Session 10: The Pre-embryonic Period and Gastrulation

Embryogenesis lasts how many weeks

8 weeks

Foetal period lasts how many weeks

32 weeks

Average gestation period in humans is ___ weeks

40 weeks

- First 8 weeks = embryogenesis

- Week 9-40 = foetal period

First trimester

0-12 weeks

What is spermatogenesis triggered by?

Testosterone produced by Leydig cells of testis during puberty

Testosterone is produced by what cells?

Leydig cells of testis

Testosterone production is under the control of what hormone?

Pituitary luteinizing hormone (LH)

Spermatogenesis process

1) Multiplication Phase

- Spermatogonia enter mitotic division to form primary spermatocyte

2) Growth Phase

- Primary spermatocyte enters meiosis 1

- Secondary spermatocyte enters meiosis 2 to form spermatid

3) Maturation Phase

- Spermatid undergoes spermiogenesis to form spermatozoa

Spermatogonia → primary spermatocyte → secondary spermatocyte → spermatid → spermatozoa

Oogenesis process

Foetal Life

- Oogonia enter mitotic division to form primary oocyte

- Primary oocyte enters meiosis 1

- Primary oocytes arrest at prophase 1 stage of meiosis 1 until puberty (beginning of menstrual cycle)

Beginning of Menstrual Cycle

- Primary oocyte becomes a secondary oocyte

- Secondary oocyte arrests at metaphase 2 of meiosis 2 until fertilisation occurs

Fertilisation

- Secondary oocyte undergoes completion of meiosis 2 to form an Ovum

Oogonia → primary oocyte → secondary oocyte → ovum

Primary oocytes arrest at what stage of meiosis 1 until beginning of menstrual cycle?

Prophase 1

Secondary oocytes arrest at what stage of meiosis 2 until fertilization takes place?

Metaphase 2

Sperm vs Ovum

Sperm

- 55µm (smaller)

- Motile (tail)

- Acrosome head full of enzymes for penetrating ovum

- Lots of mitochondria for energy (motility of sperm tail)

Ovum

- Larger (~0.1 mm) - largest cell in human body

- Immotile

- Two protective membranes = corona radiata (outermost) and zona pellucida (innermost)

First (outer) protective membrane of the ovum

Corona radiata - follicular cells

Zona pellucida is made of ___

proteoglycans

Second (inner) protective membrane of the ovum

Zona pellucida - proteoglycans

Corona radiata is made up of ___ cells

follicular

Fertilisation

Fusion of male and female gametes to form a zygote

In order to fertilise the egg, the sperm must undergo ___

Capacitation

This is a conditioning process where glycoprotein coat and seminal plasma proteins (from sperm membrane) are removed - this exposes the acrosome (enzymes) allowing penetration of ovum to take place

Describe the first phase of fertilisation

Penetration of the corona radiata

The enzyme hyaluronidase released from acrosome of sperm penetrates the corona radiata (outermost membrane of ovum) which is made of follicular cells

This process is aided by movements of the tail of the sperm

Describe the second phase of fertilisation

Penetration of the zona pellucida

Acrosome reaction

Release of proteolytic enzymes including acrosin that causes lysis of the zona pellucida (innermost membrane of ovum)

This allows sperm to come into contact with ovum plasma membrane

Describe the third phase of fertilisation

Fusion of the plasma membrane of ovum and sperm

Zona reaction

The fusion of plasma membrane of ovum and sperm leads to the hardening of zona pellucida

This hardening prevents other sperm from entering ovum

Describe the fourth phase of fertilisation

Completion of second meiotic division of the ovum

Secondary oocyte undergoes completion of Meiosis 2

Formation of two daughter cells which include = second polar body (hardly receives cytoplasm) and one mature definitive ovum

Chromatin material of the mature ovum (22, X) is arranged into the female pronucleus

Describe the fifth phase of fertilisation

Formation of the male pronucleus

Chromatin material of the sperm is organised into the male pronucleus

Sperm tail detaches, degenerates

Combination of 23 chromosomes in each pronucleus forms a zygote of 46 chromosomes

DNA replication occurs in each pronucleus - in preparation for division of zygote

Describe the sixth and final phase of fertilisation

Metabolic activation of the ovum

Breakdown of pronuclear membranes and the mixing of maternal and paternal chromosomes

Chromosomes organised onto a spindle in preparation for normal mitotic division

The first cleavage of the zygote occurs roughly ___ hours after fertilisation

30

Labelled diagram showing the second polar body which disappears slowly

What are the consequences of fertilisation process?

Describe four consequences

1) Secondary oocyte completes meiosis 2 producing a second polar body

2) Restoration of normal diploid number of chromosomes (2n = 46 chromosomes) in zygote

3) Variation in human species through mingling of paternal and maternal chromosomes - fertilisation is RANDOM!

4) Determination of the chromosomal sex of the embryo

Dizygotic twins

Fraternal twins

These siblings result from two separately fertilized eggs

Two separate placentas

Different genetic material (non-identical)

Monozygotic twins

Identical twins

These arise from single fertilisation event

Shared placenta

Identical genetic material

Risk factor in mothers for monozygotic twins

Older mothers (ageing)

What is the cleavage process of the zygote?

Series of repeated mitotic divisions of the zygote which features a rapid increase in the number of blastomere cells (totipotent cells)

After the ___ cleavage of the zygote = the compaction process takes place

After the third cleavage of the zyogte = the compaction process takes place

Compaction of a zygote

Three days after fertilisation - what is the name of the structure formed from dividing zygote?

16-cell morula

In the morula - all cells are the SAME and undifferentiated

Four days after fertilisation - what is the name of the structure formed from dividing zygote?

Blastocyst

In the blasocyst, cells are not the same - they are differentiated

Structure of a blastocyst

- Outer = trophoblast surrounding embryo

- Inside cavity = blastocoele

- Inner cell mass (ICM) = give rise to embryo (embryoblast)

The inner cell mass (ICM) is where stem cells are extracted. These embryonic stem cells are ___

Pluripotent

Pluripotent

Able to give rise to multiple, but not all, cell types.

Embryo hatching

5 days after fertilisation

Embryo liberates itself from the zona pellucida shell, leaving it behind

Implantation of the embryo

6 days after fertilisation

- The hatched blastocyst attaches itself to the endometrial epithelium (decidua)

- Trophoblast outer layer rapidly proliferates and differentiates into = cytotrophoblasts and synctiotrophoblasts

Synctiotrophoblasts erode maternal endometrium, enabling embryo to burrow and implant securely

What feature of synctiotrophoblasts allow them to burrow into the maternal endometrium?

Synctiotrophoblasts produce proteolytic enzymes that erode the maternal tissue which enables the embryo to burrow and implant securely into the endometrium

The second week post-fertilisation is characterised by the formation of what structure?

Formation of the bi-laminar embryonic disc

- Differentiation of the inner cell mass (ICM) into flat, bilaminar embryonic disc consisting of epiblast and hypoblast cells

Epiblast eventually gives rise to the...

embryo

Day 9-10 post-fertilisation

Implantation continues with the formation of a ___ plug.

The appearance of vacuoles in the synctiotrophoblasts join to form ___.

These ___ soon reach capillaries of maternal tissue and become filled with blood, allowing for the exchange of nutrients/gases.

Amnioblast cells separate from the epiblast and organise to form the ___ (a thin membrane that encloses the amniotic cavity).

Hypoblast cells proliferate to form the primary ___ sac.

Day 9-10 post-fertilisation

Implantation continues with the formation of a fibrin plug.

The appearance of vacuoles in the synctiotrophoblasts join to form lacunae.

These lacunae soon reach capillaries of maternal tissue and become filled with blood, allowing for the exchange of nutrients/gases.

Amnioblast cells separate from the epiblast and organise to form the amnion (a thin membrane that encloses the amniotic cavity).

Hypoblast cells proliferate to form the primary yolk sac.

Day 11-13 post-fertilisation

The hypoblast gives rise to extraembryonic ___ which splits to form the ___ cavity.

Transformation of the primary yolk sac to the ___ yolk sac.

Beginning of utero-placental circulation occurs as the lacunae fill with maternal blood.

Day 11-13 post-fertilisation

The hypoblast gives rise to extraembryonic mesoderm which splits to form the chorionic cavity.

Transformation of the primary yolk sac to the secondary yolk sac.

Beginning of utero-placental circulation occurs as the lacunae fill with maternal blood.

The chorion eventually gives rise to the fetal part of the ___

placenta

WEEK 2

The hypoblast becomes...

WEEK 2

The epiblast becomes...

Name two abnormalities that can occur during week two post-fertilisation in the developing embryo

Ectopic pregnancy

- Abnormal implantation (uterine tube or outside uterus)

Molar pregnancy (Hydatidiform Mole)

- Occurs when only the trophoblast layers of embryo proliferate and epiblast layer FAILS to proliferate. No embryo forms as a result

Gastrulation occurs during the third week of embryonic development.

What is gastrulation?

Gastrulation is the process by which bi-laminar embryonic disc is converted to a tri-laminar embryonic disc which has three germ layers...

Ectoderm (outer)

Mesoderm (middle)

Endoderm (inner)

Gastrulation is the beginning of ___ (the formation and structure of various organs/parts of the body)

Morphogenesis

The beginning of gastrulation is marked by the formation of a transient structure formed from epiblast cells called the ___ ___ on day 15

Primitive streak

Primitive streak formation

Primitive streak begins in caudal end of embryo (butt) and moves towards cranial end (head) elongating to form a primitive groove.

At the cranial end - epiblast cells form circular cavity known as the primitive pit

Gastrulation process

Epithelial cells at the lateral edge of the epiblast undergo epithelial-to-mesenchymal (EMT) transition

Invagination of ingression of mesenchymal cells - cells migrate down into the primitive streak.

The first set of cells to move down which integrate into hypoblast layer become the endoderm (innermost)

The second set of cells to move down primitive streak become the mesoderm (middle)

The last set of cells to move down primitive streak become the ectoderm (outer)

The consequence of gastrulation process is the formation of a ___ map

Fate map

All our tissues and structures derive from these three important layers...

Endoderm (inner)

Mesoderm (middle)

Ecotoderm (outer)

Derivatives of the three germ layers

Endoderm (inner)

- Respiratory tract

- Lining of GI tract

- Parenchyma of major digestive glands

Mesoderm (middle)

- Muscle

- Bone

- Urinary system

- Vascular system

Ectoderm (outer)

- Nervous tissue

- Sweat glands

- Epidermis

Name TWO abnormalities arising during gastrulation process

1) Caudal dysgenesis (sirenomelia)

- Insufficient mesoderm in caudal-most region of embryo

- Abnormalities of lower limbs/urogenital system

2) Sacrococcygeal teratomas

- Primitive streak persists and results in tumour in sacrococcygeal region

- Most common newborn tumour

Most common tumour found in newborns arising from persistence of the primitive streak

Sacrococcygeal teratomas

Caudal dysgenesis (sirenomelia)

Insufficient mesoderm in caudal-most regions of embryo

Abnormalities of lower limb and urogenital region

Some causes of male infertility

- Reduced motility of sperm

- Having low levels of sperm

- Having abnormally shaped sperm

Link to previous lecture = Kartagener′s syndrome causes immotile sperm leading men to be sterile

Some causes of female infertility

- Problems with ovulation

- Thickening of cervical mucus

- Scarring of ovarian tubes or cervix from surgery

- Pelvic inflammatory disease

The translucent glycoprotein shell surrounding the oocyte that dissolves prior to implantation

Zona pellucida

What is the outcome of the first cleavage of the zygote?

Two blastomeres of equal size

What is the specific name for the embryo at the 16 cell stage?

A morula

Each cell is totipotent at the 16 cell stage. What does this mean?

Cells have the potential to become any cell type

When does fate become more restricted to many rather than all lineages?

Following compaction (when cell becomes blastocyst)

What structure forms following compaction?

The blastocyst

What is the fate of the inner cell mass?

It forms the embryo and some extraembryonic structures.

The key event following implantation is the transition from histiotrophic to haemotrophic nutrition.

How is this achieved?

Establishment of maternal blood flow

Name the disorder

Poor growth of foetus due to, for example, poor supply of oxygen and nutrients.

Intra-uterine growth restriction (IUGR)

Name the disorder

An obstetric complication arising from implantation in the lower uterine segment.

Placenta praevia

Name the disorder

Incomplete differentiation of cytotrophoblast cells into endothelium results in poor blood supply to the embryo, one consequence of which is maternal hypertension.

Pre-eclampsia

A potentially life-threatening disorder resulting from implantation outside the womb.

Ectopic pregnancy

From which collection of cells does the primary yolk sac form?

Extraembryonic mesoderm

How long is the pre-embryonic period?

Two weeks

Identify the following structure in a day 9 embryo.

A = Trophoblast lacunae

B and C = Enlarged blood vessel

D = Synctiotrophoblast

E = Cytotrophoblast

F = Amniotic cavity

G = Epiblast

H = Hypoblast

I = Exocoelomic membrane

J = Fibrin plug

K = Primitive yolk sac

Identify the structures in this image depicting gastrulation

A = Epiblast

B = Primitive pit

C = Primitive streak

D = Invaginating mesodermal cells

E = Hypoblast

Identify the axes marked on this image of the anatomical position

C-D = Posterior-anterior

A-B = Cranial-caudal

E-F = Right-left

Name a disorder that is routinely detected by pre-implantation screening?

Cystic fibrosis

From which cell layer is the notochord derived?

Epiblast

Identify the transverse, coronal and sagittal planes

Coronal = Plane 1

Transverse = Plane 2

Sagittal = Plane 3

Cognitive impairment caused by teratogen...

Alcohol

Multiple defects including microcephaly, visual impairment, cognitive impairment is caused by the teratogen...

Maternal cytomegalovirus infection

Shortened limbs is caused by the teratogen...

Thalidomide

Toxoplasmosis from poorly cooked meat leads to what developmental outcome?

Hydrocephalus

Most common site for implantation

Stroma of posterior uterine wall

Which highly-invasive layer of the embryo interacts with the connective tissue of the endometrium to facilitate implantation?

Synctiotrophoblast

Identify the structures in the following diagram showing an embryo at day 13 following fertilisation.

A = Primary chorionic villi

B = Amniotic cavity

C = Epiblast

D = Cytotrophoblast

E = Secondary yolk sac

F = Remnant of primary yolk sac

In the following photomicrograph of a histological section of developing tongue, myotubes are labelled A. What type of tissue is labelled B?

Mesenchymal connective tissue

Which of one of the following is not a common source of DNA to allow genetic testing for foetal abnormalities in a pregnant woman?

A) Amniocentesis

B) Foetal DNA in the mother's blood stream

C) A chorionic villus sample

D) An isolated cell at the morula stage

An isolated cell at the morula stage

3 multiple choice options

What stage of embryogenesis does this image represent?

Embryo hatching

What stage of embryogenesis does this image represent?

Implantation of the embryo

Day 7-8 post-fertilisation

Day 9-10 post-fertilisation

Day 11-13 post-fertilisation

Formation of the tri-laminar disc during gastrulation

The beginning of gastrulation is marked by formation of transient structure from epiblast cells called the primitive streak (day 15)

- Starts at caudal end

- Moves to cranial end, elongating to form primitive groove

- At cranial end = epiblast cells form circular cavity called primitive pit

- Primitive pit cells enlarge to form primitive node

What is occurring in this image taken during gastrulation?

Epithelial cells at the lateral edge of epiblasts undergo 'epithelial-to-mesenchymal' (EMT) transition

- Invagination or ingression = cells become flask-shaped and move down into the primitive streak

- First set to move down = endoderm (innermost)

- Second set = mesoderm

- Third set = ectoderm (outermost)