Glossary of embryological terms

In this brief glossary terms are approximately in the order that they arise in the course of embryonic development, rather than alphabetical.

Gametes

gamete

Reproductive cell: e.g. ovum or sperm.

meiosis

The process where a single cell divides twice to give four cells containing half of the original genetic material; typically produces sperm or ovum.

haploid

Sperm and egg cells result from meiosis and are haploid: they have only half of the number of chromosomes of somatic (body) cells, e.g. in humans, haploid cells have 23 chromosomes, whereas somatic cells have 46 chromosomes.

polar body

Production of an ovum involves 2 rounds of cell division, giving rise to 4 (sometimes only 3) nuclei, only one of which is incorporated within an ovum. The resulting 3 (or 2) nuclei are small polar bodies which eventually degrade. One polar body may remain associated with the ovum for a while. [check]

animal pole and vegetal pole

In most ova, the nucleus is not in the centre but displaced to one end - this is called the animal pole, and the opposite end is the vegetal pole. In general there is more yolk towards the vegetal pole.

germinal disc

The embryo-forming part of the egg, e.g. excluding the body of the yolk.

zona pellucida

A clear layer surrounding the cell of the mammalian ovum (c.f. area pellucida of bird and reptile blastula).

corona radiata

The outermost layer of the mammalian ovum.

Zygote

zygote

Fertilised egg.

pronucleus

A pronucleus is the male or female polar body or ‘nucleus’ within the zygote, at the start of fertilisation.

Cleavage

cleavage

The first few cell divisions of the zygote, during which there is an increase in number of cells, but not in overall size or mass, so the cells get smaller.

holoblastic cleavage

The cell divisions pass right through the zygote.

meroblastic cleavage

The cell divisions do not extend right through the zygote (generally there is a large amount of yolk)

discoidal cleavage

A form of meroblastic cleavage where the cell divisions are restricted to the germinal disc.

rotational cleavage

A form of holoblastic cleavage where the two second cell divisions are in different planes, one meridional and one equatorial.

morula

An early stage of embryonic development, especially in mammals, when the cells are in the form of a loose clump.

compaction

A stage of embryonic development when the loose clump of cells becomes more compact, usually involving the formation of tight junctions between peripheral cells and the formation of a central blastocoel.

blastomere

One of the cells resulting from cleavage; or one of the cells of the blastoderm.

blastodisc

The germinal disc during and after cleavage, up to the stage of the blastula.

Blastula

blastula

The early embryo, after cleavage and the first differentiation of cells, just before gastrulation.

blastocyst

The name for the blastula in mammals.

blastoderm

A surface layer of cells of the blastula.

blastocoel

A space within the body of cells of the blastula, generally arising during cleavage.

subgerminal space / cavity

A space between the cells of the blastula and the underlying yolk.

yolk syncytial layer (YSL)

A layer at the surface of the yolk containing nuclei, but the cytoplasm of the cells is continuous with the yolk, i.e. the cell membranes are incomplete.

mid blastula transition (MBT)

Generally refers to the stage of the blastula when the embryonic genome becomes active and/or the cells begin to differentiate.

epiblast

In amniotes: the upper, epithelial layer of the blastula, i.e. before gastrulation, which in most cases is the source of all of the germ layers.

In anamniotes: sometimes used to refer to the upper, epithelial layer of the gastrula, i.e. after gastrulation, which usually becomes the ectoderm.

hypoblast

In amniotes: a layer of cells below the epiblast, which is substantially displaced in the course of gastrulation and does not become part of the embryo.

In anamniotes: sometimes used to refer to a lower layer of cells that have involuted in the course of gastrulation, and usually becomes mesoderm and endoderm (except amphibians).

trophoblast

The outer layer of cells of the mammalian blastocyst.

inner cell mass

Generally refers to mammals where the blastocyst comprises an outer layer of cells (the trophoblast), an inner cell mass (generally containing epiblast and hypoblast) and a blastocoel.

area pellucida

Central part of the reptile or bird blastula, that is over the blastocoel and is relatively translucent (c.f. zona pellucida of the mammalian ovum).

area opaca

Area of the reptile or bird blastula, that is around the area pellucida and above the marginal cells, and is relatively opaque.

enveloping layer (EVL)

A thin outer layer of cells of the teleost blastula, which persists through early embryonic development, but is shed at hatching.

Gastrulation

gastrulation

The overall term for the various processes through which the blastula develops into the gastrula in which all three germ layers - ectoderm, mesoderm and endoderm - are formed.

presumptive and definitive

Presumptive refers to cells which are still undifferentiated, or partly differentiated, but destined to become the final or definitive tissue.

epithelial

Surface cells are generally epithelal in nature, and move as a sheet of cells.

mesenchymal

Mesenchymal cells are usually below the surface, and migrate individually through tissues.

EMT and MET

When cells transition between epithelial and mesenchymal in nature by 'epithelail-to-mesenchymal transition' or 'mesenchymal-to-epithelial transition'.

epiboly

Spreading of a layer of cells in the course of gastrulation. This can be achieved by thinning of individual cells, and/or intercalation of cells from within the same or adjacent cell layer.

invagination

The process whereby a layer of cells indents and forms a cavity or pouch.

involution

Movement of cells from the outside to the inside, the cells remaining epithelial in nature and moving as a layer of cells.

ingression

Movement of cells, typically from the outside to the inside, the cells being (or transitioning to) mesenchymal in nature, and moving individually.

blastopore

A sac-like cavity in the outer surface of the blastula through which cells are internalised, usually by involution.

primitive streak

A depression in the epiblast of birds and mammals through which cells are internalised by ingression.

ectoderm

The outer germ layer; generally produces the outer layer of the embryo, and central nervous system via the neural tube.

mesoderm

The middle germ layer; from it forms the notochord, somites, some of the skeleton and musculature.

endoderm

The inner germ layer; it forms most of the digestive tract and associated organs.

extraembryonic membranes

These are membranes that develop along with the embryo but do not form part of the final embryo.

Usually these are the amniote amnion, chorion, and membranes of the yolk sac and allantois.

The yolk sac of teleosts is part of the embryo, and the yolk sac of chondrichthyans is reabsorbed, so these are not strictly extraembryonic.

Although the teleost enveloping layer (EVL) does not become part of the embryo, it is not usually regarded as an extraembryonic membrane.

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Page created October 2020.