The fragility of the developmental process is well known with estimates as high as 70% of conceptions lost in early pregnancy [1
]. Genetic defects are known to produce numerous changes in both metabolic and morphologic characteristics of the fetus [2
]. The choreography of genes necessary for successful fetal development is amply demonstrated by embryonic lethal [3
] and other phenotypes [4
] in knockout mouse models. Thus, genetic defects introduced in the developmental cascade contribute to structural malformations. But organogenesis requires the genetic choreography to occur not just in sequence but also during a sensitive time period. What happens when the genes regulating the developmental time clock [9
] are mutated or delayed by epigenetic factors? Can the organ continue to develop outside the fetal environment and if not, what are the consequences of retaining fetal characteristics in an adult?
The best model for addressing these questions related to organogenesis and timing is the premature infant. Modern neonatology has extended viability to 23 weeks in the 40 week human gestation, but what are the consequences of completing less than 60% of gestation on the human fetus? The obvious immediate consequences seen in any neonatal unit include poor lung function, intestinal deficiencies, immune system deficiencies, and poor renal function. These immediate problems combined with therapies used to combat them can lead to significant morbidity and mortality in the premature infant.
Infants surviving the immediate organ deficiencies and therapies of premature birth, however, do not grow into healthy adult. Rather, numerous epidemiological and clinical observations have shown that prematurity is associated with many adult onset diseases (Table ). The epidemiologic basis for late or adult-onset diseases, following either premature birth or low birth weight, includes poor pre-natal maternal nutrition and smoking during pregnancy both of which are reviewed previously [11
]. But these epidemiologic correlations do no provide a theoretical, molecular basis for explaining late onset of diseases related to delays in organogenesis.
Late-onset clinical presentations associated with premature birth.
Conceptualizing the effect of small developmental disturbances on solid organs such as the lung, liver, intestines and kidneys can be difficult because morphological differences may be minor. If one considers an organ with functional characteristics that are morphologic, however, then the concept of developmental disruption becomes obvious. A good example would be the hand. Numerous examples of impaired development through either genetic defects or blood flow interruption result in obvious deformities including webbing, polydactyl and complete and partial absences of digits. Visualization of changes in solid organ development is less obvious but the effects on function have the same impact as a hand without a thumb.
As an example, lung growth and development (Fig. ), like the hand, is precisely timed throughout gestation [13
]. In humans functional respiratory structures are complete by birth although alveolarization continues until an adult number of alveoli are reached at 2–3 years of age; however, in the mouse and rat functional alveolarization is completed after birth by day of life (DOL) 5. The embryonic human lung can be identified at 28 days with trachea and bronchi expanding from the lung bud. During the pseudoglandular stage further branching occurs and the airways are lined with multipotent stem cells. Branches of terminal bronchioles are formed and respiratory bronchioles begin as out pockets. These pockets of epithelial cells expand during the saccular and alveolar stages to form terminal alveoli which are necessary for gas exchange. Normal lung organogenesis is a carefully programmed process that can be disrupted and result in abnormal lung structure and function.
Figure 1 Lung development and the effects of developmental disruption. A-Normal lung differentiation; B-Disrupted growth; C-Premature transition resulting in abbreviated pseudoglandular stage; D-Delayed transition leading to incomplete canalicular and saccular/alveolar (more ...)
Three types of disruptions can occur in lung organogenesis (Fig. ). The most obvious are genetic mutations that completely block lung growth (Fig. ) resulting in a non-functional lung and fetal or neonatal demise [14
]. The less obvious yet equally important disruptions to normal lung function are those that impair or impede specific events in organogenesis. A genetic mutation or environmental insult to the fetus through the mother can result in either a premature (Fig. ) or prolonged (Fig. ) transition to a later stage of development. In the former case, structures, which are normally propagated, are lost along with their function. In the latter case, slower growth results in late development of important structures and functions prior to normal birth. In both cases, the histologic structure of the lung may grossly appear normal; however, the actual functional equivalency of the cells may be significantly altered.
Two questions arise from these models. First, can discontinuity between stages in a developmental cascade alter the cell type or their function? During each stage of organogenesis, stem cells are progressing through stages of transit amplifying (TA) cells in which the functional characteristics of each generation is maturing to that of a terminally differentiated cell type (Fig. ). During organogenesis the environment progresses through different temporal states (TS) in which the TA cells produce products necessary for differentiation that are not necessary needed for final functionality. As an example, in TS-A (Fig. ) may produce products that are required for development of specialized structures. Proteins produced in TS-B, however, may induce rapid cell growth and increased tissue complexity. If this process is abbreviated as proposed in Fig. , then the terminal differentiation state can not be reached and downstream functional characteristics of the cell will be missing.
Figure 2 Stem cell differentiation during organogenesis. Progression through varying transit amplifying (TA) cell stages of functional maturity (bottom gray scale) called temporal states (TS) A-C. Various geometric shapes (stars, ovals, triangles, etc) represent (more ...)
Secondly, when there is a delay in progression (Fig. ), does normal organogenesis continue postnatally? Because the fetal environment plays a significant role in differentiation, preterm birth arrests organ development with retention of fetal characteristics. Thus, all or part of the organ would remain in a fetal temporal state (e.g. TS-B; Fig. ), significantly different from that of the terminally differentiated tissue (TS-C, Fig. ).
Delay and disruption of organogenesis could result in two different phenotypes because the immature tissue would be missing different functional gene sets. Distinguishing these two would be dependent upon a developmental gene expression knowledge base. However, it is possible to get similar morphologic phenotypes but different physiology. As illustrated in Figure , if one takes a case in which a protein is required for triggering the normal final stage of complex organ architecture (Fig. ), disrupting the stage that precedes this protein could result in its premature activation without sufficient support structure (Fig. ), resulting in a hypoplastic structure. Likewise, prolonging the preceding stage by delaying activation of these proteins could lead to some gain in complexity in the supporting structure. However, similar decreased structural complexity of this final stage would result (Fig. ) due to a decrease in the time interval between activation of terminal differentiation and birth. Morphologically both tissues would appear hypoplasia of the terminal structure; because the balance among structural components is altered the physiology could be quite different. Thus, distinctions between delayed and disrupted organogenesis will be dependent upon biochemical makers of differentiation, structural characteristics of the tissue, and perhaps most importantly the physiologic functions of the modified organ.
Figure 3 Architecture and developmental programming. Normal architecture depends on building layers that are based on both quantity and timing of components with an overall structural size limitation (Panel A). Premature activation of subsequent stages disrupts (more ...)
There are numerous examples of failed differentiation due to specific gene defects leading to deficient cell types. In the lung, mutations in lamellar body formation lead to structural changes leading to chronic inflammatory diseases [17
]. Mutation of Arnt, a gene required for hepatic vascularization, results in a persistent embryonic liver phenotype that demonstrate decreased survival and changes in fat and carbohydrate metabolism after birth [20
]. Bapx1 is required for separation of the pancreas and spleen into distinct organs [21
]. In each of these single gene defects, slow or inhibited organogenesis results in a suboptimal functioning organ and a subsequent pathology.
The physical environment of the fetus can also impact the dynamics of organogenesis. One of the more common prototypes is congenital diaphragmatic hernias which result in hypoplastic lung disease. Surgical blocking of the trachea [22
] or in utero
cystic fibrosis transmembrane conductance regulator (CFTR) gene therapy [23
] to promote stretch induced lung differentiation can reverse the effect, but postnatally this condition has a poor prognosis with high morbidity and mortality. Likewise, amniotic fluid blockage to the intestines alters villus formation and function [24
Probably the least understood and most subtle effectors of developmental delay are fetal environmental insults. Maternal smoking, alcohol consumption, diabetes, and diehylstilbesterol use have all been linked to significant long term consequences on the fetus [11