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…

In this chapter, the types and functions of skin in different parts of the body are described first, followed by the microstructure of the epidermis and dermis, and the appendages of skin, including the pilosebaceous units, sweat glands and nails. The development of skin, natural skin lines and age-related changes, and clinical aspects of skin, e.g. grafts, surgical skin flaps and wound healing, are also described.…

The cardiovascular system carries blood from the heart to all parts of the body through a series of tubes, all but the smallest of which are muscular. The muscle in these tubes is of two types: smooth muscle is characteristic of the walls of blood vessels, whereas cardiac muscle provides the walls of the heart chambers with their powerful contractile pumping action. The general characteristics and…

The musculoskeletal system comprises the specialized connective tissues of the articulated bony skeleton (including bone, cartilage and tendon) and the skeletal muscles that act across the articulations. Connective tissues are characterized by a sparse population of cells within an abundant and physically durable extracellular matrix. All of the specialized cells of the musculoskeletal system are related members of the connective tissue family and are derived from…

Blood is an opaque fluid with a viscosity greater than that of water (mean relative viscosity 4.75 at 18 °C), and a specific gravity of 1.06 at 15 °C. It is bright red when oxygenated, as in the systemic arteries, and dark red to purple when deoxygenated, in systemic veins. Blood is a mixture of a clear liquid, plasma, and cellular elements, and consequently the hydrodynamic…

The nervous system has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain, spinal cord, optic nerve and retina, and contains the majority of neuronal cell bodies. The PNS includes all nervous tissue outside the CNS and consists of the cranial and spinal nerves, the peripheral autonomic nervous system (ANS) and the special senses (taste,…

Cells evolved as single, free-living organisms, but natural selection favoured more complex communities of cells, multicellular organisms, where groups of cells specialize during development to carry out specific functions for the body as a whole. This allowed the emergence of larger organisms with greater control over their internal environment and the evolution of highly specialized organic structures such as the brain. The human body contains more…