Embryology Knowledge cards

Progenesis

The prenatal period that includes gamete formation and fertilization; it prepares male and female germ cells for union and includes spermatogenesis, oogenesis, folliculogenesis, ovulation, and fertilization.

Embryogenesis

The period from fertilization to the end of the 8th week, approximately day 56; it establishes the basic body plan and includes cleavage, blastocyst formation, implantation, gastrulation, neurulation, histogenesis, and organogenesis.

Fetogenesis

The period beginning after day 56; it is mainly characterized by growth, increase in length and weight, functional maturation of organs, stronger nerve-muscle connections, active fetal movement, and reflex development.

Main periods of prenatal development

The three main periods are progenesis, embryogenesis, and fetogenesis.

End of embryogenesis

Embryogenesis ends at approximately day 56, when the embryo is about 39–40 mm long, has a human appearance, and has established buds of all major organs.

Main events of embryogenesis

Cleavage, segmentation/morula formation, blastogenesis/blastocyst formation, implantation, bilaminar disc formation, gastrulation, neurulation, histogenesis, and organogenesis.

Cleavage

A series of mitotic divisions after fertilization in which the number of blastomeres increases but the total size of the embryo does not increase.

First cleavage division

The first mitotic division of the zygote occurs about 30 hours after fertilization.

Segmentation

The process of repeated cleavage divisions that produce increasing numbers of blastomeres.

Blastomeres

The cells produced by cleavage divisions of the zygote; they become progressively smaller because the embryo does not grow during cleavage.

Morula

A compact mass of approximately 12–16 blastomeres formed around day 3; it resembles a mulberry.

Blastogenesis

The process by which a cavity appears inside the morula, forming the blastocyst.

Blastocyst

A hollow, fluid-filled embryonic structure formed around day 4; it contains the embryoblast, trophoblast, and blastocoel.

Embryoblast

The inner cell mass of the blastocyst; it gives rise to the embryo, amnion, and yolk sac.

Trophoblast

The outer cell layer of the blastocyst; it gives rise to the chorion and contributes to placenta formation.

Blastocoel

The fluid-filled cavity inside the blastocyst.

Hatching

The process by which the blastocyst escapes from the zona pellucida before implantation.

Mechanism of hatching

Trophoblast microvilli release enzymes that weaken the zona pellucida while fluid accumulation in the blastocyst cavity increases internal pressure.

Timing of free blastocyst

The free blastocyst remains in the uterine cavity for about 1–2 days, approximately days 5–6 after conception.

Implantation

The attachment and invasion of the blastocyst into the uterine mucosa; it begins when trophoblast at the embryonic pole attaches to the endometrium.

Initial implantation site

The trophoblast at the embryonic pole attaches to the uterine mucosa.

Trophoblast differentiation

During implantation, the trophoblast differentiates into cytotrophoblast and syncytiotrophoblast.

Cytotrophoblast

The inner cellular trophoblast layer; it contains individual mitotically active cells and supplies cells to the syncytiotrophoblast.

Syncytiotrophoblast

The outer invasive trophoblast layer; it is multinucleated, digests endometrial epithelium, invades the endometrium, and later contributes to the placental barrier.

Sequence of implantation

Apposition of blastocyst to endometrium, stable attachment, digestion of endometrial epithelium by syncytiotrophoblast enzymes, submergence into endometrium, and re-epithelialization over the implanting blastocyst.

Bilaminar germinal disc

The two-layered embryonic disc formed from the embryoblast during implantation; it consists of epiblast and hypoblast.

Epiblast

A layer of columnar cells in the bilaminar disc; it gives rise to the embryo proper, amniotic epithelium, and all three definitive germ layers after gastrulation.

Hypoblast

A layer of small flattened cells in the bilaminar disc; it lines the primitive yolk sac cavity and contributes to extraembryonic structures.

Amnioblasts

Cells derived from the outer marginal cells of the epiblast; they form the amniotic epithelium.

Extraembryonic mesoderm

Supportive mesoderm outside the embryo; it helps transport nutrients from trophoblast to the germinal disc and forms layers around extraembryonic cavities and membranes.

Extraembryonic coelom

A large cavity formed by fusion of spaces within the extraembryonic mesoderm; the embryo remains attached by the connecting stalk.

Connecting stalk

The structure attaching the embryo to the trophoblastic shell; it later contributes to the umbilical cord.

Visceral extraembryonic mesoderm

The extraembryonic mesoderm layer associated with yolk sac and amnion; also called splanchnopleuric extraembryonic mesoderm.

Parietal extraembryonic mesoderm

The extraembryonic mesoderm layer associated with cytotrophoblast and chorion; also called somatopleuric extraembryonic mesoderm.

Gastrulation

The process by which epiblast cells migrate to form the three definitive germ layers: ectoderm, mesoderm, and endoderm.

Main result of gastrulation

Conversion of the bilaminar germinal disc into a trilaminar germinal disc.

Primitive streak

A linear thickening on the epiblast surface that serves as the main site of epiblast cell migration during gastrulation.

Primitive groove

The depression within the primitive streak through which epiblast cells migrate inward.

Primitive node

The cranial end of the primitive streak; cells from this region migrate cranially to form the notochord.

Primitive pit

The depression within the primitive node.

Formation of definitive endoderm

Epiblast cells migrate through the primitive streak and replace the hypoblast, forming definitive endoderm.

Formation of intraembryonic mesoderm

Epiblast cells migrate through the primitive streak and spread between epiblast and hypoblast, forming embryonic mesoderm.

Formation of ectoderm

Epiblast cells that remain on the surface after gastrulation become ectoderm.

Origin of all definitive germ layers

All three definitive germ layers—ectoderm, mesoderm, and endoderm—derive from epiblast.

Notochord

A midline axial structure formed from primitive node cells migrating cranially beneath the epiblast.

Main functions of notochord

The notochord establishes the midline axis, induces neural plate formation, influences somite differentiation, and helps pattern surrounding mesoderm and ectoderm.

Notochord and neural induction

The notochord induces overlying ectoderm to form the neural plate, beginning development of the nervous system.

Notochord and sclerotome

The notochord induces cells of the somite to differentiate into sclerotome.

Induction

The developmental process in which one group of cells influences another group of cells to change its fate or differentiation pathway.

Neurulation

The process by which the neural plate folds and closes to form the neural tube, the primordium of the central nervous system.

Neural tube

The embryonic structure that gives rise to the brain, spinal cord, central neurons, central glial cells, and retina.

Neural crest

A migratory ectoderm-derived cell population that forms many peripheral and craniofacial structures.

Neural crest derivatives

Peripheral nervous system, ganglia, peripheral glial cells, adrenal medulla, melanocytes, odontoblasts, connective tissue of the head, skull bones, and cranial muscles.

Ectoderm derivatives

Epidermis, skin glands, hair, nails, lens of the eye, epithelial structures of the inner ear, mouth epithelium, salivary glands, central nervous system, and peripheral nervous system.

BMP4 in ectodermal development

BMP4 participates in regulation of neural plate and neural crest differentiation.

Mesoderm subdivisions

Mesoderm divides into paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm.

Paraxial mesoderm

The mesoderm beside the notochord; it forms somitomeres in the head and somites from the neck region caudally.

Somitomeres

Segmental whorls of paraxial mesoderm in the cephalic region that contribute to head mesenchyme.

Somites

Segmented blocks of paraxial mesoderm that differentiate into sclerotome, myotome, and dermatome.

Sclerotome

The somite component that forms cartilage and later bone, especially vertebrae and axial skeletal structures.

Myotome

The somite component that gives rise to skeletal muscle of the body wall.

Dermatome

The somite component that forms dermis and connective tissue of the skin.

Dermomyotome

The somite region that gives rise to dermatome and myotome.

Intermediate mesoderm

The mesoderm that forms excretory units of the genitourinary system and connective tissue of the gonads.

Nephrogonotomes

Segmented parts of intermediate mesoderm involved in early urogenital development.

Nephrogenic cord

Non-segmented intermediate mesoderm involved in development of the urinary system.

Lateral plate mesoderm

The mesoderm that splits into somatic/parietal and splanchnic/visceral layers, creating the intraembryonic coelom.

Somatic layer of lateral plate mesoderm

The parietal mesoderm layer that forms connective tissue of the lateral and ventral body wall.

Splanchnic layer of lateral plate mesoderm

The visceral mesoderm layer that forms connective tissue and smooth muscle of the gut wall and airways.

Intraembryonic coelom

The cavity between somatic and splanchnic layers of lateral plate mesoderm; it forms pericardial, pleural, and peritoneal cavities.

Mesothelium

The epithelial lining of body cavities derived from mesoderm.

Endoderm

The germ layer that forms the epithelial lining of the primitive gut and many organs derived from it.

Foregut endoderm derivatives

Epithelial lining of tympanic cavity, Eustachian tube, esophagus, stomach, lungs, thymus, thyroid, parathyroids, liver, pancreas, and tonsils.

Midgut endoderm derivatives

Epithelium of the small intestine and structures related to the omphalomesenteric or vitelline duct.

Hindgut endoderm derivatives

Allantois, epithelium of the large intestine, urinary bladder epithelium, and urethral epithelium.

Primordial germ cells

Embryonic precursor cells that give rise to gametes; they eventually become spermatogonia in males or oogonia in females.

Origin of primordial germ cells

Primordial germ cells form in the epiblast during the second week of embryogenesis.

PGC migration route

PGCs form in the epiblast, pass through the primitive streak during gastrulation, migrate to the yolk sac wall, then migrate to the genital ridges during weeks 4–6.

PGCs in yolk sac

Cells that give rise to gametes can be identified in the wall of the yolk sac during the 3rd to 4th weeks of gestation.

PGC migration to genital ridge

Primordial germ cells migrate from the yolk sac to the future gonads/genital ridges during weeks 4–6.

PGC role in gonad formation

PGCs stimulate coelomic epithelium in the presumptive gonad to form somatic support cells.

Somatic support cells

Cells formed under influence of PGCs that invest, nourish, and regulate developing germ cells.

Male somatic support cells

Somatic support cells in males become Sertoli cells and germinal epithelium of seminiferous tubules.

Female somatic support cells

Somatic support cells in females become ovarian follicular cells/granulosa cells.

SRY gene

The Y chromosome gene that determines testis formation by directing differentiation toward the male pathway.

Gametogenesis

The process by which mature male or female sex cells are formed from primordial germ cell descendants.

Spermatogenesis

The sequence of events by which mature male sex cells, spermatozoa, are formed in seminiferous tubules.

Site of spermatogenesis

Spermatogenesis occurs in seminiferous tubules of the testis.

Beginning of spermatogenesis

Spermatogenesis begins at puberty and continues throughout adult reproductive life.

Male germ cells before puberty

PGCs remain dormant in the gonads, then after birth differentiate into spermatogonial stem cells; approaching puberty, type A spermatogonia arise.

Four stages of spermatogenesis

Mitotic proliferation, growth, maturation, and spermiogenesis.

Mitotic proliferation in spermatogenesis

The stage in which spermatogonia divide by mitosis to maintain the stem cell population and produce cells committed to differentiation.

Type A spermatogonia

Spermatogonia that maintain the stem cell pool and produce cells that continue toward differentiation.

Type B spermatogonia

Spermatogonia that divide and give rise to primary spermatocytes; daughter cells remain connected by cytoplasmic bridges.

Cytoplasmic bridges in spermatogenesis

Incomplete cytokinesis leaves developing germ cells connected, allowing synchronous development.

Growth stage of spermatogenesis

The stage in which type B spermatogonia enter prophase of meiosis I, undergo recombination, and become primary spermatocytes.

Primary spermatocyte

A diploid germ cell undergoing meiosis I; it has the largest nucleus among spermatogenic cells and has 2N, 4C chromosome/DNA content.

Primary spermatocyte chromosome content

Primary spermatocytes are 2N, 4C because they are diploid and have duplicated DNA after S phase.

Maturation stage of spermatogenesis

The meiotic stage in which one primary spermatocyte produces two secondary spermatocytes after meiosis I and four spermatids after meiosis II.

Secondary spermatocyte

A haploid cell produced by meiosis I; it is 1N, 2C and rapidly enters meiosis II.