FHMP 023 + 024 intro to developmental biology and fertilisation and cleavage divisions Flashcards
(42 cards)
1
Q
what are the stages of human embryonic development?
A
- fertilisation
- cleavage (days 1-5)
- implantation (days 6-9)
- gastrulation and neurulation (weeks 3-4)
- body plan (week 4)
- organogenesis (weeks 4-8)
- foetal development (weeks 8-40)
- birth
2
Q
how common are congenital defects?
A
- in 20% of pregancies
- 2% of live births in england
3
Q
Name some common congenital defects
A
- anencephaly
- spina bifida
- club foot
- polydactyly
- phocomelia ( reduced or missing long bones/limbs - thalidomide survivors)
- cleft lip
- treacher collins syndrome (affects development of the head)
4
Q
describe the development process of egg cells (oogenesis)
A
- in foetus the oogonium undergoes meiosis but stops at prophase 1
- then once puberty has occurred it can continue meiosis until metaphase 2
- then it splits into a polar body and the ooctye which then stays at this point of development until ovulation and fertilisation
- after fertilisation the egg undergoes more meiosis to form a second polar body and a fertilised egg cell (which forms the embryo)
5
Q
describe the developmental processes of spermatogenesis
A
- after puberty spermatogonium undergoes mitosis into a spermatocyte which undergoes meiosis 1
- then the 2 spermatocytes undergo meiosis 2 to form 4 spermatids
- the spermatids undergo spermiogenesis (differentiation) into spermatozoa ( sperm cells)
- (no polar body produced)
6
Q
describe the structure of human eggs
A
- 100 micrometers in diameter
- surrounded by zona pellucida and cortical granules which help with fertilisation/implantation
- plasma membrane underneath surrounding cytoplasm and haploid nucleus
7
Q
describe the structure of spermatozoa
A
- 50 micrometers long
- head contains the haploid nucleus, the centriole (produce microtubules) and the acrosome ( contains enzymes that digest egg outer coating)
- midpiece contains mitochondria and microtubules to help tail move
- then the rest is the flagellum (tail)
8
Q
what happens in the first week of embryonic development?
A
- a matured oocyte is released from the ovaries during ovulation
- the fimbriae of the fallopian tubes pick of the released egg
- spermatozoa will fertilise the egg in the ampulla of the oviduct
- once fertilised the egg will begin to divide and form an embryo, moving down the oviduct into the uterus
- in the uterus, it hatches from the zona pellucida and implants into the uterine wall
9
Q
what is an ectopic pregnancy?
A
- when the embryo implants outside of the uterus
- 98% in the fallopian tubes but can also be in the cervix, ovary or abdomen
10
Q
what happens during ferilisation?
A
- the egg signals its position by secreting progesterone once a sperm has reached the egg
- this progesterone activates a calcium channel which is only present in the sperm’s tail
- the calcium influx into the sperm’s tail helps it move faster and initiate capacitation
11
Q
what happens during ferilisation?
A
- the egg signals its position by secreting progesterone once a sperm has reached the egg
- this progesterone activates a calcium channel which is only present in the sperm’s tail
- the calcium influx into the sperm’s tail helps it move faster and initiate capacitation
11
Q
what happens during ferilisation?
A
- the egg signals its position by secreting progesterone once a sperm has reached the egg
- this progesterone activates a calcium channel which is only present in the sperm’s tail
- the calcium influx into the sperm’s tail helps it move faster and initiate capacitation
12
Q
what happens during fertilisation?
A
- the egg signals its position by secreting progesterone once a sperm has reached the egg
- this progesterone activates a calcium channel which is only present in the sperm’s tail
- the calcium influx into the sperm’s tail helps it move faster and initiate capacitation
- the sperm cannot fertilise the egg without capacitation
- capacitation causes de-stability of the plasma membrane around the acrosome
- the acrosome now fuses with the plasma membrane of the ovum and secretes hydrolytic enzymes (acrosomal reaction) which allow the sperm to penetrate the zona pellucida
- the calcium increase stimulates meiosis of the egg to mature to the final stage
- cortical granules fuse with the plasma membrane (cortical reaction) releasing contents into extracellular space
- enzymes then modify the zona pellucida so no more sperm can bind to the egg
13
Q
what is capacitation?
A
- causes de-stability of the acrosome vesicle
- promotes acrosome reaction
14
Q
what is the acrosome reaction?
A
- when the acrosome vesicle of the sperm fuses with the plasma membrane of the egg
- secretes hydrolytic enzymes to penetrate zona pellucida
15
Q
what is the cortical reaction?
A
- calcium increase in the egg causes cortical granules to fuse with the plasma membrane and releasing contents into extracellular space
- enzymes then modify the zona pellucida so no more sperm can penetrate the egg
16
Q
what is parthenogenesis?
A
- where the egg develops without fertilisation
- seen in plants and invertebrates
17
Q
what are andromorphs and gynomorphs?
A
- andromorph = embryo with only male chromosomes
- gynomorph = embryo with only female chromosomes
18
Q
what are cleavage divisions?
A
- mitotic divisions without cell growth
- the cells/blastomeres get smaller
19
Q
whats the morula?
A
- 16 cell stage of the embryo
20
Q
what is compaction
A
- by the 32-cell stage, they undergo sudden compaction where the blastomeres become more adhesive to each other, forming water-tight junctions
- differentiation occurs - a fluid-filled cavity (blastocoel) forms within the embryo which is now called the blastocyst which has outer (trophoblast) and inner (inner cell mass) cell populations
21
Q
what is a blastocyst?
A
- an embryo after 32 days and after compaction
- contains the: z
- zona pellucida
- trophoblast (outer layer)
- inner cell mass (inner layer)
- blastocoel (fluid-filled cavity)
22
Q
what is blastulation?
A
- occurs after cleavage divisions
- hatches from the zona pellucida
- inner cell mass now develops another cavity (amniotic cavity)
- inner cell mass aka embryoblast
- ICM/embryoblast also differentiates into hypoblast and epiblast
- these 2 layers form the bilaminar disk
23
Q
what is the bilaminar disc?
A
- when the ICM/embryoblast differentiates into 2 layers:
- epiblast (touching outer trophoblast)
- hypoblast (inner side)
24
what are the 2 ways of dividing the embryo to form monozygotic (identical) twins?
- 70% from splitting the ICM
| - 30% from splitting during cleavage stages
25
what is tetragametic chimera?
- when 2 fertilised eggs fuse to form a single chimeric embryo
- the baby will show to have 2 genetically distinct cell types
26
what is the pathway/divisions from egg through embryo to fetus?
egg --> 16-cell morula --> blastocyst containing ICM, trophoblast and blastocoel
trophoblast --> chorion (contributes to placenta)
ICM ---> hypoblast and epiblast (bilaminar disc)
hypoblast ---> extraembryonic endoderm (heuser's membrane)
epiblast ---> extraembryonic mesoderm (vasculature of cord and plasenta), extraembryonic ectoderm (amniotic membrane) and the fetus ( ectoderm, mesoderm and endoderm)
27
how can you do genetic testing on an embryo?
- only requires a single cell from a pre-implanted embryo
- the rest of cells go on to form the blastocyst
- genetically test the single cell and if no genetic diseases are detected, the blastocyst is implanted into mother
28
what are the functions of the trophoblast?
- implantation into the uterus
- differentiates into the chorion which contributes to the placenta
- immunosuppression - as embryo expresses paternal antigens, so it protects it from being attached by mothers body
- endocrine gland - produced HCG which maintains the corpus luteum in the ovary which produces progesterone to maintain uterine lining
29
give an example of a trophoblast disease
- hydatidiform moles
- affect 1 in 500 pregnancies
- happens when trophoblast grows out of control
- most are miscarried but some will form a tumour
30
when does implantation occur?
- 6-7 days after fertilisation
31
what is the process of implantation?
- the blastocyst attaches to the uterine wall, ICM side first ( called apposition/adplantation)
- the trophoblast proliferates to form syncytiotrophoblast and cytotrophoblast (remains)
- the syncytiotrophoblast secretes enzymes to break down the uterine wall and vessels so the embryo can invade and penetrate the wall
- lacunae form within the syncytiotrophoblast and fill will maternal blood
- the stromal cells in the uterine wall undergoes a reaction and become filled with glycogen to supply the embryo until the placenta is vascularised
32
what is the cytotrophoblast?
- after trophoblast has proliferated
| - inner layer of cells around the outside on opposite side of implantation, undergoing rapid mitotic activity
33
what is the syncytiotrophoblast?
- after trophoblast has proliferated
- multinucleated with no cell boundaries as cells have fused together
- it secretes enzymes that break down and digest the uterine wall and capillaries for the blastocyst/embryo to implant
34
what are lacunae?
- small areas within the syncytiotrophoblast that fill with the mothers blood after implantation to provide blood supply (before placenta)
35
what is the structure of the embryo after implantation has occured?
- underneath top layer of uterine wall now
- surrounded by syncytiotrophoblast and lacunae
- the hypoblast layer of the bilaminar disk has now formed Heuser's membrane which surrounds the yolk sac (empty cavity atm)
- the Heuser's membrane also produces extracellular reticulum above it which will soon form chorionic cavity
- the epiblast layer has formed the amniotic sac (below yolk sac (deeper in uterine wall)
36
what is the yolk sac for?
- forms part of the gut, produces the earliest red blood cells and vessels and is a source of germ cells for the gonads
- lined by Heuser's membrane (extraembryonic endoderm)
37
what is the amniotic sac for?
- where the fetus will develop inside
- will become fluid filled
- protects fetus from trauma and maintain constant temp
38
what is the development by day 14?
- the embryo ( is now suspended in the chorionic cavity (where the extracellular reticulum was) by a connecting stalk coming from the mesoderm
- the cytotrophoblast and mesoderm extend into the syncytiotrophoblast between the lacunae, forming the stem villi
- the mesoderm forms the vasculature of the placenta
- the connecting stalk forms the umbilical cord
39
what is the stem villi?
- formed from cytotrophoblast and mesoderm extending up away from the embryo into the syncytiotrophoblast
- kinda holding it in place deeper in the uterine wall
40
what does the connecting stalk form?
- umbilical cord
41
what does the extra-embryonic mesoderm form?
- vasculature of the placenta
