We speak of organs or tissues at a stage when these cells have reached 'a significant state' of differentiation. This means that 'a significant number' of the molecules required for the function of these cells (e.g., channels or neurotransmitters in case of nerve cells) is expressed. Such a state is reached for most structures in Drosophila during stage 16 (70% of development). Before this stage we speak of primordia, anlagen, anlagen in statu nascendi. These terms are defined \-(i) in regard to the structure they give rise to \-(ii) in regard to their location (i)example: the anlage of the central brain consists of the cells (of the stage 5-7 embryo) which give rise to the central brain; see definition of central brain (ii)example: the anlage of the central brain is located between 80 and 90% egg length and 30% and 90% egg width. This is obviously very imprecise; the location of anlagen and primordial should be given graphically. Primordium The times at which we start speaking of primordium vs organ, or anlage vs primordium, are very difficult to pin down precisely. At a given stage, we might be able to pinpoint one boundary of a primordium (e.g., the boundary that separates it from the dorsally adjacent primordium), but not another boundary (e.g. the boundary that separates it from the anteriorly adjacent primordium). Therefore, definitions have a lot of slack; developmental biology is not yet mathematics. Different types of primordial: If all boundaries of a primordium are defined, and there is na 1:1 relationship of the primordium to the organ it gives rise to, we might call that a 'distinct' or 'specific' primordium (P1). Frequently, the primordia are cell populations that include more than one later cell type/organ in them. We might call that a 'complex primordium' (P2). In cases where primordia are incomplete at a given stage (we can pinpoint a structurally visible condensation or outgrowth, but it does not yet contain all the cells it will later have), we might call this 'primordium in statu nascendi' (P3). These terms may eventually prove useful for denoting details of organogenesis, but should not be incorporated in the controlled vocabulary of the in situ database. Anlage Anlagen are populations of contiguous cells, typically arranged in one plane, that are morphologically indistinct, but that already correspond in extent to a later organ/tissue. Anlagen of different organs can (or should) be defined at different stages. In theory, one could map each part of the mature organism on the blastoderm. For purposes of developmental analysis, that may not be too useful. For example, one could, very imprecisely, define where an organ like the trachea maps at the blastoderm stage. Most likely, cells of this 'blastoderm tracheal anlage' woulod not be contiguous, but intermikngled with cells that form dorsal epidermis or sensory organs. More importantly, they are (according to our current knowledge) no genes involved in tracheal specification that are expressed already at the blastoderm stage. The earliest tracheal genes, such as trachealess, come on around stage 9 in segmental domains that reflect the tracheal primordial which can be seen slightly later. Therefore, in the controlled vocabulary, the term 'tracheal anlage' appears for the stage 9/10 embryo. Different types of anlagen: The same distinctions drawn out for primordial above can be stated for anlagen. We may distinguish 'discrete' or 'specific' anlagen which contain progenitors of only one organ from 'complex' or 'inclusive' anlagen that give rise to more than one structure. 'anlagen in statu nascendi' are domains that do not yet coincide 1:1 with a later organ. Anlagen in statu nascendi are typically defined for the early blastoderm by the expression domains of genes which, in the late blastoderm or later, are expressed in specific anlagen, but initially come on in larger domains. Germ layers Germ layers are 'super complex primordia'. They arise during gastrulation and later split up into discrete organ primordia. There is a lot of slack in the nomenclature in regard to terms that refer to germ layers and to organs. For example, we speak of ventral ectoderm or the 'primordium of the ventral epidermis'. Or of 'anterior endoderm' or 'anterior midgut primordium'.
