Embryonic and Fetal Development: Stages, Structures, and Processes

Embryonic Development

The process by which the embryo forms and develops.

Zygote

1 cell

Embryo

multicell

Fetus

week 9 after fertilization

Cleavage

a period of cell division during day 1 - day 3 or 4 of embryonic development that includes rapid cell division of embryo with NO cell growth

Day 1

First cleavage begins post fertilization

Day 2

2-cell embryo formed

Day 3 & 4

rapid cell division occurs; 4 cell embryo formed; 8 cell embryo formed; Morula formed (16+ cells)

Blastulation

The process of a blastocyst (100+ cells) being formed from a morula (16+ cells); morula becomes hollowed out and develops a fluid filled cavity

Hollowed out morula

blastocyst

Fluid filled cavity

blastocyst cavity

Inner Cell Mass (ICM)

inner cell group of 20-30 cells; precursor to a bilayered embryonic disc

Epiblast

upper cells, gives rise to embryo proper & amnion

Hypoblast

lower cells, gives rise to yolk sac

Trophoblast

outer single layer of cells; precursor to chorion, which forms fetal placenta; responsible for obtaining oxygen & nutrients from mother's blood supply once implanted in uterus

Implantation

occurs around day 7-9 in the endometrium; trophoblast cells adhere to inner surface of endometrium and secrete digestive enzymes to destroy endometrial cells

Amniotic cavity

begins to form in epiblast

Extraembryonic membranes

4 membranes that serve the developing embryo: Amnion, Chorion, Placenta, Yolk sac, Allantois

Amnion

sac around embryo filled with amniotic fluid in amniotic cavity; protects embryo

Chorion

helps to form fetal part of placenta

Placenta

temporary organ composed of both maternal and fetal tissues; provides nutrients and oxygen to embryo/fetus while removing its wastes

Yolk sac

lost original function in humans (placenta has taken over), but is still source of earliest blood cells

Allantois

base for umbilical cord that links embryo to placenta

Gastrula

developed from epiblast, has three germ layers, which are precursors to various organ systems

Ectoderm

outermost layer; gives rise to epidermis, CNS, various endocrine glands

Mesoderm

middle layer; gives rise to dermis, bones, smooth muscle, blood vessels, kidneys, ureters, reproductive organs

Endoderm

innermost layer; gives rise to lining of various tracts, liver, gallbladder, pancreas, various accessory glands

Facial expression

Some features of facial bones, facial n. 3rd arch: glossopharyngeal nerve

Heart

Find left atrium and left ventricle; heart is simple and functioning at this point

Liver

Just below and dorsal to heart

Somites

Series arranged along dorsal margin of the embryo; give rise to vertebrae and associated muscles

Arm bud

Seen just above liver; will develop into arm and hand

Leg bud

Found near tail; will develop into leg and foot

Tail

All mammalian embryos have a tail at this stage, whether or not the adult has a tail

2 month embryo

Look for: Ear shape, Eye shape, Presence of limbs (arms/legs)

Fetal Development

3 months/12 weeks; during 3rd month, fetus grows rapidly and its organs enlarge and mature; kidneys begin producing urine, reproductive organs are developed enough to structurally determine sex of fetus

3-month fetus model

Look for: Eyes, ears, nose and mouth, Well developed arms/fingers, legs/toes, Amniotic sac (transparent bubble) filled with amniotic fluid that surrounds fetus, Umbilical cord between fetus and placenta

Fetal Development: Full Term

9 or 10 months / 40 weeks; further fetal development involves growth, refinement, and maturation of body organs

Sensory systems

Mature and fetus becomes aware of its surroundings (hear sounds, faintly see light, taste amniotic fluid)

Fetus movement

Fetus moves around more and more strenuously

Fetus fat reserves

Fetus accumulates fat reserves and looks much chubbier than 3-month fetus

Fetus position

Fetus assumes head-down position for childbirth

Lab Activity 1

Case Studies (2 extra credit pts); you will receive a clinician/patient scenario where you are the clinician

Lab Activity 1 details

Pt background, symptoms, clinician notes, physical exam results, other relevant information; case studies are based on organ systems we covered in this class; answer all questions to the best of your ability

Lab Activity 2

Structure Identification of Models

Models in Lab Activity 2

Full term fetus, 1 month embryo, 2.5 week embryo