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PART 5 Reproduction and Development

Learn to Predict

From page 1078

Answer

in the absence of high blood levels of estrogen and progesterone,cause the secretory units of the breasts to secrete milk. Oxytocincauses expulsion of the milk from the breast in response to suck-ling. Refraining from breastfeeding for a period of time results in noprolactin pulses. Consequently, the secretory units fail to secretemilk and quickly lose the capacity to do so. Ming can retain thecapacity to breastfeed her baby by using a breast pump while shetakes the prescribed antibiotic. The breast pump simulates suck-ling and stimulates the pulses of prolactin secretion. After finishingthe antibiotic, Ming can then continue to breastfeed her baby.

In this chapter, we learned that the production of breast milk dependson a number of hormones. Estrogen causes the duct system of thebreast to develop and is mainly responsible for the enlargementof the breasts during pregnancy. Progesterone causes the devel-opment of the breasts’ secretory alveoli, which enlarge but do notusually secrete milk during pregnancy. Other hormones necessaryfor development of the breast include growth hormone, prolactin,thyroid hormones, glucocorticoids, and insulin. Human somatotro-pin and human placental lactogen are secreted from the placenta.The high levels of estrogen and progesterone inhibit the effectof prolactin on milk secretion during pregnancy. After parturition,blood levels of estrogen, progesterone, and prolactin decrease.This chapter also explained that suckling stimulates theperiodic pulses of prolactin and oxytocin. The pulses of prolactin,

Answers to the rest of this chapter’s Predict questions are inappendix G.

29.1 Prenatal Development (p. 1079)

1. Prenatal development is divided into three parts: the germinal period,during which the germ layers form; the embryonic period, when theorgan systems form; and the fetal period, characterized by growthand maturation.2. Postovulatory age is 14 days less than clinical age.3. The stages of postnatal development are the neonatal period (birth to1 month), infancy (1 month to 1–2 years), childhood (1–2 years topuberty), adolescence (puberty to 20 years), and adulthood (20 yearsto death).

Summary

Somite Formation

Segments called somites, which develop along the neural tube, give riseto the musculature, vertebral column, and ribs.

Formation of the Gut and Body Cavities

1. The digestive tract forms as the developing embryo closes off part ofthe yolk sac.2. The coelom develops from small cavities that fuse within theembryo.

Fertilization

Fertilization, the union of the oocyte and sperm, results in a zygote.

Limb Bud Development

The limbs develop from proximal to distal as outgrowths called limb buds.

Early Cell Division

The cells of the early embryo are pluripotent (capable of making any cell ofthe body). In the very early stages of development, the cells are totipotent,meaning that each cell can give rise to any tissue necessary for development.

Development of the Face

The face develops from the fusion of five major tissue processes.

Morula and Blastocyst

The product of fertilization undergoes divisions until it becomes a masscalled a morula and then a hollow ball of cells called a blastocyst.

Development of the Organ Systems

1. The epidermis of the skin develops from the ectoderm, and the der-mis develops from the mesoderm or from neural crest cells. Melano-cytes and sensory receptors develop from neural crest cells.2. The skeletal system develops from mesoderm or neural crest cells.3. Muscle develops from myoblasts, which migrate from somites.4. The brain and spinal cord develop from the neural tube, and theperipheral nervous system develops from the neural tube and theneural crest cells.5. The special senses develop mainly as neural tube or neural crest cellderivatives.6. Many endocrine organs develop mainly as evaginations of the brainor digestive tract.7. The heart develops as two tubes fuse into a single tube, which bendsand develops septa to form four chambers.8. The peripheral circulation develops from mesoderm as blood islandsbecome hollow and fuse to form a network.

Implantation of the Blastocyst andDevelopment of the Placenta

The blastocyst implants into the uterus about 7 days after fertilization.The placenta is derived from the trophoblast of the blastocyst.

Formation of the Germ Layers

All body tissues are derived from three primary germ layers: endoderm,mesoderm, and ectoderm.

Neural Tube and Neural Crest Formation

The nervous system develops from a neural tube that forms in the ecto-dermal surface of the embryo and from neural crest cells derived from thedeveloping neural tube.