Developmental Biology: Organogenesis & Gastrulation

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Questions and Answers

What are the primary structures derived from the epiblast layer during organogenesis?

Muscle and blood vessels

Liver and pancreas

Nervous system and skin (correct)

Bone and cartilage

Which statement accurately describes the function of the primitive streak during gastrulation?

It is the site where mesodermal cells invade the underlying tissue. (correct)

It facilitates the separation of the epiblast and hypoblast layers.

It creates the ventral body cavity.

It directs the development of the amniotic cavity.

Which of the following is NOT a process involved in the formation of the bilaminar disc?

Development of the syncytiotrophoblast layer

Differentiation of the inner cell mass

Formation of the hypoblast

Invasion of trophoblast into uterine tissue (correct)

What is the key role of the notochord during early development?

It induces the overlying ectoderm to form the neural tube. Signup and view all the answers

What does lateral body folding primarily influence during organogenesis?

The development of the gut and body cavities. Signup and view all the answers

During which phase of development does the zona pellucida disappear?

Blastocyst stage Signup and view all the answers

Which structure is primarily formed from the endoderm during development?

Lungs and digestive tract Signup and view all the answers

What cellular structure actively contributes to the formation of the placenta?

Cytotrophoblast Signup and view all the answers

What is the significance of the primitive node during gastrulation?

It forms the notochord and establishes body axis. Signup and view all the answers

Which of the following processes does NOT occur during gastrulation?

Formation of the neural tube. Signup and view all the answers

What are the three primary germ layers formed during gastrulation?

Ectoderm, mesoderm, and endoderm. Signup and view all the answers

Which statement accurately describes the notochord?

It serves as a primary inductor for surrounding tissues. Signup and view all the answers

What role does the intraembryonic mesoderm play during early embryonic development?

It is involved in establishing the craniocaudal axis. Signup and view all the answers

During which weeks is the transition from a bilaminar disc to a trilaminar disc observed?

Week 2 to Week 3. Signup and view all the answers

Which structures are formed as a result of cell migration and proliferation during gastrulation?

Ectoderm, mesoderm, and endoderm. Signup and view all the answers

What happens to the notochord after embryonic development?

It forms the basis for developing the vertebral column. Signup and view all the answers

What does the neural tube develop into?

Brain and spinal cord Signup and view all the answers

Which direction does the fusion of the neural folds proceed?

Cranial and caudal Signup and view all the answers

When does the cranial opening close?

25th day Signup and view all the answers

What part of the mesoderm is located between paraxial and lateral mesoderm?

Intermediate mesoderm Signup and view all the answers

Which mesoderm division covers the visceral organs?

Visceral mesoderm Signup and view all the answers

Which factor is NOT involved in the closure of the neuropores?

Chordin Signup and view all the answers

What is the primary function of the lateral mesoderm?

Covers visceral organs and body wall Signup and view all the answers

Which structure is formed from the paraxial mesoderm?

Somites Signup and view all the answers

What is the main function of the extraembryonic membranes?

Providing nutrients and removing waste from the embryo Signup and view all the answers

Which decidua lies deep to the embryo and contributes to the placenta?

Decidua basalis Signup and view all the answers

What do the chorion laeve and chorion frondosum primarily refer to?

Smooth and villous areas formed by chorionic villi Signup and view all the answers

Which hormone is secreted by syncytiotrophoblast during the decidual reaction?

Human chorionic gonadotrophin (HCG) Signup and view all the answers

What is the role of the lacunae formed by trophoblast during placentation?

Facilitating direct exchange of nutrients with maternal blood Signup and view all the answers

What characterizes secondary villi in the placental development process?

Presence of mesoderm invading the villus Signup and view all the answers

Which of the following extraembryonic membranes is NOT involved in the direct formation of the placenta?

Amnion Signup and view all the answers

What distinguishes intraembryonic mesoderm from extraembryonic mesoderm?

Extraembryonic mesoderm contributes to structures outside the embryo Signup and view all the answers

What structure is formed from the genital ridge during the 5-6 weeks of urogenital system development?

Gonads Signup and view all the answers

Which process begins in the upper limbs during the seventh week of development?

Ossification Signup and view all the answers

What happens to the intestines during the sixth week of embryonic development?

They herniate into the umbilical cord. Signup and view all the answers

During which week does sex differentiation into male and female gonads occur?

Sixth week Signup and view all the answers

What significant change occurs in the eyes during the sixth week of development?

Eyes become prominent due to retinal pigmentation. Signup and view all the answers

In the eighth week of development, which of the following changes occurs in the limbs?

Limb regions are fully separated. Signup and view all the answers

What is formed from the auricular hillocks during embryonic development?

Auricle (external ear) Signup and view all the answers

What structure does the lateral plate mesoderm give rise to in the limbs?

Bone, ligaments, tendons, and dermis Signup and view all the answers

Study Notes

Organogenesis: Formation of the Body

Organogenesis is the process of forming organs and body shape from germ layers. It occurs from the third to eighth week of development.
Organogenesis involves numerous processes, including cell-cell interaction, cell fate determination, cell proliferation and survival, cell and tissue shape and size, and arrangement of cells into tissues and functional organs.

Gastrulation: Formation of the Three Germ Layers

Primitive Streak: A groove forms on the epiblast, guiding epiblast cell migration towards the median plane of the embryonic disc. This creates the primitive streak, which forms the notochord and endoderm. The notochord is a rod-shaped structure that defines the axis of the embryo and is essential for development of the axial skeleton.
Notochord Development: Cells migrate from the primitive node (primitive streak's cranial end) to form the notochord. The notochord lies between the ectoderm and endoderm and induces the neural plate formation (primordium of central nervous system).
Three Germ Layers: Gastrulation results in three primary germ layers: ectoderm, mesoderm, and endoderm. These germ layers give rise to all tissues and organs of the body.

Neurulation

Neural Tube Formation: The notochord induces the overlying ectoderm to thicken, forming the neural plate. The neural plate folds, creating the neural groove. The neural folds fuse along the dorsal midline to form the neural tube. The neural tube differentiates into the brain and spinal cord.
Closure of the Neuropores: The cranial opening, the rostral neuropore, closes on the 25th day. The caudal opening, the caudal neuropore, closes two days later. Syndecan4 and Vangl2 are involved in the process.

Mesoderm Specialization

The mesoderm divides into three parts: paraxial, intermediate, and lateral mesoderm.
- Paraxial Mesoderm: Located next to the neural tube; condenses into somites, which form the vertebrae, ribs, and skeletal muscles of the trunk.
- Intermediate Mesoderm: Located between paraxial and lateral mesoderm; differentiates into the excretory units (kidneys) and gonads.
- Lateral Mesoderm: Located on the lateral side of the embryonic disc; separates into two layers:
  - Somatic Mesoderm/Parietal Mesoderm: Forms inner lining of the body walls (parietal pleura and peritoneum), dermis of skin, and connective tissue of limbs.
  - Splanchnic Mesoderm/Visceral Mesoderm: Covers the visceral organs (visceral pleura and peritoneum).

Lateral and Cephalocaudal Folding

Lateral Folding: The embryonic disc folds laterally, bringing the sides closer together to form the body cavity.
Cephalocaudal Folding: The embryonic disc folds cranially and caudally to form the head and tail regions of the body.

Development of Blood and Blood Vessels

Blood Islands: Blood cells and blood vessels originate from the lateral mesoderm.
Hematopoiesis: Formation of blood cells begins in the blood islands.
Vasculogenesis: Formation of blood vessels initiates in the blood islands.

Extraembryonic Membranes and Placenta

Extraembryonic Membranes: Amnion, yolk sac, allantois, and chorion.
- Amnion: Encloses the embryo in a fluid-filled cavity protecting the fetus from trauma.
- Yolk Sac: Provides nutrients to the embryo in early stages.
- Allantois: Involved in excretion and respiration.
- Chorion: Contributes to the placenta.
Placenta: Plays a vital role in nutrient exchange and waste removal between the mother and fetus.
Umbilical Cord: Connects the fetus to the placenta.

Implantation and Placentation

Decidual Reaction: The functional layer of the endometrium (decidua) undergoes decidualization, characterized by the enlargement and vacuolation of stromal cells as the syncytiotrophoblast secretes human chorionic gonadotrophin (HCG).
Decidua Basalis: Located deep to the embryo and contributes to the placenta.
Decidua Capsularis: Encapsulates the embryo and separates it from the uterine cavity.
Decidua Parietalis: The remaining portion of the decidual reaction.

Chorion Frondosum and Chorion Laeve

Chorionic Villi: Finger-like projections that develop around the chorionic sac.
Chorion Laeve: Smooth area formed by the degeneration of villi near the decidua capsularis.
Chorion Frondosum: Extensively growing villi near the decidua basalis, forming the placental component.

Villus Development

Primary Villi: Composed of cytotrophoblast surrounded by syncytiotrophoblast.
Secondary Villi: Extraembryonic mesoderm invades the villus, forming three layers: mesoderm, cytotrophoblast, and syncytiotrophoblast.

Development of the Urogenital System

Urogenital Ridge: The intermediate mesoderm forms the urogenital ridge, differentiating into the genital ridge (forms gonads) and the nephrogenic ridge/cord (forms kidneys).

Limb Development

Upper Limbs: Elbows and hand plates with digital rays form during the sixth week of development.
Lower Limbs: Development occurs slightly later, showing similar differentiation as the upper limbs.
Digit Separation: Notches between digital rays define fingers and toes in the seventh week.
Ossification: Bone formation begins in the upper limbs during the seventh week. The lateral plate mesoderm gives rise to the bones, ligaments, tendons, and dermis of the limbs.

Eighth Week of Development

Digit Separation: Digits become fully separated.
Limb Movements: Purposeful limb movements begin.
Neck Formation: The neck becomes more defined.
Eyelid Closure: Eyelids fuse.
External Genitalia: Slight sex differences become apparent.

Key Summary Points

Organogenesis involves a series of complex processes, including gastrulation, neurulation, and mesoderm specialization, leading to the formation of the body and its organs.
The three primary germ layers (ectoderm, mesoderm, and endoderm) give rise to all tissues and organs.
The notochord plays a critical role in development.
Lateral and cephalocaudal folding contribute to the formation of the body cavity and head and tail regions.
The placenta and the extraembryonic membranes are essential for embryo and fetal development.
Limb development proceeds rapidly, leading to the formation of fingers and toes. Ossification begins in the upper limbs.

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Description

This quiz explores the intricate processes of organogenesis and gastrulation in embryonic development. It covers key concepts such as the formation of germ layers, the primitive streak, and the development of the notochord. Ideal for students of developmental biology or embryology.