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Establishment of the Basic Embryonic Body Plan

After gastrulation is complete, the embryo proper consists of a flat, three-layered disk containing the ectodermal, mesodermal, and endodermal germ layers. Its cephalocaudal axis is defined by the location of the primitive streak. Because of the pattern of cellular migration through the primitive streak and the regression of the streak toward the caudal end of the embryo, a strong cephalocaudal gradient of maturity is established. This…

Formation of Germ Layers and Early Derivatives

While it is implanting into the uterine wall, the embryo undergoes profound changes in its organization. Until the time of implantation, the blastocyst consists of the inner cell mass, from which the body of the embryo proper arises, and the outer trophoblast, which represents the future tissue interface between the embryo and mother. Both components of the blastocyst serve as the precursors of other tissues that…

Cleavage and Implantation

The act of fertilization releases the ovulated egg from a depressed metabolism and prevents its ultimate disintegration within the female reproductive tract. Immediately after fertilization, the zygote undergoes a pronounced shift in metabolism and begins several days of cleavage. During this time, the embryo, still encased in its zona pellucida, is transported down the uterine tube and into the uterus. Approximately 6 days later, the embryo sheds…

Molecular Basis of and Genetic Methods for Studying Embryonic Development

The application of new techniques in molecular biology and genetics continues to revolutionize the understanding of the mechanisms underlying both normal and abnormal embryonic development. It is impossible to have a contemporary understanding of embryonic development without integrating fundamental molecular and morphological aspects of embryology. This chapter introduces the most important families of molecules known to direct embryonic development and some important genetic techniques that help…

Transport of Gametes and Fertilization

Sexual reproduction is based upon the union of an egg and a sperm. This chapter explains how the egg and sperm cells come together in the female reproductive tract so that fertilization can occur. It then outlines the complex set of interactions involved in fertilization of the egg by a sperm. OVULATION AND EGG AND SPERM TRANSPORT Ovulation Toward the midpoint of the menstrual cycle, the…

Getting Ready for Pregnancy

Human pregnancy begins with the fusion of an egg and a sperm within the female reproductive tract, but extensive preparation precedes this event. First, both male and female sex cells must pass through a long series of changes ( gametogenesis ) that convert them genetically and phenotypically into mature gametes , which are capable of participating in the process of fertilization. Next, the gametes must be…

Discussion of Clinically Oriented Problems

Chapter 1 1 The secondary sexual characteristics develop, reproductive functions begin, and sexual dimorphism becomes more obvious during puberty. The pubertal changes are not the same in males and females. In females, the age of presumptive puberty is after 8 years, with the process largely completed by age 16 years. In males, the age of presumptive puberty is after 9 years, with the process largely completed…

Common Signaling Pathways Used During Development

During the process of embryonic development, undifferentiated precursor cells differentiate and organize into the complex structures found in functional adult tissues. This intricate process requires cells to integrate many intrinsic and extrinsic cues for development to occur properly. These cues control the proliferation, differentiation, and migration of cells to determine the final size and shape of the developing organs. Disruption of these signaling pathways can result…

Human Birth Defects

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 ,…

Integumentary System

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 Ears

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…

Nervous System

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…

Development of Limbs

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…

Muscular System

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 ,…

Skeletal System

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…

Cardiovascular System

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…

Urogenital System

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…

Alimentary System

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…

Respiratory System

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…

Pharyngeal Apparatus, Face, and Neck

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 ,…