Fertilisation, Embryology and Fetal Development

Comprehensive vocabulary flashcards covering the anatomical and physiological processes of fertilization, cleavage, implantation, blastulation, gastrulation, germ layer derivatives, neurulation, and embryonic milestones.

Menstrual Cycle Phases

The four sequential stages of the menstrual cycle: Follicular phase, Ovulatory phase, Luteal phase, and Menstrual phase, which are regulated by ovarian changes, endometrial changes, and hormonal fluctuations.

Follicular Development

The maturation process of the ovarian follicle containing the ovum inside the ovary, stimulated by GnRH (Gonadotrophin releasing hormone) from the hypothalamus and FSH (Follicle-stimulating hormone) from the pituitary gland to induce granulosa cell proliferation.

Proliferative Phase

The stage of the uterine cycle occurring after the shedding of the endometrium, characterized by the regeneration and cellular proliferation of the endometrial layer.

Secretory Phase

The phase in which the uterine endometrium regenerates and thickens in anticipation of embryo implantation.

Cleavage and Implantation Stage

The first week following fertilisation during which the fertilised ovum undergoes mitotic cleavage to form a morula and then a blastocyst, concluding with implantation.

Embryonic Period

The developmental stage spanning weeks 2 through 8 post-fertilisation, during which embryonic germ layers and the placenta develop, and the main body systems and major organs form.

Fetal Period

The developmental phase lasting from weeks 9 to 38 post-fertilisation, focused on further organ growth and tissue differentiation, as well as the locomotor system becoming functional.

Corona Radiata

An outer protective layer of follicular (granulosa) cells surrounding the unfertilised and fertilised human oocyte.

Zona Pellucida

A glycoprotein jelly-coat membrane surrounding the oocyte that prevents premature embryonic growth during early cleavage divisions and is shed ('hatched') prior to implantation.

Acrosome

An organelle located at the anterior head of a spermatozoon that secretes digestive enzymes upon contact with the zona pellucida (acrosome reaction) to penetrate the egg's outer coverings.

Polyspermy Block

The enzyme-mediated defense mechanism triggered when a single sperm makes successful contact and fuses with the oocyte plasma membrane, preventing any additional sperm from entering.

Cleavage

The rapid mitotic division of the zygote into progressively smaller cells (blastomeres) without an increase in overall external size due to confinement by the zona pellucida.

Morula

A solid ball of 16 cells formed by successive mitotic cleavage divisions, maintaining the same external size as the original zygote.

Blastocyst

A hollow sphere of 70 to 100 cells formed around Day 5 post-fertilisation, composed of an outer trophoblast layer, an inner cell mass (embryoblast), and a central fluid-filled cavity (blastocoele).

Trophoblast

The outer epithelial shell layer of the blastocyst that gives rise to embryonic support structures, including the chorion and the placenta.

Embryoblast (Inner Cell Mass)

The compact mass of cells situated internally within the blastocyst that differentiates into the embryo/fetus, amnion, and umbilical cord.

Blastocoele

The fluid-filled hollow central cavity of the blastocyst that progressively reduces in volume as adjacent embryonic structures enlarge.

Implantation

The process beginning around Day 6 post-fertilisation in which the blastocyst attaches to and burrows into the vascularized endometrium, typically at the posterior or fundal wall of the uterus.

Decidua

The specialized, vascularized uterine endometrium as designated following the successful burrowing and implantation of the blastocyst.

Syncytiotrophoblast

The outer multinucleated, invasive syncytial tissue layer derived from differentiated trophoblast cells during implantation to facilitate invasion into the maternal decidua.

Cytotrophoblast

The distinct, proliferative inner cellular layer formed by the differentiation of trophoblast cells during the second week of development.

Bilaminar Embryonic Disc

A flat, two-layered disc formed during Days 7 to 14 from the embryoblast, composed of the epiblast layer and the hypoblast layer.

Epiblast

The dorsal cell layer of the bilaminar embryonic disc that borders the amniotic cavity and gives rise to all three germ layers during gastrulation.

Hypoblast

The ventral cell layer of the bilaminar embryonic disc (primitive endoderm) situated adjacent to the blastocoel and primitive yolk sac.

Gastrulation

The developmental phase occurring during Days 14 to 21 wherein inward cellular migration transforms the bilaminar disc into a trilaminar disc composed of ectoderm, mesoderm, and endoderm.

Primitive Streak

A longitudinal groove and cellular indentation formed on the epiblast through which migrating cells invaginate to establish the middle mesodermal germ layer.

Ectoderm

The external germ layer of the trilaminar embryo that differentiates into the epidermis, hair, nails, brain, spinal cord, and peripheral nervous system.

Mesoderm

The middle germ layer of the trilaminar embryo that gives rise to muscle, bone, connective tissue, the notochord, kidneys, gonads, and the circulatory system.

Endoderm

The internal germ layer of the trilaminar embryo that gives rise to the epithelial lining of the digestive tract, stomach, colon, liver, pancreas, bladder, and lungs.

Neurulation

The folding process taking place in Week 4 that forms the neural tube, initiating along the embryonic midline and proceeding outwards toward the cranial and caudal extremities.

Embryonic Organogenesis Milestones (Weeks 4–8)

The morphological milestones of embryonic development: limb buds appear at 4 weeks, rapid brain development occurs at 5 weeks, external ears form at 6 weeks, the liver is prominent at 7 weeks, and digits separate (remaining webbed) while eyelids close by 8 weeks.