ANA 212: Embryological Development Week 2-3

Week 2-3 of Development: Bilaminar and Trilaminar Germ Layers

• During the second and third weeks of embryological development, crucial events occur, including gastrulation, which establishes the three germ layers (ectoderm, mesoderm, and endoderm) in the embryo.

Growth of the Embryonic Disc

• The embryonic disc is initially flat and round, but it becomes elongated with a broad cephalic (head) end and a narrow caudal (tail) end due to the migration of cells from the primitive streak.
• The growth and elongation of the cephalic part of the disc are caused by the continuous migration of cells from the primitive streak region in a cephalic direction.

Gastrulation

• Gastrulation is a complex process that reorganizes the embryo to generate the three embryonic germ layers (endoderm, mesoderm, and ectoderm).
• It begins with the formation of the primitive streak on the surface of the epiblast, and cells of the epiblast migrate inward (invaginate) to form the endoderm and mesoderm.
• The cells remaining in the epiblast then form the ectoderm.

Formation of Definitive Notochord

• The prechordal plate determines the cephalic end of the embryo.
• The notochordal plate forms from the thickening of endodermal cells, and cells of the notochordal plate proliferate and detach from the endoderm to form the definitive notochord.
• The notochord and prenotochordal cells extend cranially to the prechordal plate and caudally to the primitive pit.

Development of Nervous System (Neurulation)

• The development of the nervous system begins in the third week and continues until the fourth week.
• Signals from the notochord and prechordal plate induce ectoderm cells to thicken and form the neural plate.
• The neural plate cells make up the neuro-ectoderm, and their induction represents the initial event in the process of neurulation.
• The neural plate thickens and forms neural folds, which come close to each other and fuse dorsally to form a hollow tube known as the neural tube.

Fate Map Established During Gastrulation

• The regions of the epiblast that migrate and ingress through the primitive streak have been mapped and their ultimate fates determined.
• Cells that ingress through the cranial region of the node become notochord, while cells migrating at the lateral edges of the node and from the cranial end of the streak become paraxial mesoderm.

Further Development of the Trophoblast

• The trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer.
• Mesodermal cells penetrate the core of primary villi and grow toward the decidua, forming secondary villi.
• By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system.

Summary of the 3rd Week

• Gastrulation begins with the appearance of the primitive streak, which has at its cephalic end the primitive node.
• Epiblast cells move inward to form new cell layers, endoderm and mesoderm, and eventually, the three germ layers are established.
• By the end of the third week, three basic germ layers, consisting of ectoderm, mesoderm, and endoderm, are established in the head region, and the process continues to produce these germ layers for more caudal areas of the embryo until the end of the 4th week.

Embryonic Period

• The embryonic period (period of organogenesis) occurs from the 3rd to 8th weeks of development.
• It is the time when each of the three germ layers gives rise to a number of specific tissues and organ systems.
• By the end of the embryonic period, the main organ systems have been established.

Germ Layer Derivatives

• Ectodermal germ layer: gives rise to the organs and structures that maintain contact with the outside world (e.g., epidermis, nervous system, lens of the eye)
• Mesodermal germ layer: gives rise to the musculoskeletal system, circulatory system, and other tissues
• Endodermal germ layer: gives rise to the epithelial lining of the digestive and respiratory tracts, liver, and pancreas

The Ectodermal Germ Layer

• At the beginning of the third week, the ectodermal germ layer has the shape of a disc that is broader in the cephalic than the caudal region.
• The appearance of the notochord and pre-chordal mesoderm induces the overlying ectoderm to thicken and form the neural plate.

Neural Crest

• The neural crest gives rise to a variety of tissues and structures, including cranial nerve ganglia, C cells of the thyroid gland, and melanocytes.

Mesodermal Germ Layer

• The mesodermal germ layer gives rise to the paraxial, intermediate, and lateral plate mesoderm.
• Paraxial mesoderm forms somitomeres, which give rise to mesenchyme of the head and organize into somites in occipital and caudal segments.### Somites
• Give rise to skeletal muscles of trunk, limbs, head, and neck
• Give rise to extraocular muscles and intrinsic muscles of tongue
• Give rise to vertebrae and ribs, cranial bone, and dermis
• Give rise to dura mater

Intermediate Mesoderm

• Gives rise to kidneys, testes, and ovaries
• Gives rise to genital ducts and accessory sex glands

Lateral Mesoderm

• Gives rise to sternum, clavicle, scapula, pelvis, and bones of the limbs
• Gives rise to serous membranes of body cavities
• Gives rise to lamina propria, muscularis mucosae, submucosa, muscularis externae, and adventitia of the gastrointestinal tract
• Gives rise to blood cells, microglia, Kupffer cells
• Gives rise to cardiovascular system, lymphatic system, and spleen
• Gives rise to suprarenal cortex and laryngeal cartilages

Endodermal Germ Layer

• Provides epithelial lining of auditory tube and middle ear cavity
• Provides epithelial lining of posterior third of tongue, floor of mouth, palatoglossal and palatopharyngeal folds, soft palate, crypts of palatine tonsil, and sublingual and submandibular glands and ducts
• Provides principal and oxyphil cells of parathyroid glands
• Provides epithelial reticular cells and thymic corpuscles
• Provides thyroid follicular cells
• Provides epithelial lining and glands of trachea, bronchi, and lungs
• Provides epithelial lining of gastrointestinal tract
• Provides hepatocytes and epithelial lining of biliary tree
• Provides acinar cells, islet cells, and epithelial lining of pancreatic ducts
• Provides epithelial lining of urinary bladder and vagina
• Provides epithelial lining of female urethra and most of male urethra

Embryonic Development

• By the end of the fourth week, embryo has approximately 28 somites
• At the beginning of the fifth week, forelimbs and hindlimbs appear as paddle-shaped buds
• During the second month, external appearance of the embryo changes with an increase in head size and formation of limbs, face, ears, nose, and eyes

Applied Anatomy

• Most major organs and organ systems are formed during the third to eighth week
• This period is critical for normal development and is called the period of organogenesis
• Stem cell populations are establishing each of the organ primordia, and these interactions are sensitive to insult from genetic and environmental influences
• This period is when most gross structural birth defects are induced

This quiz covers the formation of bilaminar and trilaminar germ layers during week 2 and 3 of embryological development. It deals with the transformation of a single cell into a complex organism.