Embryology Lecture 1: Fertilisation and Implantation

Vocabulary practice flashcards covering terminology, gestational dating, teratogenic risks, stages of fertilisation, cleavage, blastocyst differentiation, bilaminar disc formation, and implantation milestones.

Cranial

Anatomical term meaning towards the head.

Caudal

Anatomical term meaning towards the tail.

Transverse / Axial Plane

An anatomical plane that divides the body into upper (superior) and lower (inferior) sections.

Sagittal Plane

An anatomical plane that divides the body into left and right portions.

Coronal Plane

An anatomical plane that divides the body into dorsal (back) and ventral (front) sections.

Menstrual Age

The clinical method of dating a pregnancy from the first day of the woman's last menstrual period, divided into three equal trimesters.

Fertilisation Age

The embryological method of dating prenatal development, divided into the early development period (ED), embryonic period (E), and foetal period (F).

Early Development (ED) Period

The pre-embryonic stage characterized by cell division, implantation, and bilaminar embryo formation (weeks $1\text{--}2$); it is not susceptible to teratogens but exposure can result in prenatal death.

Embryonic (E) Period

The organogenesis stage spanning weeks $3\text{--}8$ of development; it is the period of highest susceptibility to teratogens, which can induce major congenital anomalies.

Foetal (F) Period

The developmental stage from week $9$ to full term ($38\text{ weeks}$); teratogenic exposure during this time typically causes functional defects or minor congenital anomalies.

Congenital Birth Defects Incidence (UK)

The recorded frequency of congenital birth defects in the United Kingdom, occurring in $1\text{ in }44$ births.

Unknown Etiology (Birth Defects)

The classification accounting for $50\%$ of all congenital birth defect causes.

Multifactorial Etiology (Birth Defects)

The classification accounting for $25\%$ of congenital birth defects, involving combined interactions between genetic predisposition and environmental factors.

Capacitation

The preparatory conditioning process that sperm undergo within the female reproductive tract that enables them to penetrate the corona radiata and initiate the acrosome reaction.

Acrosome Reaction

The release of specialized enzymes from the sperm head cap (acrosome) that allows the sperm to penetrate the zona pellucida.

Ampulla of the Uterine Tube

The specific anatomical region of the uterine (fallopian) tube where fertilisation usually takes place.

Fimbriae

Finger-like projections at the distal end of the uterine tube that sweep the ovulated oocyte (egg) into the uterine tube.

Corona Radiata

The outer protective layer of follicular cells surrounding the oocyte that capacitated sperm must penetrate.

Zona Pellucida

The glycoprotein membrane surrounding the oocyte that is digested by acrosomal enzymes and later modified to prevent polyspermy.

Cortical Reaction

A cellular event triggered immediately following sperm penetration of the oocyte membrane that initiates the zona reaction.

Zona Reaction

The alteration of the zona pellucida that renders it impenetrable to other sperm, preventing polyspermy.

Cleavage

A sequence of mitotic divisions occurring every $24\text{ hours}$ that doubles the cell number of the zygote without increasing its overall size.

Blastomeres

The individual totipotent cells generated by cleavage divisions of the zygote.

Totipotent

The developmental potential of early blastomeres to differentiate into any cell type of the embryo proper as well as extraembryonic tissues.

Morula

A solid spherical cluster of $16\text{--}32$ totipotent cells (blastomeres) formed by cleavage of the zygote.

Blastocyst

The developmental structure formed by day $5$ when the morula develops a fluid-filled cavity and segregates into an inner and outer cell mass.

Blastocytic Cavity

The fluid-filled internal space that forms within the morula by day $5$, transitioning it into a blastocyst.

Inner Cell Mass (Embryoblasts)

The compact interior cluster of pluripotent cells in the blastocyst that gives rise to the bilaminar disc and the embryo proper.

Pluripotent

The developmental potential of embryoblasts (inner cell mass) to give rise to all cell types of the embryonic body.

Outer Cell Mass (Trophoblasts)

The thinner peripheral layer of multipotent cells in the blastocyst that contributes to the placenta.

Multipotent

The developmental potential characteristic of trophoblasts, capable of differentiating into multiple specialized cell types within a given lineage.

Epiblast

The dorsal layer of pluripotent cells formed by the differentiation of the embryoblast (inner cell mass).

Hypoblast

The ventral layer of cells formed by the differentiation of the embryoblast (inner cell mass).

Bilaminar Disc

A two-layered embryonic structure composed of the dorsal epiblast and ventral hypoblast that establishes the dorsal/ventral axis of the embryo.

Dorsal/Ventral Axis

The body orientation establishing front (ventral) and back (dorsal) surfaces, created by the differentiation of embryoblasts into hypoblasts and epiblasts.

Amniotic Cavity

A small fluid-filled space that appears within the epiblast layer around day $8$ of development.

Syncytiotrophoblast

The outer invasive multinucleated layer of the trophoblast that lacks distinct cell boundaries and erodes maternal endometrial tissue during implantation.

Cytotrophoblast

The inner mononucleated cellular layer of the trophoblast that actively divides and feeds into the outer syncytiotrophoblast.

Primary Yolk Sac

The cavity formed around day $9$ when migrating hypoblast cells spread to encase the inner surface of the blastocyst cavity.

Trophoblastic Lacunae

Isolated spaces that form within the syncytiotrophoblast by day $9$ that subsequently communicate with eroded maternal uterine vessels.

Fibrin Plug

A temporary closure plug that seals the penetration defect in the surface epithelium of the endometrium at day $9$ of implantation.