7) Embryology Flashcards
(101 cards)
1
Q
1st and 2nd stage of female gametogenesis
A
The primary oocytes is surrounded by a single layer of follicular cels from the ovarian epithelium (together they are known as the primary follicle)
2
Q
3rd stage of female gametogenesis
A
At the beginning of each monthly cycle 5-12 primary follicles begin to develop and become growing follicles
Follicular cells multiply and become several layered. They also become separated from the ovum by an acellular mucopolysaccharide layer (septum pellucidum)
3
Q
4th stage of female gametogenesis
A
All but one of the growing follicles degenerate to form a small corpus atreticum or scar
4
Q
5th stage of female gametogenesis
A
Follicular cells of the remaining growing follicle secrete a fluid which produces a fluid-filled antrum within the follicle.
The ovarian non gamete cells surrounding the follicle also become altered and form a thecal layer around the follicle
This can be divided into a vascular theta interna (produces oestrogen) and a relatively avascular inactive theca externa
5
Q
6th stage of female gametogenesis
A
In the mature of Graafian follicle the majority of the follicular cels form the stratum granulosum (will produce progesterone)
The remainder surround the oocytes as the cumulus oophorus
Now a secondary oocyte
6
Q
7th stage of female gametogenesis
A
Mature follicle ruptures to release the ovum. This retains a covering of follicular cells which form the corona radiata
Septum pellucidum has expanded to become the zona pellucida
7
Q
8th stage of female gametogenesis
A
The cells of the theca interna and the stratum granulosum enlarge, turn yellowish and form the corpus Luteum
Secretes large amounts of progesterone and oestrogen
Prior to ovulation the follicle produces is mainly oestrogen
8
Q
9th stage of female gametogenesis
A
If fertilisation does not occur the corpus luteum has a life of only 12 days after which it degenerates into the corpus albicans
The cessation of its hormonal output leads to mensturation
9
Q
10th stage of female gametogenesis
A
If pregnancy occurs, the corpus luteum is sustained by the HCG produced by the conceptus and forms a large corpus luteum of pregnancy this will eventually form a large corpus albicans
10
Q
Summarise the stages of female gametogenesis
A
1,2,3: primary oocyte to primary follicle to growing follicle
4,5: growing follicle and corpus atreticum
6: Graafian follicle and secondary oocyte
7: release of oocyte
8,9,10: corpus luteum and corpus albicans
11
Q
Define oocyte
A
A cell in the ovary that undergoes meiosis to form an ovum
12
Q
Male gametogenesis (spermatogenesis)
A
In testis, stem cells (soermatogonium 46XY) undergo mitosis
Results in primary spermatocyte which then undergoes 1st meiosis
Results in secondary spermatocytes (23 X or Y) which undergo second meiosis
Results in spermatids (23 X or Y) which mature to form spermatozoa (2X,2Y)
13
Q
Describe spermatozoon structure
A
Head consists largely of a nucleus with a sparse cytoplasmic covering. Within that cytoplasmic covering the acrosomal cap derived largely from the Golgi apparatus contains enzymes which help in the penetration of the ovum
14
Q
Why is only one egg produced in oogenesis
A
1st meiotic division results in one egg cell and 1 polar body
- suspended at this stage
- in 2nd division, polar body divides again to form 2 new ones, egg divides to form one egg and one more polar body
15
Q
Compare male and female gametogenesis
A
- different locations of meiosis
- both result in haploid cels
- male is continuous from puberty, female is discontinuous (all primary oocytes present at birth, suspended part way through meiosis)
- spermatozoa are motile, ova are not
- spermatozoa have low cytoplasmic : nuclear ratio- ova have high
- spermatogenesis involves fluids from other glands prior to ejaculation
16
Q
Describe what happens when sperm are released into the vagina
A
1% of sperm deposited penetrate the cervix
Several hours later sperm reach isthmus, become less motile
Chemoattractants released from cumulus cells on ovulation make sperm motile again- swim to ampulla- fertilisation
Sperm require capacitation- ‘conditioning’ in female reproductive tract during which acrosomal region loses glycoproteins coat
17
Q
Describe fertilisation I
A
Shed secondary oocyte halted part way through 2nd meiotic division
18
Q
Describe fertilisation II
A
Penetration by spermatozoon prompts completion of 2nd meiotic division. Acrosomal enzymes help disperse corona radiata and aid penetration of zona pellucida.
Plasma membranes of gametes fuse and male nucleus is injected
19
Q
Define zona pellucida
A
Thick membrane that develops around the mammalian oocyte within the ovarian follicle. Penetrated by at least one spermatozoon at fertilisation and persists around the blastocyst unit it reaches the uterus
20
Q
Describe fertilisation III
A
Zona pellucida undergoes reaction making it impossible for further sperm to penetrate
- enzymes are released to digest sperm proteins so they can no longer bind
- ovum shrinks, gap between zona pellucidum and cytoplasm
- all organelles come from female egg
21
Q
Describe fertilisation IV cleavage
A
1st cell with 2 nuclei divide into 2 identical cells (blastomeres) by mitosis
22
Q
Describe fertilisation V further cleavage
A
Cells continue dividing, more cytoplasm made
23
Q
Describe fertilisation VI morula
A
Further division to form a solid ball of cells (morula)
- cytoplasm : nuclear ratio has fallen to near normal
- cytoplasm differentially packaged
- zona pellucida persists to prevent implantation
24
Q
Describe blastocyst formation (from morula)
A
2nd week
- compaction: cells form tight junctions, zona pellucida breaks down
- cells begin to organise themselves: outer cells (trophoblast will form extra embryonic membrane and placenta) with tight junctions.
- push cells without tight junctions to middle (inner cell mass, going to form whole of embryo) fluid filled in the middle
25
When and where does implantation take place
At about 6 days the blastocyst adheres to the endometrium. Usually on the posterior wall of the uterus and nearer the fungus than the cervix
26
What is the morula
An early stage of embryonic development formed by cleavage of the fertilised ovum. It consists of a solid ball of cells and is an intermediate stage between the zygote and the blastocyst
27
When and where does implantation take place
At about 6 days the blastocyst adheres to the endometrium. Usually on the posterior wall of the uterus and nearer the fundus than the cervix
28
What happens to the blastocyst initially during implantation
Blastocyst gives rise to inner cell mass (5 cells) and trophoblast (55 cells)
- trophoblast is invasive, intestine and digestive
- a decidual reaction occurs in the uterine lining - increased secretory function
- possibility of ectopic implantation
- inner cell mass forms the embryo (and some membranes)
29
Summarise implantation
- blastocyst gives rise to inner cell mass and trophoblast, implant in uterine wall
- inner cell mass forms embryo (beginnings of primitive ecto and endoderm)
- primitive ectoderm surrounds amniotic cavity
- primitive endoderm surrounds yolk sac cavity, both against trophoblast
- conceptus moves into uterus wall
- trophoblast produces extraembryonic mesoderm, push embry away from trophoblast
- cavity created in mesoderm to allow for expansion, extraembryonic mesoderm Ines trophoblast
- extraembryonic mesoderm splits to produce a cavity around the embryo- extraembryonic coelom
30
In 2nd week of embryonic development what structures form?
Period of 2s:
- 2 layers (ecto and endoderm)
- 2 cavities (amniotic and yolk sac)
- 2 cavities ( cyto and syncytio trophoblast)
31
Describe what happens in the ectoderm and development of the primitive streak in week 3 of embryonic development
- in ectoderm, at head end prochordal plate forms, at tail end primitive streak forms
- intraembryonic mesoderm forms between ectoderm and endoderm
32
Describe what happens at the prochordal plate in week 3 of development
Thickening of bilaminar disc (endo and ectoderm) at prochordal plate so cells stick together
33
Describe the development of the notochord and the neural plate in week 3
- cells from the head end of the primitive streak form a midline structure, the notochord, this induces overlying ectoderm to form neuroectoderm in the neural plate
- primitive node sites at the restraints side of primitive streak, just under the notochord
- cloacal plate is under the primitive streak (becomes the anus)
- embryo lengthens, mainly because of cell production and migration from primitive streak
34
What happens to the mesoderm while the notochord is forming
- continues to be produced from primitive streak
- migrates laterally until it joins with extra-embryonic mesoderm
- also migrates forwards but cannot separate prochordal plate ectoderm form endoderm so flows in front to form septum transversum
35
Define notochord
Strip of mesodermal tissue that develops along the dorsal surface of the early embryo, beneath the neural tube. Becomes almost entirely obliterated by development of the vertebrae persisting only as part of the intervertebral discs
36
Describe the formation of the neural tube in week 3 from the primitive streak
- neural plate forms neural folds (neuroectoderm)
- eventually fuse and separate from ectoderm to form neural tube (becomes brain and spinal cord)
- notochord sits underneath neural tube
37
What structures are present at the end of the 3rd week of development
- 3 germ layers (ecto, endo, meso)
| - 3 important structures (primitive streak, notochord, neural tube)
38
What is gastrulation
Process during embryonic development that changes the embryo from a blastula with a single layer of cells to a gastrula with multiple layers of cells
- involves the blastula folding in on itself or dividing to create 2 layers of cells
39
What is the primitive streak
Transient structure which on day 15 marks the start of gastrulation (process in which the inner cell mass is converted into the trilaminar embryonic disc which is made up of 3 germ layers)
40
Describe the formation of the intraembryonic coelom
Clefts begin to appear in a horseshoe arrangement around the prochordal plate, in the lateral plant intraembryonic mesoderm
- eventually a horse shoe shaped cavity is formed which over a limited area becomes continuous with the extraembryonic coelom
41
What does the intra-embryonic coelom eventually form
- pericardial cavity
- pleural cavity
- peritoneal cavity
42
Describe how somites begin to form in the fourth week of embryonic development
Intraembryonic mesoderm forms 3 longitudinal columns
- paraxial
- intermediate
- lateral plate (continuous with extraembryonic mesoderm)
At the start of the 4th week, paraxial mesoderm begins to form paired cubodial bodies (somites)
- somites pair up either side of the notochord starting from the rostral end by the prochordal plate
- each somite receives spinal nerves
- you can tell how old the conceptus is by how many somites have formed
43
What do the medial, lateral and intermediate parts of the paraxial mesoderm become
Medial part becomes sclerotome (skeleton)
Intermediate part becomes myotome (muscle)
Lateral part becomes dermotome (skin)
44
What do the intermediate and lateral mesoderm become
- intermediate mesoderm becomes nephrotome (forms genitourinary system)
- lateral mesoderm becomes either parietal (body wall) or visceral (gut wall)
45
Describe the cranio-caudal system of somites
```
3 occipital
8 cervical
12 thoracic
5 lumbar
5 sacral
1-5 coccygeal
```
- occipital goes to form tongue musculature
- the rest form the spinal vertebrae
Each somite receives a segmental spinal nerve, which follows the tissue wherever it migrates
46
How does the neural tube close up and what can go wrong
- closes from the midline towards the head and the tail
- if it doesn’t close properly at the cranial end it causes anencephaly (exposed brain)
- if it doesn’t close towards the chordal end it can cause spina bifida
47
What are the ends of the neural tube called
Cranial is arterial neuropore
| Caudal is posterior Neuropore
48
At day 28 of development what structures are beginning to form,
- heart
- beginning of limbs
- neural tube fully closed and somites all present
- start of eye and ear
- umbilical cord
49
What happens to lateral plate in week 4
Lateral plate is split into:
somatopleuric mesoderm- (continuous with the mesoderm of the amniotic cavity) which will form striated muscle
Splanchnopleuric mesoderm- continuous with the mesoderm of the yolk sac which will form the smooth muscle of the gut
50
What are dermatomes
A specific segment of skin supplied by a single pair of spinal nerves
51
Describe the 1st stage of reversal in longitudinal folding
- differential growth causes embryo to fold both cranio-caudally and laterally
- the previously flat trilaminar disc comes up to take the general body shape
- pericardial cavity swings underneath the neuroectoderm (brain)
- rapid growth of the neural tube is especially important in producing reversal
52
Describe the 2nd stage of reversal in longitudinal folding
- rapid growth of developing brain from neural tube has pushed amniotic cavity to swing round even further to envelope embryo and yolk sac
- after reversal the amnion is only attached to the embryo over a small region - the umbillicus
- a constriction is produced at the junction of the embryo and yolk sac, part of the yolk sac has become incorporated into the gut tube
53
Describe 1st stage of lateral folding
- edges of amnion grow downwards towards yolk sac
- mesoderm becomes trapped by movement of the amnion
- yolk sac becomes elongated and moves downwards
54
Describe 2nd stage of lateral folding
- amnion continues to fold down as neural tube pinches off
- edges of yolk sac touch in the middle
- not continuous between 2 parts
- top part will form gut, bottom will be shed
- between the 2 is a small opening called the Vitello-intestinal duct
- intra and extra embryonic coelom still communicate
55
Describe the 3rd stage of lateral folding
2 edges of the amnion meet and completely surround the embryo so embryo floats in amniotic cavity
- yolk sac is shed
- gut tube held by strands of mesoderm and surrounded by splanchnopleuric mesoderm
- cavity either side of gut (intra-embryonic coelom) where gut will expand into
56
Describe formation of umbilical cord
Amnion near pericardial cavity, remnant of yolk sac and allantois all pinch together to form umbilical cord
- contains an umbilical vein, umbilical arteries, allantois, Vitello-intestinal duct, mesoderm from the amnion, yolk sac and connecting stalk and is surrounded by a covering layer of amnion
57
Describe how the body cavities form and are laid out in week 4
- horseshoe shaped intraembryonic coelom is bent around the gut tube
- future pericardial cavity lies ventrally
- pericardiaco-peritoneal canals lie dorsal to the septum transversum and eventually form pleural cavities
- pericardial and pleural cavities pinch off and the area under the peritoneal cavity constricts to form umbillicus
- extra embryonic coelom persists in the umbilical cord proximally for a time and is important in development of gut
58
What do the pericardial, pleural and peritoneal cavities house
Pericardial cavity houses the heart
Pleural cavity houses the lungs
Peritoneal cavity houses the gut tube
59
Summarise the derivatives of the 3 germ layers
Endoderm- epithelia of internal surfaces (resp system, alimentary system, urinary system)
Ectoderm- neuroectoderm: neural crest and neural tube
Mesoderm- paraxial mesoderm: muscles of head, trunk, limbs, skeleton, dermis, connective tissue, head mesoderm, intermediate mesoderm, lateral mesoderm
60
How do cells control what genes are expressed and translated into proteins
Transcription factors
Cells ability to do this is inherent
61
What does regulation development involve
The interaction of adjacent cells within embryonic fields
| - gives flexibility for differentiation
62
Totipotent cell
Can give rise to all cell types found in the adult organism, plus extraembryonic cells eg zygote
63
Pluripotent cell
Can give rise to all 3 germ layers but not extraembryonic (from where embryonic stem cells are derived)
As development proceeds, stem cells may lose their pluripotency
64
Multipotent cell
Can give rise to a restricted number of different cell types eg stem cells of blood forming tissues and adult stem cells
65
What 2 areas does the trophoblast divide into
Cytotrophoblast (immediately surrounds yolk sac and the amnion) outer coating of the conceptus contributes to extra embryonic mesoderm to create space
-syncytiotrophoblast (multinucleate, cytoplasm, with many nuclei)
66
On day 13 of development what is the chorion and what is the amnion
- extra embryonic mesoderm covers the amniotic epithelium and forms the amnion
- extra embryonic mesoderm also lines the trophoblast to form the chorion
67
What happens to the amniotic and chorionic cavity in week 4
Amniotic cavity enlarges faster than the chorionic cavity and gradually obliterates it
Eventually the amnion and chorion join and the foetus floats in amniotic fluid
68
What is the chorion made of and what does it become in week 6
Made of trophoblast and extra embryonic mesoderm
Eventually becomes
- smooth (chorion laeve) at the abembryonic Pole
- irregular (chorion frondosum) at the embryonic pole where the placenta is developing
69
What is the name of the region where the conceptus develops
The conceptus develops in the region of the decidua basalis
70
In week 8 how does the growing embryo fill the uterus
Expanding embryo and its coverings begin to obliterate the uterine cavity
Decidua capsularis eventually fuses with the decidua parietalis
71
What takes place on days 9-11 of the formation of the placenta
Blastocyst is further embedded in the endometrial stroma and the penetration site is plugged by a fibrin clot
At the embryonic pole, lacunae form in the syncytiotrophoblast
72
What happens days 12-13 in the development of the placenta
Cytotiotrophoblast mitosis produces more syncytiotrophoblast
This erodes and plugs and remodels maternal vessels and when the plug is fully removed, a functioning uteroplacental circulation is established
73
By day 13 what parts of the placenta are starting to form above the connecting stalk
- decidua basalis
- maternal blood vessel
- primary stem villus
- enlarging lacunae filled with maternal blood
74
What has taken place by day 16 in development of the placenta
Villi grow into blood filled lacunae and these come to develop fetal blood vessels which connect with the umbilical arteries and veins
Exchange of gases etc can thus take place between mother and foetus via their circulatory systems
75
What is taking place in the forming uterus in weeks 3-4
Exchange takes place mostly through the branches of the stem villi which float in the intervillous spaces
Other villi anchor the vessels in space
Feto maternal barrier initially consists of
1) syncytiotrophoblast
2) cytotrophoblast
3) mesoderm
4) endothelium of fetal vessels
76
Describe the development of villi in the placenta in weeks 2-3
When placental villi first form they are called primary stem villi have a core of cytotrophoblast covered by a layer of syncytiotrophoblast
Secondary villi have developed a core of extra-embryonic mesoderm inside the 2 trophoblast layers
Tertiary villi have developed fetal blood vessels within the mesodermal core
77
What is the intervascular barrier in placenta development
The tissue layers between maternal and fetal bloods
78
What does the intervascular barrier consist of in the 1st trimester
- villi (few but large)
- centra fetal vessels (so exchange surfaces are small and distances large)
Barrier comprises:
- layer of syncytiotrophoblast
- complete layer of cytotrophoblast
- substantial mesoderm
- endothelium of fetal vessels
79
What does the intervascular barrier consist of in the 3rd trimester
Numerous and small villi with peripheral fetal vessels (large exchange surface, small distance)
Barrier now comprises:
- syncytiotrophoblast layer
- isolated cytotrophoblast cells
- little mesoderm
- endothelium of fetal vessels
80
What is the placenta like at term
- discoidal (15-25cm diameter) weighs 450-600g
- maternal aspect is divided into : 15-20 cotyledons
Maternal blood enters cotyledons via 80-100 spiral aa and leaves via tributaries of uterine vv
Maternal blood space surrounds villus trees
- fetal blood (50ml) lies in vessels located within villi and separated physically from maternal blood
- the intervascular barrier between maternal and fetal bloods is made mainly of syncytiotrophoblast and vascular endothelium. Has a large surface area (10-15m2) but is thin
81
Placental abruption
Placenta peels away from uterine wall, partially or almost completely before delivery
- can deprive the foetus of oxygen and nutrients- poor growth
- can cause bleeding in mother
- can cause premature birth (10% of prem births)
- 1/100 pregnancies
82
What is placental previa
Low lying placenta that covers part or all of the opening of the cervix
Can block baby’s exit from uterus
- blood vessels that connect the abnormally placed placenta to the uterus may tear, resulting in bleeding at labour
- can cause premature birth
- 1/200
83
What is placenta accreta
Placenta implants too deeply and too firmly into uterine wall
- placenta increta and percreta may embed into uterine muscle or through the entire thickness of the uterus
- may cause excessive bleeding at delivery
- 1/2500
84
Name some placentally mediated conditions
- IUGR
- pre-eclampsia
- diabetic pregnancy - increased risk of malformation
- smokers pregnancy - small for date
- pregnancy at altitude
85
What is an autosomal dominant inheritance pattern
Heterozygotes with one Copy of the abnormal gene are affected
86
What is an autosomal recessive inheritance pattern
Homozygous with 2 copies of the abnormal gene affected
87
What is an X linked recessive inheritance pattern
Males with 1 copy of the abnormal gene on the X chromosome affected
88
Describe X linked recessive inheritance
No male to male transmission
- carrier females are unaffected
- all men who inherit the mutation are affected
- appears to ‘skip’ individuals
89
Describe autosomal recessive inheritance
Inherit 2 germline mutations (one from each parent) to develop disease
- equally transmitted by males and females
90
Describe duchenne muscular dystrophy
- most common and severe form of muscular dystrophy
- presentation = 3-5 years
- 33% have mild - moderate learning difficulties
- waddling gait and positive gower sign
- difficulty running and climbing stairs
- gradual deterioration - loss of mobility (wheelchair bound)
- progressive muscle weakness- cardio respiratory failure
91
What is the hardy weinberg principle
Allows the calculation of carrier rates once the incidence of a condition is known as long as the gene frequency is in equilibrium
92
How does the hardy Weinberg principle work
In a large, randomly mating population the relative proportion of different genotypes remains constant
This only holds true if there are non outside influences eg selection or assertive mating.
- if you have 2 alleles for an autosomal condition: A and a, with a frequency of p and q, p+q=1
- if this is an autosomal recessive condition and ‘’a’’ leads to a disease
P2= homozygous unaffected
2pq = carriers
Q2= affecteds
As the generations continue, these relative proportions remain the same
93
What factors can disturb the hardy Weinberg principle
- assertive mating
- consanguinity
- mutation - selection equilibrium
- selection (heterozygote advantage)
- small population size
- migration
94
How can non random mating disturb hardy weinberg
- assortive mating: tendency to choose a mate with similar characteristics eg height, IQ
- consanguinity: relationships between close relatives can lead to an increased carrier risk within a family
95
How does the mutation-selection equilibrium disturb hardy-weinberg
New mutations are arising all the time. Different genes have different new mutation rates according to their size and structure eg in duchenne muscular dystrophy and neurofibromatosis type 1. The new mutation rates are high
- usually this is balanced by the loss of alleles due to reduced reproductive fitness in affected individuals
- alteration in this balance will affect the equilibrium
96
How can small population size (founder effect) disturb hardy Weinberg
One allele can be transmitted to a large proportion of children purely by chance leading to an increased incidence of a certain condition in a population eg Tay-Sachs disease in ashkenazi Jews
97
How can migration (gene flow) disturb hardy Weinberg
Migration and intermarriage can introduce new alleles into a population eg the B blood group alleles is slowly moving westwards from Asia, the common haemochromatosis allele is distributed across Europe according to Viking invasion groups
98
Define prenatal diagnosis
Any test done in the antenatal period for the diagnosis of fetal disease
99
Describe a Michal translucency scan (NT) scan
```
Performed between 10-14 weeks gestation. Thickness of NT measured by ultrasound scan in relation to crown-rump length
Other anomalies may be detected
Causes of increased NT:
- chromosomal - Down syndrome
- major congenital heart disease
- skeletal dysplacias
- diaphragmatic hernia
```
100
Describe antenatal biochemical serum screening - T21, 18, 13
First trimester
- 10-14 weeks gestation (with NT scan)
- accurate dating important to interpret results
- free beta-human chorionic gonadotropin (B-HCG)
- placenta associated plasma protein A (PAPP-A)
- detection rate of 90% but needs CVS/amnio
Second trimester (if booked late)
- alpha-fetoprotein (AFP)
- free beta - HCG
- oestriol
101
Describe what an abnormal MSAFP assay could show
- reduced MSAFP levels (<0.5 MoM) - Down syndrome
- elevated MSAFP levels
- neural tube defects
- anterior abdominal wall defects
- missed or threatened miscarriage
- intra-uterine growth retardation
- multiple pregnancy
- congenital nephrotic syndrome
