Bilaminar and Trilaminar Embryonic Development

Comprehensive practice flashcards covering bilaminar and trilaminar embryonic development, germ layer derivatives, trophoblast differentiation, and body axis specification.

What two layers constitute the bilaminar germ disc, and when does it form?

The bilaminar germ disc consists of the Epiblast (dorsal layer) and the Hypoblast (ventral layer), forming at the end of the 1st week (around 8 days post-fertilization) under the influence of FGFs.

What are the structural characteristics and functions of the epiblast?

The epiblast consists of tall columnar cells located dorsally adjacent to the amniotic cavity. It serves as the source of all 3 germ layers and pluripotent stem cells, and forms the primitive streak, primitive node, and migratory cells.

What developmental defects can result from errors in epiblast cell migration?

Errors in epiblast cell migration can cause midline defects, including caudal dysgenesis, spina bifida, and syringomyelia.

What cell type composes the hypoblast and what structures does it form?

The hypoblast is made of cuboidal cells (primitive endoderm) that form the Exocoelomic (Heuser's) Membrane and the lining of the yolk sac.

How does the Anterior Visceral Endoderm (AVE) influence axis formation?

AVE cells migrate to become the cranial end of the embryo, signaling the epiblast for cranial-caudal axis formation and secreting nodal antagonists Cerebrus and Lefty1.

What key events mark Day 9 of embryonic development?

The blastocyst is deeply embedded, enters the LACUNAR Stage, and forms the primitive yolk sac (exocoelomic cavity) from the hypoblast and Heuser's membrane.

What is the function of the closing plug (fibrin plug) formed on Day 10?

The closing plug (fibrin plug) seals the implantation site in the endometrium where the blastocyst embedded.

When and how are maternal sinusoids formed during early development?

Maternal sinusoids form 9-10 days post-fertilization when the lacunar network from the trophoblast connects with maternal capillaries.

What is the decidual reaction and when does it take place?

Occurring on Day 11/12 throughout the endometrium, endometrial cells become polyhedral, loaded with glycogen and lipids, and edematous due to hormonal influence.

What embryonic structures of a 12-day blastocyst are depicted in this diagram?

A 12-day human blastocyst displaying the trophoblastic lacunae, maternal sinusoids, extraembryonic cavity, exocoelomic membrane, and extraembryonic mesoderm layers.

Where do both the parietal (somatic) and visceral (splanchnic) extraembryonic mesoderm layers originate?

Both parietal and visceral extraembryonic mesoderm layers originate from the cytotrophoblast.

How does the secondary (definitive) yolk sac form on Day 13?

The secondary or definitive yolk sac forms as a new wave of hypoblast cells pushes out the primary yolk sac.

What derivatives develop from the trophoblast, extraembryonic mesoderm, and connecting stalk on Day 13?

The trophoblast develops into primary chorionic villi, the extraembryonic mesoderm becomes the chorionic plate, and the connecting stalk forms the umbilical cord.

Why is uterine bleeding after Day 13 considered clinically abnormal?

Implantation bleeding only occurs within the first 6 days post-implantation; bleeding at Day 13 indicates increased blood flow to the laminar space, signaling a potential threatened abortion.

What is gastrulation and when does it take place?

Gastrulation is the process during Week 3 (starting Day 15) that transforms the bilaminar germ disc into a trilaminar germ disc with ectoderm, mesoderm, and endoderm.

How do Fibroblast Growth Factors (FGFs) regulate cell invagination during primitive streak formation?

FGFs synthesized by the primitive streak control epiblast cell movement by downregulating E-cadherin protein and regulating Brachyury expression for mesodermal cell specification.

What is the primitive node (Hensen's node) and what is its role?

The primitive node is the expanded cephalic (cranial) end of the primitive streak that serves as the organizer for body axis development.

What clinical condition results from defective mesoderm formation in the caudal region of the embryo?

Sirenomelia (mermaid syndrome), a form of caudal dysgenesis characterized by failure of mesoderm formation in lower limbs and lower trunk structures.

How are the three definitive germ layers arranged from the epiblast cells?

Epiblast cells remaining on the surface form the ectoderm, cells invaginating and displacing the hypoblast form the endoderm, and cells migrating into the intermediate space form the mesoderm.

What tissues and organ systems derive from embryonic mesoderm?

Muscles, bones, connective tissue, cartilage, cardiovascular system, blood cells, and the urogenital system.

What structures derive from embryonic ectoderm?

Epidermis, hair, nails, sweat glands, central and peripheral nervous systems (neural tube and neural crest), and sensory organs.

What body systems derive from embryonic endoderm?

Epithelial linings of the GI tract and respiratory tract, as well as glandular tissue of the thyroid, liver, and pancreas.

What is the pre-chordal plate and why is it important in brain development?

The pre-chordal plate is an area of thickened hypoblast with mesoderm cells lying between the oropharyngeal membrane and notochord; it is essential for forebrain induction using Sonic Hedgehog as a signaling molecule.

What is the structural composition and future fate of the oropharyngeal membrane?

It consists of tightly adherent ectoderm and endoderm without intervening mesoderm at the cranial end, representing the future opening of the oral cavity.

What structural progression occurs in trophoblastic villi during the third week of development?

Primary villi (cytotrophoblast core covered by syncytiotrophoblast) become Secondary villi (mesoderm penetrates primary villi core) and then Tertiary villi (mesoderm differentiates into blood vessels and capillaries).

How does the definitive notochord form?

Prenotochordal cells migrate cranially into the hypoblast to form the notochordal plate; as hypoblast is replaced by endoderm, notochordal cells proliferate, detach from endoderm, and form a solid cell cord underlying the neural tube.

What is the cloacal membrane and what structure projects from it on Day 16?

Located at the caudal end, the cloacal membrane consists of ectoderm and endoderm without mesoderm; its appearance prompts the yolk sac to form a small diverticulum called the allantois (allantoenteric diverticulum).

What major cardiovascular milestone occurs on Day 19 post-fertilization?

The appearance of cardiogenic mesoderm, which represents the first sign of heart formation.

What structures develop from paraxial mesoderm on Days 20 and 21?

The first somites appear, which form the skeletal and muscular systems.

Which body axis is established first, and what structures mark its boundaries?

The cranio-caudal (head-tail) axis is established first; it is defined by the primitive streak at the caudal end and the primitive node marking the cranial end.

Which genes and signaling molecules establish left-sided identity during left-right body axis specification?

FGF8 expresses NODAL restricted to the left side; LEFTY2 and PITX2 (master gene for left-sidedness) regulate left identity alongside 5-HT neurotransmitter activity and MAD3 activation.

What role do Hedgehog and Lefty1 play during lateral axis determination?

They act as midline genes that prevent left-sided signaling molecules from crossing over to the right side.

Which gene expression is restricted to the right lateral mesoderm?

SNAIL is restricted to the right lateral mesoderm.

What type of germ-line derived tumor is shown below, and why can it contain differentiated tissues such as hair and teeth?

A sacrococcygeal / extragonadal teratoma; it arises from primordial germ cells (PGCs) that fail to reach the gonadal ridge or undergo mutation, retaining pluripotency to form derivatives of all 3 germ layers.