Prenatal Stages Development of a human from fertilization to delivery at ‘full term’ averages 266 postfertilization days, or 9.5 lunar months (28 day units). It has long been customary to compute the length of a pregnancy, whether in a normal birth or an abortion, from the first day of the last menstrual period of the mother but, as ovulation usually occurs near the fourteenth day of…

Development of the Posterior Coelom Wall From stage 11 (29–30 days postfertilization; see Fig. 23.3 for comparison of postfertilzation days and the clinical scale of postmenstrual weeks) the dorsal region of the wall of the intraembryonic coelom is composed of a mesenchymal population termed intermediate mesenchyme. Predictive fates of epiblast cells that transform to mesenchyme on ingression through the primitive streak show temporally specified induction of…

Postpharyngeal Foregut The primitive gut is divided by head- and tail-folding into three main compartments. The foregut extends from the buccopharyngeal membrane to its continuation into the central midgut region via the cranial intestinal portal. The midgut extends between the intestinal portals and, in the early embryo, is in wide communication with the yolk sac. The hindgut extends from the caudal intestinal portal to the cloacal…

Development of the Respiratory Tree The development of the respiratory system can be conceptualized as the growth of six intertwined trees: the bronchial tree of conducting airways and terminal gas-exchanging sacs; the systemic arterial and venous trees that perfuse and drain the airways; the pulmonary arterial and venous trees that supply and drain the alveolar membrane, anastomose with the systemic and venous trees, and are pivotal…

Overarching concepts The limbs develop from permissive and instructive interactions between epithelium and mesenchyme at specified times and places along the lateral body wall. The outgrowths are initiated at defined positions along the embryonic axis where these cell lines continue to proliferate, giving rise to local thickenings that soon develop into limb buds ( ). The early limb bud elongates and, gradually, the different limb regions…

Vertebrae and their alternating intervertebral discs are one of the main manifestations of body segmentation or metamerism. A chain of segments arranged in sequence allows the overall structure to bend when it is moved by the associated muscles. The original body segments, the somites, are formed from paraxial mesenchymal populations found lateral to the neural tube and notochord in the early embryo ( Ch. 12 ).…

Head and neck development is distinct from that of the trunk, utilizing region-specific genes, signalling mechanisms and morphogenetic processes. The boundary between the head and trunk is not always clear. The neck is contiguous rostrally with the head but it also joins and shares developmental processes caudally with the back ( Ch. 18 ) and the upper limb ( Ch. 19 ). Studies of developmental gene…

Inner ear The production of a precisely positioned and functionally well-tuned inner ear depends on genetic patterning and a cascade of transcription signals expressed by numerous tissues, including the developing inner ear and its surrounding periotic mesenchyme, the adjacent hindbrain, neural crest and notochord ( , ). The first signs of inner ear development are visible shortly after those associated with the developing eyes. Two patches…

The development of the eye involves a series of inductive interactions between neighbouring tissues in the embryonic head. These are the neurectoderm of the forebrain (which forms the sensory retina and accessory pigmented structures), the surface ectoderm (which forms the lens and the corneal epithelium) and the intervening neural crest mesenchyme (which contributes to the fibrous coats of the eye and to tissues of the anterior…

The entire nervous system and the special sense organs originate from three sources, each derived from specific cell populations of the early epiblast termed neural ectoderm. The first source to be clearly delineated is the neural plate, which gives rise to the central nervous system (CNS), the somatic motor nerves and the preganglionic autonomic nerves. The second source is from cells at the perimeter of the…

The early embryonic circulation is symmetric ( Fig. 13.1 ). It is modified throughout development to produce a functioning heart and fetal circulation that is connected to the placenta, and changes rapidly at birth to accommodate disconnection from the placenta and the start of gaseous exchange in the lungs. Major restructuring of early vessels occurs as the embryo grows; anastomoses form and then disappear, capillaries fuse…

Specification of the Body Axes and The Body Plan Embryos can be thought of as being constructed with three orthogonal spatial axes (cephalocaudal, dorsoventral and laterolateral), plus a temporal axis. In mammalian embryos, axes cannot be specified at very early stages: embryonic axes can be defined only after the early extraembryonic structures have been formed and the inner cell mass can be seen. The position of…

Genes in Development Study of secondary oocytes before and after fertilization, and of the selection of embryos for implantation after in vitro fertilization, has identified the range of maternal genes ready for expression to ensure cleavage, morula and blastocyst formation. Embryonic genome activation occurs in the 4–8 cells stage ( ) when embryonic cells express polarity genes, initiating cell–cell interaction ( ); by 6 days post fertilization…

Conceptus with a Bilaminar Embryonic Disc At stage 6 the conceptus is composed of the walls of three cavities. The large chorionic cavity is surrounded by a meshwork of trophoblast and developing villi, and lined with extraembryonic mesoblast. The chorion, trophoblast and extraembryonic mesoblast enclose the extraembryonic coelom and contain the much smaller amniotic cavity and yolk sac that abut at the embryonic bilaminar disc where…

Implantation Implantation involves the initial attachment of the trophoblastic wall of the blastocyst to the endometrial luminal epithelium, stimulating the decidual response. The blastocyst is present within the uterine cavity for some 72 hours prior to implantation, and during this time there is an interactive dialogue between the implanting blastocyst and endometrial decidual stromal cells ( ). In vitro culture of human preimplantation embryos shows that normally…

Understanding the spatial and temporal developmental processes that take place within an embryo as it develops from a single cell into a recognizable human is the challenge of embryology. The control of these processes resides within the genome; fundamental questions remain concerning the genes and interactions involved in development. Staging of Embryos For the purposes of embryological study, prenatal life is divided into an embryonic period…