Development of the seed

ovule

funicle/stalk ovary

• Ovule comprising the embryo, enfosperm and testa becomes the seed

• funicle or stalk of the ovule becomes the funicle of the seed, it attaches to the seed at the hilum

• the ovary becomes the fruit [in cherries the ovary wall become sweet juicy and pigmented] [in almonds the ovary wall becomes dry and hard]

Structure of the fruit and seed

Dicotyledon

Eg. Broad beans

Its seeds have two seed leaves or cotyledons. The embryo lies between them

the endosperm (which was the food store for early embryo) is absorbed into their cotyledons, so the broad bean has a non endospermic seed

Monocotyledon

Eg. Maize

it's only has one cotyledon

the endosperm remains as the food store, so maize seeds are endospermic

the cotyledon remains small and does not develop further. The testa of a maize seed fuses with the ovary wall, so maize has one seeded fruit

seed becomes dormant, water content falls below 10% and reduces their metabolic rate

They can survive long periods and will not germinate until conditions are suitable

Seed dispersal

The movement of seeds away from the parent plant

these seeds produce plants which avoid competition

dispersal methods have been subject to natural selection

Seed dispersal method

wind

Ash and Sycamore fruits have sales to allow wind dispersal

Dandelion fruits have parachutes of stiff hair

the fruit of poppies has pores, through which the seeds are shaken out when the stem is blown in the wind

Seed dispersal method,

scarification

transport

A birds eat seeds that pass through the digestive system and are dispersed in faeces

mammals, Reptiles and fish can also disperse seeds this way

Scarification - the digestive system weakens the testa by physical attack by acid and by enzymes and the seeds of some species can only germinate when this happens

seed dispersal method

rolling

When the fruit of a horse chestnut tree breaks open, the conker, falls to the ground and rolls away from the parent tree

Seed dispersal method

bursting

When Lagoon pods dry they split and the seeds scatter

in many species the pods rotate as they burst open sending the seeds in different directions

Seed dispersal method

water

Coconut palms grow by water

coconuts are seeds and when they fall into the water, they float, because the air cavities make them buoyant and the water carries them

Seed dispersal method

carrying

Hooked seeds attached to animals coats and are carried away

Seeds and survival

• Dormant seeds

• the testa is chemically resistant

• the water content

• the testa

• the endosperm of cotyledons

• seeds can be dispersed

• dispersal allows colonization

• inhibitors

• Dormant seeds have a low metabolic rate and so they survive cold weather

• the testa is chemically resistant, so seeds survive adverse chemical conditions

• the water content of a dormant seed is produced below 10% and so seeds can survive very dry conditions

• the testa can physically protect the embryo

• the endosperm of cotyledons provide a supply of nutrients which last until the emerging seedling can photosynthesize adequately

• seeds can be dispersed great distances from the parent plant and don't compete with it

• dispersal allows colonization of new habitats

• inhibitors may allow germination at suitable times of the year. They are broken down in cold weather ( vernalization) so that the seed can germinate in spring

Germination

biochemical and physiological

The biochemical and physiological processes through which a seed becomes a photosynthesizing plant

Conditions for successful germination

• suitable temp: optimum temp for germination is the optimum for the enzymes in the process. usually between 5-30 degrees

• water: to mobilise enzymes, for transport in the xylem and phloem and to vacuolate cells, making them turgid

• oxygen: aerobic respiration releases energy, which fuels metabolism and growth

Mobilisation of food reserves and germinatiorn

when conditions are suitable

1. When conditions are suitable, water is taken up or “inbribed” rapidly by the seed through the micropyle. Water causes the tissues to swell and provide suitable conditions for enzymes activity

Mobilisation of food reserves and germination

food reserves in the seed

2. Food reserves in seeds are insoluble in water and can't be transported to the embryo

• The reserves must be broken down into soluble molecules. amylase hydrolyes starch into maltose and protease hydrolyes proteins to amino acids

• Te soluble products are transported to the embryo and carried in the phloem to the plumule and radicale, where rapid cell division occurs

• some of these sugars are converted to cellulose for cell wall synthesis

• aerobic respiration releases energy from sugars and amino acids are used for protein synthesis

Mobilisation of food reserves and germination

swollen tissues

3. The swollen tissues rupture the testa and the radicale emerges from the seed. It is positively geotropic and negatively phototropic and so it grows downwards

4. then the plumule emerges it is positively phototrophic and negatively geotropic so it grows upwards

Mobilization of food reserves and germination

the part of the plumule

5. During germination the cotyledons of the broad bean remain below ground

   • the part of the plumule above the join between the embryo and the cotyledons elongate rapidly, pushing the plumule upwards

   • the plumule is bent over in the shape of a hook as it's pushes its way up through the soil. This protects the tip from damage of soil abrasion.

Mobilisation of food reserves and germination

if the seed has been planted

If the seed has been planted at the correct depth in the soil, when the plumule emerges, the hook straightens and the leaves unfurl and begin to photosynthesize

By now the food reserves in the cotyledons will have been depleted

The brewing industry

Uses germinating barley seeds to make beer

terms “malt” and “Malting” used in brewing refers to the maltose generated when the starch in barely is digested

Gibberellin process

1. Barley embryo secretes a plant growth regulator: gibberellin, which diffuses through the endosperm to the aleurone layer. a layer has a high protein content

2. the gibberellin switches on genes in the cells of the aleurone layer, resulting in transcription and translation producing enzymes including protease and amylase

3. the proteases hydrolysed protein in the aleurone layer to amino acids, which are used to make amylase and maltase

4. the carbohydrates's diffuse out of the aleurone layer and hydrolyze the starch stored in the endosperm cells

5. the sugars produce diffuse back through the endosperm to the plumule and radicale of the embryo

6. sugars are aspired for energy which fueled bio synthesis and cell division bringing the seed out of dormancy

The male reproductive system

major organs

• A pair of testes which produce sperm. They are contained in an external sac: the scrotum.

• Vas Deferens which transports sperm from the testes to the penis.

• Glands (prostate, seminiferous tubules and the cowpers gland) which add fluid.

• Penis filled with spongy tissue which allows sperm to be placed inside the female.

testes to van deferens

each teste contain, cells lining reaches

vas efferentia

• Each testis contain seminiferous tubules. These cells lining the tubules undergo spermatozoa formation and throughout the process, cells move towards the lumen. running through the middle of the tubule

• When the sperm reaches the lumen, they move through the tubule and collect in the Vasa efferentia.

• They carry sperm to the head of the epididymis. Sperm remain in the epididymis for a short time while they become motile and then pass into vas deferens during ejaculation.

Vas deferens to penis

on the way, seminal vesicles

• Vas deferens carry sperm from the epididymis towards the penis

• on the way, the seminal vesicles secrete mucus into the vas deferens.

• The mucus contains a mixture of chemicals including fructose, respired by the sperm for energy. The sperm and seminal fluid move through the ejaculatory duct, which passes through the prostate gland.

• Where's zinc containing prostate fluid is secreted

• the fluid emerging from the prostate gland is semen: mixture of sperm and seminal and prostate fluids. It's carried through the penis in the urethra.

The secretions of accessory glands are alkaline

They:

• maintains sperm mobility

• provides nutrients to the sperm, including fructose, amino acids and zinc ions

• neutralise the acidity of any urine remaining in the urethra

• neutralise that acidity of the vaginal tract

Gametogenesis

Production of gametes in the sex organs

spermatogenesis is the formation of sperm in the testis

Spermatogenesis process

1. Cells of the germinal epithelium are diploid. They divide by mitosis to make diploid spermatogonia and more germinal epithelium cells

2. spermatogonia divide by mitosis to form diploids primary spermatocytes

3. primary spermatocytes undergo meiosis, making secondary spermatocytes which are haploid

4. secondary spermatocytes undergo meiosis ll to make haploid spermatids

5. spermatids mature into spermatozoa/ sperm

Sperm structure

enzymes used

from centrioles

• the head contains a haploid nucleus, covered at the anterior end by a lysosome {acrosome} This contains enzymes used at fertilisation

• the middle is packed with mitochondria, provided ATP for movement. they spiral around the microtubues, which extend from the centrioles into the axial filament in the tail.

• the tail/flagellum, makes lashing movements that move the sperm

Female reproductive system

ovaries

• There are two ovaries, attached to the body wall below the kidney

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

• mature follicles migrate to the surface of the ovary, from where a secondary oocyte is released at ovulation

• the ovaries alternate each month in releasing the oocyte

Female reproductive system female reproductive system

fallopian tube

• Cilia at the entrance of the oviducal funnel sweep the secondary oocyte into the oviduct/fallopian tube

• Ciliated epithelial cells lining the oviduct convey the secondary oocyte into the uterus

The uterus wall

• The perimetreum is a thin layer around the outside

• the myometrium is the muscle layer

• the endometrium is the innermost layer it is a mucous membrane which is well supplied with blood. It is the layer that builds and is shed in a monthly cycle

• the embryo implants in the endometrium

the uterus opens into the vagina, through a narrow ring of connective tissue and muscle (the cervix)

the walls are muscular and open at the vulva

Uterus diagram

ovary diagram

ovary structure

• Germinal epithelium on the outer edge, which produces the egg cells.

• Underneath the epithelium in the cortex of the ovary, which will contain eggs in various stages of maturity.

Graafian follicle

• Theca produces the female sex hormone oestrogen.

• It is stored in the developing cavity called the antrum.

• Granulosa cells surround the wall of the follicle and the maturing ovum.

• Surrounding the oocyte is a layer of carbohydrate called the zona pellucida.

Oogeesis

oocyte

the formation of secondary oocytes in the ovary

Oogenesis process

1. In the developing features, diploid Germinal epithelium epithelium divide by mitosis to make diploid oogonium

2. they divide by mitosis and enlarge making diploid Primary oocytes, and more oogonia

3. the primary oocytes begin meiosis l

4. geminal epithelium cells divide to form diploid follicle cells, which surround the primary oocytes, making primary follicles

5. secondary oocytes are formed when meiosis l completes (contains most of the cytoplasm) the other product is the first polar body, both haploid

6. the primary follicle develops into a secondary follicle (graffian follicle when it's mature) it migrates to the surface of the overary where it bursts and releases the secondary oocyte ovulation

7. the secondary oocytes begins meiosis ll but stops at metaphase ll unless fertilization takes place

8. at fertilisation meiosis ll is complete, making an ovum (containing most of the cytoplasm) the other product is the second polar body

9. after ovulation the graafian follicle becomes the corpus luteum

Zona pellucida

glycoprotien layer

secondary oocyte

periphery of cytoplasm

Is a glycoprotein layer

it surrounds the cell membrane of the secondary oocyte

the chromosomes of the secondary oocyte are at metaphase ll. They are at the equator attached to the micro tubules that make the spindle apparatus

the periphery of the cytoplasm contains cortical granules which are secretory organelles that prevent the entry of more than one sperm

Corona radiata cell surround the secondary oocytes and provide nutrients

Sexual intercourse

contraction of smooth in the wall

Physical and psychological effects caused the arterioles entering the penis to dilate and the venules leaving to constrict.

The build up of blood in the penis causes it to be erect

movements of the penis results in the contraction of smooth muscles in the walls of the epididymas, vas deferens and penis which causes the ejaculation of semen into the vagina.

The force of ejaculation is sufficient to propel some semen thru the cervix into the uterus

Ejaculation is triggered by the sympathetic nervous system.

causes of erection

Nitric oxide

blood

• Erection is caused by the parasympathetic nervous system which causes nitric oxide levels to rise in the spongy tissue of the penis.

• The result is that arteries dilate and the blood fills the cavities in the spongy tissue.

Fertilisation

Capacitation, tail, can only

Sperm can only fertilise a secondary oocyte after capacitaion has taken place

It is the removal of cholestorol and glycoproteins from the cell membrane over the acrosome.

The membrane becomes more fluid and more permeable to calcium ions. Meanwhile the tail motion changes to a whipping movement, increasing sperm motility

fertilisation. Acrosome reaction

CR

on contact with zona pellucida

the acrosome release enzymes, which digest the cells of the corona radiata.

On contact with the zona pellucida, the acrosome membrane ruptures and releases more enzymes {protease, acrosin} which hydrolyze the zona pellucida around the secondary oocyte

Fertilisation

sperm head entry

The cell membranes of the secondary oocyte and sperm fuse and the head of the sperm sinks into the cytoplasm of the secondary oocyte

Following the entry the secondary oocyte is called an ovum

Fertilisation. Cortical reaction

Reaction of oocyte

Reaction of the oocyte that produces the fertilization membrane, preventing polyspermy

when the sperm attaches to the secondary oocyte, the oocyte smooth endoplasmic reticulum releases calcium ions into the cytoplasm

They make the cortical granules fuse with the cell membrane and release their contents of enzymes by exocytosis

The zona pellucida is chemically modified and expands and hardens making a fertilization membrane which is impossible for more sperm to penetrate

Fertilisation

meiosis ll

Entry of the sperm also stimulates the completion of the second meiotic division of the ovum nucleus

It proceeds through anaphase ll and telophase ll, divides and expels the second polar body

Fertilization

zygote

Within about 24 hours, the first mitosis combines the genetic material of the parents to make the diploid cells of the embryo

The chromosomes joined the ovum chromosomes on the cells equator

the cell is now a zygote as the chromosomes have combined

Implantation definition

The sinking of the blastocyt into the endometrium

Trophoblast

Cells forming the outer layer of the blastocyt

Implantation

Has the embryo moves down the oviduct, it divides by mitosis in a sequence called cleavage

a solid ball of 16 cells called morula forms within three days

cleavage continues and the cells move in relation to each other

by 7 days, the ball of cells becomes hollow and is a blastocyt

the cells around the outside of the blastocyts are trophoblasts: they divide to make an inner cell mass on one side

the blastocyts moves from the oviduct into the uterus

After ovulation

The endometrium thickens and gets an increased blood supply to prepare it to receive an embryo

there is an “implantation window” where the endometrium is receptive between 6 and 10 days after ovulation

Trophoblastic villi

After about 9 days, protrusions from the trophoblast cells of the blastocsyt called trophoblastic villi

they penetrate the endometrium

the villi increase the surface area for the absorption of nutrients from the endometrium

The placenta

it connects the embryo and then the foetus to the uterus wall

made of tissues derived from the embryo and the mother

Formation of the placenta

• The trophoblast develops into the chorion, an outer membrane surrounding the embryo.

• cells of the chorion move into the trophoblastc villi and form chronionic villi.

• They acquire blood cappilaries,which are connected to the umbilical arteries and vein. They are the blood vessels that connect the embryo to the uterus wall thru the umbilical cord.

• Projections from the endometrium between chorionic villi are the maternal tissues of the placenta

roles of the planceta

endocrine organ

produces hormones to support the pregnancy

HCG

Oestrogen, progesterone

Roles of the placenta

exchange between mothers and foetus blood

includes nutrients, waste, products, respiratory gases.

• inter villous spaces (lacunae) containing the mothers blood, surround the chorionic villi.

• chroionic villus cells have microvili, giving a large area of contact with the mothers blood, for max exchange.

• the embryos and mothers blood do not make contact

• the concentration gradient between the two circulations is maintained by a countercurrent flow, enhancing exchange efficiency

Role of the placenta

physical barrier

Between the fetal and maternal circulation

• protect the fragile fetal capillaries from damage by the higher blood pressure of the mother

• protect the developing fetus from changes in maternal blood pressure

Role of the placenta

passive immunity

Provides passive immunity to the fetus; maternal antibodies cross the placenta and attack pathogens but do not attack the fetal cells

Role of the placenta

protection from the mother's immune system

migratory imune cells

The mother does not make an immune response against the fetus or placenta even though they contain foreign genes

the cells of the wall of the choreonic villi fuse so there are no spaces between them making a syncitium. Migratory immune cells cannot get through to the fetal blood

Why placenta may not always provide complete immunological protection

spontanous abortion

rhesus disease

2nd trimester

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

• Rhesus disease in a fetus is the destruction of its blood cells by antibodies made by a rhesus negative mother against the blood cells of a rhesus positive feotus

• in the second trimester some women develop pre-eclampsia when they have very high blood pressure one cause is an abnormal immune response towards the placenta

The menstrual cycle

detaches

• In the absence of an implanted embryo, the endometrium is shed through menstruation

• from the first period to menopause

• the endometrium detaches if a blastocyst has not been implanted

• the endometrium has a good blood supply and it appears as bleeding as it leaves the body

The hormones in the menstrual cycle

• Follicle stimulating hormone FSH

• luteinising hormone LH

• oestrogen

• progesterone

FSH

Secreted from the exterior pituarity gland

• stimulates the development of primary follicles in the ovary

• It forms a fibrous outer layer (the theca) and secretes fluid into a cavity the (antrum)

• stimulates the thecal cells to produce estrogen

LH

anterior pituraity gland

Which is its maximum concentration just before ovulation

• induces ovulation

• it's high concentration causes the graafian follicle at the surface of the ovary to release the secondary oocyte.

• Has a positive feedback effect on FSH

the remains of the graafian follicle converts into the corpus luteum- which secretes estrogen and progesterone

these inhibit further secretion of FSH and LH and their concentrations decrease

Purpose of oestrogen

• Triggers the rebuilding of the endometrium, which was shed during menstruation

• inhibit FSH secretion by negative feedback, which brings its own concentration down again

• stimulate LH production by positive feedback

Oestrogen and progesterone

The corpus luteum secretes estrogen and progesterone

the progesterone maintains the newly built endometrium so that if a secondary oocyte is fertilized, there will be suitable tissue in which the embryo can implant.

If if there is no fertilization or implantation, the falling concentrations caused the corpus luteum to degenerate so oestrogen and progesterone production declines

The endometrium is no longer being rebuilt by estrogen or maintained by progesterone and is shed

As oestrogen and progesterone are low

They know they no longer inhibit FSH production, so cycle restarts

The amnion

The embryo develops and grows in the uterus enclosed in the amnion- a membrane that is derived from the inner cell mass of the blastocyst

• amniotic fluid accumulate and increases in volume for 6 to 7 months

• the fluid is made by the mother and the fetus contributes urine to it

• the fluid pushes the amnion out

Purpose of amniotic fluid

• Maintains foetus temperature

• provides lubrication - if too little amniotic fluid circulates between fingers and toes, they may become webbed

• contributes to lung development

• allows movement so muscles and bones function before birth

• acts as a shock absorber protecting the fetus from injury, from outside the uterus

1st trimester

• 1^st trimester ~ 

  • (3 months) 

  • conception, implantation and embryogenesis.

  • All major organs are laid down.

  • Most prone to miscarriage.

2nd trimester

• Becomes called the foetus

• 3-6 months.

• Nervous system begins to function.

• Eyes begin to move

• Limbs begin to move.

3rd trimester

• 6-9 months

• all major structures are complete

• Period of growth

• fat laid down, foetus mass anf length increases

• By 28 weeks high survival rates for premature birth.

• Massive increase in scope of nervous system.

Human Chorionic Gonadotrophin (HCG)

• It maintains the follicle in the ovary form a structure called the corpus luteum. 

• HCG maintains the corpus luteum in making progesterone, which maintains the endometrium.

placental hormones

• Produced first by the corpus luteum and by three months on by the placenta.

• This maintains the endometrium.

• Increases breast development.

• Inhibits other hormones (FSH, LH, Prolactin and oxytocin)