Intra-embryonic Mesoderm, Foldings of the Embryo, Derivatives of Germ Layers

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 INTRAEMBRYONIC  MESODERM  

  • Intra-embryonic mesodermal cells are distributed as a sheet on each side of the median plane.
  • A longitudinal groove subdivides the mesoderm into 3 parts from notochord to periphery of the germ disc-
    • Paraxial mesoderm
    • Intermediate mesoderm
    • Lateral plate mesoderm

PARAXIAL  MESODERM

  • It is derived from the epiblast cells migrating through rostral part of primitive streak.
  • It extends from the primitive streak to the prochordal plate.
  • It undergoes condensations called somitomeres.
  • Caudal to the otic vesicle somitomeres further undergo segmentations called somites or metameres.
  • Rostral to the otic vesicle somitomeres remain unsegmented.

Somites :-

Count of Somites –

  • The first pair of somites appear on day 20 in the occipital region.
  • Somites continue to appear from 20th to 30th day in cranio-caudal direction.
  • 42 - 44 pairs of somites are formed, which are grouped as –
    • Occipital-        4 pairs
    • Cervical-         8 pairs
    • Thoracic-       12 pairs
    • Lumbar-          5 pairs
    • Sacral-             5 pairs
    • Coccygeal-      8-10 pairs
  • Most of the coccygeal pairs disappear and the somites become reduced to 37 pairs.

Structure of Somites –

  • Somite is a triangular mass of mesenchyme.
  • Each somite is supplied by a single spinal nerve.
  • A small cavity myocele develops in each somite and divides it into-
    • Sclerotome & Dermo-myotome
  • Later, myocele obliterates and the somite differentiates into 3 parts-
    • Sclerotome- ventromedial part
    • Dermatome- dorsolateral part
    • Myotome- middle part

INTERMEDIATE  MESODERM

  • It connects the paraxial mesoderm with lateral plate mesoderm.
  • It projects into the dorsal wall of coelomic cavity on each side of dorsal mesentery of gut.
  • It shows segmentations in the cervical and upper thoracic regions.
  • Caudally it forms unsegmented column of cells known as nephrogenic cord.

LATERAL  PLATE  MESODERM

  • It is derived from the middle of primitive streak.
  • It is unsegmented.
  • Peripherally it is continuous with extra-embryonic mesoderm.
  • Cephalic to the buccopharyngeal membrane it is continuous with pericardial bar.
  • During the somite period a series of clefts appear within the lateral plate.
  • The clefts coalesce with one another to form intra-embryonic coelom.
  • The coelomic cavity divides the lateral plate into 2 layers-
    • Somatopleuric layer
    • Splanchnopleuric layer
  • Later, intra-embryonic coelom communicates with extra-embryonic coelom at the periphery of embryonic disc, except in the cephalic part, where it is continuous with pericardial bar.
  • In the cephalic part lateral plate mesoderm forms horse-shoe shaped area adjoining the pericardial sac known as septum transversum.



FOLDINGS  OF  THE  EMBRYO  


  • At the end of 3rd week embryonic area shows development of head fold, tail fold and two lateral folds.
  • Foldings convert the flattened germ disc into cylindrical embryo.
  • During the process of embryonic folds, the growth of yolk sac is reduced and amniotic cavity enlarges, so that the embryo floats in the amniotic fluid.
  • Foldings converge on the ventral surface of embryo and form primitive umbilical ring.
  • Amniotic membrane forms a tubular investment around the connecting stalk at the umbilical ring and converts it into umbilical cord.
  • The yolk sac cavity within the embryonic folds forms primitive gut.
  • The extra-embryonic part of yolk sac cavity forms umbilical vesicle, which is connected to the primitive gut by vitello-intestinal duct.

Head  Fold :-

  • Head fold is mainly caused by the rapid and longitudinal growth of central nervous system.

The germ disc bends ventrally towards the yolk sac around the cranial end of notochord

So that the fore brain vesicle occupies the cephalic end of embryo 


  • The part of yolk sac contained within the head fold is known as fore gut.
  • Dorsal wall of the fore gut is supported by notochord and hind brain vesicle of neural tube.
  • Ventral wall of the fore gut presents buccopharyngeal membrane, pericardial sac with primitive heart tube and septum transversum.
  • Buccopharyngeal membrane lies at the bottom of a depression on the surface ectoderm known as stomodeum.
  • Buccopharyngeal membrane ruptures during the 4th week, so that the fore gut communicates with stomodeum and amniotic fluid gains entrance into the gut.

Tail  Fold :-

  • Tail fold is mainly caused by the rapid and longitudinal growth of central nervous system.

The germ disc bends ventrally towards the yolk sac around the caudal end of notochord

So that the caudal end of neural tube surrounds the caudal end of embryo 

  • The part of yolk sac contained within the tail fold is known as hind gut.
  • Dorsal wall of the fore gut is supported by notochord, neural tube, primitive node and primitive streak.
  • Ventral wall of the hind gut presents connecting stalk with allanto-enteric diverticulum and cloacal membrane.
  • Cloacal membrane lies at the bottom of a depression on the surface ectoderm known as ectodermal cloaca.

Lateral  Folds :-

  • Lateral folds are mainly caused by the formation of rapidly growing somites.

The lateral margins of germ disc bend ventrally towards the yolk sac and form lateral folds

Somatopleuric and splanchnopleuric layers of lateral plate mesoderm in each lateral fold, fuse with the corresponding layers ventrally, except at the umbilical ring.

Now the continuous somatopleuric layer lines the parietal body wall

Splanchnopleuric layers invest primitive gut and reflect dorsally as dorsal mesentery, ventrally as ventral mesentery

Thus, the intra-embryonic coelomic cavity is well defined between the somatopleuric and splanchnopleuric layers 

  • The part of yolk sac contained within the lateral folds is known as mid gut.
  • Mid gut is connected to the umbilical vesicle by vitello-intestinal duct through primitive umbilical ring. 



DERIVATIVES  OF  GERM  LAYERS  


DERIVATIVES  OF  ECTODERM

  • Derivatives of Surface Ectoderm
  • Derivatives of Neural Tube
  • Derivatives of Neural Crest Cells

Derivatives  of  Surface  Ectoderm :-

  • Epidermis, hair and nail
  • Sebaceous glands and sweat glands
  • Olfactory pit
  • Optic vesicle and lens vesicle
  • Otic vesicle
  • Branchial clefts
  • Rathke’s pouch, Pituitary glands
  • Epithelial lining of cheek, gum, teeth enamel, floor of mouth, nasal cavity and paranasal air sinuses
  • Salivary glands
  • Mammary glands

Derivatives  of  Neural  Tube :-

  • Brain
  • Spinal cord
  • Neurohypophysis of pituitary gland
  • Motor neurons
  • Retina

Derivatives  of  Neural  Crest  Cells :-

Neuronal Cells –

  • Sensory ganglia of cranial nerves V, VII, IX, X
  • Spinal ganglia
  • Ganglion cells of the autonomic nervous system

Supportive Cells of the Nervous System –

  • Glial cells of the peripheral ganglia
  • Schwann cells of peripheral nerves
  • Meninges of the forebrain

Pigment Cells – (except for pigmented retina)

Endocrine and Para-endocrine Cells –

  • Adrenomedullary cells
  • Calcitonin-producing cells
  • Type I cells of the carotid body

Mesectodermal Derivatives –

  • Visceral and facial skeleton
  • Cranial vault
  • Walls of large arteries derived from the aortic arches
  • Connective tissue of thymus and parathyroid glands
  • Dermis of neck and facial regions

DERIVATIVES  OF  MESODERM

  • Derivatives of Paraxial Mesoderm / Somites
  • Derivatives of Intermediate Mesoderm
  • Derivatives of Lateral Plate Mesoderm

Derivatives  of  Paraxial  Mesoderm / Somites :-

  •  Pre-otic somitomeres form striated muscles of Head, extrinsic muscles of the eye, base of skull, calvaria.
  • Occipital somites form muscles of the tongue
  • Sclerotome forms ribs and vertebrae
  • Myotome forms skeletal muscles
  • Dermatome forms dermis of the skin

Derivatives  of  Intermediate  Mesoderm :-

  • Genital & Urinary systems

Derivatives  of  Lateral  Plate  Mesoderm :-

Somatopleuric Layer Derivatives –

  • Parietal layers of peritoneal, pericardial and pleural cavities.
  • Dermis of skin
  • Pectoral and pelvic girdles
  • Skeletal elements of limbs

Splanchnopleuric Layer Derivatives –

  • Visceral layer of pericardial, peritoneal and pleural cavities.
  • Musculature and connective tissue of gut, respiratory tract and heart.

DERIVATIVES  OF  ENDODERM

  • Derivatives of Foregut
  • Derivatives of Midgut
  • Derivatives of Hindgut

Derivatives  of  Foregut :-

  • Lining of epithelium of tongue, floor of mouth, pharynx, oesophagus, stomach, duodenum up to the ampulla of Vater.
  • Lining epithelium of respiratory system, auditory tube, tympanic cavity.
  • Parenchyma of tonsil, thyroid, parathyroid, thymus, liver and pancreas.

Derivatives  of  Midgut :-

  • Lining epithelium of distal part of duodenum, jejunum, ileum, caecum, appendix, ascending colon, right 2/3rd of transverse colon.

Derivatives  of  Hindgut :-

  • Lining epithelium of left 1/3rd of transverse colon, descending colon, sigmoid colon, rectum, anal canal up to mucocutaneous junction.
  • Lining epithelium of urinary bladder, urethra, vagina.
  • Parenchyma of prostate, bulbourethral gland.



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