Pregnancy & Human Development

Fertilization

the process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote

an oocyte is only viable for 24 hours (at most) after ovulation

sperm can survive at most 5 days in female tract

can take place 5-7 days a month technically, probably shorter

sperm must cross both corona radiate and zona pellucida

Sperm not Reaching Uterus

millions lost immediately from the vagina

million more lost by the acidic environment of the vagina

Sperm that Reach Uterus

phagocytes kill off many more

only a few thousand survivors → reverse peristalsis push sperm towards the uterine tubes

sperm have various receptors (chemical, temp, fluid flow) to ensure thhey travel in the correct direction

Sperm Capacitation

sperm are incapable of fertilizing the oocyte immediately after entering the vagina

before fertilization can occur sperm must have this occur that enhances motility and weakening sperm cell membranes

Corona Radiata

outer protective layer of oocyte

protects and nourishes the oocyte after it has been ovulated

sperm burrow through cells here

Zona Pellucida

inner protective layer is oocyte

protects secondary oocyte and is necessary for fertilization to occur

acrosomal reaction occurs here

Acrosomal Reaction

release of enzymes from the acrosome of the sperm

sperm bind zona pellucida → Ca2+ levels in sperm rise

enzymes from acrosome digest holes in zona pellucida

Fertilization Step 1

occurs once the acrosomal reaction has been initiated and sperm has passed through both layers

sperm binds to sperm binding receptors on surface of oocyte

Fertilization Step 2

once bound the two membranes fuse together and the sperm content enter the oocyte

Polyspermy

the entry of more than one sperm cell into the oocyte

usually prevented because it doesn’t result in a viable fetus

prevented by oocyte membrane blocks and cortical reaction

Oocyte Membrane Blocks

sperm binding receptors are shed from oocyte surface

sperm unable to bind oocyte surface and fertilize oocyte

Cortical Reaction

oocyte releases Ca2+

causes zona pellucida to harden

Fertilization Step 3

sperm nucleus travels toward oocyte nucleus

as it travels sperm nucleus swells in size to form male pronucleus

surge in Ca2+ from cortical reaction causes release of zinc from oocyte which causes completion of meiosis 2 to form 2nd polar body and mature ovum

after meiosis 2 the female pronucleus forms

Fertilization Step 4

two pronuclei membranes rupture and chromosomes are released

maternal and paternal chromosomes combine to form a diploid zygote

this is actual fertilization

Twins

usually one oocyte is ovulated and fertilized to produce a single zygote and embryo → rules aren’t always obeyed and sometimes two occur

Monozygotic Twins

occurs when a single oocyte is fertilized splits into 2 identical embryos → identical twins

unknown as to why or how zygote splits

twins usually share a placenta and have a risk of unequal nutrient sharing

Dizygotic Twins

occurs when 2 oocytes are ovulated and both are fertilized → fraternal

twins have their own placentas

Zygote

a diploid cell resulting from the fusion of two haploid gametes

sometimes called a single called embryo

Embryo

stage of development from soon after the fertilization of the ovum to week 8 of development

no longer single celled

Fetus

stage of development from week 8 to birth

Infant

after birth has occurred

Beginning of Development

zygote begins to divide mitotically at fertilization

Cleavage

rapid division of the zygote

this helps create a multicellulared organism

first mitotic division produces 2 identical blastomere cells

by 72 hours after fertilization a morula is formed

Morula

contains 16 cells

Blastocyst

created when morula cells continue to divide

has two layers → trophoblast and embryoblast

External Trophoblast

aids in embryo implantation, contributes to chorion formation/function and has immunosuppressive effects

Internal Embryoblast

an inner cell mast that eventually forms the embryo proper and extraembryonic membranes

Blastocyst Implantation

implants in the endometrium

endometrium is receptive to implanting embryo due to high estrogen and progesterone levels

Trophoblast Implantation

attaches to endometrium

usually occurs high in the uterus to give it multiple opportunities to implant

once bound it releases digestive enzymes and growth factors on epithelium of endometrium

Digestive Enzyme Influence

erosion of endometrium around blastocyst

blastocyst buries itself in the blood rich lining

surrounding endometrial cells proliferate and cover the blastocyst

Growth Factor Influence

endometrium thickens to ensure trophoblast stays embedded

trophoblast proliferates and releases enzymes to facilitate implantation

Human Chorionic Gonadotropin hCG

once implantation is complete the embryo releases this hormone

what pregnancy tests measure

maintains the corpus luteum for 12 weeks and suppresses the female immune system

Placenta

a temporary organ originating from embryonic and maternal tissue that replaces the corpus luteum

maintains pregnancy, exchanges respiratory gases, provides nutrients to embryo/fetus, and disposes of waste

MATERNAL AND FETAL BLOOD DON’T MIX

Embryonic Placenta

chorion, chorionic villi, and umbilical vein and artery

Chorion

membrane that surrounds the fetus that has chorionic villi

Umbilical Vein and Artery

formed by blood vessels that extend from the chorionic villi

Parent Placenta

lacunae, decidua basalis, and decidua capsularis

Lacunae

blood filled structures that develop from the functional layer of the endometrium

Decidua Basalis

endometrium that lies underneath the embryo

forms the placenta with chorionic villi

Decidua Capsularis

endometrium that surrounds the uterine cavity face of embryo

expands the accommodate growing fetus

chorionic villi here degenerate as pregnancy progresses

Extraembryonic Membranes

membranes formed during the first few weeks of development that support and nourish developing embryo/fetus during gestation

Amnion

membrane layer that extends around the embryo

filled with amniotic fluid

Amniotic Fluid

provides buoyancy and protection

maintains consistent temperature

prevents developing parts of embryo from sticking together and fusing

allows movement of the embryo/fetus

Yolk Sac

sac like structure that eventually forms the digestive tube, earliest blood cells, and precursor gametes

Allantois

helps form the umbilical cord later in fetal development

allows for gas exchange, waste disposal, and nutrient exchange

Chorion

allows exchange of gases, nutrients, and wastes

encloses all other extraembryonic membranes

Gastrulation

early developmental process where blastocyst is reorganized into a 3 layered embryo

gives rise to all our organs and body parts

3 primary germ layers → endoderm, ectoderm, and mesoderm

Primitive Streak

a groove that will eventually form the long axis of the embryo

its formation begins gastrulation

Endoderm Formation

formed by the first cells to enter the primitive streak

the most inferior layer

Mesoderm Formation

other cells follow and push between cells of upper and lower levels and form this

the middle layer

Notochord

mesodermal cells immediately beneath the primitive streak form this

the first axial support of the embryo → not the first spinal cord

Ectoderm Formation

cells that remain at the surface

the top layer