Embryology – Early Human Development (Weeks 1-4)

This set of vocabulary flashcards summarizes key terms and definitions from early human embryology (fertilization through week 4), covering cellular stages, germ layers, extraembryonic structures, neural and mesodermal development, pharyngeal apparatus, skull growth, and germ-layer derivatives.

Gametogenesis

The process that produces haploid male (sperm) and female (ovum) gametes, each containing 23 chromosomes.

Fertilization

Fusion of sperm and ovum in the ampulla of the fallopian tube forming a diploid zygote.

Zygote

First diploid cell of the embryo formed after fertilization; contains 46 chromosomes (23 maternal, 23 paternal).

Pronuclei

Male and female haploid nuclei that migrate, fuse, and exchange chromosomes before the first mitotic division of the zygote.

Corpus luteum

Endocrine structure formed from post-ovulation follicle cells; secretes progesterone (and estrogen) to prepare endometrium.

Human chorionic gonadotropin (hCG)

Hormone from syncytiotrophoblast that rescues the corpus luteum if fertilization occurs.

Cleavage

Rapid mitotic divisions of the zygote producing smaller cells (blastomeres) within the zona pellucida.

Blastomere

One of the smaller cells produced during cleavage of the zygote.

Morula

16-cell solid ball (about day 3) consisting of inner and outer cell masses.

Inner cell mass (Embryoblast)

Central cells of the morula that form the embryo proper.

Outer cell mass

Peripheral cells of the morula that form trophoblast and extraembryonic membranes.

Blastocyst

Structure formed when fluid enters morula creating the blastocele; consists of embryoblast and trophoblast.

Blastocele

The fluid-filled cavity inside the blastocyst.

Trophoblast

Outer layer of blastocyst that contributes to placenta; differentiates into cytotrophoblast and syncytiotrophoblast.

Cytotrophoblast

Inner cellular layer of trophoblast retaining distinct cell boundaries.

Syncytiotrophoblast (and what does it produce?)

Multinucleated outer trophoblastic layer invading endometrium; produces hCG.

Implantation

Adhesion and invasion of blastocyst into endometrial lining (~day 6).

Amniotic cavity (and what day is it formed?)

Fluid space forming between embryoblast and trophoblast (~day 8).

Epiblast

Columnar cell layer of embryonic disc facing amniotic cavity; forms ectoderm and contributes to mesoderm.

Hypoblast

Cuboidal cell layer facing blastocele; forms primitive endoderm lining yolk sac.

Bilaminar embryonic disc (and when is it formed?)

Two-layered structure (epiblast + hypoblast) formed during week 2.

Heuser’s (Exocoelomic) membrane

Thin hypoblast-derived lining of the primitive yolk sac.

Primitive (primary) yolk sac

First yolk sac lined by Heuser’s membrane below the hypoblast.

Extraembryonic mesoderm

Loosely arranged cells between trophoblast and yolk/amnion; splits to form extraembryonic coelom.

Extraembryonic coelom

Fluid cavity surrounding amnion and yolk sac except at connecting stalk; becomes chorionic cavity.

Extraembryonic somatic mesoderm

Layer lining trophoblast and covering amnion; part of chorion.

Extraembryonic visceral (splanchnic) mesoderm

Layer covering yolk sac.

Connecting stalk

Band of extraembryonic mesoderm attaching embryo to trophoblast; becomes umbilical cord.

Chorion

Combination of cytotrophoblast + extraembryonic somatic mesoderm; forms chorionic sac.

Chorionic villi

Projections of trophoblast into maternal tissue; primary (cytotrophoblast), secondary (with mesoderm), tertiary (with blood vessels).

Lacunae

Maternal blood-filled spaces within syncytiotrophoblast establishing uteroplacental circulation.

Decidua

Endometrial stromal tissue transformed by implantation (glycogen/lipid rich).

Secondary yolk sac

Smaller yolk sac formed after extraembryonic coelom pinches off primary yolk sac.

Prochordal plate

Thickened endodermal area marking future mouth and cranial end of embryo.

Gastrulation

Formation of three germ layers via primitive streak (~day 15).

Primitive streak

Longitudinal midline thickening of epiblast (caudal); site of cell ingress during gastrulation.

Primitive node

Cranial swelling of primitive streak controlling notochord formation.

Primitive pit

Depression within primitive node leading to notochordal canal.

Intraembryonic mesoderm

New germ layer inserted between ectoderm and endoderm by epiblast cells during gastrulation.

Cloacal membrane

Caudal ectoderm-endoderm fusion site; future anus.

Oropharyngeal membrane

Cranial ectoderm-endoderm fusion at prochordal plate; future mouth.

Notochord

Midline mesodermal rod derived from primitive node cells; basis for axial skeleton and induces neural plate.

Sacrococcygeal teratoma

Tumor from persistent primitive streak cells; contains tissues from all germ layers.

Neurulation

Process by which neural plate folds to form neural tube (brain and spinal cord).

Neural plate

Thickened ectoderm induced by notochord; precursor to neural tube.

Neural folds

Elevated lateral margins of neural plate that fuse to close neural tube.

Anterior neuropore

Cranial opening of neural tube; failure to close causes anencephaly.

Posterior neuropore

Caudal opening of neural tube; failure to close causes spina bifida/meningomyelocele.

Paraxial mesoderm

Mesodermal columns beside notochord that segment into somites.

Somite

Paired mesodermal blocks forming sclerotome, myotome, dermatome; appear day 20 onward (≈35 pairs).

Sclerotome

Somite derivative forming vertebrae and ribs.

Myotome

Somite derivative forming skeletal muscle of trunk/limbs.

Dermatome

Somite derivative forming dermis of skin.

Prosencephalon

Forebrain; subdivides into telencephalon and diencephalon.

Telencephalon

Forms cerebral hemispheres and lateral ventricles.

Diencephalon

Forms thalamus, hypothalamus, optic structures, third ventricle.

Mesencephalon

Midbrain; forms tectum, cerebral peduncles, aqueduct of Sylvius.

Rhombencephalon

Hindbrain; divides into metencephalon and myelencephalon.

Metencephalon

Forms pons, cerebellum, upper 4th ventricle.

Myelencephalon

Forms medulla oblongata, lower 4th ventricle.

Pharyngeal (branchial) arches

Six paired mesodermal bars in head-neck (4th week) each with nerve, artery, cartilage, muscle derivatives.

Mandibular arch (First arch)

Forms maxilla, mandible, malleus, incus, muscles of mastication; contains Meckel’s cartilage.

Hyoid arch (Second arch)

Forms stapes, styloid process, lesser horn of hyoid, muscles of facial expression; has Reichert’s cartilage.

Third pharyngeal arch

Forms greater horn of hyoid and part of thymus.

Fourth pharyngeal arch

Forms thyroid & epiglottic cartilages, intrinsic palate muscles.

Sixth pharyngeal arch

Forms laryngeal cartilages (cricoid, arytenoid, corniculate) and intrinsic laryngeal muscles.

Meckel’s cartilage

Cartilage core of first arch giving rise to mandible template, malleus, incus.

Reichert’s cartilage

Second arch cartilage forming stapes, styloid process, hyoid lesser horn.

Pharyngeal pouches

Endodermal outpocketings between arches that form internal neck organs.

First pharyngeal pouch

Forms auditory (eustachian) tube, middle ear cavity, tympanic membrane.

Second pharyngeal pouch

Forms palatine tonsils and tonsillar fossa.

Third pharyngeal pouch

Gives rise to thymus and inferior parathyroid glands.

Fourth pharyngeal pouch

Forms superior parathyroid glands.

Ultimobranchial body

Derived from 4th/5th pouch; incorporated into thyroid as parafollicular (C) cells producing calcitonin.

Fontanelle

Fibrous membrane area between skull bones; allows molding and brain growth.

Anterior fontanelle

Largest fontanelle at junction of frontal and parietal bones; closes ~2 years.

Posterior fontanelle

Fontanelle at junction of parietal and occipital bones; closes ~6 months.

Skull sutures

Fibrous joints (metopic, sagittal, coronal, lambdoid, squamous) between flat bones of neonatal skull.

Ectoderm derivatives

CNS, PNS, sensory epithelium of eye/ear/nose, epidermis, hair, nails, glands, enamel.

Mesoderm derivatives

Muscle, bone, cartilage, dermis, cardiovascular system, blood cells, kidneys, gonads, spleen, adrenal cortex.

Endoderm derivatives

Epithelial lining of GI and respiratory tracts, bladder, thyroid, parathyroids, liver, pancreas, auditory tube.

Day 0: Fertilization & Zygote

Fusion of sperm and ovum in the ampulla of the fallopian tube, forming the diploid zygote, the first diploid cell of the embryo.

Days 1-2: Cleavage Begins

Rapid mitotic divisions of the zygote produce smaller cells called blastomeres within the zona pellucida.

Day 3: Morula Stage

The embryo develops into a 16-cell solid ball, known as a morula, consisting of inner and outer cell masses.

Days 4-5: Blastocyst Formation

Fluid enters the morula, creating the blastocele cavity and transforming the embryo into a blastocyst, composed of an embryoblast (inner cell mass) and trophoblast.

Day 6: Implantation Onset

The blastocyst adheres to and begins invading the endometrial lining of the uterus.

Day 8: Amniotic Cavity & Early Bilaminar Disc

A fluid-filled amniotic cavity forms between the embryoblast and trophoblast, marking the beginning of the bilaminar embryonic disc formation.

Week 2 (Days 8-14): Extraembryonic Structure Development

The bilaminar embryonic disc fully forms (epiblast + hypoblast). Heuser’s membrane lines the primitive yolk sac. The extraembryonic mesoderm develops and splits to form the extraembryonic coelom (chorionic cavity).

~Day 15 (Week 3): Gastrulation Commences

Gastrulation begins with the formation of the primitive streak, through which epiblast cells ingress to form the three germ layers (ectoderm, intraembryonic mesoderm, endoderm).

Late Week 3: Notochord Formation & Neurulation

Cells from the primitive node form the notochord, which induces the overlying ectoderm to thicken and form the neural plate, beginning the process of neurulation.

Day 20: Somite Appearance

Paired mesodermal blocks called somites begin to appear from the paraxial mesoderm, marking a significant milestone in body segmentation.

~Day 25: Anterior Neuropore Closure

The cranial opening of the neural tube, the anterior neuropore, fully closes.

~Day 28: Posterior Neuropore Closure

The caudal opening of the neural tube, the posterior neuropore, fully closes.