Embryonic Development Week 4

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Questions and Answers

What is the significance of somite formation in embryonic development?

It indicates the development of the extremities.

It is associated with the formation of major organ systems.

It contributes to the formation of the neural tube. (correct)

It marks the beginning of the body's curvature.

Which structures are primarily formed from the pharyngeal arches during the fourth week of development?

Limbs and digits.

Liver and stomach.

Heart ventricle and pulmonary artery.

Mandible and maxilla. (correct)

What developmental change is observed in the embryo due to heart formation?

It acquires additional somites.

The body becomes straight and elongated.

The head becomes larger than the trunk.

Large bulges appear on the ventral body wall. (correct)

During which week of pregnancy is the embryo's structure characterized by visible somites and curvature?

Fourth week. Signup and view all the answers

What type of interaction involves one tissue influencing the development of another during embryogenesis?

Induction. Signup and view all the answers

What primary developmental process is responsible for establishing the shape and size of organs and body parts during embryogenesis?

Morphogenesis Signup and view all the answers

During which specific embryonic folding does the brain’s primordium begin to form?

Head Fold Signup and view all the answers

What is the role of programmed cell death (apoptosis) in developmental control mechanisms?

To ensure the proper shaping of tissues and organs. Signup and view all the answers

What key feature is visible in the embryo by day 24 that contributes to the facial structure?

Pharyngeal arch formation. Signup and view all the answers

What is the term for the agents that can cause major birth defects when an embryo is exposed during the critical period of organogenesis?

Teratogens Signup and view all the answers

What structure is formed from the incorporation of the endoderm from the umbilical vesicle during the head fold?

Foregut Signup and view all the answers

What is the primary reason for the C-shaped appearance of the embryo by mid-fourth week?

Head and tail folds. Signup and view all the answers

Which of the following best describes the interaction of tissues during differentiation in the embryonic stage?

Differentiation organizes cells into tissues capable of specialized functions. Signup and view all the answers

What results from the growth of the distal neural tube in embryonic development?

Establishment of the spinal cord Signup and view all the answers

What critical change occurs in the embryo due to lateral folding?

Transformation of the flat trilaminar disc into a cylindrical shape Signup and view all the answers

Which embryological stages are involved in the development of limb buds?

Morphogenesis and Differentiation Signup and view all the answers

What role do somites play in embryonic development?

They contribute to the vertebral column and skeletal muscles. Signup and view all the answers

Which of the following structures is derived from the pharyngeal arches?

Muscles and connective tissues of the pharyngeal arch origin Signup and view all the answers

What tissue types are primarily derived from the mesoderm?

Connective tissue, cartilage, and bone Signup and view all the answers

During which stage does limb bud formation primarily occur?

Organogenesis Signup and view all the answers

What is the main function of the omphaloenteric duct?

Connects the midgut to the yolk sac Signup and view all the answers

Which germ layer is responsible for forming the central nervous system?

Ectoderm Signup and view all the answers

How does the cloaca relate to the development of the urinary bladder?

It becomes the single opening for the excretory and digestive tracts. Signup and view all the answers

Which structures arise from the endoderm during embryonic development?

Urinary bladder lining Signup and view all the answers

Study Notes

Developmental Control Mechanisms

Development is controlled by tissue interactions, regulated cell migration, controlled cell proliferation, and programmed cell death (apoptosis).
Growth occurs through cell division and the production of extracellular matrices.
Morphogenesis (shaping of organs and tissues) and differentiation (specialization of cells) shape complexity.
Inductive processes often occur in sequence, ensuring that complex structures develop orderly.
Reciprocal interactions occur where each tissue influences the other's development.

Fourth Week of Embryonic Development

Embryo is 4 mm long, with foundations of most major organ systems developing, except for limbs and urogenital system.
Embryo is C-shaped with somites (blocks of mesoderm) along the sides of the neural tube.
Head is predominantly featureless except for rudimentary eyes, ears, and partially broken down oropharyngeal membrane.

Somite Formation

Embryo initially almost straight.
4 to 12 somites are visible as surface elevations.
Somites associated with the development of the neural tube.
Neural tube remains open at the cranial and caudal ends (neuropores).

Pharyngeal Arches

By day 24, the first pharyngeal arch (mandibular arch) forms and contributes to the formation of the lower jaw (mandible) and upper jaw (maxilla).
By day 26, three pairs of pharyngeal arches are visible, the cranial neuropore closes at this time.

Embryo Curvature

Embryo begins to curve due to head and tail folds.
C-shaped appearance by mid-week.

Fourth to Eighth Week Development of Human Embryo - Organogenesis

All major external and internal structures are established.
By the end, major organ systems are developing.
Changes in the shape of the embryo occur as tissues and organs form.
Embryo has a distinctly human appearance by the end of this period.
Exposure of embryos to teratogens during this period may cause major birth defects.

Phases of Development

Growth: Cell division and production of cellular components.
Morphogenesis: Shaping of organs and body parts.
Differentiation: Cells are organized into tissues and organs, performing specialized functions.

Folding of Embryo

Transformation from flat trilaminar embryonic disc into a cylindrical shape.
Folding occurs in the median and horizontal planes due to rapid growth.
Sides grow slower than the long axis.
Simultaneous cranial, caudal, and lateral folding occur.
This process transforms the embryonic disc into a cylindrical embryo and establishes the basic body structure.

Head Fold

Occurs at the beginning of the fourth week.
Forms the brain's primordium.
Forebrain overgrows cranially, moving over the developing heart.
Incorporation of part of the umbilical vesicle’s endoderm into the embryo as the foregut, between the forebrain and heart.
Relocates the septum transversum, contributing to the formation of the central tendon of the diaphragm.

Tail Fold

Result of distal neural tube growth, forming the spinal cord.
Tail region projects over the cloacal membrane.
Part of the endoderm is incorporated as the hindgut, which leads to development of the urinary bladder and rectum.
Primitive streak moves caudally.
Connecting stalk (future umbilical cord) attaches to the ventral surface.

Lateral Folding

Rapid growth of the spinal cord and somites, forming right and left lateral folds.
Formation of the ventrolateral abdominal wall.
Incorporation of part of the endoderm as the midgut, the precursor to the small intestine.
The connection between the midgut and umbilical vesicle narrows, creating the omphaloenteric duct.
Amniotic cavity expands, forming the umbilical cord with the amnion becoming the epithelial covering.

Germ Layer Derivatives

Three germ layers (ectoderm, mesoderm, and endoderm) form the primordia of all tissues and organs.

Ectoderm

Central nervous system.
Peripheral nervous system.
Sensory epithelia of the eyes, ears, and nose.
Epidermis and appendages (hair and nails).
Mammary glands.
Pituitary gland.
Subcutaneous glands.
Enamel of the teeth.

Neural Crest Cells

Pigment cells of the dermis.
Muscles, connective tissues, and bones of pharyngeal arch origin.
Suprarenal medulla.
Meninges (coverings) of the brain and spinal cord.

Mesoderm

Connective tissue, cartilage, bone, striated and smooth muscles, heart, blood, and lymphatic vessels.
Kidneys.
Ovaries.
Testes.
Genital ducts
Serous membranes lining the body cavities.
Spleen.
Cortex of the suprarenal glands.

Endoderm

Epithelial lining of the digestive and respiratory tracts.
Parenchyma of the tonsils.
Thyroid and parathyroid glands.
Thymus, liver, and pancreas.
Epithelial lining of the urinary bladder and most of the urethra.
Epithelial lining of the tympanic cavity, tympanic antrum, and pharyngotympanic tube.