development of head and neck

Question 1

Describe how the embryo goes from egg to implantation

Answer 1

• Ovulation occurs when a maturing egg ( ovum ) is released from an ovarian follicle at the
  surface of the ovary.
  
  • Fimbriae of the uterine tube cover the ovary and guide the ovum into the uterine tube.
  • Fertilization, occurs in the distal third of the uterine tube (Ampulla)
  • A zygote forms when the sperm and egg nuclei unite.
  • Cell division results in the uterine tube’s two, four, and eight-cell stages.
  • By 3 to 4 days, a tight ball of cells (blastomeres) termed the morula is ready to enter the uterine cavity.
  • Near the end of the first week, the morula becomes the fluid-filled blastocyst with an inner cell mass (embryoblast) and outer trophoblast.
    The blastocyst adheres to the uterine mucosa (usually in the fundus on the posterior wall)
    and sinks within it during implantation.

Question 2

describe the Formation of the bilaminar germ disc

Answer 2

• The inner cell mass divides into the epiblast and the hypoblast
  
  • Epiblast: is columnar cells which are closer to the uterine lining/ventral. Forms the definitive yolk sac
  • Hypoblast: cuboidal cells further away from the uterine lining/dorsal aspect. Forms the amniotic cavity
  • A few of the hypoblast cells become elongated to create the prechordal plate-(pre-tail end)=at head end- so prechordal plate forms at the cranial end of the embryo
  • Chordal=tail
  • Cranial=head
  • Body stalk will be thee future umbilical chord
  • Epiblast cell soon proliferated nearer the chordal end of the embryo at the primitive streak
  • At the most cranial end of the primitive streak there is the primitive node
  • Inside the primitive node there is a dip which is the primitive pit

describe the Formation of the trilaminar germ disc

Question 3

describe the Formation of the trilaminar germ disc

Answer 3

• Differentiated epiblast cell from the primitive streak will migrate down and displace the hypoblast cell
  
  • The differentiated cell are called mesoderm
  • The remaining epiblast cells= ectoderm
  • The reaming hypoblast cells= endoderm
  • This is called gastrulation (3rd week) to form tri germ layer
  • Layer on top=ectoderm, middle layer=mesoderm and bottom layer= endoderm
  • ECTODERM: nervous system; sensory epithelia of the eye, ear, nose; epidermis of skin and its appendages (hair and nails); mammary glands; pituitary gland, enamel(ameloblasts)
  • MESODERM: connective tissue (cartilage, bone, blood); striated and smooth muscles; cardiovascular system; genitourinary system; spleen; serous membranes lining the body cavities (pericardial, pleural, peritoneal)
  • ENDODERM: epithelial lining of the GIT, respiratory tract, bladder and urethra; thyroid and parathyroid glands, liver, pancreas

Question 4

describe the intraembryonic mesoderm development

Answer 4

• Mesoderm cell differentiate and migrate from the primitive streak and node
  
  • The mesodermal cells migrate caudally and cranially
  • Mesoderm will not be present in the oral cavity and opening of GIT (rectum) only ectoderm and endoderm
  • Mesoderm closet to the primitive streak is called paraxial mesoderm
  • In the middle there is the intermediated mesoderm
  • To lateral end of embryo there is the lateral plate mesoderm
  • Paraxial mesoderm will be split into 3 and forms the somatic (visceral-lining of the gut), extraembryonic and splenic (parietal -dermis of the body, connective tissues etc…)mesoderm
  • Can segment the parental mesoderm can be segmented to form somite (and head)
  • The somite divide into Sclerotome , Dermatome and Myotome
  • Sclerotome forms skeleton
  • Dermatome forms dermis
  • Myotome forms muscles
  • Only from occipital region to chordal region
  • Somatomeres=in the head
  • Lateral mesoderm form the body cavities

Question 5

What are the different intraembryonic mesoderm

Answer 5

• Paraxial mesoderm - Somites give rise to most of the axial skeleton, sclerotome - the vertebral column and part of the occipital bone of the skull; dermatomyotome - the voluntary musculature of the neck, body wall, and limbs; dermatomyotome - the dermis of the body
  
  • Intermediate mesoderm - produces the urinary system and parts of the genital system
  • Lateral plate mesoderm - splits into two layers: a ventral layer associated with the endoderm and a dorsal layer associated with the ectoderm
  • The layer adjacent to the endoderm gives rise to the mesothelial covering of the visceral organs (viscera), as well as to part of the wall of the viscera; hence, it is called the splanchnic mesoderm
  • The layer adjacent to the ectoderm gives rise to the inner lining of the body wall and parts of the limbs; hence, it is called the somatic mesoderm

Question 6

What is the primitive streak and primate node

Answer 6

• Primitive streak – a linear thickened band of ectodermal cells at the caudal end of the embryo in the midline. It is formed due to the proliferation and migration of epiblast cells on the superior/ dorsal surface of the embryonic disc in the midline.
  
  • Primitive Node -the cranial end of the primitive streak presents a rounded and elevated area (Henson’snode)
  • Primitive pit - a small pit in the middle of the node
  • Determines the future craniocaudal axis of the embryo
  • Demarcates the embryo into left and right halves

Answer 7

• Primitive streak – a linear thickened band of ectodermal cells at the caudal end of the embryo in the midline. It is formed due to the proliferation and migration of epiblast cells on the superior/ dorsal surface of the embryonic disc in the midline.
  
  • Primitive Node -the cranial end of the primitive streak presents a rounded and elevated area (Henson’snode)
  • Primitive pit - a small pit in the middle of the node
  • Determines the future craniocaudal axis of the embryo
  • Demarcates the embryo into left and right halve

Question 8

describe the Development of notochord

Answer 8

1. From the primitive node the epiblast cells migrate anteriorly to form the notochordal process
   
   2. Then the primitive Pit goes inside and forms the canal
   3. The cells proliferate to fill the canal. (no more lumen present). So creates the definitive notochord
      * Forms the axis of the embryo
      * Send signal for the development of musculoskeletal structures and nervous system
      * Gives rigidity to the embryo
      * contributes to the intervertebral discs.
      * developing notochord induces the overlying embryonic ectoderm to thicken and form the neural plate, the primordium of the central nervous system (CNS).
      * Cells migrate cranially from the primitive node to form a median cord - the
      * notochordal process
      * The notochordal process grows in a cranial direction in between the ectoderm and endoderm until it reaches the prechordal plate where the ectoderm and endoderm are fused.

Question 9

describe the Development of neural tube

Answer 9

• Occurs in the 4th week
  
  • Neurulation is the process that forms the neural plate, neural folds, and then closure of the folds to form the neural tube
  • Notochord induces the overlying ectoderm to form the neuroectoderm—which gives rise to the CNS, the brain, spinal cord and other structures, including the eyes
  • Development of brain and spinal chord so any drugs injected by mother will effect the embryo
    1. Ectoderm on either on the notochord thickens to form neural plates
    2. Neural plates fold to form neural groves
    3. Neural groves fuse to form the neural tube

Question 10

• Occurs in the 4th week
  
  • Neurulation is the process that forms the neural plate, neural folds, and then closure of the folds to form the neural tube
  • Notochord induces the overlying ectoderm to form the neuroectoderm—which gives rise to the CNS, the brain, spinal cord and other structures, including the eyes
  • Development of brain and spinal chord so any drugs injected by mother will effect the embryo
    1. Ectoderm on either on the notochord thickens to form neural plates
    2. Neural plates fold to form neural groves
    3. Neural groves fuse to form the neural tube

Answer 10

• Neural crest cells give rise to the spinal ganglia (dorsal root ganglia) and ganglia of the autonomic nervous system.
  
  • Ganglia of cranial nerves V, VII, IX, and X are also partly derived from neural crest cells.
  • Neural crest cells form the neurolemma sheaths of peripheral nerves and contribute to the formation of the leptomeninges, the arachnoid mater, and pia mater.
  • Neural crest cells also contribute to the formation of pigment cells, (melanocytes), the suprarenal medulla
  • Selected neuroectodermal cells on the crest of the merging neural folds undergo epithelial-to-mesenchymal transition and migrate away (can transform to bones and cartilages of the skull)
  • neural plate border, where the neural plate meets the non-neural ectoderm (layer on the outside)
  • Before the neural tube is fused the neural crest cells detach and are now pluripotent cells that can migrate and differentiate to form different structures