Embryology and Histology: Gametogenesis, Menstrual Cycle, Fertilization, and Early Development

Comprehensive vocabulary flashcards covering gametogenesis, meiosis stages, spermatogenesis, spermiogenesis, oogenesis, ovarian follicular development, the menstrual cycle and its endocrine control, fertilization, cleavage, implantation, gastrulation, germ layer derivatives, neurulation, and embryonic folding.

Gametogenesis

A neurohormonal and molecular regulated developmental cellular process resulting in the formation of a spermatocyte or oocyte for the adult male or female, derived from primordial germ cells (PGCs).

Primordial Germ Cells (PGCs)

Cells formed in the epiblast during the second week that move through the primitive streak during gastrulation, migrate to the yolk sac wall, and travel to the developing gonads by the end of the fifth week.

Reduction Division

A type of cell division (meiosis) consisting of two successive divisions that halves the chromosome number to produce gametes containing non-identical genetic information.

Leptotene

The initial stage of prophase in the first meiotic division where chromosomes become visible, each consisting of two chromatids that cannot yet be distinguished.

Zygotene

The stage of prophase I where homologous chromosomes pair, lie parallel to each other, and become closely apposed in a process termed synapsis or conjugation.

Synapsis

The close pairing of homologous maternal and paternal chromosomes during the zygotene stage of prophase I, also known as conjugation.

Bivalent

A pair of closely apposed homologous chromosomes formed together during synapsis in zygotene.

Pachytene

The stage of prophase I where chromatids become distinct to reveal a four-chromatid tetrad, and non-sister chromatids coil, cross over, and adhere at chiasmata.

Tetrad

A bivalent during the pachytene stage of prophase I displaying four distinct chromatids (two central and two peripheral).

Crossing Over

The event occurring during pachytene and diplotene in which chromatids cross, break, and exchange genetic material between homologous chromosomes.

Chiasmata

The points of crossing and physical adherence between non-sister chromatids of a bivalent during prophase I.

Diplotene Stage

The stage of prophase I where homologous chromosomes begin to move apart while remaining connected at chiasmata, followed by breakage and exchange of genetic material.

Anaphase I

The stage of the first meiotic division where entire chromosomes move to opposite poles of the spindle without splitting their centromeres, yielding cells with 23 double-chromatid chromosomes.

Spermatogenesis

The sequence of events beginning at puberty and continuing throughout adult male life by which spermatogonia are transformed into mature spermatozoa.

Sertoli Cells

Sustentacular supporting cells derived from the surface epithelium of the testes that support, protect, nourish germ cells, and assist in spermatozoa release.

Type A Spermatogonia

Cells arising from spermatogonial stem cells whose mitotic production marks the initiation of spermatogenesis, maintaining contact with progeny via cytoplasmic bridges.

Type B Spermatogonia

Cells produced by the final mitotic division of type A spermatogonia that subsequently divide to form primary spermatocytes.

Spermiogenesis

The developmental phase transforming round spermatids into spermatozoa via acrosome formation, nuclear condensation, tail/neck development, and shedding of cytoplasm.

Acrosome

An enzyme-containing cap covering half the nuclear surface of the sperm head derived from the Golgi region, essential for penetrating the egg's outer coats.

Spermatozoon

A highly specialized, actively motile male gamete measuring approximately $60\,\mu\text{m}$ in length, comprised of a head, neck, middle piece, and principal piece with an axial filament.

Emission

The sympathetic reflex mechanism that propels semen from the epididymis into the prostatic part of the urethra.

Ejaculation

The reflex expulsion of semen from the urethra through the urethral orifice, accompanied by vesical sphincter closure and bulbospongiosus muscle contractions.

Oogenesis

The sequence of cellular events by which oogonia differentiate and mature into oocytes, initiating before birth and terminating after menopause.

Primordial Follicle

A primary oocyte arrested in prophase of meiosis I surrounded by a single layer of flat follicular epithelial cells.

Oocyte Maturation Inhibitor (OMI)

A small peptide secreted by follicular cells that maintains primary oocytes arrested in the diplotene resting stage of prophase I until puberty.

Primary Follicle

A developing ovarian follicle in which the flat follicular cells have changed into cuboidal cells and proliferated to form a stratified epithelium of granulosa cells.

Zona Pellucida

A homogeneous layer of glycoproteins secreted on the surface of the oocyte by both the oocyte and surrounding granulosa cells.

Theca Interna

The inner, highly vascularized cellular layer of the theca folliculi containing steroid-secreting cells that produce oestrogen.

Theca Externa

The outer fibrous capsule of the ovarian follicle that gradually merges with surrounding ovarian connective tissue (stroma).

Antrum

The fluid-filled cavity formed within the membrana granulosa by the coalescence of intercellular fluid spaces in a developing ovarian follicle.

Cumulus Oophorus

The cluster of intact granulosa cells that surrounds the eccentrically located oocyte within an antral or Graafian follicle.

Discus Proligerus

The specific group of granulosa cells that anchors the cumulus-enclosed oocyte to the wall of the ovarian follicle.

Graafian Follicle

A mature vesicular follicle measuring $25\text{ mm}$ or more in diameter that contains a large fluid-filled antrum and is ready for ovulation.

Secondary Oocyte

The larger daughter cell produced upon completion of meiosis I that receives almost all the cytoplasm and arrests in metaphase of meiosis II until fertilization.

Perivitelline Space

The space located between the vitelline membrane (cell membrane) of the oocyte and the surrounding zona pellucida, containing the first polar body.

Stigma

An avascular spot appearing over the most convex, bulging point of a mature ovarian follicle immediately preceding rupture and ovulation.

Corpus Luteum

An endocrine body formed from the collapsed walls of a ruptured Graafian follicle whose lutein-filled luteal cells secrete progesterone and oestrogen.

Corpus Albicans

A white mass of fibrous scar tissue formed following the degeneration and regression of an unfertilized corpus luteum or atretic follicle.

Corpus Luteum of Pregnancy

The enlarged corpus luteum maintained by human chorionic gonadotrophin (hCG) for 3 to 4 months to produce progesterone until the placenta takes over.

Corpora Atretica

Interstitial endocrine glands formed by proliferating theca interna cells of degenerating, non-ovulated ovarian follicles that secrete oestrogens.

Menarche

The onset of the first menstrual bleeding, occurring at approximately 12 years of age.

Menopause

The permanent cessation of menstruation and ovarian follicular cycles, occurring at approximately 45 years of age.

Myometrium

The thick middle muscular layer of the uterine wall composed of smooth muscle.

Endometrium

The innermost mucosal layer of the uterine wall composed of surface epithelium, stroma, uterine tubular glands, and specialized straight and spiral arteries.

Menstrual Cycle

The monthly cycle of structural and functional alterations within the uterine endometrium, divided into postmenstrual, proliferative, secretory, and menstrual phases.

Follicular Phase of Menstrual Cycle

The first half of the menstrual cycle governed predominantly by oestrogens from developing ovarian follicles, stimulating endometrial repair and proliferation.

Luteal Phase of Menstrual Cycle

The second half of the menstrual cycle dominated by progesterone secreted by the corpus luteum, causing uterine glands to secrete glycogen-rich mucoid fluid.

Hormonal Control of the Menstrual Cycle

The neuroendocrine axis involving GnRH release from the hypothalamus, stimulating FSH and LH from the anterior pituitary, which regulate ovarian steroidogenesis and endometrial remodeling.

LH Surge

A sudden elevation in luteinizing hormone release from the anterior pituitary occurring 24 to 36 hours prior to ovulation that triggers follicular rupture.

Inhibin

A protein secreted by granulosa cells when fertilization fails, acting on the anterior pituitary to inhibit gonadotrophin secretion and trigger luteal regression.

Fertilization

The fusion of male and female gametes to restore the diploid chromosome number and determine chromosomal sex, occurring typically in the ampulla of the uterine tube.

Capacitation

A required period of conditioning in the female tract lasting roughly $7\text{ hours}$ during which glycoprotein coat and seminal plasma proteins are cleared from the sperm acrosomal membrane.

Acrosome Reaction

The release of acrosomal enzymes (such as acrosin and trypsin-like substances) induced by ZP3 binding, allowing sperm to penetrate the zona pellucida.

Zona Reaction

Structural and chemical alterations of the zona pellucida triggered by lysosomal enzymes from cortical granules, preventing polyspermy and inactivating sperm receptors.

Polyspermy

The fertilization of an ovum by more than one spermatozoon, blocked physiologically by the cortical and zona reactions.

Blastomeres

Individual embryonic daughter cells produced by mitotic cleavage divisions of the zygote that become progressively smaller with each division.

Compaction

The maximization of intercellular contact and tight junction formation among blastomeres after the third cleavage (8-cell stage), separating inner cells from outer cells.

Morula

A compact ball of 16 embryonic blastomeres resembling a mulberry, formed approximately 3 days after fertilization.

Embryoblast

The inner cell mass of the blastocyst positioned at one embryonic pole, which gives rise to the tissues of the embryo proper.

Trophoblast

The outer cell mass forming the epithelial wall of the blastocyst that mediates uterine attachment and later contributes to the placenta.

Blastocyst

The embryonic stage formed when fluid enters the intercellular spaces of the morula to create a confluent central cavity (blastocele) enclosed by trophoblast and embryoblast.

Implantation

The embedding of the blastocyst into the secretory endometrium around day 6, mediated initially by L-selectin and subsequently by integrins, laminin, and fibronectin.

Gastrulation

The formative process beginning in the third week of intrauterine life that establishes the three primary germ layers and converts the bilaminar disc into a trilaminar gastrula.

Primitive Streak

A thickened linear band of epiblastic cells appearing in the third week on the caudal dorsal aspect of the embryonic disc that forms intraembryonic mesoderm.

Primitive Node

The elevated proliferation of epiblast at the cranial end of the primitive streak that surrounds the primitive pit.

Mesenchyme

Loosely organized embryonic connective tissue consisting of small spindle-shaped cells scattered within an intercellular ground substance matrix.

Prochordal Plate

A circular margin where cubical endodermal cells become columnar, establishing the central craniocaudal axis and future bucco-pharyngeal membrane without intervening mesoderm.

Notochord

A solid midline cord of cells underlying the neural tube derived from primitive streak prenotochordal cells, serving as the primary signaling center for axial skeleton induction.

Neurenteric Canal

A temporary opening at the primitive pit that transiently connects the amniotic cavity with the yolk sac cavity.

Allantois

A small diverticulum arising from the posterior wall of the yolk sac around day 16 that extends into the connecting stalk, rudimentary in humans.

Cloacal Membrane

A caudal region of the embryonic disc where ectoderm and endoderm remain in direct contact with no intervening intraembryonic mesoderm.

Neurulation

The process whereby the neural plate lengthens, elevates its neural folds, and fuses in the midline to form the closed tubular neural tube.

Convergent Extension

The lateral-to-medial movement of cells in the ectoderm and mesoderm driven by planar cell polarity signaling, lengthening the neural plate and body axis.

Neural Crest Cells

Neuroectodermal cells that undergo epithelial-to-mesenchymal transition and migrate to form melanocytes, sensory/autonomic ganglia, Schwann cells, and the craniofacial skeleton.

Head Fold

Embryonic folding in the median plane at the cranial end that establishes cephalic morphology and appropriately localizes the central nervous, gastrointestinal, and cardiovascular systems.

Tail Fold

Median plane folding resulting from distal neural tube elongation that correctly positions the hindgut and urogenital organs.

Lateral Folds

Horizontal plane folding of the right and left body walls that encloses the ventral body surface and internalizes the midgut.