Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Endocrine System
Classification, Structure, and Functions of Endocrine Glands
Solitary Hormone-Producing Cells - Morphological and Functional Development of the Endocrine System in Ontogenesis

Endocrine glands play a vital role in the body's GROWTH AND DEVELOPMENT. Their Hormones coordinate all physiological Functions and regulate the periodicity of the body's functional processes, or biological rhythms.

Endocrine glands begin functioning during the prenatal period. Hormones and BIOLOGICALLY ACTIVE SUBSTANCES already influence the growth and Development of the embryo and fetus. Most hormones start being synthesized as early as the second month of intrauterine development. As receptors for Pituitary Hormones appear in the endocrine glands, interconnections begin to form between them, reaching full maturity after birth.

In the Postnatal period of development, the Endocrine system plays a crucially important role in the body's growth and maturation. Until Puberty, the leading role in The Development of Organs and body systems belongs to Growth Hormone, THYROID HORMONES, Insulin, and subsequently Sex Hormones. Many hormones, including thyroid hormones, androgens, and estrogens, determine the onset and rate of puberty.

The Pituitary Gland begins functioning around the 9th–10th week of the prenatal period. In newborn males, it weighs 0.125 g, and in females, 0.250 g. The most significant increase in pituitary mass occurs during puberty. The Cells of the posterior pituitary mature During the first year of life. In newborns, the thyroid-stimulating hormone, adrenocorticotropic hormone, and growth hormone—produced by adenohypophyseal cells—play an exceptionally important role. Growth hormone levels are highest in newborns. After birth, its concentration in the Blood drops significantly, reaching adult normal levels by 3–5 years of age.

The Thyroid Gland is one of the first to develop during ontogenesis. At birth, its mass is 1–5 g, with maximum mass (14–15 g) reached at 15–16 years. In the postnatal period, The production of triiodothyronine and thyroxine increases, ensuring mental, physical, and sexual development. A deficiency of these hormones (especially between 3 and 6 years of age) causes intellectual disability (cretinism). During puberty, There is a surge in thyroid activity, which manifests as increased excitability of The Nervous system. A decline in gland activity is observed between 21 and 30 years of age.

The Parathyroid glands begin to form during the 5th–6th week of the prenatal period. In newborns, the mass of the glands averages 5 mg, compared to 75–85 mg in adults. Maximum gland activity is observed during the first 7 years of life, particularly in the first two years. Insufficient parathyroid hormone production causes tooth decay and Hair loss, whereas excessive production leads to increased ossification.

The Adrenal Glands in a newborn weigh about 7 g. Growth of the glands continues up to the age of 30. Development of the cortex is completed by the beginning of the second year of life. From the very first days after birth, glucocorticoids participate in mediating stress responses. Peak glucocorticoid production is noted at ages 1–3 and during puberty. The adrenal medulla begins producing catecholamines (predominantly norepinephrine) starting from the 16th week of the prenatal period. The primary growth of the medulla occurs at ages 3–8 and during puberty.

The Pineal Gland in newborns weighs about 7 mg, and in adults, 200 mg. Melatonin, produced by the pineal gland, inhibits sexual and physical development and blocks thyroid function. A decrease in the hormone-producing function of the pineal gland is observed from ages 4–7, and during puberty, the concentration of this hormone in the blood is reduced.

The Gonads develop from a common embryonic primordium. Sexual differentiation takes place during the 7th–8th week of embryonic development.

Male gonads. Between the 11th and 17th weeks, the androgen levels in a male fetus reach values characteristic of an adult Organism. Thanks to this, the development of sex hormones proceeds along the male pathway. The mass of the Testis in a newborn is 0.3 g, and its hormone-producing activity is reduced. Under METABOLISM/18.html">The Influence of gonadotropin-releasing hormone, it gradually increases from ages 12–13, reaching adult levels by 16–17 years. This surge in hormone-producing activity triggers the pubertal growth spurt, the appearance of secondary sex characteristics, and, after age 15, the activation of Spermatogenesis.

Female gonads. Starting from the 20th week of the prenatal period, primordial follicles begin to form in the Ovary. By birth, the mass of the ovary is 5–6 g, and in an adult woman, 6–8 g. Throughout Postnatal ontogenesis, three periods of ovarian activity are distinguished: neutral (from birth to 6–7 years), prepubertal (from 8 years to the first menstruation), and pubertal (from the first menstruation to menopause). At all stages, follicular cells produce varying amounts of estrogens. A low estrogen level up to age 8 allows for the female Differentiation of the Hypothalamus. Estrogen production during puberty is sufficient to drive the pubertal spurt (skeletal growth and the development of secondary sex characteristics). A gradual increase in estrogen production leads to menarche and the establishment of a regular Menstrual cycle.

REVIEW AND SELF-Control Questions:

1. Provide a Classification of endocrine glands and explain The basis of this classification.

2. Name the lobes of the pituitary gland and describe which hormones each lobe synthesizes and how these hormones affect body functions.

3. On the basis of what features is the hypothalamic-pituitary system distinguished?

4. Describe The Structure of the thyroid gland and the functions of its hormones.

5. Describe the STRUCTURE OF THE adrenal glands and their hormones.

6. What are the pancreatic islets, where are they located, and what functions do they perform?

7. Describe the Cytology/cytology/65.html">Diffuse endocrine system.