BMS221 Histology L21 - Fertilisation and Embryonic Development

Flashcards covering the pre-embryonic and embryonic periods of human development, including fertilization, germ layer formation, neurulation, and organogenesis.

Conceptus

Defined as "that which is conceived" in human development.

Gestation period

The time from the last menstrual period until birth, which is approximately $280\text{ days}$.

Pre-embryonic period

The developmental stage from fertilisation to the $2\text{ week}$ conceptus, typically occurring during $2-4\text{ weeks}$ of gestation.

Embryo

The developmental stage from $3-8\text{ weeks}$ after fertilisation, corresponding to $5-10\text{ weeks}$ of gestation.

Fetus

The developmental stage from $9\text{ weeks}$ until birth, occurring at $11+\text{ weeks}$ of gestation.

Spermatozoon Viability

Sperm are viable for $48-72\text{ hours}$ in the female reproductive tract, several weeks in the excurrent duct system, and only a few minutes outside.

First Week Chronology: Fertilization

Occurs within the uterine tube $12-24\text{ hours}$ after ovulation.

Cleavage

The process of early mitotic divisions; the first cleavage is completed about $30\text{ hours}$ after fertilization.

Morula

A solid sphere of cells (approximately $16\text{ cells}$) formed by the end of the third day after fertilization.

Blastomeres

Progressively smaller cells produced by the process of cleavage.

Blastocyst

An assembly of cells formed $4\frac{1}{2}-5\text{ days}$ after fertilization that contains a large fluid-filled cavity.

Embryoblast

The portion of the blastocyst that eventually develops into the embryo.

Trophoblast

The part of the blastocyst that ultimately develops into the outer chorionic sac and the fetal portion of the placenta.

Implantation

The process occurring about $6\text{ days}$ after fertilization where the blastocyst attaches to the endometrium.

ZP3 protein

A protein in the zona pellucida that sperm must interact with during the process of drilling holes for fertilization.

Acrosomal reaction

The swelling and exocytosis of enzymes like hyaluronidase and acrosin to facilitate sperm penetration into the oocyte.

Decidua

The term used for the endometrium following the implantation of the blastocyst.

Syncytiotrophoblast

A layer of the trophoblast that secretes enzymes to digest and liquefy endometrial cells, enabling the blastocyst to penetrate the uterine lining.

Exocoelomic membrane

A thin membrane formed by migrating cells that, together with the hypoblast, forms the wall of the yolk sac.

Bilaminar embryonic disc

The structure positioned between the amniotic cavity and the yolk sac during the second week of development.

Lacunae

Small spaces that develop within the expanding syncytiotrophoblast during day $9$.

Maternal sinusoids

Dilated endometrial capillaries around the developing embryo that eventually provide maternal blood to the lacunar networks.

Extraembryonic mesoderm

A connective tissue layer (mesenchyme) derived from the yolk sac around day $12$ after fertilisation.

Extraembryonic coelom

A single large cavity formed by the fusion of several cavities within the extraembryonic mesoderm.

Primitive streak

A structure that forms at the beginning of the $3rd\text{ week}$ of development ( $5th\text{ week}$ gestation) in the midline on the caudal portion of the dorsal surface of the embryonic disc.

Gastrulation

The process in the third week of development that establishes the three primary germ layers.

Ectoderm

The primary germ layer that gives rise to the epidermis and the nervous system.

Mesoderm

The primary germ layer that gives rise to smooth muscle, connective tissue, blood vessels, blood cells, skeleton, skeletal muscle, and reproductive/excretory organs.

Endoderm

The primary germ layer that gives rise to the epithelial linings of respiratory passages and the digestive tract.

Notochord

A rod of mesodermal cells formed beneath the primitive streak that induces the overlying ectoderm to form the neural plate.

Neurulation

The process involving the formation of the neural plate and neural folds and their closure to form the neural tube.

Neural crest cells

Cells that migrate ventrolaterally from the neural tube to form the dorsal root (sensory) ganglia of spinal and cranial nerves.

Somites

Structures derived from mesoderm that give rise to the axial skeleton, adjacent muscles, and dermis.

Amniotic fluid

A buoyant fluid that protects the embryo, maintains constant temperature, and prevents embryonic parts from adhering.

Allantois

An embryonic structure involved in gas exchange and handling liquid waste.

Chorion

The outermost membrane derived from the extraembryonic mesoderm that helps to form the placenta.

Organogenesis

The formation of body organs and systems, which begins for all major systems by the end of the eighth week.

Embryonic Folding

The process of converting the flat trilaminar embryonic disc into a three-dimensional cylinder, resulting in a C-shape.

Primitive Gut

An internal endodermal tube formed by the lateral folding of the trilaminar embryonic disc.

Foregut

The section of the primitive gut that gives rise to the pharynx, esophagus, stomach, proximal half of duodenum, liver, gallbladder, and lungs.

Midgut

The section of the primitive gut that develops into the distal half of the duodenum, jejunum, ileum, cecum, appendix, and ascending colon.

Hindgut

The section of the primitive gut that forms the distal one-third of the transverse colon, descending colon, rectum, and urinary bladder epithelium.

Pronephros

The first stage of renal development appearing at approximately $4\text{ weeks}$.

Mesonephros

The second stage of renal development functioning between $4-12\text{ weeks}$.

Metanephros

The final stage of renal development; the kidney has ascended to the posterior abdominal wall by $9\text{ weeks}$.

Wolffian duct

The mesonephric duct associated with male reproductive tract development.

Müllerian duct

The paramesonephric duct associated with female reproductive tract development.