Flat embryonic disc folds to become 3-d embryo w/ internal organ systems

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  • Introduction

    • Flat embryonic disc folds to become 3-D embryo w/ internal organ systems

    • All major organ systems begun

    • Period very sensitive to external perturbations (radiation, viruses, chemical teratogens); many congenital malformations originate during this period

    • Trilaminar Embryo Overview (longitudinal section):

      • Ectoderm/neuroectoderm dorsal to mesoderm

      • Mesoderm dorsal to endoderm

      • Oropharyngeal membrane is cranial; cloacal membrane is caudal

      • Central cardiogenic area located cranial to oropharyngeal membrane

      • Allantois forms a sm. YS diverticulum (protrusion/process) extending into connecting stalk

  • Folding -- like a mushroom cap folding over the stem (analogous to yolk sac/umbilical cord)

    • Amnion folds down over embryo & envelopes connecting stalk & yolk sac; head & tail curve ventrally into a C- shape

      • Fetus encased in amniotic cavity (amnion enlargement eventually obliterates chorionic cavity – see Placenta Lecture)

      • Results in umbilical cord ensheathed by amnion

    • Somatopleure (dorsal) folds around ventral portion of embryo & fuses to form body wall

    • Splanchnopleure (ventral) folds around ventral portion of embryo; fuses to close gut tube

    • Sagital Head Folding:

      • Neural Tube at cranial margin (forebrain) grows beyond oropharyngeal membrane; overhangs primitive heart

      • Heart, pericardial cavity, oropharyngeal membrane fold under & assume ventral position

      • Future mouth:

        • Depression of surface ectoderm forms stomodeum (primitive mouth); also endoderm, h/w endoderm is later obliterated

        • Embryonic endoderm & yolk sac form foregut which ends in oropharyngeal membrane (ruptures early in week 4)

      • Future diaphragm:

        • Septum transversum starts as a mass of mesoderm cranial to pericardial cavity

        • After folding it lies ventrally & caudally b/w heart and yolk sac

        • Eventually develops into major part of diaphragm

        • Most caudally displaced tissue (due to folding)

    • Sagital Tail Folding:

      • Cloacal membrane, connecting stalk, and allantois assume a ventral position as the posterior, caudal end of the embryo folds under

      • Embryonic endoderm & yolk sac form hindgut; hindgut ends in cloacal membrane (anus)

      • Connection of gut w/ yolk sac eventually reduced to yolk stalk becomes vitelline duct (connected at future “belly button”)

      • Allantois, connecting stalk, & yolk stalk (or vitelline duct) become closely associated, wrapped in amnion; eventually form umbilical cord

    • Transverse Folding:

      • Somatopleure (dorsal layer) folds toward midline to form ventral body wall & parietal layer; amniotic cavity now surrounds the embryo

      • Splanchnopleure (ventral layer) forms gut (part of yolk sac include into embryo at midgut) & visceral serous membranes; intra-embryonic coelomic cavity now surrounds the gut

      • Endocardial heart tubes (2) on opposite sides of embryo come together and fuse to form a single heart; due to folding, tubes are displaced medially, ventrally, & caudally

      • As umbilical cord forms (as result sagital folding), fusion of the lateral folds reduces communication b/w intraembryonic coelom (space b/w somatopleure & splanchnopleure) & chorionic cavity (surrounds amnionic cavity & yolk sac); this is because as a result of folding the amniotic cavity now lies between the intra-embryonic coelomic cavity (future pericardium/peritoneum) & the chorionic cavity (except at the umbilical cord)

  • Germ Layer Derivations (partial listing):

    • Ectodermal Derivatives

      • Surface Ectoderm

        • Epidermis (incl. hair & nails)

        • Mammary glands

        • Tooth enamel

        • Lens of eyes

        • Inner ear

        • Anterior pituitary gland (from oral ectoderm)

      • Neuroectoderm

        • Neural Tube – CNS

          • Brain & spinal cord

          • Retina

          • Posterior pituitary gland

        • Neural Crest – PNS

          • Pigment cells

          • Branchial arch cartilages

          • Head mesenchyme – facial morphology

          • Parafollicular cells of thyroid

          • Medulla of adrenal gland

    • Mesodermal Derivatives

      • Paraxial Mesoderm – Somites

        • Sclerotome – Bones & cartilage of skeleton, not skull

        • Myotome – Muscles of head , trunk & limbs

        • Dermatome – Dermis of skin

        • CT

      • Intermediate Mesoderm – Urogenital system

      • Lateral Mesoderm

        • CT & muscle of viscera

        • Serous membranes

        • Cardiovascular & lymphatic systems

        • Spleen

        • Blood & lymph cells

        • Adrenal cortex

    • Endodermal Derivatives

      • Epithelial lining of respiratory tract

      • Epithelial lining of gastrointestinal tract, liver & pancreas

      • Epithelium of urinary bladder & urachus (canal connecting the bladder of the fetus with the allantois; becomes part of umbilical cord)

      • Epithelial lining of tympanic cavity & eustacian tube

      • Parathyroid & thymus

      • Thyroid gland

  • Summary of Embryonic Period

Week 1

Week 2

Week 3

Week 4


Bilaminar Embryo




Amniotic Cavity


Neural Crest Migration


Primitive Yolk Sac (YS)


Somites Forming

2o YS


Chorionic Cavity


1o Villi

2o & 3o Villi

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