Human Anatomy (with the fundamentals of dynamic and sports morphology) - Ivanitsky, M. F. 2008

Age Morphology
Specific Indicators of Biological Age

Unlike the Integral Indicators of biological age, specific or local indicators characterize the state of individual systems, organ apparatuses, Organs, or Tissues. The division of biological age indicators into these two groups is largely conventional: it is determined not so much by their informative Specificity as by uneven study and accessibility during mass population screenings (specific indicators are less studied; their investigation often requires special laboratory equipment, microscopic techniques, and Reagents). Specific indicators of biological age are identified by examining age-related changes in Organ Systems and apparatuses.

Age-related Features of the Skeletal System. Morphological changes undergone by bone during growth (for details, see p. 46) affect its strength. The main trend of these changes involves the accumulation of Bone tissue during childhood and the replacement of Cartilage tissue with it (Stage I), followed by the bone acquiring a monolithic Structure with the disappearance of epiphyseal cartilages (Stage II), and later by a subtly initiated and age-progressive loss of bone mass (Stage III). The final stage is also characterized by additional ossification of cartilage tissue (e.g., costal cartilages). Bone exhibits maximum mechanical resistance at Stage II. During childhood, the combination of cartilage and bone tissue areas within a bone reduces its strength; injuries occur at the boundary of these biological media (e.g., a fracture of the radius in the distal third of the forearm, commonly referred to as a "typical fracture"). Bone mass loss at Stage III, leading to the disappearance of some trabeculae (Osteoporosis) and the thinning of compact substance, makes the bone more fragile. One of the most easily injured skeletal sites in individuals over 75–80 years of age is the femoral neck, where fractures frequently occur from impacts and falls.

Age-related features of bone joints. Postnatal Changes in the joint-ligament apparatus manifest as an uneven increase in the dimensions of articular surfaces, The Development of their cartilaginous cover, as well as an increase in the Collagen component of Connective Tissue and the ground substance of cartilage tissue. Upon completion of tissue Differentiation of the Joint Capsule and intra-articular structures, chondrification of the menisci, discs, and partially the intra-articular ligaments and joint capsule occurs. These structural shifts inevitably affect the mechanical Properties of the joint ligamentous apparatus: its strength increases with age. Simultaneously, the Muscles acting on the joints undergo age-related changes, resulting in age-related alterations in joint mobility. Discrepancies in the dimensions of the HEAD and socket of joints (the extent of the head's articular surface predominates) cause greater mobility in childhood than after maturity is reached, when the joint becomes more congruent. The development of bony outgrowths (osteophytes) along the edges of articular surfaces during Aging contributes to restricted joint mobility in middle-aged and elderly individuals.

Age-related features of The Muscular System. The growth of Skeletal Muscle is determined by an increase in the thickness and length of its fibers (the terminal sections of fibers at the border with the tendon serve as the zone of longitudinal growth). Intramuscular layers of connective tissue become saturated with fibrous structures with age, undergoing collagenization.

The uneven growth of individual muscle groups, which manifests in age-related differences in muscle strength, is associated with the gradual refinement of body movements. The mastery of upright posture and walking at the end of the 1st and beginning of the 2nd year of life leads to accelerated development and an increase in the strength of muscles that maintain the body in a vertical position and ensure the act of walking. Later, the gain in strength of these muscles decreases, but due to the activation of manipulative, educational, and labor activities, the strength of the hand flexors, as well as the forearm flexors and extensors, increases. The general rule is as follows: the further a muscle group has progressed in its ontogenetic development, the smaller its annual strength gains in subsequent periods. For example, the trunk extensors, which ensure human upright posture, reach 39% of their strength capabilities typical of age 17 by age 8, whereas the gastrocnemius muscles reach only 23%. Therefore, from age 8 to 17, the strength of the trunk extensors increases 1.5-fold, and that of the gastrocnemius muscles 3-fold.

Muscle aging is primarily expressed as a decrease in fiber diameter, physiological cross-section, and strength parameters. Simultaneously, the connective tissue framework and the neurovascular apparatus of the muscle undergo reorganization. Muscles subjected to insufficient loads undergo changes earlier than others. Physical training can inhibit muscle aging.

Age-related features of Internal Organs. The structure of the digestive, respiratory, and urogenital systems changes after a child's birth in accordance with the performance of new Functions associated with Separation from the maternal Organism. However, children exhibit the same variety of organ forms as adults. Age-related changes are uneven. For instance, the duodenum grows in length relatively faster than the jejunum and ileum. Clusters of lymphoid tissue in the wall of the ileum (Aggregated lymphoid follicles) reach their greatest prominence only during childhood: after 13 years of age, their number does not increase (for details on the STRUCTURE OF THE lymphoid-immune system, see p. 278). The relative dimensions of organs that play a different role in the organism during intrauterine development than after birth decrease (for example, the relative weight of the Liver, which acts as a hematopoietic organ in utero, constitutes 1/2 of the embryo's body mass, whereas in an adult it is only 1/20).

Respiratory organs do not participate in gas exchange processes during the intrauterine period. Alveolar dimensions in newborns are three times smaller than in adults. Final differentiation of the bronchial tree is completed only by age 7. However, alveolar growth continues up to 24–28 years of age.

While structural "maturation" in the Lungs stretches over years, nephron formation in the Kidney is completed by the 20th postnatal day. Further increases in renal tissue mass are coupled with the GROWTH AND DEVELOPMENT of already existing structural elements. In an area of renal tissue where up to 50 glomeruli are identified in a newborn, there are 18–20 in a 7–8-month-old infant, and only 7–8 in an adult.

Aging of internal organs is characterized by a decrease in their mass, atrophy of specific tissues (parenchyma), and proliferation of connective tissue (stroma). The thinning of the walls of certain hollow organs leads to their dilation, elongation, and the appearance of hernia-like mucosal protrusions through the muscular layer. Such formations, resembling "false diverticula," are observed, for example, in the Large Intestine. In the lungs, some interalveolar septa disappear, and small groups of alveoli merge into a single large one. In the kidney, partial Atrophy of the parenchyma occurs: between the ages of 30 and 80, the loss of nephrons ranges from 1/3 to 1/2 of their initial number. However, the remaining nephrons remain morphologically intact and functionally competent.

Age-related features of The Cardiovascular system. Absolute Heart dimensions increase after birth. Compared to the neonatal period, its mass increases 2.5-fold by 2–3 years of age, 4-fold by 4–6 years, 5.5-fold by 7–10 years, 10-fold by 11–14 years, and 12.5-fold by 15–20 years. As for the relative heart mass, it decreases during infancy and childhood compared to the newborn value (5.86 g/kg) down to 3.50–3.96 g/kg, and subsequently rises back to the initial level. During the aging process, heart mass increases (up to 60–70 years of age) due to Left ventricular myocardial hypertrophy, and then decreases. The aging heart is characterized by the proliferation of subepicardial adipose tissue, thickening of the epicardium, coarsening of the Valves with impaired closure, a decrease in the transverse striation of muscle fibers, and an increase in the size of their nuclei.

Arteries and Veins increase their lumen and wall thickness after birth. Individual layers of arterial walls change differently with age. While in the common iliac artery by age 16 the thickness of the intima and media increases 8- to 9-fold and relatively evenly compared to a newborn, the intima of the Subclavian Artery thickens more than 10-fold over the same period, whereas the media thickens less than 2-fold. As aging progresses, the arterial intima continues to thicken. The capacity of the arterial bed increases due to greater vessel tortuosity. Vein aging is accompanied by compaction of the perivascular connective tissue, thickening of the intima (predominantly at valve locations and venous confluences), and wall deformation resulting in dilations—varicosities, which are especially noticeable in the superficial VEINS OF THE lower extremities.

Lymphatic capillaries in childhood possess a larger absorptive surface area than in maturity. During the aging process, this surface area decreases further. Lymph node dimensions increase over the years while their number decreases. In parietal nodes, the connective tissue content increases, while in visceral nodes it decreases. In both types of nodes, fatty infiltration of the tissues occurs.

Age-related features of The Nervous system. Postnatal myelination of nerve fibers takes place, the mechanisms and Functional Significance of which have already been discussed (see p. 37). While the NERVES OF THE inferior oblique muscle of the head in a 4-month fetus contain 818 myelinated fibers, their number doubles in newborns and triples at 1–2 years of age. With aging, the number of myelinated fibers decreases again, the myelin sheath disintegrates, and peripheral nerves undergo sclerosis. The number of fibers within their composition declines, primarily due to large-diameter fibers. This is associated with a decrease in the number of Nerve Cells during the aging process, mainly resulting from the death of cells possessing thick processes.

Age-related features of internal secretion organs. Hormones of the Endocrine glands regulate growth and development processes (pituitary, thyroid, sex glands), participate in the body's nonspecific protective reaction—stress (Adrenal Glands), affect METABOLISM (parathyroids, the Endocrine portion of the Pancreas), and influence many other processes occurring in the body. Due to high functional significance, many glands mature early and remain morphologically relatively unchanged throughout almost the entire ontogeny. However, certain glands, whose functional activity level declines during aging, also undergo structural changes. The preservation of relative morphological stability with age is characteristic of the pituitary and adrenal glands, whereas progressive structural reorganization is characteristic of the sex, thyroid, and Parathyroid glands.