Unit 3: Reproduction and Endocrine Glands

Physiology of Reproduction

Physiology of Male Reproduction

Spermatogenesis

Spermatogenesis is the process of sperm cell (spermatozoa) formation from primordial germ cells in the testes. This intricate process ensures the continuous production of male gametes required for fertilization.

Definition: Spermatogenesis is the complete process of sperm formation, which includes cell division (mitosis and meiosis) and morphological transformation (spermiogenesis), occurring in the seminiferous tubules of the testes.

Steps of Spermatogenesis:

1. Multiplication Phase (Mitosis):
   • Primordial germ cells (2n) in the seminiferous tubules, called spermatogonia, multiply by mitotic divisions throughout life, especially after puberty.
   • Some spermatogonia continue to divide mitotically, while others differentiate into primary spermatocytes.
2. Growth Phase:
   • A spermatogonium grows in size to become a primary spermatocyte (2n), which is diploid and ready to undergo meiosis.
3. Maturation Phase (Meiosis):
   • Meiosis I: Each primary spermatocyte (2n) undergoes the first meiotic division to produce two haploid secondary spermatocytes (n). These cells are still relatively large.
   • Meiosis II: Each secondary spermatocyte (n) rapidly undergoes the second meiotic division to form two haploid spermatids (n). Thus, one primary spermatocyte yields four spermatids.
4. Spermiogenesis (Differentiation):
   • Spermatids (n) are round, non-motile cells. They undergo a process of differentiation called spermiogenesis, where they transform into elongated, motile spermatozoa (sperm).
   • During spermiogenesis, the cell develops a head (containing the nucleus and acrosome), a midpiece (rich in mitochondria), and a tail (for motility).

Hormonal Control of Spermatogenesis:

• Gonadotropin-Releasing Hormone (GnRH): Secreted by the hypothalamus, GnRH stimulates the anterior pituitary.
• Luteinizing Hormone (LH): Secreted by the anterior pituitary, LH acts on the Leydig cells (interstitial cells) in the testes to stimulate the synthesis and secretion of testosterone.
• Follicle-Stimulating Hormone (FSH): Secreted by the anterior pituitary, FSH acts on the Sertoli cells (sustentacular cells) in the seminiferous tubules to stimulate the secretion of androgen-binding protein (ABP) and other factors essential for spermiogenesis.
• Testosterone: An androgen, produced by Leydig cells, is crucial for stimulating spermatogenesis and maintaining male secondary sexual characteristics. It also exerts negative feedback on the hypothalamus and anterior pituitary.
• Inhibin: Secreted by Sertoli cells, Inhibin selectively suppresses FSH secretion from the anterior pituitary, providing negative feedback.

Exam Notes:

• Spermatogenesis begins at puberty and continues throughout a man's life.
• One primary spermatocyte produces four functional sperm.
• Spermiogenesis is a differentiation process, not a cell division.
• Sertoli cells provide nourishment and support to developing sperm.

Physiology of Female Reproduction

Oogenesis

Oogenesis is the process of formation of mature female gametes (ova or egg cells) from primordial germ cells in the ovaries. Unlike spermatogenesis, oogenesis begins during fetal development and is characterized by long periods of arrest.

Definition: Oogenesis is the process by which diploid oogonia develop into a mature haploid ovum within the ovaries, involving mitosis, meiosis, and unequal cytokinesis.

Steps of Oogenesis:

1. Multiplication Phase (Fetal Development):
   • Primordial germ cells in the fetal ovary undergo mitotic divisions to form millions of oogonia (2n). No new oogonia are formed after birth.
2. Growth Phase (Fetal Development):
   • Oogonia grow in size and differentiate into primary oocytes (2n). Each primary oocyte then gets surrounded by a layer of granulosa cells to form a primary follicle.
3. Maturation Phase (Meiosis):
   • Meiosis I: Primary oocytes begin Meiosis I but get arrested at the prophase I stage. This arrest lasts until puberty.
   • Upon puberty, typically one primary oocyte per menstrual cycle resumes meiosis I, producing a large haploid secondary oocyte (n) and a small first polar body (n) due to unequal cytokinesis. The secondary oocyte retains most of the cytoplasm and nutrients.
   • Meiosis II: The secondary oocyte begins Meiosis II but gets arrested at the metaphase II stage. It is released from the ovary during ovulation.
   • Meiosis II is completed only if the secondary oocyte is fertilized by a sperm. Completion yields a mature ovum (n) and a second polar body (n). The first polar body may also divide into two secondary polar bodies.

Comparison of Spermatogenesis and Oogenesis:

Exam Notes:

• Oogenesis produces one viable ovum and polar bodies, which degenerate.
• The primary oocyte is arrested in Prophase I until puberty.
• The secondary oocyte is arrested in Metaphase II until fertilization.

Menstrual Cycle and its Hormonal Control

The menstrual cycle is a series of cyclic changes that occur in the female reproductive system, primarily the uterus and ovaries, preparing the uterus for a potential pregnancy. It typically lasts about 28 days and is regulated by hormones.

Definition: The menstrual cycle is the approximately monthly series of changes in the female body that prepare the uterus for the possibility of pregnancy. It is marked by the cyclic growth and shedding of the uterine lining (endometrium).

The menstrual cycle is broadly divided into ovarian and uterine phases, which occur concurrently. The uterine phases are:

1. Menstrual Phase (Days 1-5):
   • If fertilization does not occur, the corpus luteum degenerates, leading to a sharp decline in progesterone and estrogen levels.
   • This hormonal drop causes the shedding of the thickened uterine lining (endometrium), accompanied by blood, mucus, and tissue. This is menstruation or the period.
2. Proliferative (Follicular) Phase (Days 6-14):
   • Following menstruation, the endometrium begins to regenerate and thicken.
   • In the ovary, under the influence of FSH, several primary follicles mature, and one typically develops into a Graafian follicle. This follicle produces increasing amounts of estrogen.
   • Estrogen stimulates the repair and proliferation of the uterine endometrium, making it thicker and more vascular.
3. Ovulation (Around Day 14):
   • High levels of estrogen from the mature follicle exert a positive feedback on the hypothalamus and anterior pituitary, causing a surge in LH secretion (the LH surge).
   • The LH surge triggers the rupture of the Graafian follicle, releasing the secondary oocyte from the ovary.
4. Secretory (Luteal) Phase (Days 15-28):
   • After ovulation, the ruptured Graafian follicle transforms into the corpus luteum under the influence of LH.
   • The corpus luteum primarily secretes large amounts of progesterone and some estrogen.
   • Progesterone further thickens the uterine lining, making it highly vascular and glandular, preparing it for implantation of a fertilized egg.
   • If pregnancy occurs, the corpus luteum persists (supported by hCG from the embryo) and continues to secrete hormones. If no pregnancy, it degenerates, leading to the menstrual phase again.

Hormonal Control of the Menstrual Cycle:

• Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH) in a pulsatile manner, stimulating the anterior pituitary.
• Anterior Pituitary:
  • Follicle-Stimulating Hormone (FSH): Stimulates follicular growth and estrogen production by ovarian follicles during the follicular phase.
  • Luteinizing Hormone (LH): Triggers ovulation (LH surge) and stimulates the formation and maintenance of the corpus luteum, as well as progesterone and estrogen production during the luteal phase.
• Ovaries:
  • Estrogen: Secreted by developing follicles. Promotes endometrial proliferation, develops secondary sexual characteristics, and exerts both negative (early cycle) and positive (pre-ovulation) feedback on GnRH, FSH, and LH.
  • Progesterone: Secreted primarily by the corpus luteum. Maintains the secretory endometrium, inhibits uterine contractions, and exerts negative feedback on GnRH, FSH, and LH.
  • Inhibin: Secreted by ovarian follicles, suppresses FSH secretion.

Exam Notes:

• FSH primarily drives follicular development, while LH primarily triggers ovulation and corpus luteum formation.
• Estrogen is dominant in the proliferative phase, progesterone in the secretory phase.
• The decline in progesterone and estrogen causes menstruation.

Endocrine Glands: Structure and Function

Endocrine glands are ductless glands that secrete hormones directly into the bloodstream to regulate various bodily functions. This section details the structure and functions of several key endocrine glands.

Pituitary Gland

Often called the "master gland", the pituitary gland is a pea-sized gland located at the base of the brain, inferior to the hypothalamus. It is anatomically and functionally connected to the hypothalamus.

Structure: The pituitary gland consists of two main lobes:

• Anterior Pituitary (Adenohypophysis): Glandular tissue, develops from Rathke's pouch. It synthesizes and secretes its own hormones.
• Posterior Pituitary (Neurohypophysis): Neural tissue, an extension of the hypothalamus. It stores and releases hormones produced by the hypothalamus.

Hormones and Functions:

• Anterior Pituitary Hormones:
  • Growth Hormone (GH): Promotes growth of body tissues, particularly bones and muscles; influences metabolism.
  • Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones.
  • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce glucocorticoids.
  • Follicle-Stimulating Hormone (FSH): In females, stimulates ovarian follicle growth and estrogen production; in males, stimulates spermatogenesis.
  • Luteinizing Hormone (LH): In females, triggers ovulation and corpus luteum formation; in males, stimulates testosterone production by Leydig cells.
  • Prolactin (PRL): Stimulates milk production in mammary glands after childbirth.
• Posterior Pituitary Hormones (synthesized by hypothalamus, released by posterior pituitary):
  • Antidiuretic Hormone (ADH) / Vasopressin: Regulates water balance by increasing water reabsorption in the kidneys.
  • Oxytocin: Stimulates uterine contractions during childbirth and milk ejection (let-down) during lactation.

Exam Notes:

• The hypothalamus controls the anterior pituitary via releasing and inhibiting hormones transported through the hypophyseal portal system.
• The hypothalamus directly communicates with the posterior pituitary via neurosecretory cells.

Thyroid Gland

The thyroid gland is a butterfly-shaped gland located in the neck, anterior to the trachea and inferior to the larynx.

Structure: It consists of two lobes connected by an isthmus. The gland is composed of numerous follicles, which are spheres of follicular cells surrounding a central lumen filled with colloid. Parafollicular cells (C cells) are located between the follicles.

Hormones and Functions:

• Thyroxine (T4) and Triiodothyronine (T3):
  • Produced by follicular cells, these iodine-containing hormones regulate the body's metabolic rate, promoting growth and development, and influencing heart rate, digestion, and body temperature.
  • T3 is more potent than T4, and T4 is often converted to T3 in target tissues.
• Calcitonin:
  • Produced by parafollicular cells (C cells).
  • Lowers blood calcium levels by inhibiting osteoclast activity (bone breakdown) and stimulating calcium excretion by the kidneys.

Exam Notes:

• Iodine is essential for the synthesis of T3 and T4.
• Thyroid hormone secretion is regulated by TSH from the anterior pituitary.
• Hyperthyroidism (e.g., Graves' disease) and hypothyroidism (e.g., Hashimoto's disease, goiter due to iodine deficiency) are common conditions.

Parathyroid Gland

The parathyroid glands are typically four small glands, usually embedded in the posterior surface of the thyroid gland.

Structure: Small, oval glands containing chief cells (which produce PTH) and oxyphil cells (function largely unknown).

Hormone and Function:

• Parathyroid Hormone (PTH):
  • The primary regulator of blood calcium levels.
  • Increases blood calcium by:
  1. Stimulating osteoclasts to break down bone and release calcium into the blood.
  2. Increasing calcium reabsorption by the kidneys.
  3. Promoting the activation of Vitamin D, which in turn increases calcium absorption from the intestine.

Exam Notes:

• PTH acts antagonistically to calcitonin to maintain calcium homeostasis.
• Secretion is directly regulated by blood calcium levels (low calcium stimulates PTH release).

Pancreas

The pancreas is a leaf-shaped gland located posterior to the stomach, extending from the duodenum to the spleen. It has both exocrine and endocrine functions.

Structure: The endocrine portion of the pancreas consists of clusters of cells called the Islets of Langerhans, which are interspersed among the exocrine acinar cells.

Hormones and Functions (from Islets of Langerhans):

• Insulin:
  • Produced by beta (β) cells.
  • Lowers blood glucose levels by promoting glucose uptake by cells, stimulating the conversion of glucose to glycogen (glycogenesis) in the liver and muscles, and promoting fat and protein synthesis.
• Glucagon:
  • Produced by alpha (α) cells.
  • Raises blood glucose levels by stimulating the breakdown of glycogen to glucose (glycogenolysis) in the liver and stimulating the synthesis of glucose from non-carbohydrate sources (gluconeogenesis).

Exam Notes:

• Insulin and glucagon are antagonistic hormones that precisely regulate blood glucose homeostasis.
• Dysfunction of insulin production or action leads to diabetes mellitus.

Adrenal Gland

The adrenal glands are a pair of triangular-shaped glands located superior to each kidney.

Structure: Each adrenal gland consists of two distinct regions:

• Adrenal Cortex: The outer region, derived from mesoderm. It produces steroid hormones (corticosteroids). It is divided into three zones: zona glomerulosa, zona fasciculata, and zona reticularis.
• Adrenal Medulla: The inner region, derived from neural crest cells. It produces catecholamines.

Hormones and Functions:

• Adrenal Cortex Hormones:
  • Mineralocorticoids (e.g., Aldosterone - from zona glomerulosa): Regulate electrolyte balance, primarily by increasing sodium reabsorption and potassium excretion in the kidneys.
  • Glucocorticoids (e.g., Cortisol - from zona fasciculata): Influence metabolism (raising blood glucose), suppress inflammation, and play a role in the stress response.
  • Adrenal Androgens (e.g., DHEA - from zona reticularis): Contribute to secondary sexual characteristics, especially in females.
• Adrenal Medulla Hormones (Catecholamines):
  • Epinephrine (Adrenaline) and Norepinephrine (Noradrenaline): Involved in the "fight or flight" response. They increase heart rate, blood pressure, blood glucose levels, and blood flow to muscles, preparing the body for acute stress.

Exam Notes:

• The adrenal cortex is regulated by ACTH from the anterior pituitary.
• The adrenal medulla is regulated by the sympathetic nervous system.
• Cortisol and adrenaline are key stress hormones.