Birth defects (anomalies) are developmental disorders present at birth. Defects are the leading cause of infant mortality (fetal outcome). They may be structural, functional, metabolic, behavioral, or hereditary. Birth defects are a global problem; close to 8 million children worldwide have a serious birth defect. Classification of Birth Defects The most widely used reference guide for classifying birth defects is the International Classification of Diseases ,…

The integumentary system consists of skin and its appendages: sweat glands, nails, hairs, sebaceous glands, arrector muscles of hairs (arrector pili muscles), mammary glands, and teeth. Development of Skin and Appendages The skin is a complex organ system, and it is the body's largest organ. The skin consists of two layers ( Fig. 19.1 ): The epidermis is a superficial epithelial tissue that is derived from…

Development of Eyes and Related Structures 17 The eyes begin to develop in 22-day embryos when optic grooves appear ( Fig. 18.1 A and B ). The eyes are derived from four sources: Neuroectoderm of the forebrain Surface ectoderm of the head Mesoderm between the previous two layers Neural crest cells The neuroectoderm differentiates into the retina, posterior layers of iris, and optic nerve. The surface…

The nervous system consists of three main regions: The central nervous system (CNS) consists of the brain and spinal cord and is protected by the cranium and vertebral column. The peripheral nervous system (PNS) includes the neurons outside the CNS as well as the cranial nerves and spinal nerves (and their associated ganglia), which connect the brain and spinal cord with peripheral structures. The autonomic nervous…

Early Stages of Limb Development 15 The upper limb buds are visible by day 24, and the lower limb buds appear 1 or 2 days later, with the activation of a group of mesenchymal cells in the somatic lateral mesoderm ( Fig. 16.1 A ). Homeobox (Hox) genes regulate patterning in the formation of the limbs. The limb buds form deep to a thick band of…

The muscular system develops from mesoderm , except for the muscles of the iris of the eye, which develop from neuroectoderm (neural crest cells) , and the muscles of the esophagus, which are thought to develop by transdifferentiation from smooth muscle. Myoblasts (embryonic muscle cells) are derived from mesenchyme (embryonic connective tissue). Three types of muscle—skeletal, cardiac, and smooth—are formed during the embryonic period. MYOD ,…

As the notochord and neural tube form in the third week, the intraembryonic mesoderm lateral to these structures thickens to form two longitudinal columns of paraxial mesoderm ( Fig. 14.1 A and B ). Toward the end of the third week, these dorsolateral columns, located in the body (trunk), become segmented into condensed blocks of mesoderm (somites) (see Fig. 14.1 C ). Externally, the somites appear…

The cardiovascular system is the first major system to function in the embryo. The primordial heart and vascular system appear in the middle of the third week ( Fig. 13.1 ). This precocious cardiac development occurs because the rapidly growing embryo can no longer satisfy its nutritional and oxygen requirements by diffusion alone. Consequently, there is a need for an efficient method of acquiring oxygen and…

The urogenital system is divided functionally into two different embryologically component parts: the urinary system and the genital system . The urogenital system includes all the organs involved in reproduction and forming and voiding urine. Embryologically, the systems are closely associated, especially during their early stages of development. The urogenital system develops from the intermediate mesenchyme (primordial embryonic connective tissue consisting of mesenchymal cells) derived from…

T he alimentary system (digestive system) is the digestive tract from the mouth to the anus, with all its associated glands and organs. The primordial gut forms during the fourth week as the head, caudal eminence (tail), and lateral folds incorporate the dorsal part of the umbilical vesicle (yolk sac; see Chapter 5 , Fig. 5.1 ). The primordial gut is initially closed at its cranial…

The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. Respiratory Primordium 9 The respiratory system starts as a median outgrowth, the laryngotracheal groove , which appears in the floor of the caudal end of the anterior foregut (primordial pharynx) ( Fig. 10.1 B and C ; see also Fig. 10.4 A ). This primordium of the tracheobronchial…

T he pharyngeal apparatus consists of pharyngeal arches, pouches, grooves, and membranes ( Fig. 9.1 ). These early embryonic structures contribute to the formation of the face and neck. Open full size image Fig. 9.1 Pharyngeal apparatus. A , Dorsal view of the upper part of a 23-day embryo. B to D , Lateral views show later development of the pharyngeal arches. E to G ,…

Early in the fourth week of development of the embryo, the intraembryonic coelom appears as a horseshoe-shaped cavity ( Fig. 8.1 A ). The bend in the cavity at the cranial end of the embryo represents the future pericardial cavity , and its limbs (lateral extensions) indicate the future pleural and peritoneal cavities . The distal part of each limb of the intraembryonic coelom is continuous…

The placenta and fetal membranes separate the fetus from the endometrium , the inner layer of the uterine wall. An interchange of substances, such as nutrients and oxygen, occurs between the maternal and fetal bloodstreams through the placenta. The vessels in the umbilical cord connect the placental circulation with the fetal circulation. The fetal membranes include the chorion , amnion , umbilical vesicle , and allantois…

The transformation of an embryo to a fetus is gradual, but the name change is meaningful because it signifies that the primordia of all major systems have formed. Development during the fetal period is primarily concerned with rapid body growth and differentiation of tissues, organs, and systems. A notable change occurring during the fetal period is the relative slowdown in the growth of the head compared…

All major external and internal structures are established during the fourth to eighth weeks. By the end of this embryonic period, the main organ systems have started to develop. As the tissues and organs form, the shape of the embryo changes, and by the end of this period, the embryo has a distinctly human appearance. Because the tissues and organs are differentiating rapidly, exposure of embryos…

Rapid development of the embryo from the trilaminar embryonic disc during the third week (see Fig. 4.3 H ) is characterized by: Appearance of primitive streak Development of notochord Differentiation of three germ layers The third week of development coincides with the week after the first missed menstrual period, that is, 5 weeks after the first day of the last normal menstrual period. Cessation of menstruation…

As implantation of the blastocyst occurs, morphologic changes in the embryoblast produce a bilaminar embryonic disc composed of epiblast and hypoblast ( Fig. 3.1 A ). The embryonic disc gives rise to the germ layers that form all the tissues and organs of the embryo. Extraembryonic structures forming during the second week are the amniotic cavity, amnion, umbilical vesicle connecting stalk, and chorionic sac. Open full…

Human development begins at fertilization when a sperm fuses with an oocyte to form a single cell, the zygote . This highly specialized, totipotent cell (capable of giving rise to any cell type) marks the beginning of each of us as a unique individual. The zygote, just visible to the unaided eye, contains chromosomes and genes that are derived from the mother and father. The zygote…