Embryology

Embryo

Development during the first 8 weeks after fertalization

Fetus

Development from week 8 until birth

Birth typically occurs from 38-40 weeks

Zygote

Fertilized egg

Oocyte

Unfertilized egg

Female gamete, women are born with all their eggs

Spermatozoa

Male gamete

Live for about 3 months

Morula

8-16 undifferentiated totipotent cells

Stage that enters the uterus

Totipotent

Capable of becoming any tissue from the embryo or placenta

Oocyte traveling to the uterus pathway

Enters peritoneal cavity, must be in contact with fallopian tube cilia

Zygote grows larger as it divides

Ectopic pregnancy

Occurs when an egg doesn’t make it to the uterus

Blastocyst

Cavitated morula

First cellular differentiation occurs during this stage → Cells are now pluripotent

Blastocoel

Cavity formed by morula cavitation

Trophoblast

Covers outside of blastocyst and becomes the placenta

Embryoblast

Only at one pole of the embryo and lies within the trophoblast

Becomes the baby

Implantation

Use chemotaxis to embed into posterior superior uterine wall at the embryonic pole

Glycoproteins on wall help blastocyst to stick

Trophoblast invades uterine lining to anchor

Trophoblast differentiation

Cytotrophoblast and syncytiotrophoblast

Secretes hCG

Syncytiotrophoblast

Moves into the uterine wall

Human Chorionic Gonadotropin (hCG)

Secreted by trophoblast into mother’s blood stream and maintains corpus luteum in ovary for 2 months

Keeps proliferative stage → Implantation can occur

Hormone detected by pregnancy tests

When is the earliest a pregnancy can be detected?

9 days after fertilization when trophoblast secretes hCG

Placenta

Formed by syncytio and cytotrophoblasts

Secretes progesterone after 2 months and corpus luteum will involute

Blighted ovum

Anembryonic pregnancy caused by chromosomal abnormalities, 50% of first trimester miscarriages

Trophoblast is normal but embryoblast fails to develop

hCG maintained → Exhibit pregnancy symptoms

Either wait to naturally miscarry or undergo D+C

Embryoblast differentiation

Endoderm and epiblast cells

Endoderm

Ventral monolayer near blastocyst cavity

Epiblast

Remaining cells of embryoblast that don’t become endoderm

Bilaminar structure

Embryoblast cells migrate away from wall

Epiblast is dorsal and exhibits slight hypertrophy

Endoderm is ventral

Amniotic cavity

Formed during the formation of the bilaminar disk

Prochordal plate

Differentiated endoderm cells at one pole of the embryo, marks the cranial end and endoderm changes from cuboidal to columnar

Becomes mouth

Gastrulation

Formation of the trilaminar embryo

Neurolation

Formation of the central nervous system

Primitive streak

Epiblast cells at caudal pole differentiate and become more columnar forming a groove

All remaining epiblast cells are now ectoderm

Mesoderm

Primitive streak cells mitose and migrate into the groove to form a third layer between ectoderm and endoderm

Primitive knot

Epiblast cells at cranial end of primitive streak

Daughter cells migrate inward through knot and make a 90 degree turn cranially → Give rise to notochord

Notochord

Derived from primitive knot cells

Produces a potent growth factor that only ectoderm has receptors for to induce gene activation/differentiation

Primitive streak cells can only give rise to…

Mesoderm

Primitive knot cells can only give rise to…

Notochord

Primitive streak is on the…

Dorsal side

Prochordal plate is on the…

Ventral side

Neuroectoderm

Differentiated ectoderm immediately dorsal to notochord

All ectoderm has the potential to become neuroectoderm if it is close enough to the notochord

Formation marks the end of gastrulation

Surface ectoderm

Undifferentiated ectoderm that did not receive growth factor from the notochord

Neural tube

Neuroectoderm cells form a deep groove and fuse, surface ectoderm covers over dorsally

Give rise to CNS

Neural tube closure

Starts in mid back and progresses simultaneously in both directions

Cranial closure occurs faster so prosencephalon can form

Anterior neuropore closes on day…

26

Posterior neuropore closes on day…

28

Neural crest cells

Neuroectoderm at crest of groove, stay separate after neural tube closes

Become dorsal root ganglion in PNS

Tissues present at weeks 3/4

• Surface ectoderm

• Mesoderm

• Endoderm

• Prochordal plate

• Neural tube

• Neural crest

• Notochord

• Syncytio- and cytotrophoblast (placenta)

What happens at the lateral aspect of the embryo while the neural tube is forming?

Moves ventrally and changes from a disk to a tube

Plate → Taco → Burrito

Nucleus pulposus of the intervertebral disc

What the notochord becomes after it fulfills its duty of inducing neuroectoderm formation

Failure to close neuropores

Caused by low folic acid levels in mother

Anecephaly

Failure to close anterior neuropore, generally incompatible with life

No prosencephalon (more cranial) → No cerebral hemispheres, still have reflexive eye movements because mesencephalon present

Mild forms can still be born because life essentials are contained in medulla

No brainstem development → Spontaneous termination

Zika virus

Prevents prosencephalon formation → No cerebral hemispheres

Babies born with anencephaly

Mesoderm differentiation

Paraxial, intermediate, lateral plate

Lateral plate

Becomes CT of muscles of viscera, pleura and pericardium, blood, cardiovascular and lymphatic system, spleen, adrenal cortex

Intermediate plate

Becomes urogenital system

Paraxial plate

Becomes trunk muscles

Somites

Somites

Paraxial mesoderm that breaks into paired cubes

Can determine how old the baby is, 32 pairs develop