Human Embryology

Human Gestation Period

~40 Weeks

First Trimester

0-14 Weeks

Major developmental process

From single cell to a fetus

Second Trimester

15-27 Weeks

Growth in length

Organogenesis

Third Trimester

28 Weeks - Birth

Growth in weight

Preparation for birth

Embryonic Period

Week 1-8

Rapid development occuring

Fetal Period

Week 8+

Week 1

Blastocyst Formation

Week 2

Implantation and embryonic disc formation

Week 3

Gastrulation and formation of 3 germ layers

Week 3-4

Neurulation, gut formation, mesoderm formation, and somitogenesis

Developmental Anomalies

Result from maternal, paternal, or external influence

Maternal Anomalies

Age

Exposure to external factors (body temp, alcohol, smoking)

Maternal effect genes

Paternal Anomalies

Age

Sperm quality

External Anomalies

Teratogens

Stress

Teratogens

Alcohol, medications, drugs, cigarettes, elevated body temperature.

Spontaneous Abortion

30% of conceptions end in the first 8 weeks

40-50% result from chromosomal abnormalities

Non-disjunction

Failure of chromosomes or sister chromatids to separate properly during cell division

Trisomy 21

Chromosomal non-disjunction can cause down syndrome

Fertilization

Sperm and oocyte come together to form a zygote

Occurs in the ampullary portion of the fallopian tube

Zona Pellucida

Oocyte covering made up of glycoprotein

Acts as block preventing polysperm, only 1 sperm per egg

Blastomere

Cel resulting from cleavage of the zygote

Polar Body

Small, haploid cell formed at the same time as the egg (meiosis II)

Morula

Forms around 4 days post fertilization and consists of ~30 cells.

Morula Biopsy

Pre-implantation genetic screening/testing

Inner Cell Mass

Embryo and extra-embryonic tissue

Become the epiblast and hypoblast during hatching

Blastocoel

Fluid-filled cavity of the blastocyst

Trophoblast

Extra embryonic tissue

Blastocyst Formation

Compaction and formation of a tight mass of cells

Blastocyst Hatching

Two cell layers of epiblast and hypoblast

Epiblast

Embryonic tissue

Hypoblast

Extra-embryonic tissue

Embryonic Stem Cells

Pluripotent cells derived from the blastocyte (derive into any type of human cell)

Implantation

Embryo (bilayer disc) invades the endometrium with th epiblast oriented towards the uterine wall

Synctiotrophoblast

Secrete proteolytic enzymes to allow the blastocyst to enter the endometrial wall

Cellular barrier between the mother and the fetus

Bi-Layer Disc Formation

Inner cell mass becomes bi-layered, with outer epiblast layer and inner hypoblast layer surrounded by trophoblast epithelium.

Cytotrophoblast

Inner layer of cells in the trophoblast that gives rise to the syncytiotrophoblast

Lacunae

Generated by syncytiotrophoblast fuse with maternal capillaries to join the maternal and embryonic circulation.

Amniotic Cavity

Formed and lined by the cells of the epiblast

Yolk Sac

Forms from the hypoblast

End of 2nd Weeks

Embryo is bi-layered and has fully implanted

Ectotopic Pregnancy

Implantation outside the uterus

Ovarian, ampullary, tubal, ishtmus, cervix, abdominal, pelvic

Gastrulation

Produces the three germ layers and the neural plate forms

Endoderm and mesoderm move internally from the epiblast through the primitive streak

Ectoderm stays on the surface

Primitive Streak

Visible thickening along the midline that marks the site of cell migration at the beginning of gastrulation

Ectoderm

Stays on the surface, giving rise to the nervous system and epidermis

Endoderm

Moves inward, gives rise to the gastrointestinal and respiratory tracts

Mesoderm

Moves inward, gives rise to the muscle, bone, blood, urogenital system, dermis

Epithelial to Mesenchymal Transition (EMT)

Epithelial cells lose polarity and cell-cell adhesion and gain migratory, invasive mesenchymal traits, allowing them to move and remodel tissue

Epiblast Movement

Toward the primitive streak, entering the primitive streak undergoing EMT and then migrate away

Cranial to Caudal Sequence

Embryo development and early vs late migrating cells will have different fates

Dynamic movements of gastrulation progresses in this sequence.

Establishing rostro-caudal and dorso-ventral body axes

Cilia

Motile cilia at the primitive node create a leftward fluid flow, activating left-side genes, that establish normal organ asymmetry

Ciliopathies

Disease caused by defects in cilia structure

Embryonic Induction

Requires organizers to allow for specification

Secondary Invagination Blastopore

Caused by grafting organizer cels onto a recipient embryo bringing secondary body axis (new body)

Xenopus Model

Vertebrate embryology model because it has large, externally developing embryos that are easy to manipulate, making it ideal for studying gastrulation, axis formation, and left–right symmetry

Embryonic Folding

Occurs during 4th week

Gives rise to the ‘tube within a tube body’ plan

Formation of the Nervous System

Notochord forms from mesoderm soon after gastrulation completion

Signals from notochord cause inward folding of ectoderm at the neural plate

Ends of neural plate fuse and disconnect to form an autonomous neural tube

Body Folding

Generates abdominal wall, contracts yolk sac, and established the embryonic body

Cranial caudal folding and lateral folding occur at the same time

Buccopharyngeal Membrane (bp)

Moves posteriorly and ventrally to oral region (ectoderm and endoderm)

Heart

Moves dorsally and posteriorly to thoracic region

Cloacal Membrane

Moves ventrally and anteriorly to form anus

Central Midgut

Remains broadly open to the yolk sac

Notochord

Provides axial support until the vertebral column forms

Paraxial Mesoderm

Forms muscles, bones, dermis (cranial tissues and somites)

Intermediate Mesoderm

Forms the urogenital system

Lateral Plate Mesoderm

Forms smooth muscle, the heart, blood vessels, and the ventral dermis

Axial Mesoderm

Forms notochord, contributes to formation of the intervertebral discs

Sclerotome

Consists of the skeleton

Myotome

Consists of striated and voluntary muscle

Dermatome

Consists of the dorsal dermis of the skin

Anterior Head Mesenchyme

Undergoes EMT with neural crest cells to form craniofacial structures

Prechordal Plate

Midline structure (anterior notochord) that contributes to the oropharyngeal membrane (endoderm)

Somites

Form in the paraxial mesoderm in an anterior to posterior progression

Migrate via EMT and differentiate into different cell types

Somitogenesis

Process of segmentation of the paraxial mesoderm to form pairs of somites

Development of Somites

1. Paraxial mesoderm gives rise to cranial tissue and somites

2. Somites form gives rise to sclerotome (skeleton) and dermamyotome

3. Dermamyotome divides into dorsal dermatome and ventral myotome

4. Dermatome dorms the dorsal dermis of the skin

5. Myotome divides into epaxial (dorsal) and hypaxial (lateral and ventral) myotome

Resegmentation

Sclerotome to form the vertebrae