fertilisation and implantation

Describe the transport of gametes to the ampullary-isthmic junction, and the changes they undergo along the way Describe the phases of fertilisation, including the role of the acrosome and how polyspermy and triploidy are prevented Describe the pre-implantation changes that the zygote undergoes Describe the process of implantation (attachment & invasion) Describe the development of the placenta in the first trimester

what are the various methods of movement of sperm through uterus

• own propulsion

• cervical and uterine contractions

• uterine cilia action

describe transport of sperm through cervix

• cervical mucus penetration

• blind-ended cervical crypts

• responsiveness to secretory molecules from reproductive tract

what happens to sperm in fallopian tube

• bind to epithelial cells

• rheotaxis (reaction to the flow of a fluid)

• thermotaxis

what group of cells surround ovum

• cumulus cells

what do cumulus cells secrete

hyaluronic acid

what does secretion of hyaluronic acid cause

chemotaxis→ acid attracts sperm towards ovum

what is chemokiness

• chemicals increase speed of sperm

describe journey of sperm from testes to fallopian tubes

• testes (where sperm are produced)

• epididymis (where sperm mature)

• vas deferens (components of ejaculatory fluid added here)

• ejaculatory duct (components of ejaculatory fluid added here)

• urethra →EJACULATION

• vagina

• cervix

• uterus

• fallopian tubes (FERTILISATION)

describe transport of oocyte to meet spermatozoa for fertilisation

• infundibulum of uterine tube moves towards ovary

• fimbriated ostium envelops ovulated oocyte with enclosing cumulus cells

• cilia and smooth muscle contraction of uterine tube→ move oocyte to ampulla

• meets spermatozoa at ampullary-isthmic junction

what changes do spermatozoa undergo to acquire ability to fertilise oocyte

• capacitation

• acrosome reaction

describe capacitation of spermatozoa

• approx. 7 hours

• occurs in uterine tube and involves epithelial interactions between sperm and mucosal surface of tube

• glycoprotein coat and seminal plasma proteins removed from plasma membrane of spermatozoa

why is capacitation of spermatozoa required

• only capacitated sperm can pass through corona cells and undergo acrosome reaction

• portions of the molecules on spermatozoa are exposed and can bind to zona pellucida of oocyte

what are the results of capacitation

• hyperactivated motility

• surface membrane changes→ allow acrosome reaction

• removal of surface glycoproteins e.g. EPPIN

• increased cytoplasmic pH

• increased calcium permeability (motility)

• loss of cholesterol

when does acrosome reaction take place

• after sperm binds to zona pellucida of oocyte

what happens during acrosome reaction

• capacitated sperms easily pass through zona cells

• when sperm makes contact with ovum→ contents of acrosome released

• enzymes loosen zona pellucida→ allows sperm to reach plasma membrane of ovum

what is the acrosome

• large membrane bound sac of enzymes in head of sperm

what does fusion of sperm and egg membranes trigger

• depolarisation of membrane→ acts as fast block to polyspermy

what is cortical reaction

• cortical granules in egg fuse with plasma membrane

• secreted contents clip off sperm-binding receptors and cause fertilisation envelope to form

• = slow block to polyspermy

where is the final destination of the sperm in fertilisation

• nucleus of egg

define fertilisation

• process by which male and female gametes fuse

where does fertilisatio occur

• occurs in ampullary region of uterine tube

what are the 3 phases of fertilisation

• penetration of corona radiata

• penetration of zona pellucida

• fusion of oocyte and sperm cell membranes

what is the zona pellucida

glycoprotein shell surrounding egg

describe process of penetration of zona pellucida

• release of acrosomal enzymes- allows sperm to penetrate zona pellucida

• contact= release of lysosomal enzymes from cortical granules lining plasma membrane of oocyte

• enzymes alter properties of zona pellucida to prevent polyspermy

• proteolytic enzymes digest a path through zona

what does release of cortical oocyte granules of the oocyte membrane

• membrane becomes impenetrable to other sperm= prevents polyspermy

how is triploidy prevented at fertilisation

• meiosis II completed

• 2nd polar body released

why does sperm not fuse to polar body

• no binding proteins on membrane overlying metaphase spindle

• prevents 2nd polar body encountering sperm

what changes occur to zygote pre-implanatation

• zygote moves to body of uterus

• zygote undergoes cleavage

• zona pellucida degraded via proteolytic enzyme action

what happens during morula stage

cells lose totipotency

how many cells present at morula stage

16 cells

what has pre-embryo developed into once it reaches uterine cavity

• blastocyst

  • inner cell mass + trophoblast cell wall

describe process of attachment in implantation

• attachment of blastocyst to uterine endometrial wall

• attachment occurs close to inner cell mass

when does attachment occur

• during ‘window of implantation’

• when uterus is more receptive to attachment (secretory, luteal phase)

what changes occur to stroma following attachments (stromal reactions)

• swelling of cells

• loss of collagen

• oedematous due to increased vascular permeability

describe arrangement of trophoblast cells

• 2 layers

  • syncytiotrophoblast (cells fused together and lose cell membranes)

  • cytotrophoblast (retain cellularity, serve as proliferative source of trophoblasts)

what cells secrete proteases, what is the consequence of this?

• syncytiotrophoblasts

• proteases break down uterine endometrium

what happens during invasion

blastocyst invades uterine tissue and is completely surrounded by it

how are lacunae formed

• cytotrophoblast and syncytiotrophoblast erodes decidual glands and spiral arteries

• their contents form lacunae→ primary villi

what cells penetrate core of primary villi? what is then formed

• mesodermal cells penetrate

• secondary villi formed

what structure develop into tertiary villi by 3rd week

blood vessels

where does stem/anchoring villi extend from

chorionic plate to decidua basalis

what are lateral branches of stem villi known as

• free/terminal villi

describe structure of placenta

• chorionic plate (foetal aspect)

• basal plate (maternal aspect)

• intervening intervillous space with maternal blood