Introduction to Embryology - Basic Science Core

Embryology

Define Embryology

Is the branch of Biology studying the prenatal development of gametes, fertilization and the development of the embryos and fetuses

Three Anatomical Planes

1) Sagittal Plane

2. Coronal Plane

3. Transverse Plane

Sagittal Plane

• Vertical plane that divides the body into left and right

Coronal Plane

• Vertical plane that divides the body into front(anterior) and back(posterior)

Transverse Plane

• Horizontal plane that divides the body into upper (superior) and lower(inferior)

Chromosomes in the human Genome

Human genome consists of 46 chromosomes total

46 chromosomes are arranged in 23 pairs (23 inherited from maternal parent & 23 inherited from paternal parent)

• 22 pairs are autosomes

• 1 pair are sex chromosomes

Chromosomes are Composed of?

• DNA complexed with proteins

• A means of packaging a large amount of information in a small space

Genes in the human genome

~23,000 genes in the human genome

• 1 gene → many different proteins

• Function of various splicing/editing mechanisms

• Proteins regulate gene expression and act as signaling molecule

Gene

Genes are segments of DNA that act as the instruction manual for your body on how to make proteins

Induction

• One group of cells or tissues causes another to change their fate

• How cells communicate to try to change

Inducer in cells

• Produces a signal

• Cells do this to communicate

Responder

• Reacts to the signal

• Cells do this to communicate

Ability for cells to respond to signals is called?

Competence

Differentiation (Cells)

• A process where unspecialized stem cells or progenitor cells develop into mature, specialized cell types with unique structures and function

INDUCTION & ORGAN FORMATION

In Cells Signaling errors may occur (2)

Either genetic or environmental factors or BOTH can result in signaling errors

2 of many examples:

• 1) Situs Inversus

• 2) Heterotaxy

Situs Inversus:

condition where the positioning of

all organs is reversed in a mirror image

arrangement

Heterotaxy

one or more organs is abnormally

placed

Gametogenesis

• The process by which gametes (germ cells) are produced in an organism

• Gametes are derived from primordial germ cells (PGCs)

  in the second week of fetal development

  • PGCs arrive in the developing gonads of the genetically

  female or male embryo by 5 th week

Female Gametogenesis

Female gamete (ova/egg cell) forms through process

called oogenesis.

Male Gametogenesis

Male gamete (sperm) forms through spermatogenesis

PGCs

Mitosis (Cell Division)

• germ cells (future mature gametes) first undergo mitosis

• Cell division → 2 daughter cells that are genetically identical

• Each cell receives the complete complement of 46 chromosomes

Meiosis

Future mature gametes then must undergo Meiosis

to reduce chromosome number (this process

completes at different times in males and

females)

• 2 successive meiotic divisions: Meiosis I and

Meiosis II

• Meiosis I reduces chromosome number to

23

• Meiosis II resembles mitosis

• Results in reduction of chromosomes from diploid

(46) to haploid (23)

Diploid

a cell or organism that has paired

chromosomes, one set from each parent

Haploid

• Cell or organism that has a single set of chromosomes

Oogenesis

• THE PROCESS BY WHICH OOGONIA BECOME MATURE OOCYTES

• PGCs undergo mitosis, enter the female gonad(5th week), and

  differentiate to oogonia

  • @ 3 months: some oogonia cells synchronously arrest

  division in prophase of meiosis 1  primary oocytes

  • All primary oocytes are formed before birth

  • Primary oocytes arrested in prophase 1 remain arrested

  until puberty

  • First meiotic division is completed at puberty

  • Each ovarian cycle: Meiosis I  Meiosis II, cell arrests

  in metaphase and only completes meiosis II if fertilized

  • 1 primary oocyte only produces 1 mature gamete

Oogenesis Extra, (she is flying make this latter)

Meiosis I completes when female starts ovulation

• Meiosis II starts but is only completed if secondary oocyte fertilization occurs

• If fertilization occurs secondary oocyte completes meiosis II and form mature oocyte +

second polar body

• OVERALL: If fertilization occurs

• 1 primary oocyte  1 mature oocyte and polar bodies

• Polar bodies are not fertilized and degenerate

• Ovarian follicles are simultaneously undergoing maturation and provide the support

for oocyte growth and maturation

• Corpus luteum secretes hormones to prepare the uterus for pregnancy

• If fertilization does not occur corpus luteum degenerates to corpus albicans

Spermatogenesis

• THE PROCESS OF SPERM CELL DEVELOPMENT

• PGCs arrive in the genetically male gonad

  (now called spermatogonia), undergo several

  cycles of mitosis, and then enter mitotic arrest

  • Spermatogonia remain dormant until puberty

  when spermatogenesis begins

  • Spermatogenesis: Spermatogonia (2n) 

  spermatocyte stages (2n  n)  spermatids (n)

   mature spermatozoa (n)

  • Process lasts about 74 days

  • No formation of polar bodies

  • 1 primary spermatocyte  4

  mature spermatozoa

Major Difierences between Oogenesis vs Spermatogenesis

• Finite population of oocytes established by birth vs Continuous stem cell population

How many pregnancies today result in a miscarriage?

As many as 50% of pregnancies end in miscarriage (spontaneous abortion)

EXTRA CHROMOSOME(S) in kids

• More common then fewer chromosomes

• Examples: Trisomy (3 copies of a chromosome), Klinefelter syndrome XXY

FEWER CHROMOSOMES

• (absence of a chromosome from a pair); Turner syndrome XO

• Death

Trisomy 21

• Down Syndrome

• Abnormality is due to the presence of an extra copy of

  chromosome 21

• Clinical features:

  • Varying degrees of intellectual disability

  • Craniofacial abnormalities, including upward slanting eyes, epicanthal folds

  (extra skin folds at the medial corners of the eyes), flat facies, and small

  ears

  • Cardiac defects

  • Hypotonia

  • Increased risk of: leukemia, infections, thyroid

  dysfunction, and premature aging

  • Head and neck manifestations: macroglossia,

  gingivitis, premature periodontitis

Cytogenetic analysis (She said she is not going to ask about it much)

• Examination of the number of chromosomes (ploidy) and

  their integrity

2. Microarrays

Small sequences of DNA on chips act as

probes to detect mutations

3. Exome sequencing

Coding regions only of DNA are sequenced

and assessed for abnormalities

FERTILIZATION AND IMPLANTATION

1) Development begins with fertilization (usually occurs in ampulla of fallopian tube)

2) Haploid (1n) gametes (sperm and oocyte) fuse to form diploid (2n) zygote

• Oocyte from female: 22 autosomal chromosomes + 1 sex chromosome (X)

• Sperm from male: 22 autosomal chromosomes + 1 sex chromosome (X or Y)

• Genetic male zygote: XY

• Y chromosome carries SRY gene that dictates testis differentiation

• Genetic female zygote: XX

• Absence of SRY gene ovary developme

Once a spearm meets an egg what is it now considered?

• A Zygote

• At this point the male cell will determine the sex of the child

Time Line of Human Development

Birth typically in 38 weeks

First Trimester is a big no/no. Do not do dental procedures at this time

The Embryonic Period Timeline

1) 0 to 8 weeks

The Embryonic Period ZYGOTE Division (3)

@ 16 cells (approx. 3 days) is called a morula

• Begins to separate into inner cell mass and outer cell mass

1) two cell stage

2) four cell stage

3) Morula

Blastomere

• Cells in Zygotic Division

Blastocyst Formation

Morula travels to uterine cavity

• Fluid begins to penetrate and forms a single

cavity called the blastocele

• Embryo at this stage is called the

blastocyst

• Blastocyst implants in the endometrium

along the anterior or posterior wall

Blastocyst parts (2)

1) Inner cell mass → embryoblast

2) Outer cell mass → trophoblas

What happens by the first week of The Embryonic Period?

• By the end of the first week the zygote  morula  blastocyst  begins implantation in the uterine mucosa

Pluripotent

• able to give rise to many different cell types

ABNORMAL IMPLANTATION

• Implantation outside of the uterus can take place and results in extrauterine pregnancy → Ectopic pregnancy

Ectopic pregnancy

Can occur any place in abdominal

cavity, ovary, uterine tube

• 95% occur in the fallopian tube

• If left to grow can cause adjacent

organ damage or life-threatening

blood loss

MATERNAL IMMUNITY

• Women, in order to not reject the embryo (which is ½ “foreign”), need to undergo an immune system shift

• Shift from cell-mediated immunity to humoral (antibody-mediated) immunity

• Not fully understood but may increase mothers' risk of infections

• Change in immune system may alter symptoms of a mother’s autoimmune disease

• Ex. Multiple sclerosis, Rheumatoid arthritis are cell-mediated and show improvement

during pregnancy

• Ex. Systemic lupus erythematosus is predominantly antibody mediated and may

worsen during pregnancy

SECOND WEEK OF DEVELOPMENT

• By second week human chorionic gonadotropic (hCG) hormone is in

significant quantities to be detected and are used for pregnancy testing

• The week of the 2’s

• Trophoblast(outer cell mass) differentiates into 2 layers:

cytotrophoblast and syncytiotrophoblast

* Embryoblast(inner cell mass) forms 2 layers: epiblast and hypoblast

• 2 cavities form: the amniotic and yolk sac cavities

• Extraembryonic mesoderm splits into two layers

Trophoblast cells during Week 1 & 2

Embryoblast (Inner Cell Mass)

Embryoblast (inner cell mass) differentiates into epiblast and hypoblast

Epiblast

Definition

• Epiblast: One of the two cell layers formed from the embryoblast; it becomes the dorsal layer of the bilaminar disc.

Answer

• Embryoblast (inner cell mass) differentiates into epiblast and hypoblast.

• These cells migrate and orient such that epiblast is dorsal.

• Together with the hypoblast, it forms the bilaminar disc.

Hypoblast

Definition

• Hypoblast: One of the two cell layers formed from the embryoblast; it becomes the ventral layer of the bilaminar disc.

Answer

• Embryoblast (inner cell mass) differentiates into epiblast and hypoblast.

• These cells migrate and orient such that hypoblast is ventral.

• Together with the epiblast, it forms the bilaminar disc.

• Some cells of the hypoblast differentiate into the anterior visceral endoderm (AVE).

Bilaminar Disc

Definition

• Bilaminar disc: A two-layered embryonic disc formed by the epiblast and hypoblast.

Answer

• Both of these layers together form the bilaminar disc.

• The epiblast is the dorsal layer.

• The hypoblast is the ventral layer

Anterior Visceral Endoderm (AVE)

Definition

• Anterior visceral endoderm (AVE): Specialized cells that develop from the hypoblast and are important for early embryo patterning.

Answer

• Some cells of the hypoblast differentiate into the anterior visceral endoderm (AVE).

• AVE cells migrate to the future cranial site of the embryo.

• AVE cells are important for signaling body axes

Amniotic cavity

Small cavity appears within the epiblast

Primitive yolk sac

Cavity forms between hypoblast and exocoelomic membrane (formed from cells of hypoblast)

THE YOLK SAC: SECOND WEEK Main Key Functions

Early Nutrition

• Gas exchange

• Hematopoiesis

• Germ cell Development

• Contributing to organ formation

• Early Immune and Metabolic Support

Amnion

refers to a membranous structure

which covers and protects the embryo

• Forms inside the chorion

Chorion

Chorion is a double-layered membrane formed by the trophoblast and the extra-

embryonic mesoderm, which eventually will give rise to the fetal part of the

placenta

Chorion & Amnion

These two membrane structures fuse later in development

THE TRILAMINAR DISC

GASTRULATION

Primitive streak appears at the caudal aspect of

embryo

• Cranial aspect of streak thickens and forms primitive

node

• Cells of the epiblast begin to invaginate

• 1st wave of cells migrating through the streak

interdigitate with the hypoblast layer  ENDODERM

• 2nd wave  MESODERM

• Remaining cells in the epiblast layer for the ECTODERM

These germ layers give rise to all organs and

tissues of the embryo

FORMATION OF THE NOTOCHORD

Prenotochordal cells migrate through the primitive streak,

become intercalated in the endoderm to form the notochordal

plate

• Plate detaches from endoderm  notochord

• Notochord forms the midline axis which serves as basis of

axial skeleton

• Allantois forms and is responsible for waste exchange

• Oropharyngeal membrane eventually breaks down

to form the opening of the mouth

• Cloacal membrane breaks down to form the opening of the

anus

WEEKS 3-8: THE EMBRYONIC PERIOD

The period of organogenesis!

• By the end of the embryonic period

main organ systems are established

• This is the time at greatest risk for

induction of birth defects

• Sensitive to genetic and environmental

impacts

Three Germ Layers

1) Ectoderm

2) Mesoderm

3) Endoderm

Ectoderm (9)

• Central nervous system (Brain & Spinal Cord)

• Peripheral nervous system

• Sensory epithelium of ears, eyes, and nose

• Epidermis

• Hair and Nails

• Subcutaneous glands

• Mammary glands

• Pituitary gland

• Enamel of the teeth

Neurulation

• Process by which the Ectoderm [guided by the notochord] forms the neural plate

• Which then bends upward and fuses → neural tube

Neurulation forms (3)

• Neurulation results in the segregation of the three ectodermal derivatives:

1) Neural tube,

2) Neural crest

3) Epidermis

Neural Tube (will form)

Neural tube will become the brain [cranial end] and the spinal cord [caudal end]

Epidermis (will form)

• the top layer of our skin

Neural Crest Cells Split to 5 Groups

1) Cranial neural crest  cartilage, bone, cranial ganglia & connective tissue of the face

2) Cardiac neural crest  septum of the outflow tract of the heart & wall of large arteries

3) Vagal neural crest  parasympathetic enteric ganglia of the gut

4) Trunk neural crest  melanocytes, dorsal root ganglia, sympathetic ganglia & adrenal medulla

5) Sacral neural crest  parasympathetic enteric ganglia of the gut

Cranial neural cres

Cardiac neural crest

Vagal neural crest

Trunk neural crest

Sacral neural crest

Mesoderm (8)

All Supporting Tissue

- 1) Muscle

• 2) Cartilage

• 3) Bone

• 4) Dermis of the skin

• 5) Vascular system

• 6) Heart, arteries, veins, lymph vessels, all blood and lymph cells

• 7) Urogenital system: kidneys, gonads, ducts

• 8) Spleen

Endoderm

Think of Lining

1) Epithelial lining of: GI tract, respiratory tract, urinary bladder, urethra, tympanic cavity, auditory tube

2) Gastrointestinal tract

3)Thyroid

4) Parathyroid

5) Liver

6) Pancreas

Placenta (two parts)

• Fetomaternal organ

• 1) Fetal component (chorion) derived from the

  trophoblast & extraembryonic mesoderm

• 2) Maternal component derived from the uterine

  endometrium

Placenta Morphology & Functions (5)

Flat like a pancake

• 1) Protection

• 2) Nutrition

• 3) Respiration

• 4) Excretion

• 5) Hormone production

Amnion

large sac containing amniotic fluid in

which the fetus is suspended by its umbilical cord

• (1) Absorbs jolts

• (2) Allows for fetal movements

• (3) Prevents adherence of the embryo to the

amnion

Amniotic fluid

Derived primarily from maternal blood

• Volume is replaced every 3 hours

• Increases throughout pregnancy from ~30mL at 10 weeks

to 1000 ml near delivery

• Either too much (hydramnios) or too little

(oligohydramnios) is associated with birth

defects

• Fetus drinks about 400ml/day beginning at 5 months

Hydramnios

• Too much Amniotic fluid

Oligohydramnios

• Too little Amniotic fluid

Dizygotic (Twin)

Dizygotic (fraternal) twins = 2 separately

fertilized eggs

• Not genetically identical

Monozygotic (Twin)

Monozygotic (identical) twins = 1 fertilized egg splits into two embryos

• Genetically identical