Embryology Chapter: Germ Layers Formation

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Questions and Answers

Which rhombomeres are responsible for the migration that leads to the formation of the jawbones and ear bones?

Rhombomeres 6

Rhombomeres 4

Rhombomeres 1 and 2 (correct)

Rhombomeres 3 and 5

What structure is formed as a result of rhombomere 4 migration?

Thyroid gland

Jawbones

Thymus

Hyoid cartilage (correct)

Which migratory agents play a role in establishing conditions for cranial neural crest cell migration?

TGF-beta proteins

Fibroblast growth factors

BMP 4 and BMP 7 (correct)

Wnt proteins

What is one of the consequences of the loss of cell adhesion molecules in migrating neural crest cells?

Promotion of amoeboid movement Signup and view all the answers

How is the path of neural crest cells controlled during their migration?

By the extracellular matrix Signup and view all the answers

What phase begins on the first day of menstruation?

Follicular Phase Signup and view all the answers

Which hormone is primarily responsible for stimulating the growth of follicles in the ovary?

Follicle-Stimulating Hormone (FSH) Signup and view all the answers

What event is triggered by a steep increase in Luteinizing Hormone (LH)?

Release of the mature oocyte Signup and view all the answers

What occurs to the follicles during the follicular phase?

Several follicles start to mature. Signup and view all the answers

What remains in the ovary after the collapse of theca interna and externa?

Corpus luteum Signup and view all the answers

What hormonal change occurs at the end of the follicular phase?

Decline in LH secretion Signup and view all the answers

How long does the follicular phase typically last?

14 days Signup and view all the answers

What primary role does estrogen play during the follicular phase?

Regulates feedback to the hypothalamus Signup and view all the answers

What is the primary role of rising estrogen levels during the menstrual phase?

Help thicken the uterine lining in preparation for a potential pregnancy Signup and view all the answers

Which phase involves the proliferation of endometrial cells and blood vessels?

Proliferative Phase Signup and view all the answers

What occurs to multiple follicles during the proliferative phase?

One or sometimes more follicles become dominant while others regress Signup and view all the answers

What is produced in high levels by the dominant follicle during the luteal phase?

Progesterone primarily with some estrogen Signup and view all the answers

What effect does the surge in Luteinizing Hormone (LH) have during the luteal phase?

It maintains the thickness of the endometrial lining Signup and view all the answers

What characterizes the secretory phase of the menstrual cycle?

Maintenance of the endometrium by high levels of progesterone and estrogen Signup and view all the answers

During which phase does the uterine lining reach its peak thickness?

Secretory Phase Signup and view all the answers

Which hormone is primarily responsible for keeping the uterine lining intact during the luteal phase?

Progesterone Signup and view all the answers

What hormone is primarily responsible for stabilizing the endometrial lining?

Progesterone Signup and view all the answers

What happens to the corpus luteum if fertilization does not occur?

It degenerates. Signup and view all the answers

During which phase of the ovarian cycle do estrogen levels rise and the uterine lining thickens?

Follicular Phase Signup and view all the answers

When is considered the safest period to avoid pregnancy?

During the proliferative days until the 11th day Signup and view all the answers

How long can ovulation occur relative to the 14th day of the cycle?

Later or earlier than the 14th day Signup and view all the answers

What role does the placenta play regarding hormone production if fertilization occurs?

It takes over hormone production to maintain pregnancy. Signup and view all the answers

Which hormone is produced by the corpus luteum besides progesterone?

Estrogen Signup and view all the answers

What marks the beginning of menstruation in the cycle?

Degeneration of the corpus luteum Signup and view all the answers

What is the role of the syncytiotrophoblast during implantation?

It secretes enzymes that break down the uterine wall. Signup and view all the answers

Which embryonic layer is primarily responsible for the formation of the nervous system?

Ectoderm Signup and view all the answers

Where does the notochord begin its formation during embryologic development?

At the primitive node Signup and view all the answers

What process leads to the formation of the embryonic germ layers during gastrulation?

Invagination Signup and view all the answers

Which component is derived from the mesodermal layer of the embryo?

Muscles Signup and view all the answers

What is the fate of the intermediate mesoderm during development?

It develops into the urogenital system. Signup and view all the answers

Which structures are formed by paraxial mesoderm?

Somites Signup and view all the answers

What role do WNT proteins play in somite differentiation?

They stimulate the expression of PAX1 gene. Signup and view all the answers

Which of the following statements about the yolk sac is accurate?

It is derived from components of the hypoblast. Signup and view all the answers

What is the function of cranially migrating prenotochordal cells?

To develop the notochord Signup and view all the answers

What is the primary outcome of epithelial-mesenchymal transition in NCCs?

Development of loose, migratory cells Signup and view all the answers

Which of the following best describes the formation of the definitive notochord?

It is formed by the mesoderm at lateral ends. Signup and view all the answers

What does the lateral plate mesoderm generate?

Visceral organs Signup and view all the answers

Study Notes

Establishing Germ Layers and Derivatives

During gastrulation, the epiblast delaminates into the embryonic epiblast and hypoblast.
The embryonic epiblast forms the amniotic ectoderm, while the hypoblast expands laterally and moves downward over the blastocoel.
The syncytiotrophoblast, a layer of trophoblast responsible for initial attachment to the uterine lining, loses its cellular membrane and forms lacunae.
Cells migrate via invagination through the primitive streak, displacing the hypoblast and forming the endoderm.
Ingressing cells into the blastocoel form the embryonic mesoderm.
The remaining cells at the top form the ectoderm.

Trilaminar Disk Formation

The inner cell mass (ICM) delaminates into the epiblast and hypoblast.
The epiblast delaminates further, forming the three embryonic germ layers — endoderm, mesoderm, and ectoderm.
The yolk sac originates from the hypoblast.
The primitive node, located at the cranial end of the primitive streak, is the site of invaginating cells.
Pre-notochordal cells migrate through the primitive streak cephalically, forming the notochordal plate.

Notochord Formation

Pre-notochordal cells intercalate into the endoderm, forming the notochordal plate.
The notochordal plate detaches from the endoderm to form the definitive notochord, located at the midline of the embryo.
Flanked on the sides by the intraembryonic mesoderm, the notochord forms cranially to caudally, starting at the cephalic region and extending towards the caudal region.

Formation of Three Mesodermal Sheets

The mesoderm expands to the sides, forming the lateral plate mesoderm and the intermediate mesoderm.
The paraxial mesoderm forms on either side of the axial mesoderm (aka epimere).
The lateral plate mesoderm splits into the parietal/somatic mesoderm (lining of the body cavities) and the visceral/splanchnic mesoderm.
The paraxial mesoderm forms the somites; intermediate mesoderm forms the urogenital system; lateral plate mesoderm forms the somatic mesoderm and splanchnic mesoderm.

Development of the Somite

Somites are derived from the paraxial mesoderm.
The cells undergo epithelialization, transforming into flat, tightly packed cells.
Somite cells lose their epithelial arrangement and migrate to form the sclerotome, myotome, and dermatome.
Sclerotome: forms the vertebral column (bones).
Myotome: forms the muscles.
Dermatome: forms the connective tissue.

Expression of Genes in Somite Differentiation

The notochord and floor plate of the neural tube express Shh and noggin, signaling the formation of the sclerotome.
The sclerotome expresses PAX1, a gene controlling chondrogenesis and vertebral formation.
WNT proteins secreted by the dorsal neural tube activate the PAX3 gene, which demarcates the dermomyotome.
Dermomyotome cells will differentiate into the dermis and muscles.
Muscle cell precursors in the dorsomedial portion of the somite express MYF5.
The dorsal neural tube expresses NT3, which signals the middorsal portion of the somite to form the dermis.
WNT and BMP4 activate MyoD expression, a muscle-specific gene.

Endoderm Formation

The endoderm forms structures such as the digestive and respiratory systems, pharynx, pharyngeal pouches, thyroid, and pharyngeal arches.
The pharynx gives rise to the pharyngeal pouches and folds.
The lining of the gut derives from the pharyngeal pouch.

Germ Layer Tracing

Endoderm: Forms the digestive and respiratory systems, pharynx, and thyroid.
Ectoderm: Forms the nervous system, skin, sense organs, and neural crest cells.
Mesoderm: Forms the circulatory system, skeletal and muscular systems, urogenital system, and components of the integumentary system.

Cephalocaudal Folding

Cephalocaudal folding affects the positions of the heart, septum transversum, yolk sac, and amnion.
The opening of the gut tube into the yolk sac narrows, forming the vitelline duct.
The vitelline duct houses vitelline blood vessels.

Lateral Plate Mesoderm

The lateral plate mesoderm forms two layers — somatic/parietal mesoderm and splanchnic/visceral mesoderm.
The splanchnic mesoderm forms linings around body cavities and gives rise to visceral organs.
The splitting of the lateral plate mesoderm forms the intraembryonic cavity.

Endodermal Structures

The digestive gut runs from the stomodeum to the proctodeum or cloaca.
Outpocketings of the digestive gut form the liver, gallbladder, and pancreas.
The vitelline duct connects the yolk sac to the midgut.
The allantois forms as an outpocketing of the endoderm.

Neural Crest Cells

Neural crest cells originate from the neuroectoderm during neurulation.
These cells migrate away from the neural tube before it fully closes.
They are pluripotent and differentiate into various cell types.
Neural crest cells undergo epithelial-mesenchymal transition.

NCC Migratory Pathways and Differentiation

Neural crest cells migrate to different targets via various pathways.
Pathway 1: NCCs travel ventrally through the anterior sclerotome, dermomyotome, and myotome, forming cartilages and bones of the vertebral column.
Pathway 2: NCCs take a dorsolateral route, forming pigment cells.

Four Overlapping Domains of NCC Differentiation

Cranial: NCCs differentiate into pigment cells, sensory ganglia, and parasympathetic ganglia.

NCC Derivatives

NCCs form the facial bones, cartilages, connective tissues, pigment cells of the skin, adrenal medulla, and sensory ganglia.
They also form the sympathetic and parasympathetic ganglia.

Cranial Neural Crest Cells (NCC) From Rhombomere Regions

NCCs, also known as the fourth germ layer, migrate from different rhombomere regions, forming distinct structures.
Rhombomeres 1 & 2 migrate to the first pharyngeal arch, contributing to the development of jawbones, ear bones (malleus and incus), and the frontonasal process.
Rhombomere 4 migrates to the second pharyngeal arch, forming hyoid cartilage.
Rhombomeres 6 contribute to the third and fourth pharyngeal arches and pouches, leading to the formation of the thymus, parathyroid glands, and thyroid.
Rhombomeres 3 and 5 do not migrate through the surrounding mesoderm, remaining on either side of the rhombomere mesoderm.

NCC Migration Mechanisms

BMP 4 & 7, proteins produced by the RhoB and Slug genes, induce migration of NCCs.
- These proteins establish cytoskeletal changes that promote cell movement.
- They also activate factors that loosen the tight junctions between cells, allowing for migration.
Loss of cell adhesion molecules enables NCCs to move freely through amoeboid movement.

NCC Migration Route Guidance

The extracellular matrix (ECM) provides a pathway for NCC migration.
Various hormones regulate the migration of NCCs.

Ovarian Cycle

The ovarian cycle is a 28-day (approximately) process involving changes in the ovaries to prepare for potential fertilization and pregnancy.
Follicular Phase (Days 1-14):
- Begins on the first day of menstruation.
- Follicles in the ovaries mature, with one becoming dominant.
  - The dominant follicle produces increasing levels of estrogen.
- Rising estrogen thickens the uterine lining (endometrium).
Ovulation (Day 14):
- A surge in Luteinizing Hormone (LH) triggers the release of the mature egg from the ovary.
Luteal Phase (Days 15-28):
- The ruptured follicle transforms into the corpus luteum, which produces progesterone and estrogen to maintain the thickened endometrial lining.
- If fertilization occurs, the corpus luteum continues producing progesterone.
- If fertilization does not occur, the corpus luteum degenerates, leading to a drop in progesterone and estrogen, causing the endometrial lining to shed (menstruation).

Uterine Cycle

The uterine cycle is synchronized with the ovarian cycle.
Menstrual Phase (Days 1-5):
- The shedding of the uterine lining (endometrium).
Proliferative Phase (Days 6-14):
- Endometrial cells, blood vessels, and glands proliferate, thickening the uterine lining under the influence of rising estrogen levels.
Secretory Phase (Days 15-28):
- The thickened endometrial lining is maintained by progesterone and estrogen produced by the corpus luteum, creating a suitable environment for implantation if fertilization occurs.

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Description

This quiz covers the key concepts of germ layer formation during embryonic development, focusing on the processes of gastrulation and the trilaminar disk formation. Students will explore the roles of the epiblast and hypoblast, as well as the formation of the ectoderm, mesoderm, and endoderm. Test your understanding of these fundamental embryological concepts.