Biology: Reproductive Process and Embryo Development

Fertilization and Cleavage

• Fertilization is the process where a sperm cell and an egg cell (oocyte) merge to form a zygote.
• Cleavage is a series of rapid mitotic divisions that the zygote undergoes, leading to the formation of a multicellular embryo.

Layers of the Blastocyst

• The blastocyst consists of two main layers: the inner cell mass and the trophoblast.
• The inner cell mass develops into the embryo.
• The trophoblast develops into the placenta and associated structures.

Implantation

• Implantation typically occurs about 6-10 days after fertilization.

Hormone Regulation

• The corpus luteum secretes progesterone (and some estrogen) until the placenta develops and takes over hormone production.

Fate of the Blastocyst Layers

• The inner cell mass develops into the embryo.
• The trophoblastic layer develops into the chorion, part of the placenta.

Gastrulation

• Gastrulation is a process during embryonic development that reorganizes the embryo into a layered structure, forming three primary germ layers.

The Three Germ Layers

• The three primary germ layers are: ectoderm, mesoderm, and endoderm.

Layers of the Blastocyst

• The blastocyst consists of two main layers.
• The inner cell mass develops into the embryo.
• The trophoblast develops into the placenta and associated structures.

Placenta Formation

• The chorionic villi, arising from the trophoblast, form the fetal portion of the placenta

Nutrient and Waste Transport

• The placenta facilitates the exchange of nutrients and waste between the mother and fetus
• The mother's blood supplies nutrients and oxygen to the fetus through the placenta
• Fetal waste products are removed through the mother's circulatory system via the placenta

Classification of Hormones

• Steroids: derived from cholesterol, lipid-soluble (e.g., cortisol, estrogen, testosterone)
• Peptides/Proteins: made of amino acids, water-soluble (e.g., insulin, growth hormone)
• Monoamines: derived from amino acids, water-soluble (e.g., epinephrine, norepinephrine, thyroid hormones)

Mechanism of Action of Hormones

Protein Hormones

• Bind to receptors on the cell membrane
• Require a second messenger system (e.g., cAMP) to relay the signal inside the cell

Steroid Hormones

• Pass through the cell membrane and bind to intracellular receptors
• Directly affect gene expression without a second messenger

Hormones Secreted by the Hypothalamus

• TRH, CRH, GnRH, GHRH, PIH, Somatostatin, Oxytocin, ADH

Hormones Secreted by the Anterior Pituitary Gland

• FSH, LH, ACTH, TSH, PRL, GH

Hormones Secreted by the Posterior Pituitary Gland

• Oxytocin, ADH (Vasopressin)
• Produced in the hypothalamus and stored and released by the posterior pituitary gland

Effects of Tumors in the Hypothalamus or Pituitary Gland

• Can lead to hyposecretion or hypersecretion of hormones
• Affects growth, metabolism, and other bodily functions
• Specific effects depend on the hormones involved

Hormones of the Anterior Pituitary Gland

• FSH (Follicle-Stimulating Hormone) is secreted from the anterior pituitary gland
• LH (Luteinizing Hormone) is secreted from the anterior pituitary gland
• ACTH (Adrenocorticotropic Hormone) is secreted from the anterior pituitary gland
• TSH (Thyroid-Stimulating Hormone) is secreted from the anterior pituitary gland
• PRL (Prolactin) is secreted from the anterior pituitary gland
• GH (Growth Hormone) is secreted from the anterior pituitary gland

Hormones of the Posterior Pituitary Gland

• Oxytocin is secreted from the posterior pituitary gland
• ADH (Vasopressin) is secreted from the posterior pituitary gland

Production of Posterior Pituitary Hormones

• Posterior pituitary hormones are produced in the hypothalamus
• Posterior pituitary hormones are stored and released by the posterior pituitary gland

Steroids

• Derived from cholesterol and are lipid-soluble
• Examples include cortisol, estrogen, and testosterone
• Pass through the cell membrane and bind to intracellular receptors, directly affecting gene expression without a second messenger

Peptides/Proteins

• Made of amino acids and are water-soluble
• Examples include insulin and growth hormone
• Bind to receptors on the cell membrane and activate a second messenger system
• Classified as hormones that interact with their target cells by binding to receptors on the cell membrane and activating a second messenger system

Monoamines

• Derived from amino acids and are water-soluble
• Examples include epinephrine and norepinephrine
• Classified as hormones that interact with their target cells by binding to receptors on the cell membrane and activating a second messenger system

Hormone Classification

• Shared characteristic of peptides/proteins and monoamines: water-solubility
• Steroids are derived from cholesterol and are lipid-soluble, whereas peptides/proteins and monoamines are derived from amino acids and are water-soluble

Growth Hormone

• Excessive growth hormone before puberty leads to Gigantism, characterized by excessive growth.
• Excessive growth hormone after puberty leads to Acromegaly, characterized by enlarged bones in hands, feet, and face.

Thyroid Hormone (TH)

• TH includes T3 and T4, which regulate metabolism, growth, and development.
• Thyroid gland functions also include the secretion of Calcitonin, which lowers blood calcium levels.

Hyperthyroidism

• Hyperthyroidism is caused by the overproduction of thyroid hormones, often due to Graves' disease or thyroid nodules.
• Graves' disease is an autoimmune disorder that leads to hyperthyroidism, characterized by goiter, exophthalmos, and other symptoms.
• In Graves' disease, autoantibodies mimic TSH, stimulating excessive thyroid hormone production.

Oogenesis

• Meiosis I is the type of cell division that occurs when a primary oocyte divides to become a secondary oocyte.
• At the completion of meiosis I, one secondary oocyte and one polar body are produced.
• The secondary oocyte is released during ovulation, paused in Metaphase II of meiosis.

Ovulation and Corpus Luteum

• After ovulation, the tissue that surrounds the oocyte transforms into the corpus luteum.
• The corpus luteum is a temporary endocrine structure that secretes progesterone and estrogen.

Hormones and Phases of Menstrual Cycle

• Estrogen causes thickening of the uterine lining during the proliferative phase.
• Progesterone causes thickening of the uterine lining during the secretory phase.
• Rising levels of LH (Luteinizing hormone) stimulate ovulation.
• Ovulation typically occurs on day 14 of a 28-day cycle.

Menstrual Cycle Phases

• Menses ( menstrual bleeding) occurs on days 1-5 of the menstrual cycle.
• The proliferative phase occurs on days 6-14, ending with ovulation.
• The follicular phase occurs from day 1 to day 14 of the menstrual cycle.

Hormonal Maintenance and Decline

• Progesterone and estrogen are necessary to maintain the uterine lining in preparation for embryo implantation.
• A decline in progesterone and estrogen levels leads to the shedding of the functionalis layer of the endometrium.

FSH Function

• FSH (Follicle-stimulating hormone) stimulates the growth of ovarian follicles.

Thyroid Disorders

• Endemic goiter is a thyroid enlargement caused by iodine deficiency, leading to insufficient thyroid hormone production.
• Iodine is a necessary mineral for producing thyroid hormones.

Hypothyroidism

• Hypothyroidism is a condition characterized by insufficient production of thyroid hormones.
• Causes of hypothyroidism include Hashimoto's thyroiditis, iodine deficiency, and other factors.

Myxedema

• Myxedema is a severe form of hypothyroidism in adults, characterized by swelling, especially in the face and extremities.

Cretinism

• Cretinism is a condition caused by congenital hypothyroidism.
• Symptoms of cretinism include stunted physical and mental growth.

Parathyroid Hormones

• Parathyroid hormones (PTH) regulate calcium levels by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and activating vitamin D.

Adrenal Cortex Hormones

• Mineralocorticoids (e.g., aldosterone) regulate electrolyte and water balance.
• Glucocorticoids (e.g., cortisol) regulate metabolism and stress response.
• Androgens are sex hormones.

Cortisol

• Cortisol is increased during stress and has anti-inflammatory properties.

Cushing's Disease

• Cushing's disease is a condition caused by excessive cortisol production or exposure to high levels of cortisol.
• Causes of Cushing's disease include pituitary adenoma, ectopic ACTH production, or long-term corticosteroid therapy.
• Symptoms of Cushing's disease include weight gain, thinning skin, hypertension, and glucose intolerance.

Cushing's Disease

• Cushing's disease is caused by three main factors: pituitary adenoma, ectopic ACTH production, or long-term corticosteroid therapy.

Symptoms of Cushing's Disease

• Weight gain is a key symptom of Cushing's disease.
• Thinning skin is another common symptom.
• Hypertension is a symptom of Cushing's disease.
• Glucose intolerance is a symptom of Cushing's disease.

Adrenal Medulla Hormones

• The adrenal medulla produces two essential hormones: Epinephrine and Norepinephrine.

Epinephrine Production

• Epinephrine is produced in the adrenal medulla.

Other Endocrine Glands and Their Functions

• Pineal Gland: Regulates sleep cycles through melatonin production
• Thymus: Involved in T-cell maturation, but its function diminishes with age
• Pancreas: Regulates blood glucose levels

Disorders of Other Endocrine Glands

• Pineal Gland: Disorders include sleep disturbances
• Pancreas: Disorders include diabetes mellitus

How Hormone Imbalance Affects Hormone Levels

Hyposecretion (Hormone Deficiency)

• Leads to deficiency symptoms, such as:
  • Hypothyroidism
  • Adrenal insufficiency

Hypersecretion (Hormone Excess)

• Leads to excess symptoms, such as:
  • Hyperthyroidism
  • Cushing's syndrome

Diabetes

• IDDM (Type 1 Diabetes) is characterized by autoimmune destruction of beta cells, requiring insulin replacement.
• NIDDM (Type 2 Diabetes) is marked by insulin resistance and can be managed through diet, exercise, and sometimes medication.

Insulin and Glucagon Functions

• Insulin lowers blood glucose levels by facilitating cellular uptake of glucose.
• Glucagon raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver.

Diabetes

• IDDM (Type 1 Diabetes) is characterized by autoimmune destruction of beta cells, requiring insulin replacement.
• NIDDM (Type 2 Diabetes) is marked by insulin resistance and can be managed through diet, exercise, and sometimes medication.

Insulin and Glucagon Functions

• Insulin lowers blood glucose levels by facilitating cellular uptake of glucose.
• Glucagon raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver.

Glucagon's Effect on Blood Glucose Levels

• Increases blood glucose levels by stimulating two key processes in the liver: glycogenolysis and gluconeogenesis.
• Glycogenolysis is the breakdown of glycogen, a complex carbohydrate stored in the liver, into glucose.
• Gluconeogenesis is the generation of new glucose molecules from non-carbohydrate sources, such as amino acids and lactate.