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Embryology

LUCY ANGELA DOCKERY

20202849@STUDENTMAIL.UL.IE

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Outline

Gametogenesis
Fertilization
Cleavage, Implantation, Blastocyst Formation
(Placental Development)
Gastrulation
Neurulation: NTDs
Somitogenesis
Facial Development and Pharyngeal Arches: Cleft Lip/Palate
Limb Development
Heart Development
Midgut formation

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Gametogenesis: Primordial Germ Cells

Teratoma

Primordial Germ Cells (A) formed in Epiblast (wk2)

Yolk Sac (wk4)

Wall gut tube(B)

Dorsal Mesentery (C)

Either side of Dorsal Body Wall

Proliferation of Somatic Support Cells

Genital Ridge (D)

*** Q: what is the first stage of embryonic development ***

Fetal Development beings with Fertilization: male gamete (sperm) and the female gamete (oocyte) fuse to form a zygote.

*** So we always start embryology with study of the gametes and gametogenesis ***

*** What cells give rise to the gametes? ***

PGCs are formed in the epiblast in week 2 and move to the yolk sac where they are found week 4
Migration(wk 4-6) PGC (A) →yolk sac → gut tube (B) → dorsal mesentery (C) → dorsal body wall → reside on either side area of future gonads → stimulate proliferation of somatic support cells to form genital ridge (D). (As they migrate they divide by mitosis)
Genital ridge is medial to each kidney
Males: PGCs undergoes a few more mitotic divisions and then remain dormant from week 6 until puberty; puberty PGCs mature and differentiate into spermatogonia. The enter meiosis and mature into spermatozoa under influence of testosterone. Process continues until death.
In females PGCs differentiate into oogonia. Month 5 oogonia begin meiosis. They arrest in prophase of meiosis I and remain dormant until puberty. Every months a few resume meiosis to monthly surge if pituitary hormones. Arrest in metaphase of meiosis II again unless fertilized. Continues until menopause.

Aberrant migration of PGCs gives rise to teratomas

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Gametogenesis: MALE

Male – Spermatogenesis

Near the basement membrane of seminiferous tubules are spermatogenic progenitor cells called spermatogonia (2n). These undergo mitosis to produce one new spermatogonia and a primary spermatocyte (2n)
The primary spermatocytes move out of the tight junctions that form the blood-testes barrier and start Meiosis I
First meiotic division produces two secondary spermatocytes (n)
Each secondary spermatocyte undergoes Meiosis II to produce four total spermatids (n)
These cells have progressively moved closer to the lumen and now undergo Spermiogenesis to change from round spermatid cells to motile cells: acrosomal cap develops, Sertoli cells absorb cytoplasm and a flagellum is formed

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Spermatogenesis & Spermiogenesis

‘Gonium’ is going to be a sperm

‘Zoon’ is ‘zooming to the egg

Spermiogenesis: converts spermatids to matures spermatozoa ready for deposition into the luman

- Cytoplasm Shrinks (Sertoli cells)

- Shape change (head, midpiece and tail)

- Acrosome formation

Spermatozoa ready for ejaculation/fertilization

Capacitation: Final maturation with female genital tract

Spermiogenesis

Spermatids converting to mature sperm via spermiogenesis; cytoplasm of spermatid shrinks and is transferred to Sertoli cell by tubulobulbar complex, spermatids change shape, connected with Sertoli cell breaks via spermiation. Mature spermatid/spermatozoa deposited into luman where it is stored in the epididymis until ejaculation. Spermatozoa has head, midpiece (mitochondria for energy), tail (motility) and acrosome (contains hydrolytic enzymes used in fertilization)
Human male spermatogenesis cycle 64 days (2 months); mitosis (16 days), meiosis I (8 days), meiosis II (18 days), spermiogenesis (24 days). Continues from puberty until death
Sertoli cells Support Sperm Synthesis
Leydig cells secrete testosterone in response to LH: Leydies (ladies) dig testosterone
Finally spermatozoon maturation is capacitation: within the female genital tract where acrosome changes in preparation to release its contents

Cells of The Testis

Fibroblasts:
Myoid cells: contractile for gentle peristaltic waves seminiferous tubules
Sertoli Cells: Support
Leydig Cells: Testosterone
Spermatogonia (Type A & B)
Primary Spermatocyte
Secondary Spermatocyte
Spermatid
Spermatozoa

Hormonal Regulation

Gonadotrophin Releasing Hormone (GnRH) released from hypothalamus → Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) released from Anterior Pituitary
FSH → Sertoli cells to maintain spermatogenesis:

- Sertoli Cells release Inhibin which negatively feedbacks onto FSH (inhibits it)

- Sertoli Cells produce Androgen Binding Protein which can bind and transport Testosterone (Dihydrotestosterone and 17-b-estradiol)

LH → Leydig cells: Leydig cells synthesises Testosterone.

- Testosterone acts on Sertoli cells to maintain spermatogenesis (with FSH)

- Testosterone negatively feeds back on GnRH (inhibits it)

(Pharmacology Note: GnRH is released in pulsatile fashion. Continuous release would result in negative feedback, ultimately supressing FSH/LH)

Blood Testis Barrier:

Tight junctions between Sertoli cells constitute a physical barrier between seminiferous tubules and blood which avoids an autoimmune response

Ejaculation Revision:

Recap: SEVEN UP (Route spermatozoa take to meet the secondary oocyte in the uterus)

- Seminiferous Tubule

- Epididymis

- Vas Deferens

- Ejaculatory Duct

- Nothing

- Urethra

- Penis

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Gametogenesis: FEMALE

Female – Oogenesis – this process begins in the female fetus

1) (before birth) Oogonia are diploid cells that undergo mitosis to produce primary oocytes (2n) – these begin meiosis I but arrest in prophase I

2) (at puberty) Primary oocytes complete meiosis I to form a secondary oocyte (n) and the first polar body. Meiosis II starts but arrests at metaphase II

3) The secondary oocyte undergoes ovulation and leaves the ovary for the fallopian tubes

4)If the secondary oocyte is fertilized, it completes Meiosis II, forms the second polar body and becomes an Ovum.

5) If it is not fertilized, the oocyte degenerates.

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Gametogenesis: MCQ

During spermatogenesis, a haploid 2N cell is produced. What is the name of this cell and from what process is it produced?

Spermatogonia arising after mitosis
Primary Spermatocyte arising after DNA synthesis
Secondary spermatocyte arising after meiosis I
Primary Spermatocyte arising after meiosis II
Secondary spermatocyte arising after meiosis II
Spermatid arising after meiosis II

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Gametogenesis: EMQ

1. At what phase is a primary oocyte arrested?

�2. After what process is the first polar body produced?

�3. At what phase is the secondary oocyte arrested?

�4. What process is completed if fertilization occurs

�

A: Prophase I

B: Anaphase II

C: Mitosis

D: Meiosis I

E: Telophase II

F: Prophase II

G: Meiosis II

H: Anaphase I

I: Metaphase II

J: Interphase

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Gametogenesis: SAQ

Using a flow chart/diagram, describe the process of oogenesis. (3 marks)

Outline spermatogenesis and it’s hormonal control (4 marks)

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Fertilization (day 1 of life)

-

= fusion of male and female gametes

OCCURS IN AMPULLA OF FALLOPIAN TUBES

Sperm penetrate the corona radiata – allowing the free passage of capacitated sperm

ZP3 (from SED1) and acrosin = penetration of the zona pellucida (ZP) – ACROSMAL REACTION

Gamete fusion occurs 30 hours post ovulation

Oocyte completes 2^nd meiotic division (ovum produced)
Female and make pronuclei (n) undergo DNA replication = followed by normal MITOTIC DIVISON (2n)

*** Q: Where does fertilization take place? ** Ampulla of the Fallopian Tube

FERTILISATION

Fusion of male and female gametes, occurs in the ampulla of the fallopian tube

Penetration of corona radiata – FREE PASSAGE OF CAPACITATED SPERM (capacitation breaks down the glycoprotein coat and seminal plasma protein from sperm head)

Penetration of zona pellucida (glycoprotein shell) - acrosomal enzymes allow the sperm to penetrate the zona pellucida (ZP3 ligand plus acrosin) ACROSOMAL REACTION

Cortical granules release lysosomal enzymes and ions, inducing depolarisation and …………..rendering the zona pellucida impermeable (prevent polyspermy)

Gamete fusion (30 hours post ovulation)

Oocyte completes the 2^nd meiotic division (ovum produced)

Female and male pronuclei (n) both replicate their DNA which is immediately …………..followed by a normal mitotic division where both cells of the zygote receives a …………..diploid chromosome set

Q: Where else can implantation happen? What is this called?

Ectopic Pregnancy

Abdominal ectopic: Oocyte fails to enter fallopian tube

Tubal ectopic: Failed migration can be due to scarring resulting from PID

Problem: Haemorrhage from placental detachment normally preventing by uterine contractions

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Morula and Blastocyst

Series of mitotic divisions then occur (following the two cell stage)

Day 3 after fertilization: Morula forms – A ball of 16 cells with an inner and outer cell mass

ICM = progenitors for embryonic tissue (embryo proper)
OCM = TROPHOBLAST = progenitors for placenta

Day 5 = BLASTOCYST FOMATION (32 cells)

Advanced morula enters the uterine cavity, undergoing increased divisions = BLASTOCYST
Fluid penetrates through ZP = blastocoele

= EMBRYOBLAST AND TROPHOBLAST

Blastocyst floats for about 3 days – implantation occurs day 6 (trophoblast invades uterine wall) – if ICM divides = monozygotic (identical twins)

The embryo goes through a series of mitotic divisions following the 2-cell stage

Blastomeres – the cells which are being consecutively divided within the embryo

Morula - 16 cells, forms 3 days after fertilization

Inner cell mass 🡪 progenitors for embryonic tissue (embryo proper) IF ICM DIVIDES + MONOZYGOTIC TWINS + SEE

ADDITIONAL SLIDES

Outer cell mass forms the trophoblast 🡪 progenitors for placenta

Advanced Morula/BLASTOCYST (32 cells) – increased blastomeres through additional divisions

Advanced morula enters the uterine cavity and becomes a blastocyst. Fluid penetrates through the zona pellucida, forming a single internal cavity called the blastocele. Zona pellucida disappears entirely (Day 4)

Embryoblast (inner cell mass) moves to one pole = pluripotent

Trophoblast (outer cell mass) flattens and forms the epithelial wall of the blastocyst

NOTE ADVANCED MORULA IS KNOWN AS BLASTOCYST ONCE ENTERS UTERINE CAVITY

BLASTOCYST floats for about three days = IMPLANTATION OCCURS 6 DAYS POST CONCEPTION

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Implantation

DAY 6 = implantation: occurs when trophoblast cells over the embryoblast pole penetrate the uterine mucosa ( L- selectin and carbohydrate receptors

OCCURS DURING SECRETORY PHASE

At this point uterine glands and arteries are coiled and tissue is spongey

DECIDUALISATION = spiral arteries are remodelled to increase diameter and straighten trajectory = increase in maternal blood

Blastocyst implants between glandular openings

At day 6, trophoblastic cells over the embryoblast pole begin to penetrate the uterine mucosa (L selectin and carbohydrate receptors), attaching the blastocyst to uterine wall (IMPLANTATION)

Implantation occurs during the secretory phase, when uterine glands and arteries are coiled and tissue is spongy

Decidualization = spiral arteries are remodelled to increase diameter and straighten trajectory so that there will be an increase in maternal blood flow to the placenta

(During pregnancy the endometrium is known as the decidua)

Blastocyst will implant in the endometrium, becoming embedded between glandular openings (Trophoblast cells invade the epithelium and underlying endometrial stroma (proteolytic enzymes)

Blastocyst usually embeds in the posterior part of either the fundus or the body. The inner cell mass points towards the endometrium

The Syncytiotrophoblast invades the endometrium by releasing proteases which digest the stroma.

The Cytotrophoblast synthesize anti-inflammatory cytokines to prevent an adverse reaction from the uterus tissue.

The embryo can embed itself within the endometrial tissue

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Bilaminar Disc (week of 2’s)

Inner cell mass - epiblast and hypoblast – BILAMINAR DISC

TROPHOBLAST – 2

CYTOTROPHOBLAST
SYNCYTIOTROPHOBLAST

EMBRYOBLAST - 2

EPIBLAST AND HYPOBLAST

CAVITIES – 2

AMNIOTIC AND PRIMARY YOLK SAC

EXTRAEMBRYONIC MESODERM – 2 LAYERS (DAY 11)

SOMATIC AND SPLANCHNIC

WEEK 2 (Bilaminar Germ Disc) – Week of 2’s

Blastocyst is becoming embedded in the endometrial stroma

NB (By end of the 2^ndweek the blastocyst is completely embedded and the mucosal surface defect is healed)

TROPHOBLAST – 2 – CYTOTROPHOBLAST AND SYNCYTIOTROPHOBLAST

Cytotrophoblast – inner layer (actively proliferates)

Syncytiotrophoblast – outer layer, no distinct cell boundaries (FORMS PLACENTA)

Erodes maternal tissue

By day 9, lacunae develop in the syncytiotrophoblast and maternal blood enters lacunae (creating a primitive uteroplacental circulation)

EMBRYOBLAST – 2 – EPIBLAST AND HYDROPLAST

Embryoblast differentiates into 2 layers, called the bilaminar disc splitting the cell to make two regions

Hypoblast (ventral) – layer of small cuboidal cells adjacent to the blastocyst cavity (large outer cavity) – continuous with the extraembryonic mesoderm creating the primitive yolk sac

Epiblast (dorsal) - layer of high columnar cells adjacent to the amniotic cavity (EMBRYO) – forms a cavity

2 CAVITIES (These two layers are sandwiched together between two balloons, the amniotic cavity within the epiblast (at the dorsal most part – appears day 8 and gives rise to the amniotic cavity (lined by amnioblasts – continuous with the embryonic disc)) and the primary yolk sac (day 9) below the hypoblast).

The amnion is the lining of the amniotic cavity (a.c lies on dorsal side of tehe bilaminar disc at the end of week 2 ).

EXTRAEMBRYONIC MESODERM – 2 - SOMATIC AND SPLANCHNIC LAYERS

Day 11 – extraembryonic mesoderm forms outside the embryo (new tissue), lies between the trophoblast and the developing bilaminar disc (outside the yolk sac, outside the amniotic cavity and within the trophoblast. Cavities form within, creating a new space – the chorionic cavity (extraembryonic coelem) (day 12). By the end of the second week, this cavity is relatively massive. It has a somatic and splanchnic layers.

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The Chorion

A layer of cells forms between the cytotrophoblast and the bilaminar disc called the extra-embryonic mesoderm (NOTE - different from mesoderm we see in gastrulation - “extra-embryonic” = not part of embryo)
Spaces form within the mesoderm to eventually become one large cavity - Extraembryonic cavity

The extraembryonic cavity will increase in size until it forms a large space surrounding the embryo.

This cavity is the Chorionic Cavity.

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Uteroplacental Circulation (wk2-3)

Primary Chorionic Villi

- Synctiotrophoblast (hCG)

- Cytotrophoblast

Secondary Chorionic Villi

- Synctiotrophoblast (hCG)

- Cytotrophoblast

- Mesoderm

Tertiary Chorionic Villi

- Synctiotrophoblast (hCG)

- Cytotrophoblast

- Mesoderm

- Blood Vessels

Fetal Heartbeat: d21

Cytotrophoblast are mitotically active and serve as stem cells for the synctiotrophoblast
Synctiotrophoblast mitotically inactive.

- Lacks major histocompatibility complex antigens → does not evoke an immune response

- Synctiotrophoblast secreted hCG (pregnancy test)

Placenta has both a maternal (decidua) and embryonic (chorion) component.

The three steps in uteroplacental circulation on the slides here are high yield for GEMs.

CHORION
Day 9: Trophoblastic lacunae form within the synctiotrophoblast
Maternal capillaries near the synctiotrophoblast expand and form maternal sinusoids
Maternal sinusoids anastomose with trophoblastic lacunae filling them with maternal blood
Cytotrophoblast extensions extend into the lacunae – primary chorionic villi
Day 16 Extra-embryonic mesoderm extends into primary villi – secondary chorionic villi
The villus mesoderm gives rise to blood vessels – tertiary chorionic villi – uteroplacental circulation is established and on day 21-22 the heart starts beating
So the maternal-fetal barrier for exchange on nutrients and gases consist of capillary endothelium, this layer extraembryonic mesoderm, cytotrophoblast, synctiotrophoblast
video: https://www.youtube.com/watch?v=xdibmSCNy6c

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Embryo: EMQ

What gives rise to all tissues of embryo?
Connected to a stalk where do primordial germ cells reside before going to gonadal ridge? (primary/secondary)
This layer has lacunae fill in it, eventually fills with maternal blood?
This accumulates fluid in mesoderm between cytotrophoblast and yolk sac eventually obliterates?
This cavity completely surrounds the embryo?

Chorionic cavity

Amniotic cavity

Blastocyst cavity

Epiblast

Hypoblast

Ectoderm

Mesoderm

Yolk sac

Synctiotrophoblast

Cytotrophoblast

Primary chorionic villi

Connecting Stalk

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Gastrulation (week of 3)

TRILAMINAR DISC FORMATION

Formation of primitive streak, node & pit (day15): midline

Epiblast cells migrate through: SERIES OF INVAGINATIONS

Invagination 1: Hypoblast displaced; endoderm formed

Invagination 2: between endoderm and epiblast; intraembryonic mesoderm

Remaining epiblast becomes ectoderm

Oropharyngeal & Cloacal membranes

Flat embryonic disc undergoes folding to become a cylindrical structure in week 3, this process brings the amniotic membrane and cavity to the ventral surface of the embryo (BECAUSE THE AMNIOTIC MEMBRANE IS CONTINOUS WITH THE EDGES OF THE EMBRYONIC DISC – as embryo folds, the amniotic cavity is dragged ending up being continuous around the embryo except for at the future umbilical cord).

Note – amniotic cavity binds with the chorionic membrane by the end of the third month, obliterating the chorionic cavity and becoming continuous with amniotic membrane).

Gastrulation occurs, beginning with the appearance of the primitive streak, with a cranial and caudal end, forming three germ layers.

Progressive cephalo-caudal process which leads to the gradual (cranio-caudal) differentiation of the three germ layers.

Characterized by invagination of epiblastic cells and migration

in the space between the epiblast and hypoblast

Entrance of the invaginating cells are primitive streak and primitive node

Gastrulation starts with migration of the invaginated cells into the cephalic regions.

This causes cranial growth of the disc

Invagination and migration (cranially and laterally) continues until the end of the fourth week 🡪 shrinkage and degeneration of the primitive streak.

In Caudal regions germ layer differentiation at the end of the fourth week.

OUTCOMES:

1) Establishment of the three embryonic germ layers:

ECTODERM, MESODERM and ENDODERM

2) Formation of the Notochord

3) Establishment of the body axes

SOMITES FORM AND BY THE END OF WEEK 3 THE NEURAL TUBE HAS FORMED (BMP4)

The primitive streak extends from cranial-caudal and determines the midline of the body

Cells from the epiblast move into the streak forming the germ layers (endoderm, mesoderm and ectoderm) INVAGINATION

Ectoderm –epidermis, hair, nails, brain, spinal cord and PNS

Mesoderm – muscle, bone, connective tissue, kidney, gonads, notochord and circulatory system

Endoderm – epithelium lining the GI tract, stomach, colon, liver, pancreas and lungs

(Epiblastic cells displace the hypoblast first forming the endoderm and then fill the space between, forming the mesoderm

Mesoderm forms a thin rod of cells known as the notochord below the primitive streak and induces the formation of the neural plate (ectoderm)))

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Somitogenesis

Somites reorganize:

- Sclerotome (ventromedial)

- Dermomyotome (dorsolateral)

Sclerotome:

- Vertebrae

- Ribs

- Sternum

Dermatome:

- Dermis of trunk

Myotome:

- Skeletal Muscle trunk and limbs

Somitomeres form from paraxial mesoderm, giving rise to the mesenchyme of the head amd organise into somites in occipital and caudal segments.

These occur in association with segmentation of the neural plate into neuromeres.

The first pair of somites form on day 20 – in the occipital region. From here, new somites form cranio-caudally, at a rate of three pairs per day, until the end of week 5 when 42-44 pairs are present (4 OCCIPITAL, 12 THORACIC, 5 LUMBAR, 5 SACCRAL AND 8-10 COCCYGEAL PAIRS) – the first occipital and last 5-7 coccygeal later disintegrate LEAVING 37 PAIRS WHICH GIVE RISE TO THE AXIAL SKELETON (because somites appear with specified peroidity, they can be used to accurately determine timeline of embryo development/age).

Formation is caused by cyclic expression of NOTCH and WNT signalling pathways – oscillating patterns, and boundaries for each somite are determined by retinoic acid (RA) and FGF8/WNT3a. RA is expressed at high concentrations cranially and decreases caudally – this along with NOTCH and WNT pathways allow for the control of somitogenesis).

�MORE DETAIL:

Somites will go on to differentiate in a process of epithelisation – forming its own sclerotome and eventually resulting in each somite having a specific dermatome and myotome – retains original innervation – dermatomes).

Signals for somite differentiation – Sonic HedgeHog (SHH), produced by notochord and floor plate of neural tube ,and noggin genes. Once differentiation is induced, PAX1 and PAX3 as well as NOTCH pathways regulate cartilage and bone formation. Two muscle forming regions differentiate – dorsomedial (WNT proteins) and ventrolateral (BMP4 and FGF).

CRANIOCAUDAL PATTERNING OF THE EMBRYONIC AXIS – HOMEOBOX (HOX) GENES

Embryonic folding which begins in week 3 and goes on until week 8 gives rise to organogenesis

Amniotic sac envelopes the embryo

Primitive gut is formed from a portion of the yolk sac and is lined with endoderm

Heart develops as a tube-like structure connected to the capillaries of the chorionic villi

Primitive heart cells – capable of electrical conduction and contraction

Doesn’t beat until the 4^th week

Doesn’t pump blood until the 5^th week

Eye pits

Limb buds

Primitive pulmonary system

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Neurulation

Process of formation of the primordium of the Central Nervous System

It starts at day 19 and leads to:

🡪 Formation of the Neural Tube

🡪 Formation of the Neural crests

FORMATION of the neural plate: Neural Induction (d18): Notochord (nucleus pulposus of spinal disc in adult) induces ectodermal cells (via BMP antagonism, Fgf8) just cranial to the primitive streak to differentiate into neuroectoderm (pseudostratified columnar). Broad cranially and tapered caudally;

NOTE: Remaining ectoderm is now called surface ectoderm

SHAPING of neural plate: Convergent extension: rapid lengthening of NP (day 22 the narrow future SC region is 25% of the length of the NP, day 24 its 50%)

Bending neural plate: neural folds consist of neuroectoderm and surface ectoderm; bend creating neural groove (median and dorsolateral hinge points). Brought in opposition with each other

Closure of Neural Groove: Day 22 closure begins at future occipital and cervical levels and proceeds cranially and caudally. Cranial neuropore close d24. Caudal neuropore close d26. Once fused two separate layers; roof plate of neural tube and overlying surface ectoderm. Cells in-between this layers undergoes Epithelial-To-Mesenchymal into Neural Crest Cells.
NCCs migrate to become many things (ganglia, adrenal medulla, melanocytes, schwann cells, autonomic neurons, skeleton of head, head mesenchyme etc.)

Secondary Neurulation (not very important):

Caudal extension of neural tube.

Tailbud (NCCs) condensation into medullary cord. Undergoes cavitation which merges with neural canal. Complete week 8

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Closure of neural tube: begins at end of week 3; complete by end of week 4 (folic acid important for this step)

Extends cranially (eventually brain) and caudally (spinal cord)

Neural crest, lateral ectodermal cells, pulled along and form sensory nerve cells and other structures

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Mesoderm begins to differentiate

Lateral to notochord, week 3
Extends cranially and caudally (from head to tail or crown to rump)

Division of mesoderm into three regions

Somites: 40 pairs of body segments (repeating units, like building blocks) by end week 4
Intermediate mesoderm: just lateral to somites
Lateral plate: splits to form coelom (“cavity”)

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Neurulation

Neural Crest Cells

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Neural Crest Cell Derivatives

Craniofacial Structures
Arachnoid and Pia Matter
Melanocytes
Odontoblasts
Tracheal Cartilage
Enterochromaffin cells (Adrenal Medulla)
Laryngeal Cartilage
Parafollicular C-cells (Thyroid)
All ganglia
Schwann cells
Spiral Septum (Aortic-pulmonary)

CAlifornia MOTEL PASS

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Reorganization of the Neural Tube

G

E

F

H

I

Primary Brain Vesicles:

- Prosencephalon (forebrain)

- Mesencephalon (midbrain)

- Rhombencephalon (hindbrain)

Secondary Brain Vesicles (5wk):

- Prosencephalon becomes Telencephalon (E) and Diencephalon (F)

- Mesencephalon (G) expands

- Rhombencephalon becomes Metencephalon (H) and Myelencephalon (I)

Neural Canal (Primitive Ventricles):

- Lateral Ventricles (Telencephalon)

- Third Ventricle (Diencephalon)

- Cerebral Aqueduct (Mesencephalon)

- Fourth Ventricle (Rhombencephalon)

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Caudal Congenital Nervous System Defects

See Langman’s Medical Embryology, Sadler, figure 17.15

Anatomy Figures: Joanna Hayes Figure 5

Normal

Spina Bifida Occulta

Meningocele

Meningomyelocele

Rachischisis

Spina Bifida Occulta: Defect in vertebral arch

- tuft of hair

Spina Bifida Aperta:

- Meningocele

- Myelomeningocele (Arnold-Chiari)

Diagnosis:

- Ultrasound (>12wks)

- Maternal Serum: ↑ AFP

- Amniocentesis: ↑ AFP

Prevention:

- 400 µg folic acid daily

Spina Bifida is a spectrum of disorders involving failure of the posterior neuropore to close

Spina Bifida Occulta: Defect in vertebral arch (bony defect)

- tuft of hair/dimple over defected site

Spina Bifida Aperta:

- Meningocele: herniation of meninges only (typically presents with soft tissue mass overlying the defect)

- Myelomeningocele: herniation of meninges and spinal cord (often associated with Arnold-Chiari Malformation)

- Rachischisis/Myeloschisis: failure of the neural folds to elevate (is this also involves cranial region then called craniorachischisis)

Anencephaly: Failure cranial/anterior neural tube to close: not compatible with life (next slide)

GEMS Note: 80-90% of NTDs also have hydrocephalous and is often related to Arnold-Chiari Malformation: herniation of cerebellum into foramen magnum obstructing CSF (Caused by tethering of the spinal cord to the vertebral column which pulls down the brain as the column grows)

Diagnosis:

- Ultrasound (From 12 weeks gestation)

- Amniocentesis: ↑ alpha-fetoprotein (AFP;16-18wks gestation) and/or ↑ acetylcholinesterase

- Maternal Serum: ↑ AFP

Prevention:

- 400 µg folic acid daily (most important in first 12 weeks post-conception and 4 weeks pre-conception)

- Recommended all women of childbearing age take 400 µg folic acid to cover unplanned pregnancies

- Recommended women with previous NTD take 400 µg folic daily (in first 12 weeks post-conception and 4 weeks pre-conception)

Neural Tube Teratogens:

- Anticonvulsants (valproate)

- Folate deficiency (Folate Antagonists)

- Diabetes mellitus

- Hyperthermia

- Hyper Vitaminosis A

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Cranial Congenital Nervous System Defects

Anencephaly: Failure of the anterior neuropore to close

Anencephaly Associations:

- ↑ AFP

- Polyhydramnios

- Gestational Diabetes

Anencephaly

Synophthalmia

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Neurulation: SAQ

With reference to the process of neurulation in the embryo:

(i) What does the notochord become in the adult? (1 mark)

(ii) State two derivatives of the neural crest cells. (1 mark)

What maternal serum test can detect a risk of neural tube defect? (1 mark)

Briefly describe two types of spina bifida. (2 marks)

List two therapeutic drugs classes that can cause neural tube defects when taken during pregnancy and name one example from each class. (2 marks)

What is spina bifida? Name and describe two examples of spina bifida. (2 marks)

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Folding (week 4):

Gastroschisis

Cranial Folding:

- OP membrane

- Cardiogenic mesoderm

- Septum Transversum

Caudal Folding:

- BP membrane

- Connecting Stalk

Lateral Folding:

- Fusion of ecto, meso and endo

- Vitelline duct and connecting stalk

Tube-within-Tube body plan

Cranial Folding (due to lengthen of neural tube) (d22)

Oropharyngeal Membrane brought to future mouth area (ruptures wk4)
Cardiogenic mesoderm brought to chest area
Septum transversum brought to chest area

(Septum Transversum (d22; mesoderm cranial to cardiac crescent): will form part of diaphragm – separates thoracic and abdominal cavities)

Caudal Folding (due to lengthening of neural tube) (d23)

Brings cloacal membrane under (ruptures wk8)
Brings the connecting stalk cranially until it merges with neck of yolk sac

(By now the neck of the yolk has begun to lengthen and constrict forming vitelline duct)

(Root of the connecting stalk contains an extension of hindgut (diverticulum) called allantois)

Lateral Fold (Simultaneously)

Embryo edges fold ventrally. Meet in the midline and the ecto, meso and endoderm fuse on either side)
All the edges of the embryo meet at the umbilicus where yolk sac and connecting stalk emerge

As a result of folding we have established our ‘tube-within-a-tube’ body plan

Fusion of endoderm: Gut tube

- Foregut: capped by oropharyngeal membrane

- Midgut: open to yolk sac but not for long (pathological ruminant Meckel’s diverticulum)

- Hindgut: capped by cloacal membrane

Fusion of mesoderm: (note it’s the lateral plate mesoderm that will fuse)

- Somatic Mesoderm (ectoderm adhered)

- Splanchnic mesoderm (endoderm adhered)

- Intraembryonic cavity (in-between)

Folding Abnormality:

Ventral Body Wall Defect: Gastroschisis

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Facial Development and Pharyngeal Arches

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Branchial/Pharyngeal Arches

Humans have 5 pharyngeal arches

Each arch has inner mesenchymal core, outer ectodermal layer and inner endodermal layer

CAP covers outside to inside: Cleft, Arch, Pouch

Within each arch: cartilage element (NCCs), muscle element (head mesoderm), cranial nerve and aortic arch artery

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Pharyngeal Arches

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Pharyngeal Pouch

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Pharyngeal Arches for GEMs

Arch 1: Trigeminal Nerve (CN V)
Arch 2: Facial Nerve (CN VII)
Arch 2: Stapedius Muscle (Hyperacusis)
Cleft 1: External Auditory meatus
Pouch 1: Middle Ear/Internal Auditory Tube
Yellow Structure: Parathyroid gland – Calcium Regulation
Arch 1: Maxillary and Mandibular prominence (facial development)

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Facial Development (wk4-10)

Frontonasal, maxillary (PA1; x2) and mandibular prominence (PA1; x2) surround stomodeum

Nasal placodes invaginate FNP to form nasal pits forming medial and lateral nasal processes

Inferior expansion of medial process forms intermaxillary process; fuses laterally with maxillary prominence

Rupture of OP membrane and fusion of maxillary and mandibular prominence creates embryonic mouth

Facial Development (wk4-10)

Week 4:

Frontonasal prominence
Maxillary prominence (x2) – pharyngeal arch 1
Mandibular prominence (x2) – pharyngeal arch 1
All surround stomodeum (primitive mouth)

Week 5:

Paired maxillary prominences enlarge and grow ventral and medial
Pair of nasal placodes form on the frontonasal prominence (ectodermal thickenings)
Initially the the 2 mandibular prominences seem separated by groove. Proliferation of underlying mesenchyme removes this groove creating the primordium of the lower lip
Rupture of OP membrane creates broad slit-like mouth (In month 2 fusion of maxillary and mandibular prominences bring to mouth to it’s proper size)

Week 6:

Nasal placodes invaginate forming nasal pits: this separates the FN prominence into lateral and medial nasal processes
Groove located between lateral nasal process and adjacent maxillary prominence is nasolacrimal groove
Medial nasal processes fuse forming primordium of bridge and septum of nose

Week 7:

(Ectoderm at floor of nasolacrimal groove invaginates into underlying mesenchyme to form nasolacrimal duct and lacrimal sac. Becomes invested by bone during ossification of maxilla. Drains excess tears from eye into nasal cavity).
Medial nasal process expand laterally and inferior to form intermaxillary process; this gives rise to philtrum of lip and primary palate containing 4 incisor teeth
Maxillary prominences fuse with the intermaxillary process.

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Palate Development:

Primary Palate: Posterior extension of intermaxillary process

Palatine shelves extend medial from maxillary prominence to fuse along the midline

Fused palatine shelves fuse anteriorly with primary palate. Now secondary palate

Point of fusion between palatine shelves and primary palate is called the incisive foramen.

Cleft Palates are named by location relative to the incisive foramen. These are anterior or posterior.

(The two nasal pits fuse dorsally forming a single ectodermal nasal sac lying. The floor of the nasal sac is separated from the oral cavity via the nasal fin; this thins to form the oronasal membrane; ruptures in week 7 forming primitive choana; at this stage the floor of nasal cavity is formed from posterior extension of intermaxillary process)

Palate Development:

Posterior extension of intermaxillary process forms primary palate (contains 4 incisors)
Medial extensions of maxillary prominence forms paired palatine shelves
These fuse with each other in the midline and the primary palate anteriorly to form secondary palate
The point where the primary and secondary palates meet is called incisive foramen
Eventually the nasal septum extends own from the roof of the nasal cavity and fuses with the secondary palate forming two nasal passages that open into the pharynx via definitive choana

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Cleft Lip and/or Palate:

Unilateral Cleft Lip

Bilateral Cleft Lip

Cleft Palate

Oblique facial cleft

Q: Cleft Palate: Is this anterior or posterior? Both,

Cleft palates are named by the location relative to the incisive foramen. Anterior cleft palates are anterior to the IF (failure of the palatine shelves to fuse with the primary palate). Posterior cleft palates are posterior to the IF (failure of palatine shelves to fuse with each other)

Cleft Lip:

Maxillary prominence fails to fuse with the intermaxillary prominence resulting in a persistent labial groove
Can be unilateral or bilateral

Cleft Palate:

Cleft Palates are names relative to their location of the incisive foramen (IF);

anterior cleft palates are anterior to IF: palatine shelves don’t fuse with primary palate
Posterior cleft palates are posterior to the IF: primary palate don’t fuse with secondary

Cleft Lip and Palate Teratogens:

Anti-epileptic medication (phenytoin, carbamazepine)
Maternal Alcohol Consumption
↓↑ RA

Oblique facial cleft

- Maxillary prominence does not fuse with lateral nasal process; nasolacrimal duct usually exposed

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Facial Development: SAQ

Describe normal embryological facial development and explain how a cleft lip or palate may occur (3 marks)

Name four therapeutic drugs that have well-documented teratogenic effects and state the organ or tissue affected by each drug (2 marks)

b) Describe normal embryological facial development and explain how a cleft lip and cleft palate may occur (3 marks)

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Heart Development

Heartbeat: d21

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Overview

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Initiation:

Primary Heart Field (d16-18)

- CPCs within VLP mesoderm

- Cardiac Crescent

Endocardial Tubes (x2)

- Fuse in midline

Primitive heart tube

- Endocardial Tube

- Cardiac Jelly

- Myocardial Tube

Secondary Heart Field (d20-21)

- RV and outflow tract

INITIATION:

Cardiac Progenitor Cells migrate cranially during gastrulation and embed into the splanchnic lateral plate mesoderm
They flank the OP membrane forming a horse-shoe cluster of cells called primary heart field (d16-18): atria, left ventricle and most right ventricl

Secondary Heart Field appear later (d20-21): remainder of RV and outflow tract

CPCs in PHF become cardiac myoblasts and blood islands: induced by underlying pharyngeal endoderm.

Blood Islands unite forming the horse-shoe shaped cardiac crescent flanking the OP membrane. During folding the limbs of the cardiac crescent fuse produce two endocardial tubes

Cranial body folding brings the endocardial tubes to the area of the future chest; now caudal to the OP/mouth

Lateral body folding brings the two endocardial tubes into opposition where they fuse to form a single tube; primitive heart tube. Failure of fusion of endocardial tubes results in cardia bifida

Day 21: endothelium surrounded by SLP mesoderm. CPCs in SLP mesoderm invade the endocardial tube forming the myocardium. Cardiac jelly is deposited separating the myocardium from the endocardium. Endocardial tube invested by cardiac jelly within myocardial tube. Epicardium will be formed lateral by an independent group of SLP mesoderm. Suspended from the dorsal body wall by the dorsal mesentery.

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Primitive Heart	Definite Heart
Truncus Arteriosus (Outflow)	Ascending Aorta Pulmonary Trunk
Conus Arteriosus (Outflow)	Smooth RV & LV
Bulbus Cordis	Smooth RV & LV
Primitive Ventricle	Trabeculated RV & LV
Primitive Atrium	Trabeculated RA & LA
Sinus Venosus (Inflow)	Coronary Sinus (left horn) Smooth RA (right horn)

Constrictions & Chambers

CONSTRICTIONS & SULCI

Over the next 5 weeks a series of constrictions and sulci form dividing the primitive heart tube into various heart chambers
Starting caudally have the inflow tract called the sinus venosus consisting of left and right sinus horns into which the common cardinal veins drain.
Primitive Atrium separated from primitive ventricle by atrioventricular sulcus
Primitive ventricle separated from bulbus cordis by bulboventricular sulcus
Next is the outflow tract which is further subdivided into the conus arteriosus and the truncus arteriosus

Cranially the TA is connected to the aortic sac which is continuous with the 1^st aortic arch establish systemic blood flow

Primitive heart tube is initially suspending in the pericardial cavity by dorsal mesocardium. When the dorsal mesocardium ruptures the heart is suspended by its vasculature. The area of ruptured mesocardium becomes the transverse pericardial sinus within the pericardial sac of the definitive heart; this space separates cardiac inflow and outflow vessels.

SCV develops from right common cardinal vein and right anterior cardinal vein
IVC develops from vitelline veins, posterior cardinal veins, subcardinal veins and supracardinal veins
Primitive Pulmonary Vein forms smooth LA

Endocardial cushions of the outflos tract forms aortic and pulmonary valves
Endocardial cushions of AV canal forms mitral and tricuspid valve

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Looping ‘to the right’

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Atria Septation

1(a): Endocardial cushion forms at the AV junction; AV septum

1(b): Thin Septum Primum extends from roof of primitive atria (foramen primum)

2(a):Septum Primum fuses AV Septum → obliterates FP.

2(b): PCD in SP forms foreman secondum

3: Thick Septum Secondum extends from roof of primitive atria (to right SP)

4: Foramen Ovale formed in SS now have right to left shunt

5: Foramen Ovale closes at birth; fossa ovalis

Intra-atrial Septum:

Thin crescent shaped septum primum extend from roof of atria, gradually separating LA and RA. The diminishing foramen is called foramen primum
Dorsal and Ventral endocardial cushions form in AV region. Fuse week 6 forming atrioventricular septum
Fusion of septum primum with AV septum obliterates foramen primum
Before this happens PCD forms foramen secondum for right to left shunting
Thick muscular septum secondum extends from roof of atrium to right of SP Growth of the SS halts before reaching AV septum leaving foramen ovale near floor of RA
Blood flow from right to left from foramen ovale to foramen secondum; collapsing of thin premium back onto thick secondum prevents backflow
Closes at birth; take first breath ↑ LA pressure, cessation umbilical flow ↓ RA pressure

Question: What shunt will form from ADS? Left-right or right-left
Atrial Septal Defects: lead to left to right shunting; common in Down Syndrome

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Ventricular Septation

1: Upward growth of muscular IV septum. Not fuse with AV cushion: interventricular foramen

2: Conotruncal cushions/Aorticopulmonary septum forming in outflow tract

3: Fusion of AP septum with the muscular IV septum; membranous IV septum; ventricular communication obliterated

VSD most common overall
ASD most common Down Syndrome
Both are left-right shunts

Persistent Ductus Arteriosus
D-Transposition of Great Vessels
Tetralogy of Fallot

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Tetralogy of Fallot

Features:

Over-riding Aorta
Pulmonary Stenosis
Right Ventricular Hypertrophy
VSD

Consequence:

-Right-to-Left Shunt

- Cyanosis ‘blue baby’ – tet spells

- Corrected surgically

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Mr X a 14 year old male is in hospital for abdominal pain in his left iliac fossa. In the patient’s notes, the house officer records an apex beat in the fifth intercostal space, in the right midclavicular line. During the ward round, the registrar asks the house officer if she is unable to distinguish left from right. She claims she can. What is the significance of her finding?

Ventricular Septal Defect
Dextrocardia
Trisomy 21
Trisomy 18
Atrial Septal Defect
Teratology of Fallot

MCQ:

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Limb Development (Wk4-8)

Limb Bud

Week 4 → 4 limbs
Limb Musculature: Paraxial Somite Mesoderm
Limb Skeleton and Vasculature: Somatic Lateral Plate Mesoderm

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Limb Development (Wk 4-8)

SOMATIC MESODERM

DAY 25 (UPPER) DAY 27 (LOWER): HOX GENES

Upper limbs: Tbx 5 (FGF10) *high 5

Lower limbs: Tbx 4 (FGF10) *4 floor

Localised thickenings: AER = outgrowth (BMPs)

ZPA: antero-postero growth = RA (from vitamin A) = SHH

AER

Limb Bud

Limb buds form: proliferations of somatic mesoderm: consist of ectoderm with an inner mesenchymal core

Begin to form approx. day 25 (upper limbs) and day 27 (lower limbs): formation is governed by HOX genes

Lower limbs: induced by TBX4 and FGF10

Upper limbs: induced by TBX5 and FGF10

Localised thickenings at the end of limb buds = AER (apical ectodermal ridge) = regulates OUTGROWTH of the limbs (through BMPs)

A ZPA (zone of polarising activity forms): governing antero-posterior growth: this releases RA (RETINOIC ACID FROM VITAMIN A) = SHH (sonic hedgehog)

Interactions between RA and SHH lead to the formation of limb paddles and digital rays (future digits) and controlled apoptosis occurs to separate these = fingers/toes

Bones laid down first as hyaline model: followed by endochondral ossification

Muscles form from somites (lateral plate mesoderm) in segmental patterning: follow along the ventral/dorsal aspect: forming flexors and extensors respectively.

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Limb Development: Axis Specification

1) Proximal-Distal Axis

- AER (fgf 4& 8)

- Progress Zone

- Hox (temporal and spatial expression)

2) Cranial-Caudal Axis

- Zone of Polarizing Activity (ZPA)

- Retinoic Acid and Sonic Hedgehog (Shh)

Dorsal Ventral Axis

ZPA

cranial

caudal

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Limb Development: Abnormalities

Abnormality	Description	Signalling Involved
Amelia	Complete Absence
Meromelia	Partial Absence of Limb
Micromelia	Short limbs (All Segments Present)
Phocomelia	Short limbs (Rudimentary hands/feet attach to trunk)
Polydactyly	Extra digits	Misexpression of Shh/RA
Ectrodactyly	Absent digits
Syndactyl	Fused digits	HOX mutations
Brachydactyly	Short digits

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Limb Development: Anatomy

Anatomy Figures: Joanna Hayes Figure 15

Q: What is this deformity called? What signalling molecules are involved?

Q: Name this deformity

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Limb Development: SAQs

When is the critical period for limb development in the embryo? (1 mark)

Outline the role of the zone of polarizing activity in relation to limb development in the embryo. (2 marks)

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Limb Development: EMQ

Lack of AER formation, no limbs develop
There’s no humerus, radius or ulna, but there was hands
Mermaid legs
Misexpression SHH and RA in rostral border of ZPA leads to
Failure of apoptosis in AER leads to

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Meromelia

Micromelia

Sirenomelia

Phocomelia

Amelia

Syndactylyl

Ectrodactyl

Polydactyl

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Teratogens

Drug Group	Adverse Effect
ACE Inhibitor (Ramipril, Captopril)	Renal Failure, Oligohydramnios
Alkylating Agents	Absence of digits
Antiepileptics (Phenytoin, Carbamazepine)	NTDs, cardiac defects, clefts, skeletal abnormalities
Folate Antagonists (Trimethoprim, Methotrexate)	NTS
Isotretinoin	Multiple Defects
Lithium	Ebstein Abnormality (cardia)
Thalidomide	Phocomelia
Warfarin	Bone deformities, spontaneous abortion

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Teratogens - Antimicrobials

Drug	Adverse Effect
Sulfonamides	Kernicterus
Aminoglycosides	Ototoxicity
Fluoroquinolones	Cartilage Damage
Clarithromycin	Embryotoxicity
Tetracyclines	Discoloured teeth, Impaired bone growth
Ribavirin	Teratogenic
Griseofulvin	Teratogenic
Chloramphenicol	Grey Baby Syndrome

SAFe Children Take Really Good Care

Most susceptible week 3-8: embryogenesis

< week 3: ‘all or none’

> week 8: growth and function effected

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References

Larsen's Embryology
Langman’s Embryology
First Aid USMLE 2019

Questions?

KEEP IT SIMPLE!!!

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By Kevin Dufendach - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=5324027

Dizygotic twins are always two zygotes.

Monozygotic twins split into two zygotes at some time very early in the pregnancy.

The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy.

isconception that two placentas means twins are dizygotic.

If monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.

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First week: BLASTOCYST FORMATION AND IMPLANTATION

Blastocyst formation with differentiation of embryoblast and trophoblast.

The zona pellucida dissolves.

Beginning of implantation

Second week: BILAMINAR EMBRYONIC DISC

Completion of implantation

Formation of the bilaminar embryonic disc with Epiblast and Hypoblast

Formation of the amniotic cavity and yolk sac

Formation of extraembryonic mesoderm (somatopleuric: future chorionic plate)

Differentiation of Cyto- and Syncytiumtrophoblast (with Lacunae filled by blood)

Third week: TRILAMINAR EMBRYONIC DISC, GASTRULATION & NEURULATION

Day 16: primitive streak and node, invagination of epiblast cells (GASTRULATION)

Day 17: migration of invaginated epiblastic cells. formation of 3 germ layers and notochord

Day 19: beginning of formation of the neural tube (NEURULATION)

Day 20: first somites appear at occipital region

Day 21: differentiation in sclerotome and dermatomyotome

Fourth week: COMPLETION OF GERM LAYERS, NEURULATION AND FOLDING

Day 25: closure of anterior neuropore

Day 27: closure of posterior neuropore

Formation of the primitive intestine

Basis for the definitive body structure

First month of life: timeline and Summary

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Embryo: EMQ

What structure is characterized by having a large mass of cells, and two layers, and a large fluid filled cavity before implantation?
What is the layer that is a part of the bilaminar disk, that differentiates into the 3 layers after the 2 week after conception, while embryo has embedded into endometrium?
What type of cell produces hCG after implantation?

Amnioblast

Epiblast

Mesoderm

Blastocyst

Embryo

Hypoblast

Synctiotrophoblast

Cytotrophoblast

Corona radiate

Zona pellucida

�

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Extreme caudal dysplasia → Sirenomelia

Teratogen: Gestational Diabetes

Anatomy: Mini-cases

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Anatomy: Mini-cases

Q: What does H become in the adult?

Q: What is the main ion that comes from D and F? and what is it coming from?

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Anatomy: Mini-cases

Q: What is B and its innervation?

Q: Damage to the muscle that arises from C would cause what?

Q: What bony structure is formed from B?

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Anatomy: Mini-cases

Q: What is E?

Q: What would the protrusion of I downward through the space a G blocking H called? Causing what?

Q: What is C? What does it join?

Q: What pathology from obstruction at E

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Anatomy: Mini-cases

Q: What is A? What is it’s innervation

Q: What does X become in adult

X

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Anatomy: Mini-cases

Q: What is B

Q: What would malformation at D result in?

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Anatomy Figures: Joanna Hayes Figure 17

Q: What is this the name of this condition?

Q: What drug is associated with this condition?

Anatomy: Mini-cases

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A

B

I

D

C

G

E

F

H

J

Q: Mesodermal derivative of limb musculature?

Q: Mesodermal derivative of limb bones and connective tissue?

Anatomy Figures: Joanna Hayes Figure 13

Anatomy: Mini-cases