Embryology Flashcards: Formation of Basic Embryonic Structures

80 practice flashcards covering key concepts from the embryology lecture notes.

What are the two important cell types in the early embryo?

Somatic cells and primordial germ cells.

What is formed when a haploid ovum and a haploid sperm fuse?

A diploid zygote.

After fertilization, what are the two main cell populations that arise in the embryo?

Embryoblast (inner cell mass) and trophoblast.

What is the purpose of meiosis in germ cell formation?

To reduce chromosome number and promote genetic recombination.

In males, how many sperm are produced from one germ cell during meiosis?

Four spermatozoa.

During oogenesis, at which stage does the first meiotic arrest occur?

Diplotene stage of prophase I.

What happens to the polar bodies during oogenesis?

They are expelled to maximize cytoplasm for the egg.

What structures surround the oocyte in the ovary (cluster with zona pellucida)?

Zona pellucida and cumulus oöphorus.

What is the process of release of the mature ovum from the ovary called?

Ovulation.

Where does fertilization most commonly occur in the female reproductive tract?

In the dilated portion of the oviduct called the ampulla.

What term describes the early mitotic divisions of the zygote?

Cleavage; the embryo is initially indeterminate in humans.

What hormone is produced by the multinucleated syncytiotrophoblast and detected in pregnancy tests?

Human chorionic gonadotropin (hCG).

What is the corpus luteum after it is maintained by hCG during early pregnancy called?

Corpus luteum graviditatis.

What happens to the corpus luteum if there is no implantation or pregnancy?

It degenerates into the corpus albicans.

Why must the trophoblast communicate chemically with the mother after implantation?

To prevent rejection of the embryo as a foreign body.

What must the embryo do to avoid expulsion when first attaching to the endometrium?

Attach to the endometrium and form placental connections while signaling with hormones.

What begins two weeks after fertilization with brain and organ rudiments forming?

Early formation of brain, spinal cord, yolk sac, gut and muscle precursors.

What is the process by which the epiblast and hypoblast give rise to the three germ layers?

Gastrulation leading to trilaminar embryo.

What are the three germ layers of the trilaminar embryo?

Ectoderm, mesoderm, and endoderm.

What is the bilaminar embryo composed of?

Epiblast (superior) and hypoblast (inferior) with the amnion and yolk sac.

What envelops the bilaminar disc and becomes part of the placenta?

The chorion, along with the chorionic cavity.

What is the yolk sac’s role in placental mammals?

Initial nourishment until the placenta forms.

What is the outer cell layer of the blastocyst that invades the endometrium?

The trophoblast (including the syncytiotrophoblast in early invasion).

What is the inner cell mass of the blastocyst called?

Embryoblast.

What is a diverticulum in embryology?

A structure that forms as a new tube branching from an existing tube (inside-out formation).

Give two examples of diverticula in embryology.

Liver and pancreas from the gut; lungs from the pharyngeal pouch.

What is the primitive streak and primitive node?

Embryonic structures through which cells migrate to form the trilaminar germ layers.

What are the three germ layers derived from migrating epiblast/hypoblast cells?

Ectoderm, mesoderm, endoderm.

What becomes the central axis of the body and organizes the body plan?

The notochord.

What is neurulation?

Formation of the central nervous system from the ectoderm (neural tube) and neural crest.

What are neural crest cells known for?

They form dentine, enamel, facial bones, ganglia, meninges, melanocytes, adrenal medulla, and more.

How is the mesoderm subdivided during neurulation?

Paraxial, intermediate, and lateral plate mesoderm.

What do paraxial mesoderm structures form?

Somites (myotome, sclerotome, dermatome) that become muscles, vertebrae, and dermis.

What do somites become after segmentation?

Muscles (myotome), vertebrae and ribs (sclerotome), and dermis (dermatome).

What is somite re-segmentalization and why is it important?

Sclerotomes split and fuse so vertebrae align with muscle segments and nerves exit between vertebrae.

What is the role of the lateral plate mesoderm?

Forms the body cavities (celoms), visceral and parietal layers, and contributes to vasculature and organs.

What structures arise from the visceral (splanchnic) and parietal (somatic) layers of the lateral plate mesoderm?

Visceral peritoneum/pleura/epicardium and parietal peritoneum/pleura; organs within celoms.

What are the primary body cavities (celoms)?

Pleural (lung), pericardial (heart), and peritoneal (abdominal).

What happens to the yolk sac as the placenta forms?

It regresses as the placenta takes over nourishment and hormone production.

What are the three arms of the embryonic folding that convert a flat disc into a tubular body?

Lateral folding, anterior-posterior folding, and heart/head movement.

What is the name given to the early heart tube and how does the heart reposition itself?

The heart starts as a tubular structure and moves ventrally and caudally into the chest as development proceeds.

What is the difference between epaxial and hypaxial muscles?

Epaxial: intrinsic back muscles; Hypaxial: body wall and limb muscles.

What are dermatomes?

Segmentation maps of skin innervation by spinal nerves.

What are the five major components of the nervous system described in the notes?

CNS, PNS, Enteric nervous system, Autonomic nervous system, and cranial nerves.

Which cranial nerves are associated with head placodes (sensory special nerves)?

CN I (olfactory), CN II (optic), CN VIII (vestibulocochlear).

Which cranial nerves are purely motor for the head (from head somites/somitomeres)?

CN III (oculomotor), CN IV (trochlear), CN VI (abducens), CN XI (spinal accessory), CN XII (hypoglossal).

Which cranial nerves belong to the pharyngeal arches (branchiomeric nerves)?

CN V (trigeminal), CN VII (facial), CN IX (glossopharyngeal), CN X (vagus).

What does the trigeminal nerve (CN V) innervate in its divisions?

V1: forehead/eye; V2: upper face/teeth; V3: lower face/teeth; V2 is sensory for midface; V3 has motor for mastication.

Which cranial nerve innervates muscles for facial expression?

CN VII (facial nerve).

What nerves innervate the tongue muscles and sensation/taste distribution pattern?

CN XII for tongue muscles; CN V (anterior 2/3 sensation via trigeminal), CN IX (posterior 1/3 sensation), CN X (posterior region); taste via chorda tympani (CN VII) anterior 2/3 and CN IX posterior 1/3.

What is the vomeronasal organ and its human status?

A pheromone-detecting organ that is reduced in humans.

Which gland originates from Rathke’s pouch and forms the anterior pituitary?

The adenohypophysis (anterior pituitary).

Where does the thyroid originate and how does it descend?

From an endodermal mass at the foramen cecum; descends to the neck via the thyroglossal duct.

Which pharyngeal pouches form the parathyroid glands?

The third and fourth pharyngeal pouches (inferior and superior parathyroids respectively).

What gland arises from the third pharyngeal pouch and is involved in immune development?

The thymus.

From which placodes do the lens and olfactory structures arise?

Lens placode for the eye; olfactory placode for the nose.

What bones form the neurocranium and through which ossification processes do skull bones form?

Neurocranium forms by intramembranous and endochondral ossification; branchiomeric cartilages contribute to viscerocranium.

What cartilage is associated with the first pharyngeal arch and what does it become in the ear?

Meckel’s cartilage; gives rise to malleus and incus in the ear.

Which ear bone arises from the second pharyngeal arch?

Stapes.

What forms the secondary palate in mammals?

Palatine shelves (medial extensions of the maxillary prominences) forming the hard palate.

Which arches give rise to mastication and facial expression muscles?

Arch 1: mastication; Arch 2: facial expression.

What is Rathke’s pouch, and what structure does it contribute to?

An invagination of oral ectoderm that forms the anterior pituitary (adenohypophysis).

How do placodes contribute to head development?

Placodes are ectodermal thickenings that invaginate to form sensory organs and ganglia; include otic, olfactory, lens, and adenohypophyseal placodes.

What is the nasolacrimal canal and its function?

A placode-derived duct that drains tears from the eye to the nasal cavity.

What is the role of the lens placode in eye development?

Gives rise to the lens vesicle, which forms the lens with lens fibers and epithelial cells.

Which germ layer contributes to the gut and its derivatives?

Endoderm (along with contributions from splanchnic mesoderm to organs).

What are the three primary body cavities and their general contents?

Pleural (lungs), pericardial (heart), and peritoneal (abdominal organs).

What is Meckel’s diverticulum?

A persistent vitelline duct remnant at the ileum; a potential Meckel’s diverticulum in adults.

What are the three kidneys in development and what is the adult one?

Pronephros, mesonephros, and metanephros; the metanephros becomes the adult kidney.

What is the urogenital sinus and what does it give rise to?

A diverticulum of the cloaca that forms the bladder and contributes to urethral and genital structures.

What causes hypospadias and epispadias?

Hypospadias: urethra opens on the ventral penis; Epispadias: urethra on the dorsal penis due to malposition of genital tubercle.

What is the default sex in mammals, and what gene(s) drive male differentiation?

Female is the default; SRY gene, Leydig cells, Sertoli cells (MIF/TDF), and DMRT1 drive male development.

What does the process of differentiation rely on, besides gene types?

Intracellular and intercellular signaling, inductive chemicals, and gene networks.

What are the four gene types that control differentiation mentioned in the notes?

Structural genes, Regulatory genes, Silenced (turned-off) genes, and Genes that regulate other genes.

What is the primary purpose of cytoplasmic determinants in early cells?

To establish axis and initial patterns of differentiation.

What is the mechanism that defines the left-right symmetry in the embryo?

Differentiation signals and migrating cells establish the midline and axis; the notochord helps set the left-right axis.

What is the term for the layer that lines organs and forms the peritoneum/pleura?

Visceral mesoderm (splanchnopleure).

What is the term for the layer lining the body wall and limbs?

Parietal mesoderm (somatopleure).

What is the role of the vitelline vessels and the vitelline duct in early gut development?

Vitelline vessels form the portal system; the duct connects yolk sac to the gut.

What is the relationship between the gut and liver/pancreas during development?

Gut diverticula give rise to liver, pancreas, and other gut derivatives.

What is the placenta’s role by end of the first trimester?

Placenta becomes the main nourishing organ and progesterone producer; corpus luteum regresses.

How does the placenta form in mammals?

From the syncytiotrophoblast invading the endometrium and establishing placental circulation.

What is the role of the corpus luteum in pregnancy maintenance?

Produces progesterone to maintain the thick endometrium; stimulated by hCG.

What is the cloaca and its fetal derivatives?

A single posterior opening that becomes the urogenital sinus and anorectal canal after septation.

What structures develop from the three pharyngeal arches?

Arteries, veins, nerves, muscles, cartilage; endoderm (pouches), ectoderm (slits), mesoderm.

What is the function of the epibranchial placodes?

Contribute to certain cranial nerves and sensory structures associated with arches.

What is the significance of the occipital region in head development?

Occipital somites contribute to tongue muscles via hypoglossal innervation; post-otic somites form neck muscles.

Which nerve innervates the intrinsic tongue muscles?

Hypoglossal nerve (CN XII) from occipital somites.

What makes up the branchiomeric muscles derived from pharyngeal arches?

Muscles of mastication (arch 1), facial expression (arch 2), stylopharyngeus (arch 3), pharyngeal constrictors (arch 4), intrinsic laryngeal muscles (arch 6).

What are the two main components of the skull’s developmental origins?

Intramembranous bones (neural crest), Endochondral bones, and branchiomeric cartilages.

What are the two ear bones formed from arch 1 and arch 2?

Malleus and incus from Meckel’s cartilage (arch 1); stapes from arch 2.

What is the role of the nasal capsule in skull development?

Forms parts of the nasal cavity and conchae; supports the external nose.

What are the two primary placodes involved in eye development?

Lens placode and optic (retinal) placode forming lens and retina.

What is Crood’s spin ball relay in the context of differentiation?

A metaphor for progressive, stepwise differentiation and signaling among cells.

What are the three types of eyes/placodes mentioned besides lens placode?

Otic placode (inner ear), olfactory placode (smell), and adenohypophyseal placode (anterior pituitary precursor).

What is the role of the lens in the eye’s development?

Lens vesicle forms from the lens placode and differentiates into lens fibers and epithelium.

What provides early nourishment before placental circulation is established in mammals?

The yolk sac initially nourishes the embryo.

What is Meckel’s cartilage’s fate in mammals?

Gives rise to the malleus and incus bones of the middle ear.

What is the main difference between intramembranous and endochondral skull bones?

Intramembranous ossify directly from mesenchyme; endochondral ossify via cartilage model.

What is the neural tube’s relationship to the brain and spinal cord?

The neural tube forms the brain and spinal cord (CNS).