Puberty Defining - Anatomy & Physiology

Introduction

Generally, puberty can be defined in both sexes as the ability to accomplish reproduction. The fundamental requirement for the onset of puberty is the secretion of GnRH at the appropriate frequency and quantity to stimulate gonatotrophin release by the anterior pituitary gland. Gonadotrophins promote:

• Gametogenesis
• Steroidogenesis
• Development of reproductive organs

The degree of function of neurons in the hypothalamus that secrete GnRH increases as puberty begins. The function of these neurons may be influenced by:

• Plane of nutrition
• Exposure to certain environmental or social cues
• Genetics

Male

Age when Behavioural Traits are Expressed

• Generally acquire reproductive behavioural traits such as mounting and erection a long time before they acquire the ability to ejaculate and produce spermatozoa.
• Behavioural traits are easy to determine, as mounting behaviour and erection can be easily observed.

Age at First Ejaculation

• Ejaculation is a complex process,requiring coordinated development of nerves, specific muscles and secretion of seminal fluids from the accessory sex glands.
• When development of all of these components occurs, ejaculation can take place.
• Generally the ability to ejaculate preceeds the ability to produce sufficient spermatozoa to achieve fertilization.

Age when Spermatozoa First Appear in the Ejaculate

• Males acquire the ability to produce seminal fluid and ejaculate before spermatozoa are available to be ejaculated.
• To determine when spermatozoa first appear, the ejaculate must be collected at least once per week.
  • Easy to collect using an artificial vagina from the the boar, bull, dog, ram or stallion.
• After behavioural characteristics have developed and the male is willing to mount a receptive female/surrogate, frequent seminal collection is possible.
  • Enables determination of the age at which spermatozoa first appear in the ejaculate.

Age when Spermatozoa First Appear in the Urine

• Most spermatozoa produced are lost in the urine during periods of sexual abstinence.
• The presence of sperm in the urine indicates that spermatogenesis is occuring.
• Frequent urine collection is difficult in large domestic species and requires special equipment, so this method of determining pubertal onset has limitations.

Age when the Ejaculate Contains a Threshold Number of Spermatozoa

• Even if the ejaculate contains spermatozoa, there may be insufficient numbers for optimum fertilization.
• The presence of a threshold (minimum number) of spermatozoa is required. These thresholds vary between species.
• Reflect minimum seminal characteristics required to achieve pregnancy following copulation.
• Practically, this is the most valid criterion for puberty in the male.

Female

Age at First Oestrus

• The age that the female becomes sexually receptive and displays the first oestrus.
• Easy to determine because females show outward behavioural signs of sexual receptivity, especially in the presence of a male.
• The first ovulation is usually not accompanied by behavioural oestrus in heifers and ewes and is hence termed 'silent ovulation'. Thus, the age at first oestrus may not accurately reflect acquisition of puberty.

Age at First Ovulation

• The age at which ovulation first occurs.
• Manual or visual validation is required. This can be accomplished by palpation of the ovary per rectum in animals that are large enough to permit insertion of a hand/arm into the rectum (cow and camelids).
• Ultrasound can be used most successfully in the mare to determine ovarian status.
• When ovulation has occured, a soft depression on the surface of the ovary can be palpated.
• In smaller animals (sow, ewe, bitch, queen) surgical procedures are necessary to allow visualisation of the ovary in order to determine ovulation. In addition laparoscopic observation can be used to determine when ovulation occurs.
• All techniques require frequent observation of the ovary, so although age at ovulation is a good criterion for puberty, it is difficult to determine.

Age at which the Female can Support Pregnancy without Deleterious Effects

• Most practical definition in all domestic animals.
• Acquisition of a threshold body size is important in controlling onset of puberty.
• Energy requirements for follicular development and ovulation are small, however the metabolic cost of pregnancy and lactation are high so the female must reach a 'metabolic threshold' before puberty can occur.