Reproduction and Embryology

Question 1

What are the advantages and disadvantages of sex?

Answer 1

Advantages= Each new individual has a new mix of genes that give it immunity to pathogens. Also, only the strongest gametes will become humans. 
Disadvantages= need to find a partner and there may be a dilution of a 'perfect set' of genes or the gene mix may not work well

Question 2

What are the different cells in reproduction?

Answer 2

Early in embryonic development, the germ line is established which goes on to create gametes only. The other cells used to create the rest of the body are somatic

Question 3

How is the inside of the testis organised?

Answer 3

Many seminiferous tubules all drain to a network in the middle of the tests called the rete testis, then out by the vasa efferent (efferent ducts) to a structure called the epididymis

Question 4

What is the anatomy of an individual tubule?

Answer 4

The thick outer layer of each tubule consists of myoid cells. Around the outside of the cell there is stem cells. between the tubules is a general connective tissue called stroma.

Each seminiferous tubule of the adult testis has a central lumen, or cavity, which is connected to the epididymis and then the spermatic duct

Question 5

What is the purpose of the myloid cells?

Answer 5

The cells are muscle-like and help the tubules contract so that sperm is cast into the lumen of the tube and moved along the tube.

Question 6

What is the process of meisos in the testis?

Answer 6

Where stem cells differentiate and half their genes to produce gametes.

Starts on the outside of the tubules where the stem cells rapidly reproduce, then get reduced to a haploid state (half chromosomes) as they move inwards.

Afterwards they are differentiated into mature spermatozoa.

Process begins in puberty.

Question 7

What are the different stages of meiosis in the testes?

Answer 7

Question 8

What are the roles of somatic cells in the tubules of the testes?

Answer 8

Leydig’s cells= produce testosterone

Sertoli’s cells= stimulate spermatogenesis and release inhibin (It inhibits the synthesis and release of the follicle-stimulating hormone in the pituitary gland)

Question 9

What changes occur in a sperm as it matures?

Answer 9

Nucleus becomes small and compact

Mitochondria becomes columns

Golgi apparatus becomes the head of the sperm.

Question 10

What are the roles of the continuous tight junctions between Sertoli cells?

Answer 10

It stops cells from testes leaving (body does not recognise sperm cells as its own and so will trigger an immune response)

Stops inflammatory mediators entering.

Mumps can cause this to happen however

Question 11

How do female gamates get into the fallopian tube?

Answer 11

Released by the ovary into the pelvic space and are then swept into the fallopian tube by fluid flow, and the cilia.

Question 12

What is different about a female’s germ line cells?

Answer 12

An adult women has no germ line cells. Germ line stem cells exist only in foetal life. They undergo meiosis in foetal life, then there is a pause and during puberty meiosis is completed.

Question 13

What are follicles in the ovary? And what happens during puberty?

Answer 13

Follicles are oocytes in the ovary that are surrounded by granulosa cells.

During puberty, the pituitary gland produces follicle stimulating hormone. in response to this, some follicles resume development.

Question 14

What are the steps in an oocyte maturing?

Answer 14

The granulosa cells grow and share the metabolic load of looking after the oocyte. The oocyte synthesises lots of RNA and mRNA to later help produce lots more protein

The granulosa then makes multiple layers which become a zone known as the zone pellucida (jelly like)

Next granulosa cells secrete follicular fluid that form the antrum. Stromal cells also form an outer layer called the thecal layer.

Question 15

What determines if a mature oocyte dies or gets released?

Answer 15

At a critical point, the developing follicle depends on recieving a surge of Luteinizing hormone (LH). If it does not recieve this, it dies.

If it recieves it, it matures futher to a graafian follicle at the surface of the ovary.

Only follicles that are exactly mature enough to be released will be. The other die.

The mature follicle ends up at the edge of the ovary, and the oocyte ruptures the follicle and it bursts.

Question 16

What is different between male and female daughter cells in meiosis?

Answer 16

In females, one of the daughter cells is discarded.

In males, both of the daughter cells can be used as sperm

Question 17

What is left behind after an oocyte has been released?

Answer 17

Corpus lutea- full of thecal and granulosa cells.

Produces hormones such as progesterone and oestrogen that prepares the lining of the uterus to receive an oocyte. If there is no pregnancy it will die and the lining will shed.

Question 18

What happens to sperm in the cervix?

Answer 18

Capacitation

The glycoprotein and sterol coat that was aquired in the epididymis is removed by proteases in the cervical fluid.

This causes the cell membrane to become more permeable to calcium ions which indirectly activates strong tail lashing.

Question 19

How do sperm reach an oocyte in the fallopian tube?

Answer 19

Moves by tail movement and female tract movement.

If sperm arrive days before they are needed they can bind to the wall of the fallopian tube (bind to epithelial cells) and wait.

Question 20

What happens when the sperm meets the zona pellucida of the egg?

Answer 20

An acrosome reaction

(acrosome= membrane covering sperm head)

The reaction dissolves a hole through which the sperm swim to fuse with the oocyte

Fusion causes a wave of calcium to enter oocyte= cortical granules are released and change the zona pellucida and make it impenetratable.

Question 21

What happens after the acrosomal reaction has taken place?

Answer 21

The acrosomal and plasma membrane fuse at many points.

The acromal contents spill out and can digest the zona pellucida

The acrosome reacted sperm burrows towards the egg. Another membrane fusion occurs from the sperm to give its genes up.

Question 22

What mitochondria do the fertilised oocytes have?

Answer 22

Sperm mitochondria is so burnt out that is rarely passed on, so majority of mitochondria we have is from our mothers which is important in mitochondrial disease

Question 23

What are the reasons for assested fertilisation?

Answer 23

Blocked/ absent fallopian tubes due to STIs or congenital absence

Blocked vasa deferntia

Low male fertility

Female age (viable oocyte production decreases)

Elective ligation of either sex’s tubes.

Question 24

What are the typical stages of assisted fertilisation?

Answer 24

• Super-ovulation, using hormones such as FSH or antagonists of feedback systems to cause far more oocytes being produced.
• Oocyte are harvested by a thin aspiration needle guided by ultrasound
• Sperm harvesting usually harvested by masturbating, or if that does not work through aspiration of the epididymis or testes.
• The sperm are capacitated- given salts, human serum albumin, lactate, pyruvate, glucose, bicarbonate and others used to mimic the female cervix.
• Next, mixing of sperm and oocytes into the same medium. If sperm is so damaged, ICSI (intra-cytoplasmic sperm infection) can take place- more abnormalities found in offspring to this, maybe due to a damaged oocyte or badly damaged sperm.
• Next stage is observation of development - make sure meiosis takes place and here is where genetic testing takes place.
• Then embryo transfer takes place into the womb.
  *

Question 26

What initially happens in a fertilised cell?

Answer 26

The two nucleis fuse, ready for mitosis. There is a quick succession of mitosis, but no growth occurs and cells become smaller and smaller

Question 27

What happens to a cell for it to be called a blastocyst?

Answer 27

The cells on the outside of the embryo differentiate into an epithelium called a trophoblast, made up of trophectoderm cells which pumps fluid into the cell, making it similar to a cyst.

When the blastocyst arrives at the uterus, the trophoblast invades the uterine epithelium

Question 28

What is human chorionic gonadotrophin?

Answer 28

hGC

Produced by blastocysts

Maintains the uterus for the embryo

Question 29

What is the next step of a developing embryo after the blastocyst?

Answer 29

Cells facing the fluid filled cavity are triggered to turn into the hypoblast and move round the sides of the trophoblast to form a yolk sac.

The cells directly touching one area of the trophoblast turn into the epiblast area.

The cavity between the epiblast and trophoblast is called the amniotic cavity

Question 30

What are the different ways monozygotic twins can be created?

Answer 30

1) cells can seperate at the two cells stage, producing two complete embryos with seperated placenta
2) the blastocyt can have two seperate cell masses within the cavity- this ends up with two foetuses with seperaqte amniotic cavities but the same placenta.
3) there can be two distinct sites of hex-expressing cells which causes the twins to share the amniotic sac and placenta.

Question 31

What is a hex-expressing site of the embryo?

Answer 31

One area of the hypoblast changes by switching on new genes such as hex, which then sticks together and and move to the outside of the hypoblast ‘disk’

These cells secrete proteins that inhibit progress in the epiblast layer above. There are some cells in the epiblast far enough away to escape inhibition- these will form the bottom half.

Question 32

What 3 layers are humans made of?

Answer 32

The outer skin ECTODERM

The inner skin (lines the guts) ENDODERM

The middle layer between these (Muscles ect) MESODERM

Question 33

What is the term given to a embryo changing from a flat disk to a 3D model?

Answer 33

Gastrulation

Question 34

How does gastrulation occur in a embryo?

Answer 34

Cell of the epiblast stream down the middle to the hypoblast; the first cells move the hypoblast layer until the whole of the central region becomes replaced. This forms the endoderm- the lining of the gut.

Once that’s complete, the next lot of cells start making a middle layer of loose packed cells with lots of matrix forming the mesoderm. The cells that did not manage to dive through remain in the epiblast and become the ectoderm.

The middle area of the new endoderm detaches to become the notochord

Question 35

What is the notochord?

Answer 35

Sits in the middle of the endoderm and ectoderm.

Runs through the body from the neck to the end of the spinal cord.

Question 36

What happens after the 3 layers have been formed in the embryo?

Answer 36

The embryo grows significantly lengthways, however the yolk sac never growths causing the endoderm to become kinked into a folded structure. The endoderm has been dragged out into a long tube.

Question 37

How does the embryo make a CNS?

Answer 37

Derived from the ectoderm- the dorsal part starts to fold inwards and create a valley. Eventually this valley becomes a sealed tube.

When the cells touch, cells change neighbours. Within the ‘valley’ the cells have stopped producing E-cadherin (cell adhesion molecule for epithelia) and switch on N-cadherin. N and E prefer to stick to one another so cells will undergo a change around the gap and will seal the gaps.

Question 38

What can go wrong in the sealing of the neural tube?

Answer 38

When it fails the condition is called spina bifida where the spinal cord has not closed and there will be half vertebrae which cannot link at the back. Smaller versions are relatively common until it was understood that folic acid (green vegetable) is useful in stopping this. If it happens in the brain region, called anencephaly.

Super rare things can happen in neural tube closure such as a twin getting enveloped in the neural tube and creates a foetus in another. (foetus in fetu)

Question 39

Where is segmentation first seen in an embryo?

Answer 39

Seen in the mesoderm- each side of the midline (neural tube) divides itself into somites; precursors of vertebrae

The notochord then produces a molecule called SHH which spreads and intrusts some of the neural tube to become floorplate

Question 40

After the floorplate and somites have been formed, what happens next?

Answer 40

the SHH travels to the somite and fates it to turn into bone, the more distal parts go on to form other things such as muscle or dermis. As soon as these have specialised, they go on to make their own specialised molecules.

Question 41

How does the gut grow in the embryo and what are some rare consequences?

Answer 41

the gut doesn’t have enough space to twist into how it finally ends up. What happens is the gut ends up leaving the peritoneal cavity, physiologically ‘herniating’ into the umbilical cord space, having done this its twists around and re-enters twisted.

If it doesn’t go back in, there will but gut left outside the cavity- 1 in 4000 births it happens. Another rare consequence is a second twin can develop in that and create fetus-in-fetu.

Question 42

What happens in the secondary palate in the embryo?

Answer 42

Softer area on the roof of the mouth- needs to be sealed

That separation happens because 3 shelves of cells grow from the front and sides of the mouth.

If these don’t joint together, causes cleft lip and cleft palate also known as orofacial cleft

Question 43

What is hypospadias?

Answer 43

A birth defect in boys in which the opening of the urethra is not located at the tip of the penis

Question 44

What is associated with gigantism?

Answer 44

Pituitary tumours

Question 45

How can people be smaller but in proportion?

Answer 45

Pituitary gland secretes growth hormone to the circulation to the whole body. The tissues then pick this hormone up and release IGFI and IGFII, insulin growth factor. If people cannot make growth hormone of receptors for growth hormone, they will be smaller but in proportion.

Question 46

Why will an leg inhibited for a time period catch up with the other leg when freed?

Answer 46

Insulin growth factor controls stem cell proliferation of the growth plates

CNP (a hormone) is released from dying cells and given to their younger siblings, making them mature. The unmatured cells signal to the stem cells to proliferate (increase in number rapidly) by releasing IHH to the sheath of the bone, which release PTHrP (proliferate) which is picked up by stem cell receptors.

When the bone is small, the distance to bone sheath will be small, so the loop is efficient, however when the bone has enlarged, the signalling loop takes longer and bone growth slows, until they stop and the stem cells run ou

Question 47

What is a non-genetic cause of non-vitruvian phenotype?

Answer 47

Thalidomide was given to pregnant women in the 1950s to help mood swings, then there was an increase in the babies with phocomelia, ‘seal limb’ where limbs are super short. It is now known thalidomide slows blood vessel growth

Question 48

What is a genetic cause of non-vitruvian phenotype?

Answer 48

Achondroplasia

Fibroblast growth factors (FGF) usually inhibits proliferation and differentiation of chondrocytes. Activating mutations in FGFR3 can cause premature closing of growth plates. People with achondroplasia have normal sized trunks and head.

Question 49

Where do the germ line cells and gonads develop?

Answer 49

The germ line comes from epiblast cells that were removed from the body around the time of gastrulation. It therefore ended up outside the body in the yolk sac.

To get back in, they crawl along the gut, leave the gut through the mesentery ( a fold of membrane that attaches the intestine to the abdominal wall)and develop into structures. They leave gut as there are particular molecules that are attractive the cells where they need to be.

The gonads develop in the trunk of the body in the gonadal ridge, half way between shoulder and pelvis.

Question 50

What determines the sex of the gonad?

Answer 50

Females have XX and males have XY. Y chromosomes have an important gene SRY (sex-determining region) which determines the sex of the gonad

Question 51

How does the body outside of the gonads know their sex?

Answer 51

The rest of the body pays no attention to whether it has a Y chromosome. It has to take its cue from the testis which excretes androgenic hormones. The hormones are anti-Mullerian hormone and testosterone. If the soma do not receive these, will become female.

Question 52

What are the differences in male and female sex determination?

Question 53

What is andorgen insensitivity syndrome?

Answer 53

There can be cases in XY humans that the gonad is produced to be male, but does not release the hormones or the hormone receptors did not pick up the signals to change the rest of the body to male. Anti-Mullerian hormone can be produced however, so in some cases they will not have ovaries.

Question 54

What are guevedoces syndrome?

Answer 54

Testosterone stimulates androgen receptors only weakly- tissues convert it using the enzyme 5-alpha-reductase to 5-alpha-dihydrotestosterone which stimulates androgen receptors strongly.

XY children with deficient 5-alpha-reducatase makes female bodies.

However, in puberty, is enough testosterone to produce a response for the receptors and the body changes to a male phenotype

Question 55

What are the different body cavities in the human?

Answer 55