Human Prenatal Development and Embryology

Vocabulary practice flashcards covering human prenatal development, gametogenesis, fertilization stages, blastocyst formation, gastrulation, embryonic germ layer derivatives, and placentation.

Pre-embryonic Period

The developmental phase extending from fertilization up to the end of the third week of development, characterized by cleavage, blastocyst formation, implantation, and gastrulation.

Embryonic Period

The developmental stage spanning from the 4th to the 8th week of development, involving intense organogenesis, morphogenesis, and the activity of homeotic genes.

Fetal Period

The stage of development from the 9th week until the end of the 38th week (or 40 weeks from the last menstrual period / 266 days post-fertilization), characterized by rapid body growth and organ maturation.

Acrosome

A cap-like organelle located on the sperm head containing proteolytic enzymes necessary for penetrating the zona pellucida of the oocyte.

Sperm Centrioles

Structures located in the neck/midpiece of the sperm that are introduced into the oocyte during fertilization to reconstitute the centrosome and form the mitotic spindle.

Mitochondrial Sheath (Spiral)

The spiral arrangement of mitochondria in the sperm midpiece responsible for generating ATP/energy required for flagellar movement.

Axoneme

The central cytoskeletal core of the sperm flagellum that enables motility.

Primary Oocyte

An immature female germ cell ($46,XX$) present at birth within primordial ovarian follicles, arrested in the diplotene stage of prophase I (dictyotene stage) until puberty by oocyte maturation inhibitor (OMI).

Secondary Oocyte

The female gamete ($23,X$) released at ovulation accompanied by the first polar body, arrested in metaphase of meiosis II by cytostatic factor (CSF) until fertilization.

Capacitation

A species-specific physiological maturation process lasting $3\text{--}7\text{ h}$ in the female reproductive tract that unmasks sperm surface receptors for the oocyte and prepares sperm for hyperactivation and the acrosome reaction.

Thermotaxis

The directed migration of capacitated sperm guided by a temperature gradient of $1\text{--}2\text{ }^\top C$ from the cooler oviductal isthmus toward the warmer ampulla.

TRPC2 Channel

A transient receptor potential canonical channel involved in sperm thermosensitivity and directed thermotaxis in the female reproductive tract.

Hyperactivation

A change in sperm flagellar kinetics characterized by increased flagellar beat frequency, high thrust, and asymmetrical, curved swimming trajectory, allowing sperm to detach from oviductal reservoirs and penetrate the oocyte vestments.

Zona Pellucida Glycoprotein 3 (ZP3)

A primary ligand ($25\%$ of the zona pellucida) that binds species-specifically to capacitated sperm via N-glycans, triggering intracellular signaling ($G$-proteins, PLC, $IP_3$, and $Ca^{2+}$ influx) that induces the acrosome reaction.

Zona Pellucida Glycoprotein 2 (ZP2)

The most abundant zona pellucida glycoprotein ($65\%$) that acts as a secondary ligand, binding to the inner acrosomal membrane of acrosome-reacted sperm to facilitate zona penetration.

Zona Pellucida Glycoprotein 1 (ZP1)

A structural glycoprotein making up $10\%$ of the zona pellucida that cross-links filaments of ZP2 and ZP3.

Cortical Reaction

The calcium-dependent exocytosis of cortical granules from the oocyte into the perivitelline space following sperm entry, causing zona hardening and alteration of ZP2 to establish the block to polyspermy.

Block to Polyspermy

The mechanism that prevents multiple sperm from fertilizing a single oocyte, aided by oviductal proteins such as oviductin, osteopontin, and ANP to ensure diploid embryo development.

Ootid

The fertilized oocyte containing two distinct haploid pronuclei (female and male) prior to their fusion.

Epigenetic Asymmetry of Pronuclei

The phenomenon where the paternal genome undergoes rapid active demethylation and histone replacement in the male pronucleus, whereas the maternal genome is passively demethylated during subsequent cleavage divisions.

Karyogamy

The fusion of the parental genomes from male and female pronuclei, occurring approximately $20\text{ hours}$ after fertilization to establish the diploid zygote nucleus.

Cleavage (Bruzdkowanie)

A series of rapid mitotic divisions of the zygote with a minimally reduced G1 phase, producing progressively smaller cells called blastomeres without an increase in overall embryo size.

Morula

A solid ball of $12\text{--}16$ blastomeres surrounded by the zona pellucida that enters the uterine cavity on days 3 to 4 after fertilization.

Compaction

A process occurring in the late morula stage in the middle of the first week where blastomeres adhere tightly via junctions, initiating separation into embryoblast and trophoblast cell lineages.

Human Blastocyst (Day 5)

A stage of pre-implantation development consisting of an inner cell mass (embryoblast), an outer epithelial shell (trophoblast), and a fluid-filled cavity (blastocoel), enclosed by the zona pellucida.

Blastocyst Hatching

The shedding of the surrounding zona pellucida by the late blastocyst around days 5 to 6, enabling direct contact and implantation into the uterine endometrium.

Totipotent Cells

Stem cells (such as the zygote and blastomeres up to the 8-cell stage) capable of forming an entire organism, including both embryonic and extraembryonic tissues.

Pluripotent Cells

Cells of the inner cell mass (embryoblast) that can differentiate into any of the three embryonic germ layers (ectoderm, mesoderm, endoderm), but cannot form extraembryonic tissues or a complete organism alone.

Multipotent Cells

Stem cells committed to giving rise to multiple specialized cell types within a specific germ layer or lineage (e.g., hematopoietic stem cells).

Unipotent Cells

Stem cells that can only produce one mature cell type while retaining self-renewal capacity, such as basal layer epidermal keratinocytes.

Implantation

The attachment and embedding of the blastocyst by its embryonic pole into the upper and posterior wall of the uterine body, beginning around day 6.5 and completing by days 11–12.

Decidual Reaction

The morphological and functional transformation of endometrial stromal cells in response to progesterone and estrogen during the secretory phase, forming the decidua of pregnancy.

Decidua Basalis

The maternal endometrial layer situated directly beneath the implanting embryo and adjacent to the chorion frondosum, which forms the maternal component of the placenta.

Syncytiotrophoblast

The outer, multinucleated, non-cellular trophoblastic layer with high invasive and enzymatic activity that erodes maternal capillaries to form blood-filled lacunae and secretes hCG.

Cytotrophoblast

The inner, mononucleated, mitotically active cell layer of the trophoblast that supplies cells to the syncytiotrophoblast and forms the core of primary chorionic villi.

Hydatidiform Mole (Zaśniad Groniasty)

An abnormal pregnancy marked by trophoblastic proliferation caused by paternal genomic imprinting abnormalities, presenting as complete (androgenetic, uniparental disomy without an embryo) or partial (triploid, usually two sperm fertilizing one oocyte).

Genomic Imprinting

An epigenetic mechanism affecting approximately $1\%$ of genes where DNA methylation establishes parent-of-origin monoallelic gene expression, erased in primordial germ cells and reset during gametogenesis.

Bilaminar Embryonic Disc

The flat, two-layered structure formed in the second week consisting of a dorsal epiblast (columnar cells lining the amniotic cavity) and a ventral hypoblast (cuboidal cells lining the primary yolk sac).

Amnioblasts

Epiblast-derived cells that line the amniotic cavity, forming the thin amniotic membrane.

Heuser's Membrane (Exocoelomic Membrane)

A layer of flattened hypoblast-derived cells that lines the inner surface of the cytotrophoblast, forming the primitive yolk sac (exocoelomic cavity).

Gastrulation

The process beginning in early week 3 by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc composed of ectoderm, mesoderm, and endoderm.

Primitive Streak

A linear groove formed on the caudal dorsal midline of the epiblast through which epiblast cells migrate and invaginate to generate intraembryonic mesoderm and endoderm.

Primitive Node (Hensen's Node)

An elevated organizer region at the cranial end of the primitive streak with a central depression (primitive pit) that initiates the formation of the notochordal process.

Notochord

A midline axial mesodermal rod formed from the primitive node that induces the overlying ectoderm to form the neural tube and establishes the primary longitudinal axis; its adult remnant is the nucleus pulposus.

Paraxial Mesoderm

Thickened longitudinal columns of intraembryonic mesoderm flanking the notochord that segment into somites beginning around day 20.

Somites

Segmented blocks of paraxial mesoderm that differentiate into sclerotome (vertebrae and ribs), myotome (skeletal muscle of limbs and trunk), and dermatome (dermis and subcutaneous tissue).

Intermediate Mesoderm

The longitudinal column of intraembryonic mesoderm situated between paraxial and lateral plate mesoderm that gives rise to the urogenital system.

Lateral Plate Mesoderm

The thin lateral mesodermal sheet that splits into a somatic (parietal) layer lining the ectoderm and a splanchnic (visceral) layer lining the endoderm, enclosing the intraembryonic coelom.

Neurulation

The developmental process starting on day 18 wherein the neuroectoderm thickens into a neural plate, folds into a neural groove, and fuses to form the neural tube.

Neuropores

The temporary cranial and caudal openings of the neural tube that close between days 25–26 (anterior neuropore) and days 27–28 (posterior neuropore).

Neural Crest Cells

Ectodermal cells migrating from the margins of the neural folds that give rise to sensory and autonomic ganglia, Schwann cells, oligodendrocytes, melanocytes, thyroid C cells, and APUD cells.

Primitive Gut

An endoderm-lined tube created during week 4 by embryonic folding, subdivided on day 20 into the foregut, midgut, and hindgut.

Allantois

A diverticulum of the posterior wall of the primitive gut/yolk sac appearing on day 16 that extends into the connecting stalk, carries umbilical blood vessels, and later regresses to form the urachus.

Primary Chorionic Villi

Early placental villous projections consisting exclusively of a cellular cytotrophoblastic core surrounded by a syncytiotrophoblast layer.

Secondary Chorionic Villi

Placental villi that have acquired a mesenchymal core of extraembryonic mesoderm within the cytotrophoblastic layer.

Tertiary Chorionic Villi

Definitive placental villi containing functional capillaries and venules differentiated within the central mesodermal core.

Chorion Frondosum (Bushy Chorion)

The portion of the chorion related to the decidua basalis characterized by profusely branched villi that forms the fetal component of the placenta.

Chorion Laeve (Smooth Chorion)

The abembryonic part of the chorion where villi compress and degenerate, producing a smooth surface.

Cotyledon (Liścień)

The structural and functional unit of the mature placenta (numbering 10 to 40 per placenta), consisting of a main stem villus and its branching capillary network exposed to maternal intervillous blood.

Umbilical Cord

The vascular conduit connecting the fetus to the placenta, normally containing two umbilical arteries, one umbilical vein, and mucoid connective tissue (Wharton's jelly), measuring 50–60 cm in length.

Physiological Umbilical Herniation

The normal temporary herniation of intestinal loops into the extraembryonic coelom of the umbilical cord between weeks 6 and 11 due to rapid gut growth and liver enlargement.

Nuchal Translucency (NT)

An ultrasound measurement of the thickness of subcutaneous fluid on the posterior aspect of the fetal neck used as a screening parameter for congenital anomalies.

Dizygotic Twins

Fraternal twins resulting from the independent fertilization of two distinct oocytes by two separate sperm ($70\%$ of twin pregnancies), possessing separate amnions, chorions, and placentas (which may secondarily fuse).

Monozygotic Twins

Identical twins derived from a single fertilized zygote ($30\%$ of twins) that can split at the 2-cell stage, inner cell mass stage, or bilaminar disc stage, depending on the timing of division.

Quickening

The initial maternal perception of fetal movements occurring typically during the 5th month of pregnancy, when the fetus is also covered with fine lanugo hair.