Week Three of Development

The development of the trilaminar disc is the highlight of the third week of development and this occurs through the process of gastrulation.

A developed trilaminar disc gives rise to three primary germ layers:

  1. Endoderm

  2. Mesoderm

  3. Ectoderm

The appearance of the primitive streak begins the process of gastrulation.

As a result of cell signaling, a thickening begins to form on the surface of the epiblast layer of cells and this is known as the primitive streak. Simultaneously, on the cranial end of the developing embryo, a primitive node begins to develop.

The primitive streak also begins to give us an idea of the body axes of the developing embryo.

Some of the cells in the center of the primitive streak begin to apoptose (programed cell death) and begin to form a cavity and deepening and in doing so, the primitive streak becomes a primitive groove.

At the same time, some of the cells in the center of the primitive node begin to apoptose and form a cavity, and deepening as well, and in doing so, the primitive node becomes a primitive pit.

As a result of further cell signaling, the cells of the epiblast layer are stimulated to separate from each other, move and make their way towards the primitive groove and begin to migrate through it. In doing so, these cells reach the previously established hypoblast layer of cells and begin to displace them. As a result, they produce a new layer: the endoderm layer of cells.

Once the endoderm layer of cells has been formed, more epiblast cells are stimulated to separate from each other and migrate through the primitive groove, this layer starts to form the mesoderm layer of cells. Cells that migrate from lateral regions of the primitive node and cranial regions of the primitive groove become the paraxial mesoderm, cells migrating through the middle of the primitive groove become the intermediate mesoderm, and cells migrating through the caudal most regions of the primitive groove become the lateral plate mesoderm.

Finally, the remaining epiblast cells differentiate into the ectoderm layer of cells.

This describes the process of gastrulation, where the bilamiar disc becomes the trilaminar disc during week three of development.

Ectoderm

The ectoderm layer of cells will eventually give rise to structures on the external surface of the embryo amongst other structures. The derivatives of the ectoderm layer of cells (including neuroectodermal cells) includes:

  1. Epidermis of skin

  2. Central nervous system

  3. Neural crest cells

  4. Epithelial tissue

  5. Lens placode

    • Accommodation of the lens of the eyes

  6. Otic placode

    • Inner ear structures (cochlea,vestibule, semicircular canals)

  7. Olfactory placode

    • Olfactory epithelium and receptor cells

  8. Surface ectoderm

    • Epithelial tissue

      • Epidermis of the skin

      • Oral cavity

      • Nasal cavity

      • Nails

      • Hair

      • Sweat gland

      • Inferior anus

      • Ear canal

  9. Inferior third of anal canal

  10. Rathke’s pouch

    • Anterior pituitary gland

Mesoderm

The mesoderm layer of cells can be further categorized as the paraxial mesoderm, intermediate mesoderm, and the lateral plate mesoderm which itself has a somatic and splanchnic layers. The entire mesoderm layer of cells contributes greatly to the developing embryo.

The derivates of the various parts of the mesoderm layer of cells includes:

  1. Paraxial mesoderm derivatives:

    • Somites

      • Dermomyotomes

        • Dermatome

          • Spinal meninges

          • Dermis of the skin

          • Subcutaneous tissue

        • Myotome

          • Skeletal muscles

      • Sclerotome

        • Vertebrae

        • Intervertebral discs

        • Ribs

  2. Intermediate mesoderm derivatives:

    • Renal system

      • Kidneys

      • Ureters

    • Reproductive system

      • Gonads (Testes and ovaries)

      • Male and female reproductive ductal systems

        • Epididymis

        • Ductus deferens

        • Uterine tubes

      • Uterus

  3. Lateral plate mesoderm derivatives:

    • Somatic layer

      • Parietal layer of body cavities

      • Sternum

      • Bones and cartilages of the upper and lower limbs

    • Splanchnic layer

      • Visceral layer of body cavities

      • Adrenal cortex

      • Spleen

      • Lymphatic system

      • Smooth muscles of the gastrointestinal system

      • Most of the cardiovascular system

Endoderm

The endoderm layer of cells contributes to structures within the developing embryo. The derivatives of the endoderm layer of cells includes:

  1. Gastrointestinal System (epithelial lining of gut tube):

    • Foregut

      • Pharynx

        • Pharyngeal apparatus (refer to head and neck embryology)

          • Middle ear

          • Tonsils (tubal, pharyngeal, lingual, palatine)

          • Superior and inferior parathyroid glands

          • Thyroid gland

      • Esophagus

      • Stomach

      • First two parts of the duodenum

      • Respiratory diverticulum

        • Respiratory system

      • Hepatic diverticulum

        • Liver

        • Gall bladder

      • Pancreatic diverticulum

        • Pancreas

    • Midgut

      • Last two parts of the duodenum

      • Jejunum

      • Ileum

      • Cecum

      • Ascending colon

      • Proximal two thirds of the transverse colon

    • Hindgut

      • Distal third of the transverse colon

      • Descending colon

      • Sigmoid colon

      • Rectum

      • Anal canal (superior two thirds)

  2. Respiratory system (outpouching from the foregut)

  3. Pancreas (outpouching from the foregut)

  4. Liver (outpouching from the foregut)

  5. Epithelia of urethra, prostate gland, and bladder