Section 3 — Extraembryonic Membranes, Embryogenesis, Germ Layers, Organogenesis & Pregnancy Adjustments

Extraembryonic membranes and when do they form

structures that form in the first 2-3 weeks of development

membranes that develop around embryo + helps support it
doesnt become aprt of embyro

What are the four extraembryonic membranes

amnion, yolk sac, allantois, chorion

Amnion

forms amniotic sac

yolk sac

forms part of digestive tube to feed
+ first blood cells

Allantois

umbilical cord

Chorion

Forms the placenta

Embryogenesis

During implantation, blastocyst begins converting to gastrula (inner cell mass develops into bilaminar embryonic disc)

Bilaminar Embryonic disc is subdivided into what

epiblast (upper layer) and hypoblast (lower layer)

What happens at week 3 of development (2)

gastrulation → development of 3 primary germ layers (trilaminar embryonic disc)
epiblast → 3 layers

hypoblast→

+ development of extraembryonic membranes

Gastrulation

bilaminar embryonic disc → 3 layered embryo (ectoderm, mesoderm, endoderm)

What appears during gastrulation

primitive streak / dorsal groove

Notochord and what does it do

temporary rod like structure made from mesodermal cells
provides axial support

What are the three germ layers from top to bottom
and what turns into them

Ectoderm, mesoderm, endoderm

epiblast gives rise to all 3

What are the 3 layers here

Blue - ectoderm , red endoderm, yellow endoderm (where the hypoblast used to be)

Ectoderm + function (2)

external layer → nervous system and skin epidermis

epidermis skin cells, neurons of brain, pigmented cells

Endoderm + functions (3)

Internal layer

epithelial lining of digestive, respiratory, and urogenital systems

lung thryoid pancreatic

Mesoderm _ functions

middle layer → forms all other tissues

all 3 muscles, RBCs,

What germ layers are considered epithelia

ectoderm and endoderm

ectoderm - outer epithelia

endoderm - inner epithelia

What is the first major event of organogenesis

neurulation

Neurulation

specialization of ectoderm

Neural plates fold inward → neural groove → fuse into neural tube → brain & spinal cord

neural crest cells and what can they turn into (4)

ectoderm cells that form along neural tubes during neurulation → break away and migrate to different parts of the embryo to form

• cranial ganglia

• spinal ganglia

• sympathetic ganglia

• andrenal medulla

Organogenesis

formation of body organs and systems

involves all 3 germ layers

What happesn at the end of the 8th week (2)

organogenesis mainly ends → all organ systems are present but not fully functional

end of embryonic stage → embryo → fetus

Fetal development

ffrom week 9-birth

rapid growth of body structures established in embyro

Fetal development the trimesters

week 9 → first trimester fetus

week 13 → second trimester fetus

week 28 → third trimester fetus

week 40 → end of third trimest

adjustments to pregnancy (4)

1. placenta releases relaxin → relaxes pubic symphysis and pelvic ligaments → ease birth passage

2. reproductive organs become engorged with blood

3. lordosis → inward curve of lower back

4. uterus expands to fill up most of abdominal cavity

Pregnancy on GI tract

estrogen and progesterone levels are high → morning sickness

Pregnancy on urinarysystem

increased urine output

• caused by increased metabolism and waste from fetus

Stress incontinence

urine leaks when pressure in abdomin increases because bladder is squeezed

pregnancy on respiratory system (2)

tidal volume increases → needs more oxygen for mother and fetus

dyspnea (shortness of breath) can happen later in pregnancy becasue uterus pushes diaphragm up → less space for lungs to expand

Pregnancy on cardiovascular system (3)

• blood volume increases 25-40% → supply fetus and mother + prepare for blood loss at pregnancy

• blood pressure and pulse increase (heart working harder)

• venous return from lower limbs impaired (uterus can block vena cava)