Basics of embryology 😍

Gametogenesis

Periods	GA
Pre-embryonic period	Up to 2 weeks
Embryonic period	3rd to 8th week
Fetal period	9th week to birth

Primordial Germ Cells (PGCs)

• Origin:
• From epiblast (primitive ectoderm)
• by 2nd week.

• Migration:
• Move to wall of yolk sac by 4th week.
• Reach gonadal ridge (future testes/ovary) by 5th week.

• Differentiation:
• PGCs become gametogonium.
• Gametogonium forms:
• Spermatogonium (in testes).
• Oogonium (in ovary).

Abnormal migration of PGC

Site	Conditions
To Neck	Craniopharyngeal Teratoma
To Sacrum & coccyx	Sacrococcygeal Teratoma

Anorectal malformation associations

1. VACTERL
• Vertebral, Anorectal, Cardiac (m/c), Tracheoesophageal fistula, Renal, Limb defects

2. CURARINO syndrome
• ARM + Sacrococcygeal teratoma

Similar

• Goldenhar Syndrome
• Oculoauriculovertebral spectrum
• Features
• Hypoplasia of:
• Malar
• Maxillary
• Mandibular regions
• Macrostomia
• Microtia
• Preauricular and facial skin tags
• Hemivertebrae
• Usually cervical
• Mental handicap
• Cardiac, renal, and CNS anomalies
• Ocular
• Dermoid
• Upper lid notching / coloboma
• Microphthalmos
• Disc coloboma

Spermatogenesis and Oogenesis

Mnemonic:

• At birth → Put DP → Diplotene, Prophase

• In egg → Mutta → Metaphase

Sperm production

• Begins at: Puberty.

• Takes place in: Seminiferous tubules.

• Time for Spermatogenesis: 72 days (70-74).
• In testis

• Spermiation
• Release of spermatozoa from Sertoli cells

• Epididymis:
• Storage, maturation, and motility of spermatozoa

• Spermiogenesis:
• Transformation of spermatids to sperm.
• No mitosis or meiosis.
• In caudal part of Epididymis
• Time for Spermiogenesis: 10-14 days.
• Spermatogonia - In basal part
• Rest all - Adluminal compartment

• Sperms attain:
• Maturity: Proximal part of epididymis.
• Motility: Distal part of epididymis.

• Fertilizable life span: 48-72 hours.

• Sperm
• Acrosome is derived from Golgi Apparatus
• Mitochondrial sheath is in Middle piece
• Nucleus is in head

Arrangement of Cells in Seminiferous Tubules

Basal layer → lumen:

• Cells mature from basal layer (stem cells) to lumen (mature sperm).
• Myoepithelial cells
• Spermatogonia
• Primary spermatocytes
• Spermatids
• Spermatozoa

Ovum

• Secondary follicle = Antral Follicle

• Size of follicle just before ovulation : 18-20 mm.

• Size of ovum/female pronucleus : 120 microns.

• Fertilisable span of ova : 12-24 hours.

Gastrulation and Germ Layers

Overview

• Occurs in 3rd week of development in craniocaudal sequence

• Epiblast → forms 3 definitive germ layers:
• Ectoderm
• Mesoderm
• Endoderm

Primitive Streak

• Tail end of embryo

• Formed by proliferation of epiblasts

• First sign of gastrulation.

• Forms by day 14 at caudal end.

• Germ layers form in cephalocaudal direction.

• Mnemonic: Primitive people have tail

Prechordal plate

• Future head end

• Columnar hypoblast cells

• Forms Oro/buccopharyngeal membrane → ruptures → form oral cavity

• Mnemonic: Prechordal → Head

Epiblast migration steps:

• Cells invaginate into primitive pit.

• First cells replace all hypoblast → form endoderm

• Next layer between epiblast and endoderm → mesoderm

• Remaining epiblast → ectoderm.

Formation of Neural Tube

• Formed by stimulation of ectoderm by notochord.

• Begins in 3rd week from the cervical region

• Process: Ectoderm → Neural plate → Neural groove → Neural tube.
• The neural plate invaginates → neural fold
• The neural folds → multiply and differentiate → neural crest cells.
• separates from the ectoderm → neural tube.
• Neural crest cells (NCC) → junction of neural tube & ectoderm.
• Neural tube → brain and spinal cord

Remnants of Notochord

1. Apical ligament of DENS (part of intervertebral disc).

2. Nucleus pulposus (central part of intervertebral disc).

3. Tectorial membrane (continuation of posterior longitudinal ligament).

”Mneumonic:

• Notorius (notocord) DON (DENS) ,

• Tutoryil (tectorial) keriyapo idich Pulp (pulposes) aaki)“

Persistence of Notochord:

• Chordoma (Congenital tumour)

Derivatives of Neural Tube

• Two pores are present:
1. Anterior neuropore
• Also called cranial neuropore or rostral neuropore.
• Closes on 25th day.
• (anterior - adyam, posterior - pinne)
2. Posterior neuropore
• Also called caudal neuropore.
• Closes on 27th or 28th day.

Derivatives of Neural Tube (Neuroectoderm):

• Forms CNS Structures
• Brain & spinal cord.
• Oligodendrocytes (myelination).
• Astrocytes (blood-brain barrier).
• Ependymal cells (lining of ventricles).

• Eye
• Retina
• Retinal pigment epithelium
• Optic nerve
• Pigmented epithelium of iris
• Dilator & sphincter pupillae

• Others
• Posterior pituitary
• Pineal gland

• Anencephaly
• Failure of closure of Anterior/cranial/rostral neuropore.

• Spina bifida.
• Failure of closure of Posterior/caudal neuropore.

• Cranial Rachischisis
• Cause: Anencephaly + failure of the spinous process to fuse.

Neural Crest Cells (NCC)

• Present at the junction of neural tube & ectoderm.

Structures formed by NCC:

1. PNS structures:
• ”Mneumonic: Ente (Enteric Plexus) NCC Gang Shawarma (Schwann cells) Medikkan (adrenal medulla, melanoblast) Paravur Odi (Odontoblasts) - kamannu veenu mugam idichu - Mugathe bone (skull), cartilage (pharyngeal cartilage) and skin (dermis of head and neck) poi. pinne Cone (conotruncal) ice-cream vangi kazhichu“
• All ganglion.
• Enteric plexus.
• Schwann cells.
• Adrenal medulla.
• Melanoblast/cyte.
• Parafollicular-C cells.

2. Derivatives of head & neck:
• Conotruncal septum (in heart) → spiral and bulbar septum
• Skull bones (most).
• Odontoblasts (Dentine)
• Pharyngeal arch cartilage.
• Dermis of head & neck.
• Ciliary muscle

Note

• Adrenal cortex:
• Derivative of intermediate mesoderm.

• Intervertebral disc has 2 parts:
• Outer: Annulus fibrosus.
• Central: Nucleus pulposus.

Surface ectoderm derivatives

• Epidermis (skin)

• Hair

• Nails

• Sweat glands

• Sebaceous glands

• Mammary glands

• Enamel of teeth

• Lens of eye

• Corneal epithelium

• Conjunctiva

• Lacrimal gland

• Salivary glands
• Parotid
• Submandibular
• Sublingual

• Anterior pituitary (from Rathke’s pouch)

• Sensory epithelium of inner ear & nose

NOTE:

Mesoderm

• Soman chutiipatii nadakkum (Somites → Paraxial)
• Muscle (myotome) ondu, scooter (sclerotome) ond, back nnu kuthum (dermatome of back)
• Rest by LPM

• Exceptions in smooth m/s -

M/s	Derived from
Pupillary m/s • Sphincter pupillae • Dilator pupillae	Neuroectoderm (Neural tube)
Ciliaris m/s	NCC
Smooth m/s of Ascending Aorta, Pulmonary Trunk, Coronary & Outflowing Tract	NCC → Conotruncal septum (TOF?)
Smooth m/s of Sweat Gland, Mammary gland & Myoepithelial cells	Surface Ectoderm

• Mesoderm is the middle germ layer in the embryo.

• Intraembryonic Mesoderm Is divided into three main parts:
1. Lateral plate mesoderm (LPM)
2. Intermediate mesoderm
3. Para-axial mesoderm

Lateral Plate Mesoderm (LPM)

• LPM splits due to intra-embryonic coelom.

• Intra-embryonic coelom forms
• Pleural cavity
• Pericardial cavity
• Peritoneal cavity

• LPM splits into two layers:
• Somatopleuric LPM:
• All parietal layers are derived
• Forms dermis of front & limb.
• Forms appendicular skeleton.
• Splanchnopleuric LPM/cardiogenic layer:
• All visceral layers are derived
• Forms cardiac muscles (cranial end)
• Forms smooth muscles
• Forms Heart tube

SEPTUM TRANSVERSUM

• Undifferentiated LPM at the cranial end of the embryo

• Most cranial structure in the embryonic plate before Folding

• DERIVATIVES
1. Central Tendon of diaphragm
2. Fibrous Pericardium
3. Stroma of Liver including Kupffer cells
4. Ventral Mesogastrium — Lesser omentum.

Intermediate Mesoderm

• Intermediate mesoderm is all about the urogenital system

Para-Axial Mesoderm

• Forms Somites:
• Spherical ball like structures
• Develop into sclerotome, dermatome and myotome.
• Somite Period: = D20 - D30
• Total Somites = 42 - 44 Pairs
• Definitive Somites = 36-38 pairs

• 20 → 30 → 40
• 20 - 30 days
• 40 +/- 4 = 36/44

 

• Sclerotome:
• Forms skeletal muscles of axial skeleton

• Dermatome:
• Forms dermis of back.

• Myotome
• Epiaxial myotome
• Erector spinae
• Multifidus
• Hypoaxial myotome
• Rest of skeletal muscles
• intercoastal muscles
• abdominal muscles
• Cervical myotome
• Muscular part of diaphragm
  (bodywall of mesoderm) (phrenic N: C3, C4, C5)
• Occipital somite
• All muscles of tongue
• except Palatoglossus