Lecture 48: Fertilisation Implantation Gastulation Flashcards
what day does ovulation occur in a menstrual cycle
day 14 of 28
when does sperm need to arrive in female reproductive tract in order to be successful
ovulation +/- 1-2 days
of the 4-5mil sperm ejaculated how many actually reach site of fertilisation
300-500 (1%)
what happens to the cervical mucus during ovulation and why
- ^ in amount
- becomes less thick
- becomes less acidic
- allows for easier transport of sperm
how long can sperm survive in the female reproductive tract
5 days
outline the process of capacitation
- sperm membrane molecules masked by plasma proteins
- spermatozoa cannot fertilise oocytes when newly ejacualted due to plasma prots
- in uterus and oviduct plasma prot covering is removed
- binding portions of sperm head exposed
- process can take 5-7hrs
where on the oocyte does spermatozoa bind to
zona pellucida
what does passage of sperm through cumulus layer of oocyte depend on
- hyaluronidase released from sperm acrosome
- tubal mucosal enzymes
- flagellar action
what is the cloudy layer of cells around the oocyte called
cumulus layer of cumulus cells (or corona radiatus)
outline the binding of sperm to the zona pellucida
- zona pellucida is a glycoprotein shell (facilitates and maintains sperm binding and induces acrosome reaction)
- capacitated sperm can penetrate cumulus oophorus and adhere to ZP
- multiple adhesions to ZP triggers acrosome reaction
- ZP binding triggers opening of Ca2+ channels in sperm cell membrane driving exocytosis of acrosomal contents
- acrosome reaction is prerequisite for sperm to fuse w/ oocyte plasma membrane
what is an acrosome
modified lysosome located around anterior of sperm head
outline what happens when a sperm binds to the oocyte plasma membrane
- sperm binding to ZP results in cortical reaction
- exocytosis of cortical enzymes into ZP
- destroys sperm receptors on zona pellucida
- causes hardening of zona pellucida (zona reaction)
- slow block to polyspermy (fertilisation envelope)
what happens when more than one sperm manages to enter ovum
foetus nearly always aborts
what happens at stages 3 and 4 of fertilisation
- fusion of plasma membranes of oocyte and sperm
- triggers completion of meiosis 2 (mature ovum + second polar body)
- head and tail of sperm enters cytoplasm of oocyte
what is the 5th stage of fertilisation
formation of male and female pronuclei
outline 5th stage of fertilisation
- pronuclei each contain 23 chromosomes
- male and female pronuclei indistinguishable
- membranes of pronuclei breakdown
- chromosomes become arr. for mitotic cell division (first cleavage division)
- fertilisation complete
what is a fertilised egg called
zygote
describe the results of fertilisation
- stimulates secondary oocyte to complete meiosis
- restoration of diploid number of chromosomes (46)
- variation of human species as maternal and paternal chromosomes intermingle
- determination of sex
- initiation of cleavage
what is cleavage
- series of mitotic divisions
- resulting cells (blastomeres) become smaller w/ each division
- after 3 divisions blastomeres undergo compaction
- compact blastomeres divide to form 16 cell morula
what is the name of the cells resulting from cleavage
blastomeres
describe a blastocyst
- embryo between 5-7days following fertilisation
- 2 distinct cell types
- central cavity filled w/ fluid (blastocoel)
- hatches from zona pellucida prior to implantation
name the outer and inner cell mass of the blastocyst
outer = trophoblast inner = embryo blast
describe what happens to the blastocyst in implantation
- when implantation begins:
- trophoblast cells divide and differentiate
- -> outer syncytiotrophoblasts
- -> inner cytotrophoblasts
- embryo blast cells divide and differentiate
- -> epiblast
- -> hypoblast
what phase is the uterus in at time of implantation
secretory phase
describe gastrulation
- formation of the 3 embryonic germ layers
- at week 2, embryo consists of 2 flat layers of cells; epiblast and hypoblast (bilaminar disc)
- beginning of week 3, embryo enters gastrulation
- bilaminar disc converted into trilaminar disk and formation of 3 germ layers
- -> ectoderm
- -> mesoderm
- -> endoderm
(groups of cells that will give rise to specific organ systems and tissues are moved into the right position both externally and internally, and group of cells that will influence each other positioned so they can have affect on each other - begins w/ formation of primitive streak)
name the 3 germ layers
- ectoderm
- mesoderm
- endoderm
what does the primitive streak include
- groove
- node (Hensen’s node)
- pit
what does the primitive streak define
- anterior –> cranial
- posterior –> caudal
- R and L –> lateral
describe formation of 3 germ layers from primitive streak in gastrulation
- epiblast cells roll over the primitive ridge and involute into the groove
- the cells lose contact with each other and migrate inwards by ingression
- first cells through the streak integrate and displace hypoblast cells –> endoderm
- some lie in the middle mesoderm
- cells remaining in the epiblast –> ectoderm
- 3 germ layers established
describe the results of gastrulation
- primary germ layers formed and organised in proper locations
- creation of body axes
- -> anterior-posterior
- -> dorsal-ventral
- -> left-right
describe the derivatives of the primary germ layers
ectoderm:
- CNS
- PNS
- sensory epithelia of eye/ear/nose
- epidermis and appendages (hair and nails)
- mammary glands
- pituitary gland
mesoderm:
- CT
- striated and smooth muscle
- heart
- blood and lymphatic vessels
- kidneys
- ovaries and testes
- spleen
- serous membranes lining body cavities
endoderm:
- epi lining of GIT
- resp tract
- bladder and urethra
- thyroid and parathyroid glands
- liver
- pancreas
