INTRODUCTION TO EMBRYOLOGY

Presenter: Dr Pashi V. MModerator: Prof Munkonge

INTRODUCTIONEmbryology (Gr. έμβρυον, embryo + logos, study) general embryology (embryogenesis) special embryology (organogenesis) prenatal development 280 days, 10 months:

embryonic period (embryo)ofrom fertilization to 8th week of development

preembryonic period (early development) – from the fertilization to 2nd week of gesta

embryonic period (late development) – from the 3rd week to

the end of 2nd month fetal period (fetus) – from the 9th developmental week to the birth

Sex cells (gametes)• Reproductive tissue:

–A separate tissue–Kind of epithelial tissue

• Composition:–Sex cells (gametes) – male and female–“somatic” cells

• Embryonic origin–Premordial germ cells (gonocytes)

• Formation of epiblast – 2nd week of gestation• Movement to the wall of the yolk sac – 3rd week

• Migration toward the developing gonads – 5th week• Formation of primary sex cords• Sex differentiation – male and female• gametogenesis

GAMETOGENESISConversion of germ cells into male and female

gametesThe cell division that take place in the germ

cells to generate male and female gametes is meiosis

CytodifferentiationTypes of gametogenesisSpermatogenesisoogenesis

spermatogenesisSpermatogonium > mature spermatozoonProcess duration – 64 daysSpermatocytopoiesis – in seminiferous tubules of the testis:Dividing of spermatonia by mitosis

(spermatocytogenesis)Growth and maturation by meiosisCytodifferentiation of spermatids into

spermatozoa (spermiogenesis)

Peculiarities of spermatogenesisMaturation of sperms begins at pubertyWavy and continuously course to a ripe old

ageTwo meiotic divisions without interphaseFour mature spermatozoa are formed from

one spermatogoniumOnly after their separation from the residual

bodies can the spermatozoa be considered isolated cells

SpermiogenesisSpermatid > mature spermatozoonProcess duration – 24 days3 phasesGolgi phase

Proacrosomal granuleAcrosomal phase

Acrosomal vesicle Acrosome – hydrolytic enzymes:

Hyaluronidase Neuraminidase Acid phosphatase Acrosin (zonalysin)

Maturation phase Residual bodies are shed Formation of spermatozoa Release of mature spermatoza > spermiation

SpermatozoonMature male gamateTotal length – 58-67umStructure:Head

NucleusAcrosomal cap

Neck Covered by plasmalemaBasal body - proximal centriole

Tail – flagellum Middle piece Principal piece End piece

Female gametesFollicular cells

Secretory role Endocrine secretion – estrogens Ovulation > lutein cells – progesterone

Theca cellsBuild up the theca interna

Interstitial cellsHilus cells

Produce testosterone

OocytesOogonia – mitotically active cellsPrimary oocyteSecondary oocyteovum

OogenesisPrenatal stage

Period of proliferation – gonocytesOogonia - 7 million /5th monthPrimary oocyte – 700000 – 2 million

Postnatal stageGrowth - primary oocyte, remain in P of meiosis 1

Percliarities of oogenesisfirst miotic division begins during fetal life and

is completed just before ovulationMeiosis II is completed only if the oocyte is

fertilizedOne mature oocyte (ovum) and three polar

bodies are formed from one oogonium

Fertilization • Fertilization is the fusion of spermatozoa with

the mature ovum• Capacitation and acrosome reaction• Capacitation enables the sperm cell to interact

with uterine cells• Acrosome reaction is important for

penetration of the spermatozoa through the zona pellucida

Corona radiataZona pellucida

(ZP-1, -2, and -3)Cortical granules

The egg (and corona radiata) at ovulation

Fertilization continues• Fertilization has 3 phases• Penetration of the Corona Radiata• Penetration of the Zona Pellucida• Fusion of the Oocyte and Sperm Cell

Membranes– Cortical and zona reactions– Resumption of the second meiotic division

Fertilization continues• The main results of fertilization are as follows:• Restoration of the diploid number of

chromosomes• Determination of the sex of the new individual• Initiation of cleavage

1. Acrosome Rx sperm bind to ZP proteins in the zona pellucida; this initiates the release of enzymes from the sperm allowing it to burrow through the zona pellucida.

2. Zona Rxbinding of the sperm and egg plasma membranes initiates Ca+ influx into the egg and release of cortical granules from the egg that block other sperm from fertilizing the egg.

Fertilization is a multi-step process whereby multiple sperm bind to the corona radiata, but only a single sperm usually fertilizes the egg

This so-called cortical reaction prevents other sperm from fertilizing the egg (aka “polyspermy”)

Cortical granule enzymes digest ZP proteins so other sperm can no longer bind.

Hyaluronic acid and other proteoglycans are also released that become hydrated and swell, thus pushing the other sperm away.

Meiosis II complete

Formation of male and female pronuclei

Decondensation of male chromosomes

Fusion of pronuclei

Zygote

Fertilization

Transport through the oviductAt around the midpoint of the menstrual cycle (~day 14), a single egg is ovulated and swept into the oviduct.

Fertilization usually occurs in the ampulla of the oviduct within 24 hrs. of ovulation.

Series of cleavage and differentiation events results in the formation of a blastocyst by the 4th embryonic day.

Inner cell mass generates embryonic tissuesOuter trophectoderm generates placental tissues

Implantation into the uterine wall occurs ~6th embryonic day (day 20 of the menstrual cycle)

Week 1: days 1-6

• Fertilization, day 1• Cleavage, day 2-3• Compaction, day 3• Formation of blastocyst, day 4• Ends with implantation, day 6

Fertilized egg2 polar bodies2 pronuclei

Day 10.1 mm

Fertilized egg (zygote)

Cleavage

Cleavage = cell division

Goals: grow unicellular zygote to multicellular embryo.Divisions are slow: 12 - 24h eaNo growth of the embryo- stays at ~100 um in diameterDivisions are not synchronous

Cleavage begins about 24h after pronuclear fusion

2 Cell Stage

Individual cells = blastomeres

Mitotic divisions maintain 2N (diploid) complement

Cells become smaller

Blastomeres are equivalent (aka totipotent).

4 cell; second cleavage

4 equivalent blastomeres

Still in zona pellucida

8 Cell; third

cleavageBlastomeres still

equivalent

Embryo undergoes compaction after 8-cell stage:first differentiation of embryonic lineages

Caused by increased cell-cell adhesion

Cells that are forced to the outside of the morula are destined to become trophoblast--cells that will form placenta

The inner cells will form the embryo proper and are called the inner cell mass (ICM).

Formation of the blastocyst

Sodium channels appear on the surface of the outer trophoblast cells; sodium and water are pumped into the forming blastocoele. Note that the embryo is still contained in the zona pellucida.

Early blastocystDay 3

Later blastocystDay 5

blastocoele

inner cell mass

Monozygotic twinning typically occurs during cleavage/blastocyst stages

“Hatching” of the blastocyst:preparation for implantation

Hatching of the embryo from the zona pellucida occurs justprior to implantation. Occasionally, the inability to hatchresults in infertility, and premature hatching can result in abnormal implantation in the uterine tube.

Ectopic Implantation

Implantation somewhere other than upper portion of uterus

“Rupture” can lead to life-threatening hemorrhage

Tubal pregnancy

Week 2: days 7-14implantation

• Implanted embryo becomes more deeply embedded in endometrium

• Further development of trophoblast into placenta

• Development of a bi-laminar embryo, amniotic cavity, and yolk sac.

Implantation and placentation (day 8)

Trophoblast further differentiates and invades maternal tissues– Cytotrophoblast: stem cell population– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood

Inner cell mass divides into epiblast and hypoblast:– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity– Hypoblast contributes to forming the underlying yolk sac.

Implantation and placentation (day 9)

Trophoblast further differentiates and invades maternal tissues– Cytotrophoblast: stem cell population– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood

Inner cell mass divides into epiblast and hypoblast:– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity– Hypoblast contributes to forming the underlying yolk sac.

Implantation and placentation (day 12)

Trophoblast further differentiates and invades maternal tissues– Cytotrophoblast: stem cell population– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood

Inner cell mass divides into epiblast and hypoblast:– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity– Hypoblast contributes to forming the underlying yolk sac.

Implantation and placentation (day 13)

Trophoblast further differentiates and invades maternal tissues

– Cytotrophoblast: stem cell population

– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast

– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood

Inner cell mass divides into epiblast and hypoblast:

– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity

– Hypoblast contributes to forming the underlying yolk sac.

Week 3: Days 14-21

• Two layer germ disc• Primitive streak forms• Gastrulation forms tri-laminar embryo• Neural induction• Left-right asymmetry• 0.4mm - 2.0mm

Gastrulation

At gastrulation the two layered epiblast is converted into the three primary embryonic germ layers:

– Ectoderm: outside, surrounds other layers later in development, generates skin and nervous tissue

– Mesoderm: middle layer, generates most of the muscle, blood and connective tissues of the body and placenta

– Endoderm: eventually most interior of embryo, generates the epithelial lining and associated glands of the gut, lung, and urogenital tracts

The human embryo at gastrulation

At gastrulation, primitive endoderm is replaced by definitive or embryonic endoderm then

mesoderm is formed

Cell movements during gastrulation

Axial mesoderm: passes through the node and migrates along the midline –forms the notochord

Paraxial mesoderm: passes just caudal to the node and migrates slightly laterally –forms cartilage, skeletal muscle, and dermis

Lateral plate mesoderm: passes more caudal and migrates more laterally –forms circulatory system and body cavity linings.

Extraembryonic mesoderm: passes most caudal and migrates most laterally –forms extraembryonic membranes and associated connective tissue & blood vessels.

Mesoderm is patterned in a cranial to caudal gradient

The notochord and pre-chordal plate develops from mesoderm arising from cells that passed directly through the node and migrated cranially along the midline

The notochord and pre-chordal plate are important signaling centers that pattern the overlying ectoderm and underlying endoderm.

Fate of the “axial” mesoderm

Major signaling centers at gastrulation: the node and the anterior visceral endoderm (AVE)

• Primitive node positions primitive streak for gastrulation, induces neural differentiation• AVE from primitive endoderm secretes factors that position primitive streak in posterior, induce head

formation

The node also sets up the neural plate

Left-Right asymmetry is established at gastrulation

Leftward beating of cilia at node moves secreted molecules sonic hedgehog (Shh) & FGF-8 to the left side of embryo.

Causes left side genes Nodal and Pitx2 to be expressed which then pattern developing organs. If cilia are defective, Shh and Fgf8 can randomly end up on right side, resulting in reversal of symmetry, aka situs inversus (liver on the left, spleen on the right, etc.)

Situs can be complete (everything reversed) or partial (only some organs reversed).

Situs Inversus

What happens if there is “not enough” gastrulation?Caudal agenesis (sirenomelia)

Premature regression of the primitive streak leads to widespread loss of trunk and lower limb mesoderm.

VATeR association:Vertebral defectsAnal atresiaTracheo-esophageal fistulaRenal defects

VACTeRL association:those above plus…Cardiovascular defectsLimb (upper) defects

If the primitive streak fails to regress, multipotent primitive streak cells can develop into multi-lineage tumors (containing ecto-, meso-, and endodermal tissues).

What happens if there is “too much” gastrulation?Sacrococcygeal teratoma

THE EMBRYONIC PERIOD The period in which each of the three germ layers

will give rise to a number of specific tissues and organs

Ectodermal germ layers Initially the ectodermal germ layer has shape of a

disc, not equal at caudal and cranial points Notocord and precordal mesoderm induces

overlying ectoderm to thicken and form the neuro plate

Induction of neuroectoderm is the initiation of neurulation

Neurulation Process by which neuro tube is formed from

neuro plate Lateral edges of neuro plates are elevated are

elevated to form the neuro fold while the depression makes the neuro groove

Neuro folding results in formation of a neural tube

Communication with amniotic cavity through neuropores

Closure of neuropores marks end of neurulation

Neural crest cells These are cells at the lateral border of the

neuroectoderm During fusing of the neuro folds the cells will

undergo the epithelial to mesenchymal transition

Migrates from neuroectoderm to underlying mesoderm

Neuro crest cells of the trunk region start migrating in 2 directions following closure of the neuro tube• Dorsal and ventral

• Ectoderm germ layer gives raise to organs and structures that maintain contact with the outside world

Mesodermal germ layer Mesodermal germ layer tissue under goes

proliferation Midline form a thickened tissue known as

paraxial mesoderm Laterally remain thin and called lateral plate Lateral plate divides into somatic or parietal

mesoderm layer continuous with amnion The layer continuous with the yolk sac is the

splanchnic or visceral mesoderm layer

• Paraxial mesoderm• Segmental organization known as somitomeres

and form in a cephalocaudal manner• Somitomeres further organise into somites• First pair arise from the occipital region at

approximately 20th day of development then they appear at the rate of 3 pairs per day until week 5

• 42 to 44 pairs are present, 4 occipital, 8 cervical, 12 thoracic, 5 lumber, 5 sacral and 8 to 10 coccygeal pairs

• Age determination

Intermediate mesoderm• Forms segmental cell clusters, future

nephrotome• More caudally it forms an unsegmented mass of

tissue, the nephrogenic cord• Excretory units of the urinary system and gonads

formLateral plate mesoderm• Differentiate into visceral and parietal mesoderm

Blood and blood vessels Blood vessels form in two ways,

vasculogenesis, where by blood vesselsarise from blood islands and angiogenesis, sprouting from existing blood vessels

The first blood island appear in mesoderm sorrounding the wall of yolk sac at 3 weeks

Islands arise from mesodermal cell that are induced to form hemangioblasts

Definitive hematopoeitic stem cells > aorta-gonad-mesonephros region (AGM)

Endodermal germ layer The gastrointestinal tract is the main organ

system derived from the endodermal germ layer the epithelial lining of the respiratory tract the Parenchyma of the thyroid, parathyroids,

liver, and pancreas the reticular stroma of the tonsils and thymus the epithelial lining of the urinary bladder and

urethra the epithelial lining of the tympanic cavity and

auditory tube

References

• Langmans’s Medical Embryology, 12th ed• Phillip M. Ecker et al, An animated tour of

human development, version 1.1.• www.slideshare.net/bindmadhuli/embryology