H-P-O Axis
Human Reproduction and Development in Health and Disease (BI1HRD2)
Instructor and Course Overview
Delivered by Phil Knight
Week 2 Lecture 1-2
Learning Objectives
Describe the components of the H-P-O axis.
Describe and illustrate the gross anatomy of the female external and internal genitalia.
Describe and illustrate ovarian histology including the different stages of folliculogenesis.
Describe and illustrate the anatomy of the mammary gland.
Understand the physiology of the female reproductive system.
Overview of Anatomy
H-P-O axis
External genitalia
Internal genitalia
Vagina
Uterus
Uterine tubes
Ovaries
Breasts
H-P-O Axis
The hypothalamic-pituitary-ovarian (H-P-O) axis is crucial for regulating female reproductive functions.
Components of the H-P-O Axis
Hypothalamus: Produces Gonadotrophin-releasing hormone (GnRH).
GnRH: Stimulates the pituitary gland to release FSH and LH.
Pituitary Gland: Secretes Luteinising Hormone (LH) and Follicle Stimulating Hormone (FSH).
FSH: Stimulates follicle maturation in the ovaries.
LH: Triggers ovulation and promotes the formation of the corpus luteum.
Ovary: Produces sex hormones including Estradiol and Progesterone.
Estradiol: Influences the menstrual cycle; high levels can have both negative and positive feedback effects on FSH and LH.
Progesterone: Low levels allow ovulation while high levels suppress FSH and LH.
External Genitalia (Vulva)
The external features of the female reproductive system, collectively termed the vulva, include:
Labia majora: Hair-covered folds that contain fat, sebaceous, and sweat glands.
Labia minora: Hairless folds that also contain sebaceous and sweat glands.
Vestibule: The area surrounding the vaginal and urethral openings.
Vestibular Glands (Bartholin’s Glands): Secrete fluid to moisten the vaginal opening.
Vaginal Orifice: The opening of the vagina.
Urethral Orifice: The opening of the urethra.
Perineum: The area between the vaginal opening and the anus.
Clitoris: Composed of erectile tissue and contains numerous sensory nerve endings (over 8,000). Internally shaped like an upside-down wishbone.
Clitoral Anatomy
Visible Portion (Glans): Approximately the size and shape of a pea, rich in nerve endings; erectile tissue beneath.
Internal Genitalia
Comprises structures located within the pelvic cavity:
Vagina
Uterus
2 Uterine tubes (Fallopian tubes)
2 Ovaries
Vagina
Description: A fibromuscular tube lined with stratified squamous epithelium, running upwards and backwards.
Length: About 7-9 cm, shorter anterior wall than posterior.
Vagina Structure
Three layers of tissue:
Outer connective tissue layer
Smooth muscle layer
Inner stratified squamous epithelium:
Forms rugae (folds)
Lacks secretory glands, moistened by cervical mucus.
Maintained pH by Lactobacillus acidophilus (4.9 to 3.5).
Uterus
Description: A hollow muscular organ resembling a pear (7.5 cm in length, 2.5 cm walls).
Positioning: Typically anteverted; can be retroverted.
Uterus Anatomy
Three parts:
Fundus: The dome-shaped upper portion.
Body: The main section that narrows at the internal os.
Cervix: The lower section that opens into the vagina.
Uterus Histology
Three layers of tissue:
Perimetrium: Outer connective tissue.
Myometrium: Thick smooth muscle layer responsible for contractions.
Endometrium:
Columnar epithelium with a rich blood supply and dozens of tubular glands.
Contains two layers:
Functional layer: Shed during menstruation.
Basal layer: Remains intact and regenerates the functional layer monthly.
Uterine Tubes (Fallopian Tubes)
Length: Approximately 10 cm, connecting the uterus to the ovaries, encased in the peritoneum.
Diameter: Varies from ~1 mm to 1 cm.
Ends in fimbriae (funnel-like projections near the ovaries).
Uterine Tubes Structure
Three layers of tissue:
Encased in peritoneum
Smooth muscle layer
Inner ciliated epithelium, facilitating movement of the ovum and early conceptus via ciliary action and peristalsis.
Ovaries
Description: Female gonads responsible for producing gametes and sex hormones (about 2.5 x 2 x 1 cm).
Attachment: Connected to the uterus via the ovarian ligament but are loosely associated with the uterine tubes.
Ovarian Structure
Outer Cortex: Contains maturing ovarian follicles and the corpus luteum, covered by surface epithelium.
Inner Medulla: Comprises central fibrous tissue, blood vessels, and nerves.
Follicle Development
Stages:
Primordial Follicle
Primary Follicle
Secondary Follicle
Antral follicle
Preovulatory (dominant) follicle
Ovarian Reserve
Females are born with a finite number of primordial follicles (~1.5 million) which declines over time, leading to approximately 0.3 million at puberty, with few remaining by age 50 leading to menopause.
Female Germ Cell Development
During fetal life:
Primordial Germ Cells (PGCs) become oogonia and subsequently primary oocytes, peaking at ~7 million at 5 months of gestation.
By birth, only ~1.5 million follicles remain, with a further decline by puberty.
Development Stages of Follicles
Terminology:
Primordial Follicle: Containing primary oocyte.
Primary Follicle: Surrounded by a single layer of granulosa cells.
Secondary Follicle: Features multiple layers of granulosa cells.
Antral (Tertiary) Follicle: Characterized by a fluid-filled antrum.
Breasts (Mammary Glands)
Function: Accessory glands of the female reproductive system, primarily for milk production for infant nourishment.
Description: Structurally modified sweat glands, present rudimentarily in males.
Breast Anatomy
Glandular Tissue:
Composed of ~20 lobes, each with lobules responsible for milk production and lactiferous ducts (15-20) channeling milk to the nipple.
Adipose & Fibrous Connective Tissue: Provides structural support and anchors the breast to the chest wall.
Nipples and Areolas
Nipples: Conical protrusions with multiple ducts exiting.
Areolas: Pigmented skin surrounding nipples with smooth muscle and many sebaceous glands to lubricate during lactation.
Key Physiological Functions of the Female Reproductive System
Formation of Ova: Regular ovulatory cycles.
Fertile Environment: Uterine cycle prepares for potential fertilization.
Sex Hormone Production: Estrogen from ovaries.
Sperm Reception: Vagina serves as a receptacle for coitus.
Pregnancy: Uterus provides an environment for fetal growth and development.
Parturition: Childbirth process facilitated by uterine contractions.
Lactation: Milk production for offspring.
Functions of Specific Structures
External Genitalia: Protection, lubrication, and attraction.
Vagina: Receptacle for penis; elastic for childbirth; maintains an anti-microbial environment with low pH.
Cervix Functions
Canal linking vagina to uterine cavity for sperm passage, menstrual discharge, and childbirth.
Secretes mucus with fluctuating pH levels during menstrual cycle:
Alkaline during ovulation to facilitate sperm survival.
Acidic during luteal phase (sperm-hostile).
Uterus Functions
Provides an environment for fertilized ovum and nourishment.
Endometrium: Cycles in thickness in preparation for ovum and provides blood supply for placental development.
Myometrium: Capable of contractions during childbirth.
Uterine Tubes Functions
Typical site for fertilization.
Moves ovum via peristalsis and ciliary action toward the uterus.
Ovaries Functions
Female gonads producing ova, estrogen, and progesterone.
Store and mature follicles with a monthly release of an ovum from a dominant follicle, forming the corpus luteum after ovulation.
Conclusion
A comprehensive understanding of female reproductive anatomy, physiology, and related endocrine functions is essential for studying human reproduction and development within the context of health and disease.